PROJECT SUMMARY Acute inflammatory signals associated with infection lead to HSC proliferation thus generating large numbers of immune effector cells. However, more severe and prolonged inflammation has been linked to bone marrow failure states, e.g. complications following bone marrow transplantation. Infectious as well as non-infectious inflammatory signals affect HSC homeostasis. Such effects can be mediated directly on HSC or indirectly via niche cells. CXCL12-expressing niche cells represent an important niche cell population forming distinct niches that support hematopoietic stem and/or progenitor cells. Utilizing complementary novel imaging approaches for 2D and 3D analysis of the bone marrow cavity, we show that inflammatory stress following administration of TNF-α and/or lipopolysaccharide (LPS) rapidly reduces the number of CXCL12 expressing niche cells and causes defects in niche cell cytoplasmic structure within 1-4 hours. We propose that understanding how inflammatory signals modulate HSCP niche cells may implicate a pathogenic role of niche cells in disorders of dysfunctional hematopoiesis associated with inflammation in the peri-transplant setting. In Aim 1 we will delineate the consequences of inflammation on CXCL12-niche cell number and/or growth factor expression in specific CXCL12 expressing niche cell populations. The analyses will be done at various time intervals following administration of TNF-α, LPS. In aim 2, the dynamic alterations in CXCL12 expressing niche cell cytoplasmic structure will be visualized and quantified at different time intervals by three complementary imaging techniques. The dynamic structural changes will be correlated with hematopoietic cellularity in BM and egress into the circulation. In aim 3, we will examine whether LPS and TNF-α mediate their effect on hematopoiesis directly or indirectly (e.g. by acting first on hematopoietic cells such as macrophages). To this end, 4 separate reconstituted chimeric mice in which the hematopoietic compartment/bone marrow microenvironment is either WT or deficient in TNFR/TLR4, will be generated. We propose that understanding the dynamic relationship between stromal cells and HSPC following inflammatory stress may lead to new therapeutic approaches for hematopoietic regeneration in bone marrow dysfunction states in the peri- transplant setting, which require significant blood product support.