Functional CXCR4 Expressing Microparticles and SDF-1 Correlate with Circulating AML Cell Counts.

Stromal cell-derived factor-1 (SDF-1/CXCL12) and its receptor CXCR4 are implicated in the pathogenesis and prognosis of AML. Cellular microparticles (MPs), the submicron vesicles shed from the plasma membrane of various circulating cells, are associated with numerous human disorders. In the present study, we studied the putative relationships between CXCR4/SDF-1 axis and MPs in AML. We detected CXCR4 expressing MPs (CXCR4+MPs) in the peripheral blood and bone marrow plasma samples of normal donors (n=24) and newly diagnosed adult AML patients (n=26). The majority of CXCR4+MPs in AML patients were CD45+ whereas in normal individuals they were mostly CD41+. In samples from AML patients, the levels of CXCR4+MPs and total SDF-1 were significantly elevated as compared to normal individuals. Importantly, we found a strong correlation between the levels of CXCR4+MP and white blood cell (WBC) counts in the peripheral blood and bone marrow plasma obtained from the AML patients. Of interest, functional, non-cleaved SDF-1 levels were reduced in these patients compared to normal individuals, and also strongly correlated with the WBC counts. Furthermore, our data indicate N-terminal truncation of the CXCR4 molecule in the MPs of AML patients. This was found also in MPs obtained from the conditioned media of normal human CD34+ progenitors lentiviarlly transduced with CXCR4 vector in vitro. Appearance of MPs possessing N-terminally truncated CXCR4 in AML patients is likely to be dependent on proteolytic enzymes, such as elastase, which was elevated. However, MPs isolated from AML patients were capable of transferring functional CXCR4 molecule to the AML-derived HL-60 cells, enhancing their migration to SDF-1 in vitro and increasing their homing to the bone marrow of irradiated NOD/SCID mice. The CXCR4 antagonist AMD3100 reduced the increased migration and homing of MP treated HL-60 cells. Taken together, these findings suggest that functional CXCR4+MPs and SDF-1 are involved in the progression of AML. We propose that their levels are potentially valuable as an additional diagnostic AML parameter. Moreover, our findings suggest also the need for CXCR4- and SDF-1-target therapeutic approaches, clinically relevant in AML in the near future.