Immunologic Strategies Directed Toward HIV Infection

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The use of effective antiretroviral therapy (ART) to treat HIV-infected individuals has dramatically changed the clinical outcome in many patients and has led to a substantial decline in the incidence of AIDS and in AIDS-related mortality. However, it is now clear that prolonged suppression of plasma viremia by ART is not likely to eradicate HIV in most infected individuals. In addition, long-term ART may lead to drug-induced toxicities, difficulties in adhering to drug regimens, and development of drug-resistant virus. Thus, development of alternative immunologic strategies aimed at efficient suppression of HIV in infected individuals is urgently needed. Over the past year, we conducted three research projects, two basic and one clinical, to better understand how the host immune system controls HIV replication and to develop alternative treatment strategies. Characterization and identification of CD8+ T cell-associated antiviral factors in HIV-infected long-term non-progressors At least two types of CD8+ T cell-mediated antiviral activities play an important role in controlling HIV infection. The first is a suppressive activity against HIV involving lysis of infected cells in an antigen-specific, HLA-restricted fashion, while the second mechanism inhibits viral replication in the absence of cell killing, in a process that includes CC-chemokines. Whereas these CC-chemokines exert their antiviral activity on the binding and ultimate entry of HIV into CD4+ T cells and macrophages, another class of currently unidentified antiviral factor (s) suppresses post-entry viral replication by down-regulating transcription of HIV in infected cells. We have constructed a cDNA library from CD8+ T cells isolated from three HIV-infected long-term non-progressors and have begun screening for genes involved in the inhibition of HIV replication. Upon identification of such antiviral candidates, we plan to characterize their biological properties in vitro and pursue a clinical strategy aimed at utilizing these CD8+ T cell-derived antiviral factors in vivo. Analysis of anti-HIV activities of plasmacytoid dendritic cells Plasmacytoid dendritic cells (pDCs) represent a small fraction of total blood cells. However, they exhibit very potent antiviral activities, in part through the production of large quantities of the antiviral compound interferon (IFN)-a. A severe reduction in numbers of blood pDCs has been observed in HIV-infected individuals, and the levels of pDCs do not return to normal following initiation of effective antiviral therapy. Previous in vitro studies have suggested that pDCs may exert an anti-HIV effect, but work addressing the ability of patient-derived pDCs to suppress HIV replication has been lacking. During the past year, we began to examine the ability of pDCs from HIV-infected individuals to suppress endogenous viral replication in autologous CD4+ T cells ex vivo. We are investigating 1) the mechanism by which pDCs suppress HIV replication, 2) the effect of active viral replication on pDC functions, 3) the effect of plasma viremia on differential expression of genes in pDCs, and 4) the level of apoptosis and necrosis in pDCs induced by active HIV replication in vitro. Role of an immunosuppressive drug in HIV replication Despite the development of successful therapeutic strategies, it has not been possible to eradicate HIV in infected individuals, mainly due to the persistence of various viral reservoirs. In particular, replication-competent virus, HIV-1 proviral DNA including 2 LTR circles, and spliced and unspliced HIV-1 RNA in CD4+ T cells have been found in the majority of infected individuals in whom plasma viremia is below the limit of detection. This persistent viral reservoir has emerged as the major obstacle preventing the eradication of HIV. In addition, we find a statistically significant inverse correlation between the frequency of CD4+ T cells carrying HIV-1 proviral DNA and the CD4+/CD8+ T cell ratios in aviremic infected individuals receiving ART (with plasma virus levels below the limit of detection for prolonged periods of time [>2.5 years]). Strategies aimed at minimizing cellular activation may further diminish residual viral replication in patients receiving ART. To address this question, we have begun a pilot clinical trial to examine the safety and tolerability of a mildly immunosuppressive agent, daclizumab, a humanized IgG1 monoclonal antibody that binds specifically to the alpha subunit of the human high-affinity IL-2 receptor expressed on the surface of activated lymphocytes. Considering the intimate relationship between the degree of immune activation and HIV replication, we aim to determine whether daclizumab can normalize immunologic profiles and reduce plasma viremia in study subjects.