Background/Objectives: Cholera remains an important global health problem. Inactivated oral cholera vaccines (OCVs) are essential in the WHO/GTFCC (World Health Organization/Global Task Force on Cholera Control) strategy to end cholera by 2030; however, global supply is insufficient, they require partial cold-chain storage, and their formulation and antigen contents leave room for improvement. We describe here the development and preclinical evaluation of DuoChol OCV, a next-generation thermostable oral vaccine designed to address these gaps. Methods: DuoChol is a lyophilized dry-powder formulation in enteric capsules containing formalin-inactivated Vibrio cholerae O1 El Tor Ogawa and Inaba isogenic bacteria, recombinant cholera toxin B subunit (rCTB), and sucrose as stabilizer. Methods describe the construction of the novel vaccine strains, processes for the preparation and characterization of vaccine components, and the final dry formulation in enteric capsules, and in vitro and in vivo vaccine stability analyses. Results: The newly engineered vaccine strains, together with a high-yield mixed-mode chromatography process for rCTB purification, enabled efficient and cost-effective vaccine production. Stability studies demonstrated complete preservation of O1 LPS and rCTB antigens for at least 21 months across temperatures of 4–40 °C. Moreover, regardless of storage duration or temperature, oral immunization of mice with DuoChol elicited strong serum and mucosal antibacterial and antitoxin responses that were similar to those induced by the licensed Dukoral® OCV. Conclusions: Its heat stability, practical enteric capsule formulation, and potential for improved efficacy compared to inactivated whole-cell only OCVs support positioning DuoChol as a promising next-generation OCV, suitable for national cholera control programs and particularly advantageous for outbreak response, where rapid deployment and early, robust protection are essential.

BACKGROUND:Cholera and typhoid fever are often co-endemic, making vaccine co-administration practical. However, due to lack of immunogenicity data, current guidelines advise against co-administration of the oral inactivated whole-cell recombinant cholera toxin B-subunit vaccine (WC-rCTB) and the oral live Salmonella Typhi Ty21a vaccine. METHODS:Healthy adults (18-65 years) were randomized 1:1:1 to receive WC-rCTB with Ty21a (group Ch + Ty), WC-rCTB alone (group Ch) or Ty21a alone (group Ty). Peripheral blood mononuclear cells (PBMCs) were isolated on Days 0, 5 and 7 from all, plus on Days 12 and 14 from WC-rCTB recipients, to assess antibody-secreting cells (ASCs) specific to rCTB and to typhoidal O9,12-structures by enzyme-linked immunosorbent spot (ELISPOT) assay. Vibriocidal antibodies were assessed, and anti-rCTB IgA/IgG and anti-S. Typhi lipopolysaccharide (LPS) IgA/IgG/IgM were measured by enzyme-linked immunosorbent assay (ELISA) in Day 0 and 28 ± 3 serum samples. Adverse events (AEs) were recorded during one month. RESULTS:The final study population included 63 volunteers, 21 per group. A non-significant trend towards stronger rCTB-specific ASC (IgA + IgG + IgM) peak responses was observed in group Ch + Ty compared to group Ch (geometric mean, GM 94 vs 32 ASC/106 PBMC, P = 0.096). Serum anti-rCTB IgA and IgG fold rises (post-vaccination vs pre-vaccination) were higher in group Ch + Ty than in group Ch (IgA P = 0.039, IgG P = 0.028), whereas vibriocidal fold rises were comparable between the two groups (P = 0.847). ASC (IgA + IgG + IgM) peak responses to typhoidal O9,12-structures were comparable between groups Ch + Ty and Ty (GM 183 vs. 210 ASC/106 PBMC, P = 0.684). Serum anti-S. Typhi LPS IgA, IgG and IgM fold rises were also similar across Ch + Ty and Ty groups (all P-values ≥0.145). AEs were comparable in single and co-administration groups. CONCLUSIONS:Co-administration of the oral cholera and typhoid vaccines demonstrated favourable safety and robust immunogenicity for both vaccines, supporting their simultaneous use without spacing precautions.

Background: Enteric disease caused by Shigella, Campylobacter, and enterotoxigenic Escherichia coli (ETEC) represents a significant global health burden, particularly among children in low-resource settings. However, no licensed vaccines are currently available for these bacterial pathogens. Given the wide range of enteric pathogens and the constraints posed by an increasingly crowded infant immunization schedule, the development of combination vaccines or combined administration of individual oral vaccines may offer a practical approach to address this unmet need. Objectives: In this study, we evaluated the combined administration of two multicomponent oral vaccine candidates: ETVAX, targeting ETEC, and a trivalent whole-cell vaccine targeting Shigella. Methods: The vaccine candidates were administered orally in mice, both individually and in combination, with and without the inclusion of the double-mutant heat-labile toxin (dmLT) adjuvant. Results: The results demonstrated systemic and intestinal-mucosal immune responses to the key protective antigens following both individual and combined vaccine administration. Importantly, the combination of the two vaccines did not compromise the elicitation of specific antibody responses. The inclusion of dmLT as an adjuvant significantly enhanced immune responses to several antigens, highlighting its potential to improve vaccine efficacy. Conclusions: These findings underscore the feasibility of combining ETEC and Shigella vaccine candidates into a single formulation without compromising immunogenicity. This combined approach has the potential to provide broad protective coverage, thereby mitigating the global impact of enteric diseases and streamlining vaccine delivery within existing childhood immunization programs. Our results support further development of this combination vaccine strategy as a promising tool in combating enteric infections and improving health outcomes, particularly among young children in endemic regions who are vulnerable to enteric disease.

Introduction:Airway mucosa represents the main entry point for several human pathogens, and as such vaccines against respiratory diseases should ideally elicit protective immune responses in the airways. We have previously reported two immunomodulatory adjuvants based on non-toxic derivatives of Cholera toxin (CT), namely mmCT and CTB-CpG with strong ability to mount mucosal immune responses. Methods:Herein, we aimed to pinpoint the potential of prime-boost immunization approaches using the fusion-protein based Mycobacterium tuberculosis subunit vaccine candidate H56 as a model antigen, combined with adjuvants CAF01, mmCT, and CTB-CpG in mice. This included a parenteral H56+CAF01 priming followed by an intranasal boost with H56+CAF01, H56+mmCT, or H56+CTB-CpG, compared with repeated homologous intranasal administrations of H56 with each adjuvant. Results:We observed that a parenteral prime with H56+CAF01 followed by an intranasal H56+CTB-CpG booster immunization triggered a Th1-skewed immune response. Conversely, combining the parenteral H56+CAF01 prime with an intranasal H56+mmCT boost resulted in a mixed Th1/Th17-skewed immune response. Notably, the latter combination also engendered anamnestic, long-lived T-cell responses in the lungs which homologous intranasal H56+mmCT immunizations failed to induce. Discussion:These results suggest that an immunization regimen consists of parenteral priming with H56+CAF01 followed by an airway boosting with H56 protein and mucosal adjuvants holds promise in mounting combined systemic and mucosal immune responses to Mycobacterium tuberculosis, and as such warrants further exploration. Given the rising interest in mucosal vaccines for respiratory pathogens, these findings offer an important immunological framework for future translational studies.

Background: Shigella infections remain endemic in places with poor sanitation and are a leading cause of diarrheal mortality globally, as well as a major contributor to gut enteropathy and stunting. There are currently no licensed vaccines for shigellosis but it has been estimated that an effective vaccine could avert 590,000 deaths over a 20-year period. A challenge to effective Shigella vaccine development has been the low immunogenicity and protective efficacy of candidate Shigella vaccines in infants and young children. Additionally, a new vaccine might be less immunogenic in a highly endemic setting compared to a low endemic setting (“vaccine hyporesponsiveness”). The use of a potent adjuvant enhancing both mucosal and systemic immunity might overcome these problems. InvaplexAR-Detox is an injectable Shigella vaccine that uses a novel combination of conserved invasion plasmid antigen proteins and a serotype-specific bacterial lipopolysaccharide attenuated for safe intramuscular administration. The adjuvant dmLT has been shown to enhance Shigella immune responses in mice, has safely been administered intramuscularly, and was shown to enhance immune responses in healthy volunteers when given in combination with other antigens in phase I trials. This article describes the protocol of a study that will be the first to assess the safety, tolerability, and immunogenicity of InvaplexAR-Detox co-administered with dmLT in healthy adults in low-endemic and high-endemic settings. Methods: In a multi-center, randomized, double-blind, and placebo-controlled dose-escalation phase Ia/b trial, the safety, tolerability, and immunogenicity of three intramuscular vaccinations administered 4 weeks apart with 2.5 µg or 10 µg of InvaplexAR-Detox vaccine, alone or in combination with 0.1 µg of the dmLT adjuvant, will first be assessed in a total of 50 healthy Dutch adults (phase Ia) and subsequently in 35 healthy Zambian adults (phase Ib) aged 18–50 years. The primary outcome is safety, and secondary outcomes are humoral and cellular immune responses to the adjuvanted or non-adjuvanted vaccine. Discussion: This trial is part of the ShigaPlexIM project that aims to advance the early clinical development of an injectable Shigella vaccine and to make the vaccine available for late-stage clinical development. This trial addresses the issue of hyporesponsiveness in an early stage of clinical development by testing the vaccine and adjuvant in an endemic setting (Zambia) after the first-in-human administration and the dose-escalation has proven safe and tolerable in a low-endemic setting (Netherlands). Besides strengthening the vaccine pipeline against a major diarrheal disease, another goal of the ShigaPlexIM project is to stimulate capacity building and strengthen global North-South relations in clinical research. Trial registration: EU CT number: 2023-506394-35-02, ClinicalTrials.gov identifier: NCT05961059.

The feared diarrheal disease cholera remains an important global health problem. Use of oral cholera vac-cine (OCV) from a global stockpile against both epidemic and endemic cholera is a cornerstone in the World Health Organisations (WHOs) global program for "Ending cholera by 2030". Three liquid inacti-vated whole-cell OCVs (Dukoral (R), ShancholTM, and Euvichol-Plus (R)) are WHO prequalified and have proved to be safe and effective. However, their multicomponent composition and cold-chain requirement increase manufacturing, storage and transport costs. ShancholTM and Euvichol-Plus (R) OCVs used in WHOs global vaccine stockpile also lack the protective cholera toxin B-subunit (CTB) antigen present in Dukoral (R), which results in suboptimal efficacy.WHOs Global Task Force on Cholera Control (GTFCC) has identified a thermostable, dry formulation vaccine as a priority for further OCV development. We describe here the development of such a vaccine, based on a lyophilized mixture of a single strain of formalin-killed Hikojima bacteria together with a low-cost, recombinantly produced CTB. The new vaccine, which is easy and inexpensive to manufacture, could be stored for at least 26 months at 25 degrees C and for at least 8 months at 40 degrees C with preservation of cell mor-phology and with no loss of protective Ogawa and Inaba lipopolysaccharides or CTB. It also proved to be well tolerated and to have equivalent oral immunogenicity in mice as ShancholTM and Dukoral (R) OCVs with regard to both serum and intestinal-mucosal antibody responses.(c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

The heat-labile enterotoxins of Escherichia coli and cholera toxin of Vibrio cholerae are related in structure and function. Each of these oligomeric toxins is comprised of one A polypeptide and five B polypeptides. The B-subunits bind to gangliosides, which are followed by uptake into the intoxicated cell and activation of the host's adenylate cyclase by the A-subunits. There are two antigenically distinct groups of these toxins. Group I includes cholera toxin and type I heat-labile enterotoxin of E. coli; group II contains the type II heat-labile enterotoxins of E. coli. Three variants of type II toxins, designated LT-IIa, LT-IIb and LT-IIc have been described. Earlier studies revealed the crystalline structure of LT-IIb. Herein the carbohydrate binding specificity of LT-IIc B-subunits was investigated by glycosphingolipid binding studies on thin-layer chromatograms and in microtiter wells. Binding studies using a large variety of glycosphingolipids showed that LT-IIc binds with high affinity to gangliosides with a terminal Neu5Acα3Gal or Neu5Gcα3Gal, e.g. the gangliosides GM3, GD1a and Neu5Acα3-/Neu5Gcα3--neolactotetraosylceramide and Neu5Acα3-/Neu5Gcα3-neolactohexaosylceramide. The crystal structure of LT-IIc B-subunits alone and with bound LSTd/sialyl-lacto-N-neotetraose d pentasaccharide uncovered the molecular basis of the ganglioside recognition. These studies revealed common and unique functional structures of the type II family of heat-labile enterotoxins.

Patients suffering from systemic lupus erythematosus (SLE) are at increased risk of developing cardiovascular disease (CVD) and traditional therapies including statins provide insufficient protection. Impaired removal of apoptotic material is a common pathogenic mechanism in both SLE and atherosclerosis and is considered to be a key factor in the development of autoimmunity. Since oxidized LDL and apoptotic material bind to the same receptors, we aimed to investigate if targeting the oxidized LDL autoimmunity can affect atherosclerosis in SLE. To investigate the possible role of oxidized LDL autoimmunity in the accelerated atherosclerosis associated with SLE we used a hypercholesterolemic SLE mouse model (B6.lpr.ApoE-/- mice). Promoting LDL tolerance through mucosal immunization with an apolipoprotein B-100 peptide p45 (amino acids 661-680) and cholera toxin B-subunit fusion protein increased regulatory T cells and B cells in mesenteric lymph nodes and reduced plaque development in the aorta by 33%. Treatment with the oxidized LDL-specific antibody Orticumab reduced aortic atherosclerosis by 43%, subvalvular plaque area by 50% and the macrophage content by 31%. The present study provides support for oxLDL as a possible target for prevention of cardiovascular complications in SLE.

Cholera toxin (CT) is widely used as an effective adjuvant in experimental immunology for inducing mucosal immune responses; yet its mechanisms of adjuvant action remain incompletely defined. Here, we demonstrate that mice lacking NFκB, compared to wild-type (WT) mice, had a 90% reduction in their systemic and mucosal immune responses to oral immunization with a model protein antigen [Ovalbumin (OVA)] given together with CT. Further, NFκB−/− mouse dendritic cells (DCs) stimulated in vitro with CT showed reduced expression of MHCII and co-stimulatory molecules, such as CD80 and CD86, as well as of IL-1β, and other pro-inflammatory cytokines compared to WT DCs. Using a human monocyte cell line THP1 with an NFκB activation reporter system, we show that CT induced NFκB signaling in human monocytes, and that inhibition of the cyclic AMP—protein kinase A (cAMP-PKA) pathway abrogated the activation and nuclear translocation of NFκB. In a human monocyte-CD4+ T cell co-culture system we further show that the strong Th17 response induced by CT treatment of monocytes was abolished by blocking the classical but not the alternative NFκB signaling pathway of monocytes. Our results indicate that activation of classical (canonical) NFκB pathway signaling in antigen-presenting cells (APCs) by CT is important for CT's adjuvant enhancement of Th17 responses. Similar findings were obtained using the almost completely detoxified mmCT mutant protein as adjuvant. Altogether, our results demonstrate that activation of the classical NFκB signal transduction pathway in APCs is important for the adjuvant action of both CT and mmCT.

Cholera Toxin (CT) as well as its related non-toxic mmCT and dmLT mutant proteins have been shown to be potent adjuvants for mucosally administered vaccines. Their adjuvant activity involves activation of cAMP/protein kinase A (PKA) signaling and inflammasome/IL-1β pathways in antigen presenting cells (APC). To get a further understanding of the signal transduction and downstream pathways activated in APCs by this group of adjuvants we have, employing quantitative proteomic analytic tools, investigated human monocytes at various time points after treatment with CT. We report the activation of three main biological pathways among upregulated proteins, peaking at 16 hours of CT treatment: cellular organization, metabolism, and immune response. Specifically, in the further analyzed immune response pathway we note a strong upregulation of thrombospondin 1 (THBS1) and integrin β1 (ITGB1) in response to CT as well as to mmCT and dmLT, mediated via cAMP/PKA and NFKB signaling. Importantly, inhibition in vitro of THSB1 and ITGB1 in monocytes or primary dendritic cells using siRNA abrogated the ability of the treated APCs to promote an adjuvant-stimulated Th17 cell response when co-cultured with peripheral blood lymphocytes indicating the involvement of these molecules in the adjuvant action on APCs by CT, mmCT and dmLT.

Antibiotic selection for the maintenance of expression plasmids is discouraged in the production of recombinant proteins for pharmaceutical or other human uses due to the risks of antibiotic residue contamination of the final products and the release of DNA encoding antibiotic resistance into the environment. We describe the construction of expression plasmids that are instead maintained by complementation of the lgt gene encoding a (pro)lipoprotein glyceryl transferase essential for the biosynthesis of bacterial lipoprotein. Mutations in lgt are lethal in Escherichia coli and other Gram-negative organisms. The lgt gene was deleted from E. coli and complemented by the Vibrio cholerae-derived gene provided in trans on a temperature-sensitive plasmid, allowing cells to grow at 30°C but not at 37°C. A temperature-insensitive expression vector carrying the V. cholerae-derived lgt gene was constructed, whereby transformants were selected by growth at 39°C. The vector was successfully used to express two recombinant proteins, one soluble and one forming insoluble inclusion bodies. Reciprocal construction was done by deleting the lgt gene from V. cholerae and complementing the lesion with the corresponding gene from E. coli The resulting strain was used to produce the secreted recombinant cholera toxin B subunit (CTB) protein, a component of licensed as well as newly developed oral cholera vaccines. Overall, the lgt system described here confers extreme stability on expression plasmids, and this strategy can be easily transferred to other Gram-negative species using the E. coli-derived lgt gene for complementation.IMPORTANCE Many recombinant proteins are produced in bacteria from genes carried on autonomously replicating DNA elements called plasmids. These plasmids are usually inherently unstable and rapidly lost. This can be prevented by using genes encoding antibiotic resistance. Plasmids are thus maintained by allowing only plasmid-containing cells to survive when the bacteria are grown in medium supplemented with antibiotics. In the described antibiotic-free system for the production of recombinant proteins, an essential gene is deleted from the bacterial chromosome and instead provided on a plasmid. The loss of the plasmid becomes lethal for the bacteria. Such plasmids can be used for the expression of recombinant proteins. This broadly applicable system removes the need for antibiotics in recombinant protein production, thereby contributing to reducing the spread of genes encoding antibiotic resistance, reducing the release of antibiotics into the environment, and freeing the final products (often used in pharmaceuticals) from contamination with potentially harmful antibiotic residues.

Objective: Intranasal immunization with a fusion protein of the ApoB100-derived peptide p210 and the cholera toxin B subunit (CTB-p210) has previously been shown to induce mucosal tolerance and reduce atherosclerosis development, but the exact mode of action remains to be elucidated. Recent studies have indicated an important role for B cells in mucosal tolerance, in particular by induction of regulatory B (Bregs) and T cells (Tregs). In this study, we aimed to investigate if transfer of B cells pulsed with CTB-p210 can protect against atherosclerosis. Method and results: First, we studied if CTB-p210 can induce Bregs and Tregs in vitro. After pulsing B cells from Apob(tm2gy)ldlr(-/-) or Apoe(-/-) mice with CTB-p210 for 1 h and co-culturing them with naive T cells for 48 h, we observed increased expression of membrane bound TGF beta/latency-associated peptide (mTGF beta/LAP) on B cells and an increased proportion of CD25(hi)FoxP3(+) Tregs. Adoptive transfer of B cells pulsed with CTB-p210 into high-fat diet-fed Apoe(-/-) mice at 8, 10 and 12 weeks of age, reduced the plaque area in the aorta at 20 weeks of age as compared with control-treated (CTB-pOVA treated B cells or PBS) mice. Moreover, mice receiving p210-CTB treated B cells had increased levels of anti-p210 IgG antibodies. Conclusion: Our observations suggest that CTB-p210 pulsed B cells acquire a regulatory phenotype and induce Tregs in vitro. Adoptive transfer of CTB-p210, but not control-treated, B cells into Apoe(-/-) mice decreased atherosclerosis development.

The calcium-binding protein S100A4 has been described to promote pathological inflammation in experimental autoimmune and inflammatory disorders and in allergy and to contribute to antigen presentation and antibody response after parenteral immunization with an alum-adjuvanted antigen. In this study, we extend these findings by demonstrating that mice lacking S100A4 have a defective humoral and cellular immune response to mucosal (sublingual) immunization with a model protein antigen [ovalbumin (OVA)] given together with the strong mucosal adjuvant cholera toxin (CT), and that this impairment is due to defective adjuvant-stimulated antigen presentation by antigen-presenting cells. In comparison to wild-type (WT) mice, mice genetically lacking S100A4 had reduced humoral and cellular immune responses after immunization with OVA plus CT, including a complete lack of detectable germinal center reaction. Further, when stimulated in vitro with OVA plus CT, S100A4-/- dendritic cells (DCs) showed impaired responses in several CT-stimulated immune regulatory molecules including the co-stimulatory molecule CD86, inflammasome-associated caspase-1 and IL-1β. Coculture of OVA-specific OT-II T cells with S100A4-/- DCs that had been pulse incubated with OVA plus CT resulted in impaired OT-II T cell proliferation and reduced production of Th1, Th2, and Th17 cytokines compared to similar cocultures with WT DCs. In accordance with these findings, transfection of WT DCs with S100A4-targeting small interfering RNA (siRNA) but not mock-siRNA resulted in significant reductions in the expression of caspase-1 and IL-1β as well as CD86 in response to CT. Importantly, also engraftment of WT DCs into S100A4-/- mice effectively restored the immune response to immunization in the recipients. In conclusion, our results demonstrate that deficiency in S100A4 has a strong impact on the development of both humoral and cellular immunity after mucosal immunization using CT as adjuvant.

There is an urgent need for new adjuvants that are effective with mucosally administered vaccines. Cholera toxin (CT) is the most powerful known mucosal adjuvant but is much too toxic for human use. In an effort to develop a useful mucosal adjuvant we have generated a novel non-toxic mutant CT molecule that retains much of the adjuvant activity of native CT. This was achieved by making the enzymatically active A subunit (CTA) recalcitrant to the site-specific proteolytic cleavage ("nicking") required for toxicity, which was found to require mutations not only in the two residues rendering the molecule resistant to trypsin but also in neighboring sites protecting against cleavage by Vibrio cholerae proteases. This multiple-mutated CT (mmCT) adjuvant protein could be efficiently produced in and purified from the extracellular medium of CT-deleted V. cholerae. The mmCT completely lacked detectable enterotoxicity in an infant mouse model and had >1000-fold reduced cAMP inducing activity compared to native CT in a sensitive mammalian target cell system. It nonetheless proved to have potent adjuvant activity on mucosal and systemic antibody as well as cellular immune responses to mucosally co-administered antigens including oral cholera and intranasal influenza vaccines. We conclude that mmCT is an attractive novel non-toxic mucosal adjuvant for enhancing immune responses to co-administered mucosal vaccines.

We have examined the molecular pathways involved in the adjuvant action of cholera toxin (CT) and two novel nontoxic molecules, multiple-mutated CT (mmCT) and double-mutant heat-labile toxin (dmLT) on human T cell responses. Human PBMCs or isolated monocytes were stimulated in vitro with CT, mmCT, or dmLT plus a polyclonal stimulus (staphylococcal enterotoxin B) or specific bacterial Ags, and effects on expression of cytokines and signaling molecules were determined. CT, mmCT, and dmLT strongly enhanced IL-17A and to a lesser extent IL-13 responses, but had little effect on IFN-γ production or cell proliferation. Intracellular cytokine staining revealed that the enhanced IL-17A production was largely confined to CD4+ T cells and coculture experiments showed that the IL-17A promotion was effectively induced by adjuvant-treated monocytes. Relative to CT, mmCT and dmLT induced at least 100-fold lower levels of cAMP, yet this cAMP was enough and essential for the promotion of Th17 responses. Thus, inhibition of cAMP-dependent protein kinase A was abolished, and stimulation with a cAMP analog mimicked the adjuvant effect. Furthermore, CT, mmCT, and dmLT induced IL-1β production and caspase-1 activation in monocytes, which was associated with increased expression of key proinflammatory and inflammasome-related genes, including NLRP1, NLRP3, and NLRC4. Inflammasome inhibition with a specific caspase-1 inhibitor, or blocking of IL-1 signaling by IL-1 receptor antagonist, abrogated the Th17-promoting effect. We conclude that CT, mmCT, and dmLT promote human Th17 responses via cAMP-dependent protein kinase A and caspase-1/inflammasome–dependent IL-1 signaling.

The survival of pediatric patients with cancer entities including osteosarcoma and Ewing's sarcoma (ES), remains extremely low hence novel treatment approaches are urgently needed. Therefore, based on the concept of targeted therapy, numerous potential targets for the treatment of these cancers have been evaluated pre-clinically or in some cases even clinically during the last decade. In ES the CD99 protein is an attractive target antigen. In this respect, a new entry site for therapeutic intervention may derive from specific human antibodies against CD99. Human scFvC7 was isolated from a semi-synthetic ETH-2 antibody phage library panned on the extracellular portion of recombinant human CD99 protein. The scFvC7 was genetically sequenced, tested for CD99 recognition on an array of recombinant CD99 fragments and measured for binding affinity by ELISA. Finally, it was tested for staining CD99 antigen on a large panel of tumor and normal cells and tissues by cytofluorimetric and immunohistochemical assays. The new antibody scFvC7 recognizes the CD99 extracellular domain included between residues 50 and 74 with a binding affinity of 2.4 x 10(-8) M. In contrast with all other antibodies to CD99 so far isolated, scFvC7 shows a unique specificity in cancer cell recognition: It stained prevalently ES cells while no or weak reactivity was observed on the majority of the other tumor and normal cells and tissues. Thanks to its properties the new anti-CD99 antibody here described represents the first step towards the construction of new selective ES therapeutics.