Human immunodeficiency virus (HIV), the etiological agent of acquired immunodeficiency syndrome (AIDS), enters cells by binding its envelope glycoprotein (Env) to receptor molecules and fusing its membrane with the cell membrane. The infection cycle then proceeds through a series of steps which may result in virus spread and AIDS. Understanding how receptor molecules mediate HIV entry and what determines the kinetics of virus infection is critical for elucidating the mechanisms of HIV pathogenesis and design of new antiretroviral treatments. By using a variety of experimental and mathematical approaches we discovered important interactions of the receptor molecules with the HIV-1 Env as well as relationships between parameters of virus infection kinetics and how they affect response to treatment. We found that a monoclonal antibody, CG10, which is strictly specific for the CD4-gp120 complex, enhanced HIV-1 Env-mediated fusion and infection, possibly due to increased exposure of intermediate structures interacting with coreceptor molecules. We also developed a simple model of the kinetics of the Env-CD4-coreceptor interactions which predicted cooperative effects. Currently, we are further characterizing the interactions in the oligomeric HIV-1 Env-CD4-coreceptor complex leading to entry which is our major research goal for the next year. In order to understand the mechanisms of HIV-1 infection kinetics in vivo, and how it affects pathogenesis and responses to antiretroviral treatment, we worked in several directions: 1) kinetic analysis of fluctuations in virus concentration - we found that HIV-1 RNA exhibits diurnal variations in pediatric patients and developed a mathematical model which allowed us to estimate virus clearance rates (corresponding to half-life on the order of several hours); 2) regeneration rates of CD4 and CD8 cells - our mathematical analysis of data for T lymphocyte regeneration in pediatric cancer patients after treatment with cyclophosphamide demonstrated several distinct patterns of regeneration and relatively low rate constants of CD4 cell regeneration, corresponding to an effective mean doubling time of about 43 days; 3) turnover rates of T cells - we used telomere lengths as a marker for the cell replication history and found the baseline for uninfected children: interestingly during the first 3 years of life telomeres shorten several fold faster than in adults; based on these results we proposed that not only the telomeres but also their rate of shortening is age dependent. Our preliminary data for monkeys indicate that virus infection does not change significantly the CD4 cell telomere length suggesting that the CD4 cell turnover is not significantly affected by the infection; 4) efficacy of antiretroviral treatment - an analysis of the kinetics of response to treatment may indicate lack of complete inhibition of the infection; and 5) analysis of correlations between long- and short-term response to treatment with antiretroviral drugs - the kinetics of initial viral and immune response in HIV-1 infected children treated with ritonavir correlated with long term responses, which would allow predictions of viremia and CD4 cell increase, and possibly clinical outcome, based on observations made during the first week on therapy. An overall conclusion from these results is that the mechanisms of HIV-1 pathogenesis may not be related to a massive direct killing of cells by the virus but rather directly or indirectly may involve impairments in the regeneration ability of CD4 cells, and that only a combination of virological and immunological parameters could determine the heterogeneous response to treatment and probably the natural course of infection. These findings have implications for understanding the mechanisms controlling the HIV-1 life-cycle and the development of AIDS, and for a rationale design of antiviral drugs. AIDS title: HIV-1 Infection / Note: Part of this project is supported by the Pediatric AIDS Foundation Grant # 50663-22-PG (Dr. X. Xiao)