Mobilization of CD34 + Progenitor Cells in Association with Decreased Proliferation in the Bone Marrow of Macaques after Administration of the Fms-Like Tyrosine Kinase 3 Ligand

Fms-like tyrosine kinase 3 ligand (FLT3-L) is critical for the differentiation and self-renewal of CD34+ progenitor cells in primates and has been used therapeutically to mobilize progenitor and dendritic cellsin vivo . However, little is known regarding the expansion of progenitor cells outside of peripheral blood, particularly in bone marrow (BM), where progenitor cells primarily reside. Evaluation of FLT3-L-mediated cell mobilization during lentivirus infections, where the numbers of CD34+ progenitor cells are reduced, is limited. We enumerated frequencies and absolute numbers of CD34+ progenitor cells in blood and BM of naive and SIV- or SHIV-infected macaques during and after the administration of FLT3-L. Flow cytometric analyses revealed that, while CD34+ cells increased in the circulation, no expansion was observed in BM. Furthermore, in the BM intracellular Ki67, a marker of cell proliferation, was downregulated in CD34+ progenitor cells but was upregulated significantly in the bulk cell population. Although the exact mechanism(s) remains unclear, these data suggest that CD34+ cell mobilization in blood was the result of cellular emigration from BM and not the proliferation of CD34+ cells already in the periphery. It is possible that the decreased progenitor cell proliferation observed in BM is evidence of a negative regulatory mechanism preventing hyperproliferation and development of neoplastic cells.