PROJECT SUMMARY: Hematological abnormalities, such as thrombocytopenia, anemia, lymphocytopenia, monocytopenia, and neutropenia, frequently occur in patients infected with Human Immunodeficiency Virus (HIV)-1. Of note, Pancytopenia has been considered as the hallmark feature of patients with advanced Acquired immunodeficiency syndrome (AIDS) (Group IV). In addition, patients with HIV infection are susceptible to HIV-associated opportunistic infections and bone marrow neoplasms. More importantly, antiretroviral treatments result in incomplete rescue of hematopoiesis and are known to impair normal hematopoiesis in patients with HIV infection. Emerging evidence establish a strong link between HIV-1 and hematopoietic stem cells (HSCs) of the bone marrow. Overall, these studies provide a compelling rationale for further investigations into mechanisms through which HIV-1 dysregulates hematopoiesis. However, the cellular mechanisms responsible for pancytopenia and bone marrow failure following HIV-infection remain incompletely understood. More importantly, it remains unclear if HIV infection is directly affecting the physiology and functions of HSCs. To this end, the research proposed here aims to unravel the physiologic impact of HIV infection on HSCs and to identify the progenitors that are susceptible to HIV infection. The major hypothesis of this study is that HIV infection impairs HSC maintenance in the bone marrow and their differentiation into myeloid, erythroid, and lymphoid lineages, which ultimately results in immunodeficiencies. The rationale is that HIV infection alters the physiological properties of HSCs, such as quiescence, self-renewal and multilineage differentiation, therefore suppressing their vital functions. To test our hypothesis, we will use a combination of xenotransplantation, in-vitro culture, molecular cell biology, biochemical and fluorescence correlation spectroscopy studies. In the first specific aim, we would investigate the impact of HIV infection on the physiology of HSCs. In the second specific aim, we would identify the HSPC subsets that are susceptible to HIV-1 both in-vitro and in-vivo. Studies proposed in Aims 1 and 2 are complimentary and informed by the data of each other, but not dependent on the outcome of the other Aim. We believe that the proposed research will provide detailed insights into the mechanisms through which HIV infection impairs HSC biology and causes hematopathology. Knowledge obtained from the proposed research would be useful to design novel and more effective therapies for human hematologic diseases that are caused by viral infections.