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Abstract 1750 Background: Polycythemia vera (PV) and chronic idiopathic myelofibrosis (CIMF) are both clonal disorders of CD34+ hematopoietic stem and progenitor cells (HSPC) that undergo inappropriate expansion. The pathological CD34+ clones from both disorders are found at increased levels in the peripheral blood of patients, indicating that the interactions between these CD34+ cells and tissue HSPC niches are different from normal HSPC. Other studies have implicated deficiencies in the CXCR4 signaling axis as a possible reason for the abnormal trafficking of HSPCs in PV and CIMF. We set out to functionally test the importance of CXCR4 for the interactions of HSPCs collected from peripheral blood of patients with PV and CIMF with the bone marrow and other tissue microenvironments. Methods: We purified CD34+ cells from peripheral blood of patients with PV or CIMF, labeled them with fluorescent trackers, and engrafted them into unmanipulated SCID mice. Using video rate, scanning confocal microscopy we tracked CD34+ cell interactions with bone marrow, spleen and liver. To functionally test the role of CXCR4 in the homing of patient CD34+ cells to different microenvironments, we treated CD34+ cells with a blocking antibody prior to engraftment. Results: Flow cytometry revealed that PV CD34+ cells had significantly lower surface expression of CXCR4 than control HSPCs (PV mean: 32.93% positive, range: 14.8–55.1; Control mean: 58.85% positive, range: 33.5–78.7; p value = 0.018) while CIMF samples had scattered CXCR4 expression (mean 61.2% positive range: 0.6–95.4) PV CD34+ cells migrated toward SDF-1 in migration assays, demonstrating that although they had lower surface CXCR4 expression, they were still able to respond to signals from the environment through this receptor. In vivo imaging of mice after engraftment revealed that PV and CIMF CD34+ cells had decreased BM homing compared to control HSPCs. However, CD34+ cells from PV and CIMF patients homed to similar areas of bone marrow vasculature as control cells. Treating CD34+ cells with a blocking antibody against CXCR4 before engraftment significantly reduced the number of PV and CIMF cells that homed to the bone marrow, indicating that their bone marrow homing still relied on CXCR4. Interestingly, there were an increased number of PV and CMF cells that homed to the spleen at baseline compared to control HSPC, suggesting that a distinct homing mechanism favored PV and CMF cell trafficking to the spleen over the BM. The number of cells that homed to the spleen increased upon antibody blockade of CXCR4, indicating that splenic homing of CD34+ cells did not depend on CXCR4/SDF-1 interactions. Conclusion: CD34+ HSPC from PV and CIMF patients and controls homed to similar areas of the BM, although PV and CIMF cells had significantly decreased BM homing. Conversely, PV and CIMF CD34+ cells homed to the spleen in significantly higher numbers than control cells. PV, CIMF and control CD34+ cells were all dependent on CXCR4 for maximal BM homing. Splenic homing was not CXCR4 dependent, however, and PV and CIMF cells homed to the spleen in higher numbers when BM homing was inhibited by CXCR4 blockade. Taken together, we conclude that unique signaling mechanisms regulate trafficking of CD34+ cells to the BM vs. the splenic microenvironments. PV and CIMF CD34+ cells have an abnormal response to these tissue signals, resulting in their preferential homing to the spleen. Molecularly targeted therapies with the Jak2 inhibitors significantly decrease splenomegaly in CIMF patients, although they do not reverse the BM disease process. Recent evidence in a mouse model suggests that the BM is in fact a sanctuary site for disease during treatment with these agents. Whether PV and CIMF CD34+ cells in patients can escape apoptosis during treatment by trafficking to more favorable microenvironments is unknown. Our study suggests that an increased understanding of the mechanisms that these cells use to engage different tissue niches could aid the treatment of these diseases. Disclosures: No relevant conflicts of interest to declare.