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We have previously shown that the biologically active Tat protein targets and efficiently enters dendritic cells, and increases the proteolytic activities of the immunoproteasome, thereby favoring the generation and presentation of the subdominant MHC-I binding CTL epitopes of heterologous antigens. In the present study, we demonstrate that Tat broadens in vivo epitope-specific T cell responses directed to heterologous antigens including HIV structural proteins. Specifically, co-immunization of mice with OVA and Tat proteins induces CTL responses against subdominant and cryptic OVA-derived epitopes, which are not detected in mice vaccinated with OVA alone. Similarly, mice vaccinated with the HIV-1 Gag, Env or V2-deleted Env antigens in combination with Tat show Th1-type and CTL responses directed to a larger number of T cell epitopes, as compared to mice vaccinated with these proteins in absence of Tat. In contrast, Tat did not affect Th2-type responses to these structural HIV proteins. These results indicate that Tat is not only an antigen but also a novel Th1-type adjuvant capable of broadening in vivo the spectrum of epitopes recognized by T cells, and suggest that Tat can be considered an optimal co-antigen in the development of novel vaccination strategies against AIDS.

We have investigated the molecular mechanisms underlying the peculiar cross-presentation efficiency of human dendritic cells (DCs) differentiated from monocytes in the presence of Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) and Interferon (IFN)-α (IFN-DCs). To this end, we evaluated the capability of IFN-DCs to present and cross-present epitopes derived from Epstein–Barr Virus (EBV) or human melanoma-associated antigens after exposure to cell lysates or apoptotic cells. In an autologous setting, IFN-DCs loaded with Lymphoblastoid Cell Lines (LCL) lysates or apoptotic LCL were highly efficient in expanding, respectively, EBV-specific class II- or class I-restricted memory T cell responses. Of note, IFN-DCs loaded with apoptotic LCL were more potent than fully mature DCs in triggering the cytotoxicity of CD8 + T lymphocytes recognizing a subdominant HLA-A*0201-restricted epitope derived from EBV latent membrane protein 2 (LMP2). In addition, IFN-DCs loaded with apoptotic human melanoma cells were highly efficient in cross-presenting the MART-1 27–35 epitope to a specific CD8 + cytotoxic T cell clone, and this functional activity was proteasome-dependent. These IFN-DC properties were associated with an enhanced expression of all the immunoproteasome subunits as well as of TAP-1, TAP-2 and tapasin, and, interestingly, to a higher proteasome proteolytic activity as compared to immature or mature DCs. Altogether, these results highlight new mechanisms by which IFN-α influences antigen processing and cross-presentation ability of monocyte-derived DCs, with potentially important implications for the development of DC-based therapeutic vaccines.

BackgroundAs HIV-specific cytotoxic T cells play a key role during acute and chronic HIV-1 infection in humans, the ability of potential anti-HIV vaccines to elicit strong, broad T cell responses is likely to be crucial. The HIV-1 Gag antigen is widely considered a relevant antigen for the development of an anti-HIV vaccine since it is one of the most conserved viral proteins and is also known to induce T cell responses. In the majority of studies reporting Gag-specific cellular immune responses induced by Gag-based vaccines, only a small number of Gag T cell epitopes were tested in preclinical mouse models, thus giving an incomplete picture of the numerous possible cellular immune responses against this antigen. As is, this partial knowledge of epitope-specific T cell responses directed to Gag will unavoidably result in a limited preclinical evaluation of Gag-based vaccines.ResultsIn this study we identified new Gag CD8+ T cell epitopes in BALB/c mice vaccinated with the HIV-1 Gag antigen alone or in combination with the HIV-1 Tat protein, which was recently shown to broaden T cell responses directed to Gag. Specifically, we found that CTL responses to Gag may be directed to nine different CTL epitopes, and four of these were mapped as minimal CTL epitopes.ConclusionThese newly identified CTL epitopes should be considered in the preclinical evaluation of T cell responses induced by Gag-based vaccines in mice.