Progressive HIV-1 disease is accompanied by high viral loads and depletion of circulating CD4+ T cells, leading to a state of profound immunodeficiency. Such cell loss may be caused by accelerated destruction of mature T cells; it may also be due to the fact that HIV-1 interferes with the ability of the body to make new T cells from the bone marrow and/or thymus. Consistent with the latter hypothesis (of "regenerative failure") is the fact that many HIV-1-infected patients have deficits not only of CD4+ T cells but of other blood lineages as well. It is the central hypothesis of this proposal that HIV-1 infection leads to destruction and/or dysfunc:ion of the bone marrow and thymic microenvironments which enable multilineage and lineage-committed hematopoiesis. We will take advantage of a novel system for the analysis of human hematopoiesis in vitro, the human bone marrow microaggregate (or "MAG") culture, which has been developed during the previous funding period of this grant. In preliminary experiments, MAG cultures have been found to support multilineage and lineage-committed human hematopoiesis for periods of time as long as 8 days and to be permissive for HIV-1 infection. Results from these experiments will be correlated with those obtained from studies of bone marrow cell turnover in HIV-1-infected subjects, using a novel technique for the quantitation of cell production in vivo, the deuterated glucose technique. We will specifically address the hypothesis that HIV-1 suppresses multilineage hematopoiesis in the bone marrow microenvironment by inhibiting proliferation, differentiation, or survival of hematopoietic progenitor cells. Using this two-pronged approach, the following specific aims will be addressed: Specific Aim 1. To optimize HIV-1 infection of human bone marrow microaggregate cultures and to characterize HIV-1-induced effects on cell proliferation, differentiation, and survival. Specific Aim 2. To determine whether hematopoietic cell turnover in the bone marrow is altered by HIV-1 infection. Specific Aim 3. To investigate potential mechanisms of HIV-1-inducedhematosuppression in the bone marrow microaggregate cultures, including (a) modulation of hematopoietic cytokines and (b) interruption of cell-cell contacts via adhesion molecules.