Population-specific HLA impact in immune control of HIV in Mexico and non-Mexican HIV infected cohorts

HIV-1 persists to be a major health problem worldwide. A prophylactic or therapeutic vaccine offers the best hope to restrain the HIV-1 epidemic, however a consistent correlate of immune protection is yet to be found.
HLA class I expression and their restricting HIV-1 specific CD8+ T cell responses have been shown to play a vital role in the control of HIV-1 infection. The interactions between HIV-1 and CD8+ T cell responses are complex and the mechanisms involved in the success or failure to control viraemia remain uncertain. Thus, the aim of these studies was to help define what CD8+ T cell responses a vaccine needs to induce to achieve durable immune control of HIV-1 infection. Focusing initially on HLA-B*35, an allele that has consistently been associated with rapid HIV-1 disease progression in the context of B clade infection, this study shows substantial differences in markers of HIV-1 disease outcome associated with different HLA-B*35 subtypes. Preliminary data suggest that effective targeting of a single epitope in Gag may be associated with HLA-B*35 mediated control of HIV-1 disease progression. Increased breadth of the Gag- specific CD8+ T cell responses is found to be associated with decreasing viral loads. These data therefore support the Gag hypothesis, and suggest that targeting of certain regions of the HIV-1 genome may have a positive effect in disease outcome, even for individuals carrying “detrimental” alleles.
The extensive diversity of the HIV-1 genome and rapid viral adaptation are the main chal- lenges to vaccine design. Previous studies have suggested that effective CD8+ T cell responses drive selection of escape mutations that reduce viral replication capacity (VRC). There is also evidence that certain escape mutations can be transmitted from one host to another allow- ing for its accumulation in a population. The second study looked at the impact of HLA driven evolution of HIV-1 in VRC at a population level. This study compared two ART-naïve HIV-1 B clade infected cohorts, in Mexico and Barbados, in which protective HLA alleles (HLA- B*27/57/58:01/81:01) are expressed at 10% and 35% respectively, to analyze differences in VRC at a population level. Viral loads (VL) were found to be significantly higher in Mexico compared to Barbados and median CD4+ T cell counts significantly lower. Analysis of VRC in a subset of subjects in each cohort matched by CD4+ T cell counts between 300-500 cells/μl revealed that VL and VRC was significantly higher in the Mexican subset. This VRC difference was associated with accumulations in Barbados of eight previously described Gag escape mutation where fitness cost has previously been implicated. Accumulation remained significant in mismatched subjects. These data suggest that VLs and disease progression rates may differ between distinct populations as a result of the frequency of protective alleles in the respective populations, and that CD4+ T cell count-based guidelines to initiate antiretroviral therapy (ART) may need to be modified accordingly, to optimize the effectiveness of treatment-for-prevention strategies and reduce HIV-1 transmission rates to the absolute minimum.
The final project aimed to improve the HIV-1 replication fitness assays currently used in the context of C clade infection. In order to achieve this, we attempted to design a clade C infectious molecular clone for the testing of gal-pol gene regions. However, the clones produced were not replication competent. Sequence analysis showed a large quantity of stop codons, most located within env which may explain the lack of infectivity. Chapter 5 describes the methodology used in the construction of the clade C isolate and suggests future work. Although we were unsuccessful in producing a replication competent virus, the construction of a C clade backbone which replicates efficiently remain an aim due to its importance for research directed to the analysis of genetic determinants of C clade virus.
Data presented in this thesis suggest that vaccine-induced immune responses should aim to focus on vulnerable regions of the virus. These are conserved regions that can not escape without a high fitness cost and with a complex and difficult selection of compensatory mutations. Although much work remains to be done to achieve an effective CD8+ T cell based vaccine, hope remains that the induction of HIV control may be possible.

Keywords: AIDS
HLA
Immunology
HIV
Virus
Advisor: Goulder, P