Macaque models for early immunosuppression and T cell loss during acute immunodeficiency virus infections

The interval of acute infection with immunodeficiency viruses is critically important for determining the long-term rate of disease progression. The steps of initial infection, systemic dissemination, and explosive replication of pathogenic SIV or SHIV in macaques are being mapped to show the mechanisms responsible for remodeling host immunity, for establishing the persistent infection, and for promoting disease progression. Here, we describe recent studies on two ways in which CD4+ T cell populations are depleted during acute infection. Initially, we discuss recent work on the mechanisms for CD4+ T cell-mediated, MHC-unrestricted cytolysis. This mechanism shows how even soluble viral antigens such as the envelope glycoprotein, can prime CD4+ lymphocytes to be both effector and target cells in an unrestricted cytolysis mechanism. The consequence of unrestricted cytolysis is a more rapid destruction of the CD4+ T cell population. Secondly, we discuss the broader issue of T cell hyperactivation during acute infection. Inappropriate activation of this lymphocyte population renders cells susceptible to activation induced cell death and also increases the rate of virus replication. Macaque immunization studies have shown a clear role for extracellular Tat in hyperactivation. These two mechanisms, unrestricted cytolysis and T cell hyperactivation, are components of the acute infection that remodel host immunity and dictate the rate of progression to AIDS.