The mechanisms underlying initiation and maintenance of CD4 T cell responses after DNA vaccination were studied using a construct coding for nonsecreted fifth component of complement (C5) protein, thus restricting the availability of antigen. The only cell types to express C5 were keratinocytes at the site of DNA application and a small number of dendritic cells present in the draining lymph nodes. Antigen expression persisted for up to 12 wk in keratinocytes, but dendritic cells did not express C5 beyond 2 wk after vaccination. Cross-priming of dendritic cells by C5 expressed in keratinocytes did not occur unless keratinocyte death was induced by irradiation in vitro. CD4 T cells were activated in the draining lymph nodes only and subsequently migrated to the spleen, where memory T cells persisted for longer than 40 wk despite the absence of a source of persistent antigen. While DNA vaccination resulted in transfection of a small proportion of dendritic cells only, it led to general activation of all dendritic cells, thus providing optimal conditions for effective T cell activation and maintenance of memory.

One of the difficulties in understanding the complex pathology of human immunodeficiency virus (HIV) infection is to explain the progressive depletion of the CD4 helper T cell population and consequently the destruction of the immune system. Although cytopathic effects of HIV are observed in vitro, they cannot in vivo account for CD4 T cell depletion because relatively few cells are productively infected. Thus immunological mechanisms must be envisaged. We have found that peripheral blood lymphocytes (PBLs) from asymptomatic HIV-infected individuals are primed for a suicide process known as apoptosis or programmed cell death (PCD). DNA fragmentation characteristic of apoptosis was enhanced by stimulation of lymphocytes with ionomycin, a known inducer of apoptosis in suitably primed cells. Identification of the T cell subpopulations programmed for apoptosis indicated that both CD4+ and CD8+ cells died when cultured without stimulation or when polyclonally stimulated with ionomycin. Activation-induced cell death was also observed after stimulation with self-MHC class II-dependent superantigens, namely bacterial toxins from Staphylococcus (SEB), Streptococcus (ETA), and Myocoplasma (MAM) and under these conditions the CD4+ T cells were preferentially affected. To explore whether new macromolecular synthesis were required for apoptosis, various known inhibitors of apoptosis such as cycloheximide, cyclosporin A, Zn2+, or EGTA were tested. Activation-induced apoptosis was found sensitive to these inhibitors, indicating an active mechanism, but apoptosis observed in nonstimulated cultures was not, suggesting that these cells already contained the complete machinery for death. Prevention of apoptosis could be obtained in the presence of a mixture of cytokines and the minimal signal necessary for this prevention was IL-1 alpha and IL-2. Finally, a correlation between PCD and AIDS-pathogenesis was suggested by the comparison of lymphocytes from lentivirus-infected primates suceptible (SIV-infected macaques) and resistant (HIV-infected chimpanzees) to AIDS. Altogether our results suggest that, during HIV or SIV infection, PCD may contribute in vivo to the deletion of reactive T cells after antigenic stimulation.

The mechanisms by which immunological memory is maintained after infection or vaccination are still a matter of debate. Long-term survival of memory T cells does not require major histocompatibility complex (MHC) contact. We show here that compared with memory CD4+ T cells that maintain contact with MHC class II, memory CD4+ T cells deprived of MHC class II contact show distinct functional defects upon antigen re-encounter. Thus, in contrast to their survival, maintenance of the typical quality of memory T cells crucially depends on MHC-derived signals.

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