T cells play a primordial role in antiviral immunity. Virus-specific T-cell responses can be characterized by a number of independent variables. These include the magnitude of the response; the functional quality of the response, i.e. the types of cytokines secreted after stimulation and the proliferative or lytic potential; the tissue distribution of the T cells; the breadth of the response; and the avidity of the response. All of these together constitute the T-cell response to antigen (Ag) and comprise potential variables that may correlate with antiviral protective immunity. Substantial advances have recently been obtained in the characterization of virus-specific T-cell responses. These studies have shown that the quality (in term of functional profile) rather than the quantity of Ag-specific T cells was associated with protection. Recently, the term polyfunctional has been used to define T-cell responses that, in addition to typical effector functions such as secretion of IFN-g, TNF-a and MIP-1b and cytotoxic activity, comprise distinct T-cell populations, also able to secrete IL-2 and retaining Ag-specific proliferation capacity. The term \only effector" defines T-cell responses/ populations able to secrete cytokines such as IFN-g, TNF-a and MIP-1b and endowed with cytotoxic activity but lacking IL-2 and proliferation capacity. Several models of virus infections (HIV-1, cytomegalovirus [CMV], Epstein Barr virus [EBV], influenza [Flu] and Herpes Simplex virus) exclusively differentiated on the basis of Ag exposure and persistence, were investigated: 1) antigen clearance, 2) protracted Ag exposure and persistence and low Ag levels, 3) Ag persistence and high Ag levels, and 4) acute Ag exposure/re-exposure. These analyses have demonstrated that polyfunctional and not \only effector" T-cell responses were associated with protective antiviral immunity. However, the factors and mechanisms governing the generation of functionally distinct T-cell populations remain to be elucidated. Recently, several studies have shown a major influence of HLA genotype in the evolution of HIV and the progression of HIV-associated disease. In particular, certain HLA-B alleles were most closely associated with non-progressive disease and low viral load or disease and had a dominant involvement on the clinical course of HIV-associated diseases. In this study, we have investigated the relationship between HLA restriction and the functional profile of Tcell responses in order to determine whether HLA-B influenced the generation of polyfunctional CD8 T-cell responses. To be able to address this issue, we studied CD8 T-cell responses against HIV-1, CMV, EBV and Flu in 128 subjects. These analyses enabled us to demonstrate that HLA-Arestricted epitopes were mostly associated with \only effector" T-cell responses while, in contrast, polyfunctional CD8 T-cell responses were predominantly driven by virus epitopes restricted by HLA-B alleles. We then characterized eventual differences in the responsiveness of CD8 T-cell populations restricted by different HLA-A and HLA-B alleles. For this purpose, we investigated the T-cell receptor (TCR) avidity for the cognate epitope of polyfunctional and \only effector" CD8 T-cell populations. Our results indicated that overall virus-specific CD8 T-cell populations recognizing virus epitopes restricted by HLA-B alleles were equipped with lower avidity TCR for the cognate epitopes when compared to those recognizing epitopes restricted by HLA-A alleles.
In conclusion, these results provide the rationale for the observed protective role of HLA-B genotypes in HIV-1- infection and new insights into the relationship between TCR avidity and functional profile of virus-specific CD8 Tcells.