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Vaccination against Salmonella Typhi using the Vi capsular polysaccharide, a T-cell independent antigen, can protect from the development of typhoid fever. This implies that antibodies to Vi alone can protect in the absence of a T cell-mediated immune response; however, protective Vi antibodies have not been well-characterized. We hypothesized that variability in the biophysical properties of vaccine-elicited antibodies, including subclass distribution and avidity, may impact protective outcomes. To interrogate the relationship between antibody properties and protection against typhoid fever, we analyzed humoral responses from participants in a vaccine efficacy (VE) trial using a controlled human infection model (CHIM) who received either a purified Vi polysaccharide (Vi-PS) or Vi tetanus toxoid conjugate (Vi-TT) vaccine followed by oral challenge with live S. Typhi. We determined the avidity, overall magnitude, and vaccine-induced fold-change in magnitude from before immunization to day of challenge of Vi IgA and IgG subclass antibodies. Amongst those who received the Vi-PS vaccine, Vi IgA magnitude (FDR p = 0.01) and fold-change (FDR p = 0.02) were significantly higher in protected individuals compared with those individuals who developed disease (“diagnosed”). In the Vi-TT vaccine group, the responses of protected individuals had higher fold-change in Vi IgA (FDR p = 0.06) and higher Vi IgG1 avidity (FDR p = 0.058) than the diagnosed Vi-TT vaccinees, though these findings were not significant at p < 0.05. Overall, protective antibody signatures differed between the Vi-PS and Vi-TT vaccines, thus, we conclude that although the Vi-PS and Vi-TT vaccines were observed to have similar efficacies, these vaccines may protect through different mechanisms. These data will inform studies on mechanisms of protection against typhoid fever, including identification of antibody effector functions, as well as informing future vaccination strategies.

Typhoid Vi vaccines have been shown to be efficacious in children living in endemic regions; however, a widely accepted correlate of protection remains to be established. We applied a systems serology approach to identify Vi-specific serological correlates of protection using samples obtained from participants enrolled in an experimental controlled human infection study. Participants were vaccinated with Vi-tetanus toxoid conjugate (Vi-TT) or unconjugated Vi-polysaccharide (Vi-PS) vaccines and were subsequently challenged with Salmonella Typhi bacteria. Multivariate analyses identified distinct protective signatures for Vi-TT and Vi-PS vaccines in addition to shared features that predicted protection across both groups. Vi IgA quantity and avidity correlated with protection from S. Typhi infection, whereas higher fold increases in Vi IgG responses were associated with reduced disease severity. Targeted antibody-mediated functional responses, particularly neutrophil phagocytosis, were also identified as important components of the protective signature. These humoral markers could be used to evaluate and develop efficacious Vi-conjugate vaccines and assist with accelerating vaccine availability to typhoid-endemic regions.

Nonhuman primate studies conducted after RV144 supported the importance of non-neutralizing antibodies to V2 for protection from experimental challenge. Vaccine-elicited V2 IgG correlated with delayed SIV 9-13 and SHIV acquisition 14 , and with viremia control after SIV infection 13. Passive transfer of 830 A, a V2-specific monoclonal antibody (mAb), resulted in improved viremia control in nonhuman primates 15. Many studies have reported antiviral functions that include antibody Fc effector functions and direct neutralization mediated by antibodies that recognize V2 7,16-22. V2-specific mAbs recognizing aa169, which were isolated from RV144 vaccine recipients, were shown to mediate tier 1 neutralization, bind to infectious virions 23 , and mediate killing of primary HIV-1 isolate infected cells 16. Aa169 is located within the linear epitope sequence bound by RV144 vaccine-elicited antibodies that correlated with decreased HIV-1 risk 4. The other known site of RV144-induced immune pressure in V2, aa181, is also part of the leucine-aspartic acid-isoleucine/valine (LDI/V) aa179-181 sequence motif reported to mediate HIV-1 envelope interaction with the gut mucosal homing integrin receptor α4β7 to facilitate cell-to-cell spread 19,20. It has been postulated that V2-specific antibodies may block or interfere with the interaction between α4β7 and the HIV-1 envelope to prevent viral transmission 19,20. In participants from the RV144 trial and the RV305 trial, in which delayed boosters were given to RV144 participants, a group of V2-specific mAbs were shown to be capable of blocking AE.92TH023 V2 peptide binding to α4β7 24. The α4β7-blocking antibodies included both those targeting the linear V2 hotspot and those targeting the conformational epitopes 25. V2-specific antibodies can mediate phagocytosis 18,26,27 and can synergize with C1-C2 specific IgG for enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) 28. Furthermore, quaternary epitopes involving the V2 loop were identified as a target for broadly neutralizing antibodies 21,29. The Pox-Protein Public-Private Partnership (P5) program was established to develop a subtype C-directed vaccine based on the RV144 regimen 30. As part of this program, two phase 1/2 trials were designed in South Africa: HIV Vaccine Trials Network (HVTN) 097 31 (ClinicalTrials.gov NCT02109354) determined immune responses of an African population to the RV144 regimen, and HVTN 100 32 (ClinicalTrials.gov NCT02404311) investigated the immunogenicity of the regimen adapted to subtype C. The selection criteria for the subtype C envelope immunogens included binding and affinity by V2-specific mAbs 33,34. The HVTN 100 primary immunological data were evaluated against prespecified immunological criteria and guided the decision to proceed with the vaccine regimen into an efficacy trial, HVTN 702. Vaccine-elicited responses in HVTN 100 met all four prespecified go/no-go criteria for the continuation of the HVTN 702 trial, including the envelope-binding and V1V2-binding IgG response 32. Our stu...

RTS,S/AS01 is an advanced pre-erythrocytic malaria vaccine candidate with demonstrated vaccine efficacy up to 86.7% in controlled human malaria infection (CHMI) studies; however, reproducible immune correlates of protection (CoP) are elusive. To identify candidates of humoral correlates of vaccine mediated protection, we measured antibody magnitude, subclass, and avidity for Plasmodium falciparum (Pf) circumsporozoite protein (CSP) by multiplex assays in two CHMI studies with varying RTS,S/AS01B vaccine dose and timing regimens. Central repeat (NANP6) IgG1 magnitude correlated best with protection status in univariate analyses and was the most predictive for protection in a multivariate model. NANP6 IgG3 magnitude, CSP IgG1 magnitude, and total serum antibody dissociation phase area-under-the-curve for NANP6, CSP, NPNA3, and N-interface binding were also associated with protection status in the regimen adjusted univariate analysis. Identification of multiple immune response features that associate with protection status, such as antibody subclasses, fine specificity and avidity reported here may accelerate development of highly efficacious vaccines against P. falciparum.