The influence of HIV gp120 structure on the helper T-cell and antibody response
Description
The development of an effective vaccine against HIV has been hampered by an incomplete understanding of the correlates of protection against the virus. It is generally accepted that robust antibody and cytotoxic T-lymphocyte (CTL) responses both contribute to the control of disease progression. Both of these arms of the immune system require help from CD4+ helper T-cells. However, several important aspects of the CD4+ helper T-cell response remain poorly defined, namely, (1) the factors that determine epitope immunodominance in the CD4+ response, (2) the relationship of specificity in the CD4+ T-cell response to specificity in the antibody and CD8+ responses, and (3) the investment made by HIV (or any pathogen) to control the CD4+ T-cell response Previous studies in mice showed that antigen structure modulates antigen processing and presentation of CD4+ helper T-cell epitopes. Immunodominant CD4+ helper T-cell epitopes raised in response to immunization with the HIV envelope glycoprotein gp120 were found adjacent to flexible loops between elements of secondary structure. This was rationalized by the fact that flexible loops more readily conform to protease active sites and therefore are preferentially cleaved by proteases during antigen processing. In this study, we show that helper T-cell epitopes against the gp120 envelope glycoprotein in humans infected with HIV are also found flanking flexible loops. Immunodominant epitopes were found to be located primarily in the outer domain, an average of 12 residues C-terminal to flexible loops. In the less immunogenic inner domain, epitopes were found an average of 5 residues N-terminal to conserved regions of the protein, once again placing the epitopes C-terminal to flexible loops. These results suggest that antigen structure plays a significant role in the shaping of the helper T-cell response against HIV gp120 in both mice and humans Mapping of the helper T-cell response to the gp120 envelope glycoprotein also revealed a marked absence of CD4+ T-cell responses to regions of gp120 that coincided with the location of disulfide bonds. It was hypothesized that these disulfide bonds stabilized these regions of the protein, protecting them from proteolysis. This results in the exclusion of these regions from the helper T-cell epitope repertoire. It was further hypothesized that the deletion of these disulfide bonds would create new helper T-cell epitopes, which in turn would alter the antibody response To test these hypotheses, three disulfide-bond variants of gp120 were constructed by substituting paired cysteines with alanines. Characterization of the mutants suggested that gp120dss296 was partially unfolded, while gp120dss378 and gp120dss385 retained overall WT-like structures, albeit with slight modifications. Groups of 10 Balb/c mice immunized with these proteins produced very different patterns of helper T-cell response from each other and from a group of 10 Balb/c mice immunized with WT gp120. In general, the T-cell response was reduced in mice immunized with the variant proteins. Surprisingly, anti-gp120 antibody titers increased in these same mice. Interestingly, antibody produced in response to immunization with gp120dss378 inhibited CD4-binding to WT gp120 Statistical analysis of the data from mice immunized with the variant proteins revealed that some helper T-cell responses were positively correlated with the antibody response against either the intact protein or linear epitopes, while others were negatively correlated. Surprisingly, no helper T-cell responses were correlated to the antibody response against the intact protein for mice immunized with WT. Positive correlations were observed however between the helper T-cell response and the antibody response against three linear epitopes in this group of mice. The presentation of particular subsets of helper T-cell epitopes by B-cells in the germinal center presumably enables the B-cells to recruit T-cell help for development into antibody-producing plasma cells. We propose that deletion of disulfide bonds within gp120 altered its processing and changed the timing of the appearance of some CD4+ epitopes, improving the ability of B-cells to recruit T-cell help. These results may be useful in the rational design of an effective vaccine against HIV