Abstract Opportunistic infections and unusual tumors have been reported in an unprecedented outbreak of community-acquired cellular immune deficiency among homosexual and drug-abusing men. We repor...

"Plasmacytoid dendritic cell and CD4 + T cell deficiencies in untreated Hodgkin disease: implications for susceptibility to opportunistic infections." Leukemia & Lymphoma, 55(11), pp. 2656–2657

Myeloid and plasmacytoid dendritic cells (DCs) are important mediators of both innate and adaptive immunity against pathogens such as HIV. During the course of HIV infection, blood DC numbers fall substantially. In the present study, we sought to determine how early in HIV infection the reduction occurs and whether the remaining DC subsets maintain functional capacity. We find that both myeloid DC and plasmacytoid DC levels decline very early during acute HIV in-fection. Despite the initial reduction in numbers, those DCs that remain in circulation retain their function and are able to stimulate allogeneic T-cell responses, and up-regulate maturation markers plus produce cytokines/chemokines in response to stimulation with TLR7/8 agonists. Notably, DCs from HIV-infected subjects produced significantly higher levels of cytokines/chemokines in response to stimulation with TLR7/8 agonists than DCs from uninfected controls. Further examination of gene expression profiles indicated in vivo activation, either directly or indirectly, of DCs during HIV infection. Taken together, our data demonstrate that despite the reduction in circulating DC numbers, those that remain in the blood display hyperfunctionality and implicates a possible role for DCs in promoting chronic immune activation.

The primate T-cell lymphoma viruses (PTLV) are divided into six distinct species. The biology and epidemiology of PTLV-1 and PTLV-2 are very well understood. However, that of PTLV-3, 4, 5, and 6 are not. Recently, in Cameroon, three and one humans were shown to be infected with HTLV-3 and HTLV-4, respectively. We undertook a study to ascertain whether any of these two retroviruses were present in the peripheral blood mononuclear cell DNA of New York State subjects deemed at risk for PTLV infection. Samples were analyzed by PTLV-3 and PTLV-4 specific PCR assays from the following human and simian subject types: African-American medical clinic patients; HTLV EIA+, WB indeterminate blood donors; intravenous drug users; patients with leukemia, lymphoma, myelopathy, polymyositis, or AIDS; and African chimpanzees. None of the 1200 subjects was positive for HTLV-3 or 4. The data indicate that, at the time of sample collection, no evidence exists for the dissemination of HTLV-3 or 4 to New York State. Continued epidemiological studies are warranted to explore the worldwide prevalence rates and dissemination patterns of HTLV-3 and 4 infections, and their possible disease associations.

American Journal of HematologyVolume 83, Issue 12 p. 937-938 Correspondence and Letter to the EditorFree Access Therapy-related leukemia in patients with human immunodeficiency virus infection after treatment for non-Hodgkin lymphoma Jed A. Katzel, Jed A. Katzel Department of Hematology and Oncology, Saint Vincent's Hospital, Manhattan and New York Medical College, New YorkSearch for more papers by this authorSanford J. Kempin, Sanford J. Kempin Saint Vincent's Comprehensive Cancer Center, New York, New YorkSearch for more papers by this authorPortia Lagmay-Fuentes, Portia Lagmay-Fuentes Saint Vincent's Comprehensive Cancer Center, New York, New YorkSearch for more papers by this authorWilliam A. Cook, William A. Cook Saint Vincent's Comprehensive Cancer Center, New York, New YorkSearch for more papers by this authorFrederick P. Siegal, Frederick P. Siegal Comprehensive HIV Center, Saint Vincent's Hospital, Manhattan, New YorkSearch for more papers by this authorLizette E. Henriquez, Lizette E. Henriquez Department of Pathology, Saint Vincent's Hospital, Manhattan, New YorkSearch for more papers by this authorMoon H. Lee, Moon H. Lee Department of Pathology, Saint Vincent's Hospital, Manhattan, New YorkSearch for more papers by this authorDavid H. Vesole, David H. Vesole Division of Hematology and Oncology, Loyola University Chicago, Stritch School of Medicine, Maywood, IllinoisSearch for more papers by this author Jed A. Katzel, Jed A. Katzel Department of Hematology and Oncology, Saint Vincent's Hospital, Manhattan and New York Medical College, New YorkSearch for more papers by this authorSanford J. Kempin, Sanford J. Kempin Saint Vincent's Comprehensive Cancer Center, New York, New YorkSearch for more papers by this authorPortia Lagmay-Fuentes, Portia Lagmay-Fuentes Saint Vincent's Comprehensive Cancer Center, New York, New YorkSearch for more papers by this authorWilliam A. Cook, William A. Cook Saint Vincent's Comprehensive Cancer Center, New York, New YorkSearch for more papers by this authorFrederick P. Siegal, Frederick P. Siegal Comprehensive HIV Center, Saint Vincent's Hospital, Manhattan, New YorkSearch for more papers by this authorLizette E. Henriquez, Lizette E. Henriquez Department of Pathology, Saint Vincent's Hospital, Manhattan, New YorkSearch for more papers by this authorMoon H. Lee, Moon H. Lee Department of Pathology, Saint Vincent's Hospital, Manhattan, New YorkSearch for more papers by this authorDavid H. Vesole, David H. Vesole Division of Hematology and Oncology, Loyola University Chicago, Stritch School of Medicine, Maywood, IllinoisSearch for more papers by this author First published: 19 September 2008 https://doi.org/10.1002/ajh.21290Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat References 1 McMaster M,Greer J,Greco A, et al. Effective treatment of small-noncleaved-cell lymphoma with high-intensity, brief-duration chemotherapy. J Clin Oncol 1991; 9: 941–946. 2 Magrath I,Adde M,Shad A, et al. Adults and children with small non-cleaved-cell lymphoma have a similar excellent outcome when treated with the same chemotherapy regimen. J Clin Oncol 1996; 14: 925–934. 3 Clarkson B,Gaynor J,Little C, et al. Importance of long-term follow-up in evaluating treatment regimens for adults with acute lymphoblastic leukemia. Haematol Blood Transfus 1990; 33: 397–408. 4 Wogan GN. Does perinatal antiretroviral therapy create an iatrogenic cancer risk? Environ Mol Mutagen 2007; 48: 210–214. 5 Olivero OA,Fernandez JJ,Antiochos BB, et al. Transplacental genotoxicity of combined antiretroviral nucleoside analogue therapy in Erythrocebus patas monkeys. J Acquir Immune Defic Syndr 2002; 29: 323–329. Citing Literature Volume83, Issue12December 2008Pages 937-938 ReferencesRelatedInformation

Therapy related secondary malignancies after NHL are well characterized in the HIV negative population. The increased risk of secondary leukemia is most commonly associated with alkylating agents, topoisomerase II inhibitors and radiation therapy. We describe 2 patients with HIV-associated Burkitt's lymphoma who subsequently developed acute leukemia.

Optimal HIV vaccines should elicit CD8 + T cells specific for HIV proteins presented on MHC class I products, because these T cells contribute to host resistance to viruses. We had previously found that the targeting of antigen to dendritic cells (DCs) in mice efficiently induces CD8 + T cell responses. To extend this finding to humans, we introduced the HIV p24 gag protein into a mAb that targets DEC-205/CD205, an endocytic receptor of DCs. We then assessed cross-presentation, which is the processing of nonreplicating internalized antigen onto MHC class I for recognition by CD8 + T cells. Low doses of αDEC-gag, but not control Ig-gag, stimulated proliferation and IFN-γ production by CD8 + T cells isolated from the blood of HIV-infected donors. αCD205 fusion mAb was more effective for cross-presentation than αCD209/DC-SIGN, another abundant DC uptake receptor. Presentation was diverse, because we identified eight different gag peptides that were recognized via DEC-205 in 11 individuals studied consecutively. Our results, based on humans with highly polymorphic MHC products, reveal that DCs and DEC-205 can cross-present several different peptides from a single protein. Because of the consistency in eliciting CD8 + T cell responses, these data support the testing of αDEC-205 fusion mAb as a protein-based vaccine.

Dendritic cells (DCs) undergo maturation during virus infection and thereby become potent stimulators of cell-mediated immunity. HIV-1 replicates in immature DCs, but we now find that infection is not accompanied by many components of maturation in either infected cells or uninfected bystanders. The infected cultures do not develop potent stimulating activity for the mixed leukocyte reaction (MLR), and the DCs producing HIV-1 gag p24 do not express CD83 and DC-lysosome-associated membrane protein maturation markers. If different maturation stimuli are applied to DCs infected with HIV-1, the infected cells selectively fail to mature. When DCs from HIV-1-infected patients are infected and cultured with autologous T cells, IL-10 was produced in 6 of 10 patients. These DC-T cell cocultures could suppress another immune response, the MLR. The regulation was partially IL-10-dependent and correlated in extent with the level of IL-10 produced. Suppressor cells only developed from infected patients, rather than healthy controls, and the DCs had to be exposed to live virus rather than HIV-1 gag peptides or protein. These results indicate that HIV-1-infected DCs have two previously unrecognized means to evade immune responses: maturation can be blocked reducing the efficacy of antigen presentation from infected cells, and T cell-dependent suppression can be induced.

Antigen presenting dendritic cells (DCs) can serve as sites for HIV replication and as vehicles for transmission of the virus to T cells. It is known that the numbers of DCs in blood is reduced during HIV-1 infection. Here we monitored the two major subsets of blood DCs in 12 individuals undergoing a change, primarily initiation, of highly active antiretroviral therapy. The numbers of plasmacytoid DCs were reliably higher on therapy, although in the 1–3 month interval we followed, these numbers did not return to those seen in HIV uninfected controls. An increase in plasmacytoid DCs was accompanied by an increase in IFN-α production in response to a standard challenge in culture with UV-inactivated herpes simplex virus. The levels of myeloid DCs also demonstrated an increase while on HAART, and these numbers become comparable to the HIV uninfected controls. The numbers of plasmacytoid and myeloid DCs varied inversely with the levels of plasma HIV viremia. These longitudinal studies extend prior work showing that virus infection with HIV leads to a decrease in the number of dendritic cells in blood, and that this can be reversed at least in part by therapy.

Chronically HIV‐1‐infected patients fail to contain their viremia despite high frequencies of HIV‐1‐specific, IFN‐γ‐producing CD8+ T cells. However, these cells are known to exhibit both phenotypic and functional defects. We tested if mature dendritic cells (DC) could correct defective HIV‐1 gag‐specific T cell responses and if responses to other viral antigens were comparably affected. The circulating gag‐specific CD8+ T cells in fresh blood reliably produced IFN‐γ but lacked IL‐2 and high perforin levels and failed to expand significantly during culture with mature DC presenting HIV‐1 gag peptides. In contrast, CD8+ T cells from long‐term nonprogressors contained gag‐specific IFN‐γ and IL‐2 double producers, and the numbers of IFN‐γ producers expanded ∼15‐fold during culture with DC. DC from chronically infected patients could expand IFN‐γ‐ and IL‐2‐producing cells specific for influenza, cytomegalovirus and Epstein Barr virus, and the expansions were comparable to those in healthy donors. When the proliferative capacity of CD8+ T cells from progressor patients was assessed by CFSE dilution, proliferation to other viral antigens was more vigorous than to HIV‐1 gag. Therefore, monocyte‐derived DC from HIV patients present viral antigens effectively, but there is a selective inability to expand CD8+ IFN‐γ‐producing and IFN‐γ and IL‐2 double‐producing T cells when challenged with HIV‐1 gag.

Clinical observations in the natural history of acquired immunodeficiency syndrome (AIDS) and other immunodeficiencies have suggested a role for certain interferon (IFN)-producing cells (originally termed NIPC) in the host defense against opportunistic infection (OI). Identification of these cells with the previously described enigmatic cells resident in thymus and T cell areas of lymphoid tissues has led to improved understanding of mechanisms of induction of Th-1 immunity. The NIPC, now referred to as plasmacytoid pre-dendritic cells or plasmacytoid dendritic cells (pDC), may carry human immunodeficiency virus (HIV)-1 from the periphery into contact with immature T cells in lymphoid tissue, leading to infection of the T cells and selective ablation of the Th-l pathway. Progressive losses of pDC numbers and function during the course of HIV infection may eventually deprive the Th-1 pathway of essential IFN-α signaling, in turn needed for an interleukin-12 (IL-12) mediated IFN-γ response. Infection of the thymus by HIV-1 is both resisted, and later probably enhanced by IFN-α locally generated by HIV-stimulated and HIV-infected pDC. The resulting thymic pathology would then lead to failure of peripheral T cell repopulation. These pDC-related processes probably contribute to the pathogenesis of AIDS and explain the original clinical observations relating the IFN-production deficit to susceptibility to OI.

Fifty-one patients with human immunodeficiency virus infection and acute coronary syndromes were identified. Nearly all patients (98%) had traditional coronary risk factors. Revascularization procedures were performed safely with low in-hospital mortality.

Introduction The innate immune system is composed of many different recognition and effector components including natural killer (NK) cells, interferon-producing cells (IPC), the complement system, lysozyme, dendritic cells (DC), phagocytic cells and many others (Table 1)[1]. The main function of these components is to prevent and control infection by microorganisms. These innate immune mechanisms are activated directly by microorganisms through broadly specific recognition systems; components of many of these systems are thought to have developed earlier in evolution than antigen-specific T-cell and B-cell receptor molecules that characterize adaptive immunity. Innate immune responses may be especially important in controlling pathogens during the early period of infection, before primary or secondary adaptive immune responses (i.e., specific clonal selection and expansion) have developed. However, innate immune responses continue to be active after the development of antigen-specific cells and during chronic infections. They may be expanded during adaptive immune responses [e.g., NK cells responding to helper T-cell-derived interleukin (IL)-2]. Some innate immune responses (e.g., plasmacytoid IPC) may also provide a milieu supportive of the development of specific types of adaptive immune responses and, as such, provide an essential bridge between the natural and adaptive cellular immune responses.Table 1: Components of the innate immune systema.The effects of innate immunity during HIV infection have been extensively studied in three areas: NK and gamma/delta T cells; IPC; and the complement system. Many other areas of study of how innate immunity functions during HIV infection remain relatively unexplored. For example, at the time this review was prepared, only one study of toll receptors and HIV, and three studies of defensins and HIV had been published. The role played by NK and gamma/delta T cells during HIV infection was extensively reviewed in these pages [2]. This review therefore focuses specifically on recent studies of the role of the complement system and of IPC during HIV infection. HIV and the complement system The complement system is an important component of both innate and adaptive immune responses to viruses and other microorganisms, and is composed of approximately 30 proteins found in blood and on cell surfaces. There are three complement activation pathways: classical, alternative, and lectin. The alternative pathway is activated spontaneously at the surface of microbes due to a lack of regulatory factors, whereas the lectin pathway is activated by binding of the C-type lectin, mannose binding lectin (MBL), to carbohydrates on microbes. These two pathways are therefore classified as innate immune responses, whereas the classical complement pathway, in contrast, is not since it is generally activated by antibody. Homologs of proteins from the mammalian alternative and lectin pathways are found in echinoderms and protochordates, suggesting that these pathways evolved long before adaptive immunity [3]. Many of the complement proteins are normally present in an inactive form in blood and are activated in a cascade after exposure to microbes. Activation results in three important effector functions: opsonization of microorganisms for uptake by phagocytic cells; stimulation and chemotaxis of phagocytic cells; and lysis of microorganisms. The interactions of viruses with the complement system during infection are numerous and complex (reviewed in [4]). A comparison of HIV with other viruses highlights several interesting points. There is evidence that all three complement pathways participate in inactivation of certain viruses, although many viruses have evolved mechanisms to avoid complement activation. While some viruses, such as herpes simplex, encode proteins that inactivate complement, HIV and SIV instead incorporate host cell complement regulatory proteins during budding from cells, which increases their resistance to complement-mediated destruction. Some viruses, such as Epstein-Barr, infect cells primarily through cell-surface complement receptors. In contrast, as reviewed later, HIV utilizes CD4 cells as a receptor, but replication of HIV is increased after interaction with complement receptors on certain cells. The interaction of human complement with both animal and human viruses has importance for gene therapy with virus vectors and, since HIV is under intense study as a vector, this field of is of great interest. Finally, based on evidence in mouse models, complement is likely to be involved in the development and maturation of antibody responses to HIV in humans. The complement system is highly activated during HIV infection, as determined by high levels of complement breakdown products in blood (reviewed in [5]). Some of this activation is probably due to microorganisms associated with opportunistic infections (OI). However, at least a portion of the complement activation during HIV infection is due to the interaction of HIV or HIV-infected cells with complement since virus in infected individuals is coated with complement activation products, notably C3 fragments [6-9]. The complement pathway (or pathways) that leads to the in vivo coating of virus with complement has not been defined. Also, all of the effects that in vivo coating with complement has on virus are not known, although there is evidence that activation results in destruction and opsonization of some of the virus [6,8]. In vitro, T-cell-adapted strains of HIV are destroyed by complement under conditions where sufficient levels of antiviral antibodies are present [6,10,11]. Primary isolates of HIV are much more resistant to destruction by complement, probably due to both the relatively low amount of antibody that binds to primary isolate virions [11] and incorporation of three different types of complement control proteins [4]. In the absence of specific antiviral antibodies, HIV virions activate the alternative and lectin pathways in vitro[5,12,13]. Also, in vitro, HIV appears to activate the classical pathway independent of antibody by direct binding of C1 to the virus [14,15], thus representing a fourth way in which HIV activates the complement system (Table 2). This antibody-independent mechanism for complement activation has also been observed with other retroviruses [16,17].Table 2: Complement pathways activated by HIV.Activation of alternative or lectin complement pathways in plasma or serum by HIV generally does not lead to inactivation/neutralization of HIV [18]. A recent study showed, however, that pseudotyping of HIV with the vesicular stomatitis virus-G envelope protein resulted in inactivation by complement in the absence of antibodies [19], suggesting that the vesicular stomatitis virus-G protein activates an innate complement pathway. Activation of the classical pathway in the presence of antiviral antibodies can cause virus lysis and inactivation [5]. Speth et al.[20] recently showed that exposure of brain astrocytes to HIV results in production of complement components. They speculated that complement production by these cells could contribute to HIV-associated neurodegeneration. The importance of complement receptor 2 on cells for pathogenesis Several recent studies reveal that an important consequence of activation of complement by HIV is formation of HIV immune complexes (HIVIC) that are infectious for T cells when bound to cells that express complement receptors. Jakubik et al.[21,22] showed that B lymphocytes isolated from blood or tonsils bound relatively high amounts of HIVIC. HIVIC capable of binding to B cells were formed both in the absence and presence of anti-HIV antibodies. The principal receptor on B cells for HIVIC was complement receptor 2 (CR2, CD21), which binds an activation fragment of complement component C3 that covalently attaches to microbes during complement activation. After binding to B cells, the majority of the HIVIC remained on the cell surface for as long as 3 days, as determined by virus sensitivity to protease. Importantly, as long as they were bound to the B cells, HIVIC were infectious for T cells. Infection of T cells by HIVIC that were bound to B cells resulted in five to 10 times more virus replication than an equivalent amount of cell-free HIVIC. This enhanced infection of T cells appeared to be due to B cell-T cell adhesion mediated by lymphocyte function associated-1/intercellular adhesion molecule-1 interactions that resulted in efficient transfer of HIV to the T cells [23]. Studies by Doepper et al.[24] also showed that tonsil B cells bind HIVIC and that binding is mainly dependent on CR2. Binding of HIVIC to tonsil B cells enhanced the infection of activated autologous tonsil T cells over infection of cell-free HIVIC. Surprisingly, infection of unstimulated T cells was also enhanced. Moir et al.[8] provided evidence that HIV is in fact associated with B cells isolated from peripheral blood and lymph nodes of HIV-infected persons. The virus was located at the B cell surface and could be eluted from cells by incubation with antibody to CR2, strongly suggesting it was in the form of an immune complex bound to CR2. The virus bound to B cells was infectious since co-culture of B cells with T cells resulted in virus replication. Numerous studies have shown that follicular dendritic cells (FDC) in lymph nodes of HIV-infected persons have HIV bound to their surface. In fact, it is estimated that the majority of virus in the body is attached to FDC and, as such, this is thought to be a potentially important reservoir of virus for infection of T cells in lymph nodes. However, the identity of receptors on FDC that were critical for binding HIV was not previously known. Kacani et al.[9] isolated tonsil cells from HIV-infected persons and showed that virus bound to FDC was released from the cell surface after incubation with antibody to CR2 but not with antibody to other complement receptors. Antibody to C3 fragments also released virus from FDC. These findings showed that, in infected persons, FDC bind HIV through CR2 and provided evidence that the FDC-bound virus is in the form of an immune complex that has activated complement. Another recent study [25] indicated that HIVIC bound to FDC can remain infectious for an extraordinarily long time. HIV and antibody were injected into mice and at various times after injection, FDC were isolated from the draining lymph nodes and cultured with T cells. Infectious virus was observed as long as 9 months after injection of virus. Since mouse cells do not become infected and consequently do not produce HIV, these observations indicate that infectious HIV can remain bound to the surface of FDC for this long period. The aforementioned studies showed that CR2 on B cells and FDC was important for binding of HIVIC. A study with a T-cell line showed that soluble complement receptors, specifically complement receptor 1 (CD35), can be utilized to inhibit formation of HIVIC and subsequent infection [26]. HIV gp120 carbohydrates; interaction with complement protein MBL, DC-SIGN and other human lectins Several recent studies highlight the importance of carbohydrates on the surface of HIV for interaction with complement and other components of the innate immune system. The HIV gp120 is highly glycosylated with up to 26 N-linked carbohydrate groups. This extensive glycosylation has been suggested to protect virus from immune responses [27] but may also provide sites for interaction with endogenous lectins. Previous studies have shown that MBL, the initiating protein for the lectin pathway of complement, interacted with virions or gp120 from cell-line adapted HIV [28,29]. More recently, Saifuddin et al.[13] showed that MBL also bound intact HIV primary isolate virions. Additionally, MBL bound to both R5 and X4 primary isolate virions. Primary isolates of HIV are thought to be a better model for study than cell-line adapted virus. It appeared that MBL bound primarily to the glycans of gp120 on the surface of virions since particles that lacked gp120 did not bind to MBL. Taken together, these results indicate that the high mannose glycans of gp120 are important for stimulating innate immunity. In a retrospective study of HIV-infected persons [30], there was a significantly higher frequency of subjects with low serum MBL that had pneumonia than a group without pneumonia, suggesting that, in immunocompromised persons, MBL helps protect against some OI. Another mammalian lectin that can be classified as part of the innate immune system was also found to interact with HIV carbohydrates. DC-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), a C-type lectin expressed on the surface of DC, was shown to capture infectious HIV [31]. Virus captured by DC was not internalized but was efficiently transferred to T cells, and it was postulated that DC in the periphery could capture virus via this receptor and carry it to lymph nodes that are rich in activated T cells. Another C-type lectin, called DC-SIGNR, was also recently described [32]. This lectin is expressed in liver and lymph nodes [33], and has 73% homology to DC-SIGN. When expressed on cells, DC-SIGNR also binds HIV-1 as well as HIV-2 and SIV, and it can help in transferring virus infection to T cells [34]. The aforementioned studies show that N-linked glycans of gp120 are one of the important motifs found on HIV that activate innate immune pathways. These carbohydrates were analyzed in several recent studies. Plant lectins and surface plasmon resonance were used to probe gp120, providing evidence that the high-mannose glycans on gp120 are highly accessible [35]. A structural model of the completely glycosylated form of gp120 was composed using spectrometry data [36]. Interestingly, this model highlighted that high-mannose and complex glycans are clustered separately on gp120. A study of the SIV envelope protein, which like the HIV gp120 is extensively glycosylated, showed that while carbohydrates on soluble monomeric gp120 were accessible to glycosidases, intact gp120 on virions was highly resistant to digestion [27]. Plasmacytoid IPC: AIDS as an experiment of nature Interferons (IFN) have been known to play a role in the host defense against viral infections since their initial definition in 1957 by Isaacs and Lindemann. The next year, Lennert and his pathologist colleagues described peculiar plasmacytoid cells clustered in paracortical areas of reactive lymphoid tissues. The two fields were not to converge for 40 years. IFN have been categorized as type-I (including IFN-α, IFN-β, IFN-ω) [37] and type 2 (IFN-γ). Type 1 IFN have been generally associated with natural host defenses, while type 2 IFN-γ is, among other sources, a principal product of activated T helper cell (Th)-1 biased CD4 T cells participating in the adaptive immune system and NK cells. IFN-γ has been known as an important macrophage activating cytokine, necessary for the final effector mechanisms of cellular immunity [38]. IFN have been known to be involved in resistance to HIV; antibodies to IFN were incorporated into the initial culture systems utilized for the isolation of HIV (lymphadenopathy-associated virus) [39]. IFN-α, used to treat Kaposi's sarcoma, had an effect on HIV viremia, although notably only in patients having sufficiently high numbers of CD4 T cells [40]. Their involvement in the pathogenesis of AIDS was recognized by the presence of IFN in serum during the latter stages of HIV infection [41,42]; these IFN were considered acid-labile alpha-IFN because, despite their neutralization with anti-alpha antisera, they were destroyed by acidification, a characteristic of IFN-γ. IFN was known to be produced by mononuclear cells (MNC) in response to virus-infected target cells [43-47]; this response was not dependent on prior immunization (i.e., it was 'natural'). The cellular source of these IFN was, at the time, poorly defined, but by the early 1980s the IPC were known to be CD4, non-T cells. This cell type was ultimately designated 'natural' IPC (NIPC) to distinguish them from monocytes and B cells, both capable of IFN-α production in response to stimuli other than herpes simplex virus (HSV) (and other enveloped viruses), which chiefly stimulated this subset [42] (reviewed in [48]). Ulcerative HSV infection as a manifestation of AIDS [49] drew attention to HSV-specific immunity. Initially, it was thought that deficits of HSV-directed NK (distinguishable from NK activity directed at K562 target cells) might be involved. When NK deficits did not appear to be an important mechanism of susceptibility, Lopez et al.[50] investigated IFN-α generation by MNC responding to HSV-infected target cells. Deficiencies of IFN generation were noted to be associated with OI in AIDS patients in cross-sectional studies. Further prospective serial studies, somewhat surprisingly, indicated that loss of MNC IFN-α generation predicted the development of OI in general, not just those related to herpesviruses [51]. Additionally, both deficits of IFN-α generation and a paucity of CD4 T-cell numbers had to be present before subjects with HIV infection developed OI. Freedom from OI and overall survival were dramatically enhanced when patients' MNC maintained their capacity to produce IFN-α in response to HSV (Fig. 1)[51]. These results, and the non-T-cell nature of the NIPC, suggested their importance to the biology of cellular immunity [51].Fig. 1: The 'natural history' of HIV infection: Kaplan-Meier survival of patients with HIV infection before the introduction of prophylaxis for Mycobacterium avium complex and antiretroviral drugs (∼1981-1985). More than one-half of the participants in this study were receiving prophylaxis for Pneumocystis carinii infection using trimethoprim-sulfamethoxazole. (a) The probability of freedom from systemic opportunistic infection; (b) the probability of overall survival. The upper two curves in each plot represent survival after interferon (IFN)-α generation, CD4 T-cell count, or both exceeded cut-off values associated with avoidance of opportunistic infections. The lower curve in each plot reflects the outcomes when both IFN-α generation and CD4 T-cell count had fallen to below those cut-off values by time zero. For IFN-α generation, the cut-off value was 300 IU/ml, while for CD4 T-cell counts it was 250 cells/mm3. The striking difference in survival when the measures of both adaptive immunity (CD4 T-cell count) and innate immunity (IFN-α generation) were simultaneously compromised suggested that the IFN-producing cells had a crucially important role in host defense. (Reproduced from [51] with permission of the Journal of Clinical Investigation in the format Journal via Copyright Clearance Center.)On discontinuous Percoll gradients, NIPC appeared in the lowest density fraction, separable from NK cells and most of the other, denser MNC; thus, they were a very low-frequency MNC subset of unknown lineage [48]. NIPC could be enriched through depletion of cell types in blood whose lineage could be defined by monoclonal antibodies (i.e., B cells, T cells, NK cells, and monocytes). Such crude mixtures of cells included DC at different stages of differentiation and marker characteristics, CD34 hematopoietic stem cells, and other low frequency MNC co-separating in the mixtures [52-55]. In addition, NIPC were dramatically labile with manipulation, disappearing functionally during enrichment and culture. The pitfalls inherent in purification and sustaining viability in vitro made detailed study of the NIPC difficult, and definition of their cytokine products nearly impossible. Nonetheless, by the late 1990s, such cells had been defined by flow cytometry as a lineage-negative, HLA-DR, IL-3 receptor-α (CD123) and CD4-positive subset of blood MNC, whose IFN generation could be sustained by IL-3 and IFN-α [52-59]. Their cell surface phenotype and co-isolation with cells of DC morphology suggested to many that they were related to a type of immature DC [42,52-61]. In contrast, others who had employed differential adherence techniques to isolate a plasmacytoid IPC believed the NIPC were a previously unknown cell type [62,63], distinguishable by several criteria from DC. NIPC were shown to generate IFN in response to a variety of microbial stimuli other than HSV and cytomegalovirus, including various strains of HIV, other enveloped viruses [42,43,45,53-55,64-66], Corynebacterium parvum and Staphylococcus aureus. Such studies suggested they could engage in natural immune responses to non-viral microbial agents. Further studies in HIV infection confirmed that NIPC function (to a lesser degree than monocyte IFN production in response to Sendai virus) became progressively deficient with disease progression [42,67,68]. However, heavily HIV-exposed but uninfected gay men did not show particularly exuberant IFN-α generation [69]. Studies in primary immunodeficiency disorders and hairy cell leukemia appeared to confirm the association between NIPC function and the host defense against microbial opportunists [70-72]. IFN production by NIPC from healthy controls varied substantially over ∼2.5 log (100-40 000 IU/ml). Yet the precise nature of the NIPC, their tissue localization, and a clear mechanism of their contribution to the host defense remained elusive. The cells recognized in lymphoid tissues in 1958 by Lennert and colleagues had plasmacytoid morphology and tended to cluster in and around high endothelial venules, especially in states of active immunization. These cell aggregates contained a high frequency of pycnotic nuclei. They were designated 'enigmatic' plasmacytoid T cells (later, plasmacytoid monocytes) [55,73,74] by pathologists because of their localization to the T-cell area of lymphoid tissue and their staining characteristics. Their function was unknown for many years. Because their cell surface phenotype suggested they might be DC precursors, an efficient sorting strategy was developed, capitalizing on their lack of CD11c, which is expressed on the other major HLA-DR-positive, CD4 cell, lineage-negative MNC subset, immature myeloid DC [74]. This unique approach permitted purification to near homogeneity (98-99%) from human tonsillar tissue. Similar cells were found in blood. Their pycnotic nuclei in tissue reflected a high rate of spontaneous apoptosis in vitro, which could be reduced to permit cell cycling and survival uniquely by IL-3, among all cytokines tested. Despite their microscopic and ultrastructural appearance as secretory cells, the purified plasmacytoid cells had no discernible secretory product; efforts to detect mRNA or secreted protein had been unsuccessful (Y. J. Liu, personal communication, 1998). Nonetheless, the sorted cells underwent a dramatic metamorphosis to fully formed DC after stimulation with monocyte-conditioned medium, as previously described in the DC literature, or with IL-3 and CD40-ligand transfected cells [74]. The DC progeny, designated DC2 to distinguish them from monocyte-derived (myeloid) DC1, were reported to foster Th-2 cytokine production (including IL-4 and IL-10, capable of downregulating Th-1 [75]). The plasmacytoid cells were referred to as 'pre-DC2'. DC1 tended to foster Th-1 biasing. The by-then striking similarities between NIPC and pre-DC2 prompted studies revealing the identity of the two cell types [76,77], which were quickly confirmed [78,79]. These investigations brought together two disparate lines of study. The pre-DC2 were capable of producing huge amounts of type-1 IFN, chiefly IFN-α, their previously unrecognized, non-constitutive secretory product, but only on triggering with microbial stimuli. Viral stimulation, as well as IL-3 plus CD40-ligand contact, was also shown to drive the IPC to differentiate into DC2 [79,80]. Pre-DC2 were thus defined as 'professional IPC', while the revelation of a site of tissue localization for the blood-borne NIPC to T-cell areas provided a possible explanation for their clinically defined role in cellular immunity. The NIPC/pre-DC2 have also been referred to in the literature as plasmacytoid T cells, plasmacytoid monocytes, plasmacytoid precursor DC, CD11c-negative DC, and plasmacytoid dendritic cells (pDC). Although an oxymoron (since the plasmacytoid cells have no dendrites and appear smooth-surfaced in scanning electron micrographs), the latter terminology (pDC) has the greatest currency. We will utilize the term pDC to refer to these cells for the remainder of this review. Concurrent with the clarification of the NIPC/pre-DC2 connection, advances in cytokine research have shown a complex role for type-1 IFN in induction and maintenance of T-cell functions and differentiation [81-84]. IFN-α was shown to support the survival of activated T cells in vitro and, in humans, through phosphorylation of the transcription factors, STAT2 and STAT4 [83,85-87], to induce upregulation of IL-12 receptors on immature T cells and to initiate their production of IFN-γ. IFN-α can also, however, downregulate IL-12 production by DC (reviewed in [88]). The essential role of IL-12 in the biasing of immature T cells towards Th-1 is generally accepted [89]. These findings provided the additional information necessary to explain, hypothetically, how the pDC could enhance cellular immunity to a wide variety of microbial stimuli. If pDC, interacting with blood-borne microorganisms, were able to produce IFN-α in T-cell areas, they could participate in the early steps of T-cell biasing towards Th-1 cytokine production. The same stimuli could cause further differentiation of the pDC towards DC2, which might provide negative feedback control of Th-1 (Fig. 2).Fig. 2: Simplified scheme showing the partial role of plasmacytoid dendritic cells (pDC) at the interface of innate and adaptive immunity. The pDC circulate in the blood at low frequency and reach peripheral tissues, where they encounter pathogenic microbes through their pattern recognition receptors (e.g., Toll-like receptor-9). Interaction with the pathogen leads to secretion of interferon (IFN)-α, which may inhibit viruses directly (their innate immune function). Interaction with the pathogen also alters expression of adhesion molecules and chemokine receptors on the pDC, favoring their migration via high endothelial venules into T-cell (paracortical) areas of secondary lymphoid organs where, carrying their microbial passenger, they encounter naive CD4 T cells. The pDC present the microbial antigens via their strongly-expressed MHC class II, while secreting large amounts of IFN-α locally, thus providing signals that activate and sustain the naive CD4 T cells and induce them to express interleukin (IL)-12 receptors. During this process, the pDC also produce tumor necrosis factor-α, granulocyte-macrophage-colony stimulating factor, IL-6 and IL-8, and upregulate expression of CD40, CD80 and CD86, co-stimulatory molecules reflecting their differentiation and permitting further interaction with other cells in their environment. IL-3 provided locally by the activating T cells, as well as the IFN-α, prevent apoptosis of the pDC. The antigen-activated CD4 T cells, now expressing IL-12 receptors (IL-12R) and beginning to produce IFN-γ, receive IL-12 from type 1 DC (DC1) and other signals, leading to biasing of the developing antigen-reactive T-cell clones towards T helper cell (Th)-1. Once the pDC have differentiated, under the influence of CD40 ligand (CD40-L) expressed on activated T cells and other cells in their milieu, into fully formed type-2 dendritic cells (DC2), they both present antigen and bias other immature T cells down a Th-2 pathway. The resulting production of Th-2 cytokines including IL-4 and IL-10 either cause apoptosis of, or otherwise downregulate, the remaining undifferentiated plasmacytoid cells, shutting off the process. The relative amounts of IFN-α and other cytokine generation, versus differentiation of the plasmacytoid cells, which appears substantially to influence the Th-1 or Th-2 outcomes, is in turn dependent on the nature of the original microbial stimulus. IPC, interferon-α-producing cells.Most recently, the reality of this scenario has been demonstrated experimentally, in studies showing that virally stimulated pDC migrate from blood to lymphoid tissue through the high endothelial venules, finding their way to T-cell areas and locally producing IFN-α [90]. This traffic pathway appears to differ from that of other DC, which are thought to migrate to lymphoid organs via afferent lymphatics on encountering antigen in the periphery [91], and to sites of inflammation. This distinction appears to be mediated in part via discrete expression of chemokine receptors on the two major immature DC types [92]. Further studies recently provided additional cell surface markers characteristic of the pDC [55,78,91,93], and showed significant alterations in their circulating numbers and/or function in response to corticosteroids [94], Flt3 ligand and GCSF [95]. Other data show that pDC produce not only type-1 IFN, but also (depending on the stimulus) significant quantities of certain other cytokines (tumor necrosis factor-α, IL-6 and granulocyte-macrophage-colony stimulating factor but, notably, not IL-12) [78,80,88,90,91,96] and chemokines (IL-8) [96] while retaining their plasmacytoid morphology (P. A. Fitzgerald-Bocarsly, personal communication, 2000), and that they express the chemokine receptor, CxCR3 [78]. The pDC express the pre-T-receptor alpha chain, suggesting their inclusion in a lymphoid lineage, and they can be derived in vitro from CD34 precursors through response to growth factors, suggesting a lymphoid (rather than myeloid) differentiation pathway [97-99]. The pDC actually appear to be involved in a variety of immune responses including nasal allergic reactions, thought to involve dominantly Th-2 biasing [100]. They are found in cerebrospinal fluid from patients with multiple sclerosis and Borrelia burgdorferi infection [101] as well as in the thymus [102,103], where their function is not yet defined. The nature of the recognition of microbial structures by pDC has also recently been elucidated. Cells of the innate immune system have evolved germline-encoded receptors capable of broadly recognizing macromolecular patterns characteristic of pathogenic microbes such as the Toll-like receptors (TLR) [104]. Bacterial DNA sequences consisting of non-methylated DNA carrying the CpG motif signals via TLR9, while bacterial lipopolysaccharide binds in association with CD14 to TLR4. Ligation of both of these TLR leads to intracellular signaling through the IL-1R/TLR signal transduction pathway (see references in [96]). Later elements of this pathway are common to CD40-mediated, TLR-9-mediated and TLR4-mediated intracytoplasmic signaling, possibly an explanation for their similar effects in activating and differentiating DC. The pDC carry TLR9 and respond dramatically to CpG DNA and CpG oligonucleotides [96] with both the production of type-1 IFN, tumor necrosis factor-α, IL-6, granulocyte-macrophage-colony stimulating factor and IL-8, and differentiation to DC. In contrast, IL-3 and CD40 ligation lead to differentiation but not to cytokine production (other than the chemokine IL-8). Certain CpG oligodeoxynucleotide sequences appear to be especially stimulatory of type-1 IFN generation, similar to the magnitude induced by HSV-1 [105]. Th-1 biasing appears to be tied closely to the production of type-1 IFN; differentiation towards DC2 may favor Th-2-type responses. It is becoming clear that the effect of an inducing signal on these cells, and their subsequent influence on neighboring immune cells, will be heavily dependent on the nature of the inducer. Two recent studies in HIV infection extend the earlier clinical observations that suggested the biological importance of this cell in human host defenses. The avoidance of OI during successful antiviral therapy of AIDS is closely associated with repair of the deficit of IFN-α generation [106,107]. A cohort of 294 subjects with HIV infection was followed prospectively during highly active antiretroviral therapy. Regression analysis showed equally loose, but significant, negative correlations for both CD4 cell count and IFN-α generation to virus load in this population. Eighty-two patients who had had an AIDS-defining OI showed reconstitution to OI-protective cut-off values for IFN-α generation ∼6 months earlier than recovery of CD4 cell counts to protective levels, as previously defined [51]. Moreover, reconstitution of IFN-α generation alone, with or without CD4 cell rises, was consistently associated with absence of recurrent or new OI [106,107]. These data are consistent with the idea that recovering IFN-α generation is an early marker of immune reconstitution and confirm the earlier observation [51] that concurrent deficits of IFN generation and CD4 cell counts are necessary to permit OI. Both IFN-α generation and the number of circulating pDC were studied cross-sectionally in 54 patients with HIV infection, including 23 long-term non-progressors [108]. There was a significant relationship between pDC IFN-α generation and stage of disease; using two-color flow cytometry, the number of circulating lineage-negative, CD11c-negative, CD4 pDC correlated linearly with function, indicating progressive losses in number as well as in function, in contrast to the previous negative findings by other workers based on less-well defined flow cytometric definitions of the pDC subset [42,109]. Much earlier work with ELISpot assays [67,68] had suggested reduction in IPC numbers, but could not exclude simple failure of IFN production by individual cells. Notably, the long-term non-progressors had significantly higher numbers and functions of pDC compared not only with patients with progressive HIV infection, but also with uninfected controls [108]. The correlation noted between virus burden and pDC suggests that pDC function is, perhaps not surprisingly, involved in the suppression of HIV itself, either directly by IFN production acting on infected cells, or through a Th-1 mechanism. This study [108] also showed that development of susceptibility to both OI and active Kaposi's sarcoma depends on the simultaneous depression of both CD4 cell counts and pDC numbers below certain critical levels, confirming earlier observations [51]. The pDC appear not only to carry the appropriate cell surface structures to fuse with HIV-1, but actually to be more susceptible to infection with these viruses and to produce more virions [110] than do myeloid DC. The pDC, as part of the innate immune system, appear to initiate the development of adaptive, antigen-specific, Th-1 immunity in normal humans. They distinguish microbial molecular patterns through TLR (and perhaps other receptors [111]) in the periphery and migrate through the blood to secondary lymphoid organs. There, in T-cell areas, they provide the type-1 IFN that directly enhance IFN-γ production by immature T cells, and increase the receptiveness of these T cells to subsequent contact with IL-12, delivered by DC1 or other cells. The progressive losses of IFN-α generation observed during HIV infection would be expected to selectively suppress this pathway, ordinarily devoted to Th-1 induction. Additionally, since pDC can themselves interact and be infected with HIV [110], they could serve as a vector for the virus (for example [112-116]), secondarily infecting and aborting the nascent Th-1 pathway (Fig. 3).Fig. 3: Hypothetical scheme showing alterations in the normal functions of plasmacytoid dendritic cells (pDC) when HIV is the microbial pathogen (with reference to Fig. 2). HIV-1 binds to and actively infects pDC in the periphery through their chemokine receptors (CCR5, others) and CD4 cells. Carried into the node, the infected pDC infect the naive CD4 T cells they are activating via their secretion of cytokines, thus preferentially aborting the clonal expansion and T helper cell (Th)-1 biasing of HIV-specific T cells. Since other microbial immunizations are occurring concurrently in the same paracortical areas, they will be similarly affected over the long run. Ultimately, functional and numerical depletion of pDC, consistently observed in the progression to AIDS, will deprive the node of the Th-1-favorable influence of interferon-α (IFN-α) and the antigen-presenting, immune activating functions of the cells themselves. Deficits of pDC will thus contribute to the pathogenesis of AIDS.

OBJECTIVES:To quantify the effect of HIV infection and HIV-suppressive therapy on interferon-alpha (IFN-alpha) production by human blood mononuclear cells; to compare, in parallel, effects on CD4+ T-cell numbers; and to ascertain the relationship of these interferon and CD4 parameters to resistance to opportunistic infections. DESIGN:Serial studies of 294 unselected patients with HIV infection during therapy, with outcomes analysis. METHODS:Determination of IFN generation by blood mononuclear cells via bioassay, and T-lymphocyte subset analysis via flow cytometry; serial studies of individual patients; linear regression and chi2 contingency table analysis. RESULTS:HIV burden is inversely related to interferon-alpha generation, much as it is to CD4+ T-cell counts. Both of these recover during HIV-suppressive therapy. Reconstitution of IFN-alpha generation to levels commensurate with protection against opportunistic infection occurs prior to similar restoration of CD4 counts. In the outcomes analyses, such immune reconstitution was associated with protection from recurrent or new opportunistic infection. Conversely, viral suppression without such immunologic recovery was not protective against opportunistic infection. CONCLUSIONS:Rapidly responding IFN-alpha generating cells appear to participate in resistance to opportunistic intracellular infection. Recovery of IFN-alpha generation may be an early marker of immune reconstitution in AIDS.

Glucocorticoids are strongly immunosuppressive and are associated with reactivation of some intracellular infections. The plasmacytoid dendritic cell is a rare blood mononuclear cell detected through its production of IFN-alpha in response to herpes simplex virus and by surface immunophenotyping. We here report that steroid administration results in a decrease of IFN-alpha generation of approximately 25-fold, accompanied by reduction in circulating plasmacytoid dendritic cell numbers. Both parameters return to normal within days after steroid cessation.

Respiratory papillomas, caused by human papillomaviruses, are benign tumors that recur following removal. We evaluated immune function and major histocompatibility complex (MHC) phenotype and expression in these patients. MHC-independent immune function appeared normal. The frequency of peripheral blood MHC class II phenotypes was highly enriched for DQ3 and DR11, one split of DR5. Class I MHC antigen expression on papilloma tissue was markedly reduced. Together, these phenomena may facilitate papillomavirus evasion of the cellular immune response.