Functional and phenotypic characterization of virus-specific CD8 T cells against cytomegalovirus, Epstein-Barr virus, influenza (flu), and HIV-1 were performed on the basis of the ability of CD8 T cells to secrete IFN-γ and IL-2, to proliferate, and to express CD45RA and CCR7. Two functional distinct populations of CD8 T cells were identified: ( i ) dual IFN-γ/IL-2-secreting cells and ( ii ) single IFN-γ-secreting cells. Virus-specific IFN-γ/IL-2-secreting CD8 T cells were CD45RA - CCR7 - , whereas single IFN-γ CD8 T cells were either CD45RA - CCR7 - or CD45RA + CCR7 - . The proportion of virus-specific IFN-γ/IL-2-secreting CD8 T cells correlated with that of proliferating CD8 T cells, and the loss of HIV-1-specific IL-2-secreting CD8 T cells was associated with that of HIV-1-specific CD8 T cell proliferation. Substantial proliferation of virus-specific CD8 T cells (including HIV-1-specific CD8 T cells) was also observed in CD4 T cell-depleted populations or after stimulation with MHC class I tetramer-peptide complexes. IL-2 was the factor responsible for the CD4-independent CD8 T cell proliferation. These results indicate that IFN-γ/IL-2-secreting CD8 T cells may promote antigen-specific proliferation of CD8 T cells even in the absence of helper CD4 T cells.

The EuroVacc 02 phase I trial has evaluated the safety and immunogenicity of a prime-boost regimen comprising recombinant DNA and the poxvirus vector NYVAC, both expressing a common immunogen consisting of Env, Gag, Pol, and Nef polypeptide domain from human immunodeficiency virus (HIV)-1 clade C isolate, CN54. 40 volunteers were randomized to receive DNA C or nothing on day 0 and at week 4, followed by NYVAC C at weeks 20 and 24. The primary immunogenicity endpoints were measured at weeks 26 and 28 by the quantification of T cell responses using the interferon γ enzyme-linked immunospot assay. Our results indicate that the DNA C plus NYVAC C vaccine regimen was highly immunogenic, as indicated by the detection of T cell responses in 90% of vaccinees and was superior to responses induced by NYVAC C alone (33% of responders). The vaccine-induced T cell responses were (a) vigorous in the case of the env response (mean 480 spot-forming units/106 mononuclear cells at weeks 26/28), (b) polyfunctional for both CD4 and CD8 T cell responses, (c) broad (the average number of epitopes was 4.2 per responder), and (d) durable (T cell responses were present in 70% of vaccinees at week 72). The vaccine-induced T cell responses were strongest and most frequently directed against Env (91% of vaccines), but smaller responses against Gag-Pol-Nef were also observed in 48% of vaccinees. These results support the development of the poxvirus platform in the HIV vaccine field and the further clinical development of the DNA C plus NYVAC C vaccine regimen.

The most common human viruses have different abilities to establish persistent chronic infection. Virus-specific T-cell responses are critical in the control of virus replication and in the prevention of disease in chronic infection. A large number of phenotypic markers and a series of functions have been used to characterize virus-specific CD4+ and CD8+ T-cell responses, and these studies have shown great phenotypic and functional heterogeneity of the T-cell responses against different viruses. The heterogeneity of the T-cell response has been proposed to be specific to each virus. However, over the past 2 years, several studies have provided evidence that the phenotypic and functional heterogeneity of CD4+ and CD8+ T-cell responses is predominantly regulated by the levels of antigen load. The levels of antigen load modulate the phenotypic and functional patterns of the T-cell response within the same virus infection. Furthermore, the functional characterization of virus-specific CD4+ and CD8+ T-cell responses has identified signatures of protective antiviral immunity. Polyfunctional, i.e. interleukin-2 and interferon-gamma (IFN-gamma) secretion and proliferation, and not monofunctional, i.e. IFN-gamma secretion, CD4+ and CD8+ T-cell responses represent correlates of protective antiviral immunity in chronic virus infections.