Escherichia coli is a leading cause of neonatal sepsis, with infection occurring in approximately one in every 1,000 live births1,2. However, with E. coli colonization beginning soon after birth3-5 and defects in neonatal host defence maturation6-9, an alternative consideration is why infection does not occur even more frequently. Here we show that newborn babies with E. coli sepsis have selectively reduced vertically transferred natural antibodies that recognize E. coli, mechanistically explaining their susceptibility to infection. Complementary preclinical studies show that preconceptual intestinal colonization with probiotic E. coli Nissle 1917 (EcN)10 primes anti-E. coli immunoglobulin G (IgG) antibodies with broad cross-reactivity to clinical isolates responsible for neonatal sepsis that override the inherent susceptibility of neonatal mice. Outer membrane protein A (OmpA) is a target of maternal IgG and is also essential for EcN colonization-induced serological immunogenicity. Upon vertical transfer to neonates, colonization-primed anti-E. coli IgG uniquely protects against infection via opsonization, requiring both complement and IgG Fc receptors. Compared with specimens from sex and gestational age-matched healthy control babies without infection, dried blood spot specimens collected one day after birth from 100 babies with E. coli sepsis show consistently reduced IgG titres to pooled E. coli clinical isolates and OmpA, along with impaired IgG-dependent antibacterial opsonization. Together, these results demonstrate that natural infection susceptibility of neonates is efficiently rescued by anti-E. coli IgG and identify defects in pathogen-targeted vertically transferred immunity as a primary risk factor for severe invasive infection in newborn babies.

Organ transplant recipients require continual immune-suppressive therapies to sustain allograft acceptance. Although medication nonadherence is a major cause of rejection, the mechanisms responsible for graft loss in this clinically relevant context among individuals with preceding graft acceptance remain uncertain. Here, we demonstrate that skin allograft acceptance in mice maintained with clinically relevant immune-suppressive therapies, tacrolimus and mycophenolate, sensitizes hypofunctional PD1hi graft-specific CD8+ T cells. Uninterrupted immune-suppressive therapy is required because drug discontinuation triggers allograft rejection, replicating the requirement for immune-suppressive therapy adherence in transplant recipients. Graft-specific CD8+ T cells in allograft-accepted mice show diminished effector differentiation and cytokine production, with reciprocally increased PD1 expression. Allograft acceptance-induced PD1 expression is essential, as PDL1 blockade reinvigorates graft-specific CD8+ T cell activation with ensuing allograft rejection despite continual immune-suppressive therapy. Thus, PD1 sustained CD8+ T cell inhibition is essential for allograft acceptance maintained by tacrolimus plus mycophenolate. This necessity for PD1 in sustaining allograft acceptance explains the high rates of rejection in transplant recipients with cancer administered immune checkpoint inhibitors targeting PD1/PDL1, highlighting shared immune suppression pathways exploited by tumor cells and current therapies for averting allograft rejection. The Editors have selected this article as a highlight of the issue.

Background Lower respiratory infections are a leading cause of severe morbidity and mortality among older adults. Despite ubiquitous exposure to common respiratory pathogens throughout life and near universal seropositivity, antibodies fail to effectively protect the elderly. Therefore, we hypothesized that severe respiratory illness in the elderly is due to deficient CD8 + T cell responses. Results Here, we establish an aged mouse model of human metapneumovirus infection (HMPV) wherein aged C57BL/6 mice exhibit worsened weight loss, clinical disease, lung pathology and delayed viral clearance compared to young adult mice. Aged mice generate fewer lung-infiltrating HMPV epitope-specific CD8 + T cells. Those that do expand demonstrate higher expression of PD-1 and other inhibitory receptors and are functionally impaired. Transplant of aged T cells into young mice and vice versa, as well as adoptive transfer of young versus aged CD8 + T cells into Rag1 −/− recipients, recapitulates the HMPV aged phenotype, suggesting a cell-intrinsic age-associated defect. HMPV-specific aged CD8 + T cells exhibit a terminally exhausted TCF1/7 − TOX + EOMES + phenotype. We confirmed similar terminal exhaustion of aged CD8 + T cells during influenza viral infection. Conclusions This study identifies terminal CD8 + T cell exhaustion as a mechanism of severe disease from respiratory viral infections in the elderly.

Abstract CD8+ T cell dysfunction contributes to severe respiratory viral infection outcomes in older adults. CD8+ T cells are the primary cell type responsible for viral clearance. With increasing age, CD8+ T cell function declines in conjunction with an accumulation of cytotoxic tissue-resident memory (TRM) CD8+ T cells. We sought to elucidate the role of PD-1 signaling on aged CD8+ T cell function and accumulation of CD8+ TRM cells during acute viral respiratory tract infection, given the importance of PD-1 regulating CD8+ T cells during acute and chronic infections. PD-1 blockade or genetic ablation in aged mice yielded improved CD8+ T cell granzyme B production comparable to that in young mice during human metapneumovirus and influenza viral infections. Syngeneic transplant and adoptive transfer strategies revealed that improved granzyme B production in aged Pdcd1−/− CD8+ T cells was primarily cell intrinsic because aged wild-type CD8+ T cells did not have increased granzyme B production when transplanted into a young host. PD-1 signaling promoted accumulation of cytotoxic CD8+ TRM cells in aged mice. PD-1 blockade of aged mice during rechallenge infection resulted in improved clinical outcomes that paralleled reduced accumulation of CD8+ TRM cells. These findings suggest that PD-1 signaling impaired CD8+ T cell granzyme B production and contributed to CD8+ TRM cell accumulation in the aged lung. These findings have implications for future research investigating PD-1 checkpoint inhibitors as a potential therapeutic option for elderly patients with severe respiratory viral infections.

Intestinal colonization by antigenically foreign microbes necessitates expanded peripheral immune tolerance. Here we show commensal microbiota prime expansion of CD4 T cells unified by the Kruppellike factor 2 (KLF2) transcriptional regulator and an essential role for KLF2+ CD4 cells in averting microbiota-driven intestinal inflammation. CD4 cells with commensal specificity in secondary lymphoid organs and intestinal tissues are enriched for KLF2 expression, and distinct from FOXP3+ regulatory T cells or other differentiation lineages. Mice with conditional KLF2 deficiency in T cells develop spontaneous rectal prolapse and intestinal inflammation, phenotypes overturned by eliminating microbiota or reconstituting with donor KLF2+ cells. Activated KLF2+ cells selectively produce IL-10, and eliminating IL-10 overrides their suppressive function in vitro and protection against intestinal inflammation in vivo. Together with reduced KLF2+ CD4 cell accumulation in Crohn's disease, a necessity for the KLF2+ subpopulation of T regulatory type 1 (Tr1) cells in sustaining commensal tolerance is demonstrated.

Pregnancy confers partner-specific protection against complications in future pregnancy that parallel persistence of fetal microchimeric cells (FMcs) in mothers after parturition. We show that preexisting FMcs become displaced by new FMcs during pregnancy and that FMc tonic stimulation is essential for expansion of protective fetal-specific forkhead box P3 (FOXP3)-positive regulatory T cells (Treg cells). Maternal microchimeric cells and accumulation of Treg cells with noninherited maternal antigen (NIMA) specificity are similarly overturned in daughters after pregnancy, highlighting a fixed microchimeric cell niche. Whereas NIMA-specific tolerance is functionally erased by pregnancy, partner-specific resiliency against pregnancy complications persists in mothers despite paternity changes in intervening pregnancy. Persistent fetal tolerance reflects FOXP3 expression plasticity, which allows mothers to more durably remember their babies, whereas daughters forget their mothers with new pregnancy-imprinted immunological memories.

Adaptive immune components are thought to exert non-overlapping roles in antimicrobial host defence, with antibodies targeting pathogens in the extracellular environment and T cells eliminating infection inside cells(1,2). Reliance on antibodies for vertically transferred immunity from mothers to babies may explain neonatal susceptibility to intracellular infections(3,4). Here we show that pregnancy-induced post-translational antibody modification enables protection against the prototypical intracellular pathogen Listeria monocytogenes. Infection susceptibility was reversed in neonatal mice born to preconceptually primed mothers possessing L. monocytogenes-specific IgG or after passive transfer of antibodies from primed pregnant, but not virgin, mice. Although maternal B cells were essential for producing IgGs that mediate vertically transferred protection, they were dispensable for antibody acquisition of protective function, which instead required sialic acid acetyl esterase(5) to deacetylate terminal sialic acid residues on IgG variable-region N-linked glycans. Deacetylated L. monocytogenes-specific IgG protected neonates through the sialic acid receptor CD22(6,7), which suppressed IL-10 production by B cells leading to antibody-mediated protection. Consideration of the maternal-fetal dyad as a joined immunological unit reveals protective roles for antibodies against intracellular infection and fine-tuned adaptations to enhance host defence during pregnancy and early life.

Pregnancy makes women especially vulnerable to infection. The most common cause of bloodstream infection during pregnancy is by a bacterium called Escherichia coli .

Tacrolimus is widely used to prevent graft rejection after allogeneic transplantation by suppressing T cells in a non-antigen-specific fashion. Global T-cell suppression makes transplant recipients more susceptible to infection, especially infection by opportunistic intracellular pathogens. Infection followed by secondary challenge with the opportunistic intracellular bacterial pathogen, Listeria monocytogenes, was used to probe when tacrolimus most significantly impacts antimicrobial host defense. Tacrolimus-treated mice showed no difference in innate susceptibility following primary infection, whereas susceptibility to secondary challenge was significantly increased. Modifying the timing of tacrolimus initiation with respect to primary infection compared with secondary challenge showed significantly reduced susceptibility in tacrolimus-treated mice where tacrolimus was discontinued prior to secondary challenge. Thus, tacrolimus overrides protection against secondary infection primed by primary infection (and presumably live attenuated vaccines), with the most critical window for tacrolimus-induced infection susceptibility being exposure immediately prior to secondary challenge. These results have important implications for strategies designed to boost antimicrobial T-cell-mediated immunity in transplant recipients.

Human metapneumovirus (HMPV) is a leading cause of acute lower respiratory tract illness in children and adults. Repeated infections are common and can be severe in young, elderly, and immunocompromised persons due to short-lived protective humoral immunity. In turn, few protective T cell epitopes have been identified in humans. Thus, we infected transgenic mice expressing the common human HLA MHC-I allele B*07:02 (HLA-B7) with HMPV and screened a robust library of overlapping and computationally predicted HLA-B7 binding peptides. Six HLA-B7-restricted CD8+ T cell epitopes were identified using ELISPOT screening in the F, M, and N proteins, with M195–203 (M195) eliciting the strongest responses. MHC-tetramer flow cytometric staining confirmed HLA-B7 epitope-specific CD8+ T cells migrated to lungs and spleen of HMPV-immune mice. Immunization with pooled HLA-B7-restricted peptides reduced viral titer and protected mice from virulent infection. Finally, we confirmed that CD8+ T cells from HLA-B7 positive humans also recognize the identified epitopes. These results enable identification of HMPV-specific CD8+ T cells in humans and help to inform future HMPV vaccine design.

Vaccines against Zika virus (ZIKV) infection that target CD8(+) T cells are of considerable interest because Abs may enhance infection susceptibility. However, whether CD8(+) T cells are protective or promote susceptibility to clinical infection symptoms remains uncertain. To more precisely investigate ZIKV-specific CD8(+) T cells in isolation, we engineered a Listeria monocytogenes-based vector to express a single MHC class I-restricted immune dominant peptide, E294-302, from ZIKV envelope protein. We show accumulation of activated ZIKV-specific CD8(+) T cells primed by recombinant L. monocytogenes is associated with reductions in circulating virus levels after ZIKV challenge in type I IFN receptor-deficient mice and wildtype mice administered neutralizing Abs against type I IFN receptor. Interestingly, susceptibility to ZIKV clinical infection including weight loss and mortality each persists and is neither significantly improved nor worsened compared with isogenic L. monocytogenes-primed control mice. These data demonstrating persistent ZIKV clinical susceptibility despite reduced viral burden in mice with expanded virus-specific CD8(+) T cells highlights the need for targeting other adaptive immune components in developing vaccines against ZIKV infection.

Viruses are frequent causes of lower respiratory infection (LRI). Programmed cell death-1 (PD-1) signaling contributes to pulmonary CD8(+) T cell (TCD8) functional impairment during acute viral LRI, but the role of TCD8 impairment in viral clearance and immunopathology is unclear. We now find that human metapneumovirus infection induces virus-specific lung TCD8 that fail to produce effector cytokines or degranulate late postinfection, with minimally increased function even in the absence of PD-1 signaling. Impaired lung TCD8 upregulated multiple inhibitory receptors, including PD-1, lymphocyte activation gene 3 (LAG-3), T cell Ig mucin 3, and 2B4. Moreover, coexpression of these receptors continued to increase even after viral clearance, with most virus-specific lung TCD8 expressing three or more inhibitory receptors on day 14 postinfection. Viral infection also increased expression of inhibitory ligands by both airway epithelial cells and APCs, further establishing an inhibitory environment. In vitro Ab blockade revealed that multiple inhibitory receptors contribute to TCD8 impairment induced by either human metapneumovirus or influenza virus infection. In vivo blockade of T cell Ig mucin 3 signaling failed to enhance TCD8 function or reduce viral titers. However, blockade of LAG-3 in PD-1-deficient mice restored TCD8 effector functions but increased lung pathology, indicating that LAG-3 mediates lung TCD8 impairment in vivo and contributes to protection from immunopathology during viral clearance. These results demonstrate that an orchestrated network of pathways modifies lung TCD8 functionality during viral LRI, with PD-1 and LAG-3 serving prominent roles. Lung TCD8 impairment may prevent immunopathology but also contributes to recurrent lung infections.

Acute viral infections typically generate functional effector CD8(+) T cells (TCD8) that aid in pathogen clearance. However, during acute viral lower respiratory infection, lung TCD8 are functionally impaired and do not optimally control viral replication. T cells also become unresponsive to Ag during chronic infections and cancer via signaling by inhibitory receptors such as programmed cell death-1 (PD-1). PD-1 also contributes to TCD8 impairment during viral lower respiratory infection, but how it regulates TCD8 impairment and the connection between this state and T cell exhaustion during chronic infections are unknown. In this study, we show that PD-1 operates in a cell-intrinsic manner to impair lung TCD8. In light of this, we compared global gene expression profiles of impaired epitope-specific lung TCD8 to functional spleen TCD8 in the same human metapneumovirus-infected mice. These two populations differentially regulate hundreds of genes, including the upregulation of numerous inhibitory receptors by lung TCD8. We then compared the gene expression of TCD8 during human metapneumovirus infection to those in acute or chronic lymphocytic choriomeningitis virus infection. We find that the immunophenotype of lung TCD8 more closely resembles T cell exhaustion late into chronic infection than do functional effector T cells arising early in acute infection. Finally, we demonstrate that trafficking to the infected lung alone is insufficient for TCD8 impairment or inhibitory receptor upregulation, but that viral Ag-induced TCR signaling is also required. Our results indicate that viral Ag in infected lungs rapidly induces an exhaustion-like state in lung TCD8 characterized by progressive functional impairment and upregulation of numerous inhibitory receptors.

ABSTRACTType I IFN signaling, which is initiated through activation of the alpha interferon receptor (IFNAR), regulates the expression of proteins that are crucial contributors to immune responses. Paramyxoviruses, including human metapneumovirus (HMPV), have evolved mechanisms to inhibit IFNAR signaling, but the specific contribution of IFNAR signaling to the control of HMPV replication, pathogenesis, and adaptive immunity is unknown. We used IFNAR-deficient (IFNAR−/−) mice to assess the effect of IFNAR signaling on HMPV replication and the CD8+T cell response. HMPV-infected IFNAR−/−mice had a higher peak of early viral replication but cleared the virus with kinetics similar to those of wild-type (WT) mice. However, IFNAR−/−mice infected with HMPV displayed less airway dysfunction and lung inflammation. CD8+T cells of IFNAR−/−mice after HMPV infection expressed levels of the inhibitory receptor programmed death 1 (PD-1) similar to those of WT mice. However, despite lower expression of inhibitory programmed death ligand 1 (PD-L1), HMPV-specific CD8+T cells of IFNAR−/−mice were more functionally impaired than those of WT mice and upregulated the inhibitory receptor Tim-3. Analysis of the antigen-presenting cell subsets in the lungs revealed that the expansion of PD-L1lowdendritic cells (DCs), but not PD-L1highalveolar macrophages, was dependent on IFNAR signaling. Collectively, our results indicate a role for IFNAR signaling in the early control of HMPV replication, disease progression, and the development of an optimal adaptive immune response. Moreover, our findings suggest an IFNAR-independent mechanism of lung CD8+T cell impairment.IMPORTANCEHuman metapneumovirus (HMPV) is a leading cause of acute respiratory illness. CD8+T cells are critical for clearing viral infection, yet recent evidence shows that HMPV and other respiratory viruses induce CD8+T cell impairment via PD-1–PD-L1 signaling. We sought to understand the role of type I interferon (IFN) in the innate and adaptive immune responses to HMPV by using a mouse model lacking IFN signaling. Although HMPV titers were higher in the absence of type I IFN, virus was nonetheless cleared and mice were less ill, indicating that type I IFN is not required to resolve HMPV infection but contributes to pathogenesis. Further, despite lower levels of the inhibitory ligand PD-L1 in mice lacking type I IFN, CD8+T cells were more impaired in these mice than in WT mice. Our data suggest that specific antigen-presenting cell subsets and the inhibitory receptor Tim-3 may contribute to CD8+T cell impairment.

Abstract CD8-positive T cells (CTLs) confer protective immunity against many infectious diseases, and thus the development of safe and efficient CTL-targeted vaccination is of high clinical significance. Although many respiratory pathogens invade the host through the lung mucosa, most licensed human vaccines elicit limited and systemic T cell responses, rather than strong local pulmonary responses. We therefore wondered whether elicitation of pulmonary CTL response with microbe-free CTL-targeted subunit vaccine might benefit the host and protect against respiratory infections. We discovered superior protection of humanized mice from lethal respiratory challenge with vaccinia virus with intranasal, but not intra-peritoneal, vaccination. Critically, this superior protection did not correlate with the magnitude of the systemic CTL response but was dependent on in situ elicitation of robust pulmonary epitope-specific CTLs, which were able to limit viral replication and prevent disease. Global transcriptome analysis revealed that pulmonary CTLs elicited by vaccination possessed distinct phenotypic and functional properties when compared to CTLs that homed to the spleen. To extend this finding, we developed an approach for eliciting protective CTL responses that utilizes nanoparticle-based mucosal delivery of epitope-targeted vaccine formulated with an optimal adjuvant.

Reinfections with respiratory viruses are common and cause significant clinical illness, yet precise mechanisms governing this susceptibility are ill defined. Lung Ag-specific CD8(+) T cells (T-CD8) are impaired during acute viral lower respiratory infection by the inhibitory receptor programmed death-1 (PD-1). To determine whether PD-1 contributes to recurrent infection, we first established a model of reinfection by challenging B cell-deficient mice with human metapneumovirus (HMPV) several weeks after primary infection, and found that HMPV replicated to high titers in the lungs. A robust secondary effector lung T-CD8 response was generated during reinfection, but these cells were more impaired and more highly expressed the inhibitory receptors PD-1, LAG-3, and 2B4 than primary T-CD8. In vitro blockade demonstrated that PD-1 was the dominant inhibitory receptor early after reinfection. In vivo therapeutic PD-1 blockade during HMPV reinfection restored lung T-CD8 effector functions (i.e., degranulation and cytokine production) and enhanced viral clearance. PD-1 also limited the protective efficacy of HMPV epitope-specific peptide vaccination and impaired lung T-CD8 during heterotypic influenza virus challenge infection. Our results indicate that PD-1 signaling may contribute to respiratory virus reinfection and evasion of vaccine-elicited immune responses. These results have important implications for the design of effective vaccines against respiratory viruses.

ABSTRACT Human metapneumovirus (HMPV) is a leading cause of respiratory disease in infants, children, and the elderly worldwide, yet no licensed vaccines exist. Live-attenuated vaccines present safety challenges, and protein subunit vaccines induce primarily antibody responses. Virus-like particles (VLPs) are an attractive alternative vaccine approach because of reduced safety concerns compared with live vaccines. We generated HMPV VLPs by expressing viral proteins in suspension-adapted human embryonic kidney epithelial (293-F) cells and found that the viral matrix (M) and fusion (F) proteins were sufficient to form VLPs. We previously reported that the VLPs resemble virus morphology and incorporate fusion-competent F protein (R. G. Cox, S. B. Livesay, M. Johnson, M. D. Ohi, and J. V. Williams, J. Virol. 86:12148–12160, 2012), which we hypothesized would elicit F-specific antibody and T cell responses. In this study, we tested whether VLP immunization could induce protective immunity to HMPV by using a mouse model. C57BL/6 mice were injected twice intraperitoneally with VLPs alone or with adjuvant and subsequently challenged with HMPV. Mice were euthanized 5 days postinfection, and virus titers, levels of neutralizing antibodies, and numbers of CD3+ T cells were quantified. Mice immunized with VLPs mounted an F-specific antibody response and generated CD8+ T cells recognizing an F protein-derived epitope. VLP immunization induced a neutralizing-antibody response that was enhanced by the addition of either TiterMax Gold or α-galactosylceramide adjuvant, though adjuvant reduced cellular immune responses. Two doses of VLPs conferred complete protection from HMPV replication in the lungs of mice and were not associated with a Th2-skewed cytokine response. These results suggest that nonreplicating VLPs are a promising vaccine candidate for HMPV. IMPORTANCE Human metapneumovirus (HMPV) is a leading cause of acute respiratory infection in infants, children, and the elderly worldwide, yet no licensed vaccines exist. Live-attenuated vaccines present safety challenges, and protein subunit vaccines induce primarily antibody responses. Virus-like particles (VLPs) are an attractive alternative vaccine approach. We generated HMPV VLPs by expressing the viral matrix (M) and fusion (F) proteins in mammalian cells. We found that mice immunized with VLPs mounted an F-specific antibody response and generated CD8+ T cells recognizing an F protein-derived epitope. VLP immunization induced a neutralizing-antibody response that was enhanced by the addition of either TiterMax Gold or α-galactosylceramide adjuvant. Two doses of VLPs conferred complete protection against HMPV replication in the lungs of mice and were not associated with a Th2-skewed cytokine response. These results suggest that nonreplicating VLPs are a promising vaccine candidate for HMPV.

Abstract Immunological memory, including both persistent and recall antibody (Ab) responses, enables more efficient and effective protection against infection, but key molecular mechanisms regulating emergence and persistence of memory B cells and plasma cells remain unclear. Poly-ADP-Ribose Polymerase14 (PARP14) is highly expressed in B cells and identified as a Stat6-interacting protein with intrinsic mono-ADP-ribosyltransferase (mART) activity. Although there are 17 PARP family members, the physiological function of most PARPs are not known well. Here we show that recall IgA and IgE production are impaired in PARP14-deficient mice. Further analysis revealed that, although PARP14 regulation of IgE involves B cell intrinsic function, IgA regulation by PARP14 was predominantly B cell extrinsic. Of note, PARP14 regulates the levels of CD103+ dendritic cells and T helper 17 (Th17) cells which might influence IgA generation in B cells. Moreover, PARP14 regulates the expression of RORα, Runx1 and Smad3 during Th17 differentiation, and mART activity of PARP plays a critical role in Th17 differentiation. Collectively, these results shed new light on differential mechanisms of Ig regulation, provide evidence that an endogenous mono ADP-ribosyltransferase regulates recall Ab production, and may have important implications for a new therapeutic target.