MOBILIZATION STUDIES IN COMPLEMENT-DEFICIENT MICE REVEAL THAT OPTIMAL AMD3100 MOBILIZATION OF HEMATOPOIETIC STEM CELLS DEPENDS ON COMPLEMENT CASCADE ACTIVATION BY AMD3100-STIMULATED GRANULOCYTES

We reported that complement cascade (CC) becomes activated in bone marrow (BM) during mobilization of hematopoietic stem/progenitor cells (HSPCs) induced by granulocyte-colony stimulating factor (G-CSF) and C5 cleavage plays an important role in optimal egress of HSPCs. In the current work, we explored whether CC is involved in mobilization of HSPCs induced by the CXCR4 antagonist, AMD3100. To address this question, we performed mobilization studies in mice that display a defect in the activation of the proximal steps of CC (Rag(−/−), SCID, C2.Cfb(−/−)) as well as in mice that do not activate the distal steps of CC (C5(−/−)). We noticed that proximal CC activation-deficient mice (above C5 level), in contrast to distal step CC activation-deficient C5(−/−) ones mobilize normally in response to AMD3100 administration. We hypothesized that this discrepancy in mobilization could be explained by AMD3100 activating C5 in Rag(−/−), SCID, C2.Cfb(−/−) animals in a non-canonical mechanism involving activated granulocytes. To support this granulocytes i) as first egress from BM and ii) secrete several proteases that cleave/activate C5 in response to AMD3100. We conclude that AMD3100-directed mobilization of HSPCs, similarly to G-CSF-induced mobilization, depends on activation of CC; however, in contrast to G-CSF, AMD3100 activates the distal steps of CC directly at the C5 level. Overall, these data support that C5 cleavage fragments and distal steps of CC activation are required for optimal mobilization of HSPCs.