Mapping HIV-1 Vaccine Induced T-Cell Responses: Bias towards Less-Conserved Regions and Potential Impact on Vaccine Efficacy in the Step Study

T cell directed HIV vaccines are based upon the induction of CD8+ T cell memory responses that would be effective in inhibiting infection and subsequent replication of an infecting HIV-1 strain, a process that requires a match or near-match between the epitope induced by vaccination and the infecting viral strain. We compared the frequency and specificity of the CTL epitope responses elicited by the replication-defective Ad5 gag/pol/nef vaccine used in the Step trial with the likelihood of encountering those epitopes among recently sequenced Clade B isolates of HIV-1. Among vaccinees with detectable 15-mer peptide pool ELISpot responses, there was a median of four (one Gag, one Nef and two Pol) CD8 epitopes per vaccinee detected by 9-mer peptide ELISpot assay. Importantly, frequency analysis of the mapped epitopes indicated that there was a significant skewing of the T cell response; variable epitopes were detected more frequently than would be expected from an unbiased sampling of the vaccine sequences. Correspondingly, the most highly conserved epitopes in Gag, Pol, and Nef (defined by presence in >80% of sequences currently in the Los Alamos database www.hiv.lanl.gov) were detected at a lower frequency than unbiased sampling, similar to the frequency reported for responses to natural infection, suggesting potential epitope masking of these responses. This may be a generic mechanism used by the virus in both contexts to escape effective T cell immune surveillance. The disappointing results of the Step trial raise the bar for future HIV vaccine candidates. This report highlights the bias towards less-conserved epitopes present in the same vaccine used in the Step trial. Development of vaccine strategies that can elicit a greater breadth of responses, and towards conserved regions of the genome in particular, are critical requirements for effective T-cell based vaccines against HIV-1

Selection and Characterization of Murine Monoclonal Antibodies to Staphylococcus aureus Iron-Regulated Surface Determinant B with Functional Activity In Vitro and In Vivo▿ †

In an effort to characterize important epitopes of Staphylococcus aureus iron-regulated surface determinant B (IsdB), murine IsdB-specific monoclonal antibodies (MAbs) were isolated and characterized. A panel of 12 MAbs was isolated. All 12 MAbs recognized IsdB in enzyme-linked immunosorbent assays and Western blots; 10 recognized native IsdB expressed by S. aureus. The antigen epitope binding of eight of the MAbs was examined further. Three methods were used to assess binding diversity: MAb binding to IsdB muteins, pairwise binding to recombinant IsdB, and pairwise binding to IsdB-expressing bacteria. Data from these analyses indicated that MAbs could be grouped based on distinct or nonoverlapping epitope recognition. Also, MAb binding to recombinant IsdB required a significant portion of intact antigen, implying conformational epitope recognition. Four MAbs with nonoverlapping epitopes were evaluated for in vitro opsonophagocytic killing (OPK) activity and efficacy in murine challenge models. These were isotype switched from immunoglobulin G1 (IgG1) to IgG2b to potentially enhance activity; however, this isotype switch did not appear to enhance functional activity. MAb 2H2 exhibited OPK activity (≥50% killing in the in vitro OPK assay) and was protective in two lethal challenge models and a sublethal indwelling catheter model. MAb 13C7 did not exhibit OPK (<50% killing in the in vitro assay) and was protective in one lethal challenge model. Neither MAb 13G11 nor MAb 1G3 exhibited OPK activity in vitro or was active in a lethal challenge model. The data suggest that several nonoverlapping epitopes are recognized by the IsdB-specific MAbs, but not all of these epitopes induce protective antibodies