A novel role of complement in mobilization: Immunodeficient mice are poor granulocyte-colony stimulating factor mobilizers because they lack complement-activating immunoglobulins

Complement ( C) and innate immunity emerge as important and underappreciated modulators of mobilization of hematopoietic stem/progenitor cells (HSPC). We reported that ( a) C becomes activated in bone marrow ( BM) during granulocyte-colony-stimulating factor (G-CSF)-induced mobilization by the classic immunoglobulin (Ig)-dependent pathway and that (b) C3 cleavage fragments increase the responsiveness of HSPC to a stromal derived factor-1 gradient. Since patients suffering from severe combined immunodeficiency ( SCID) mobilize poorly, we hypothesized that this could be directly linked to the lack of C activating Ig in these patients. In the current study to better elucidate the role of C activation in HSPC mobilization, we mobilized mice that lack Ig (RAG2, SCID, and Jh) by G-CSF or zymosan, compounds that activate C by the classic Ig-dependent and the alternative Ig-independent pathways, respectively. In addition, we evaluated mobilization in C5-deficient animals. Mobilization was evaluated by measuring the number of colony-forming unit-granulocyte macrophage and leukocytes circulating in peripheral blood. We found that ( a) G-CSF- but not zymosan-induced mobilization was severely reduced in RAG2, SCID, and Jh mice; (b) impaired G-CSF- induced mobilization was restored after infusion of purified wild-type Ig; and ( c) mobilization was severely reduced in C5-deficient mice. These data provide strong evidence that the C system plays a pivotal role in mobilization of HSPC and that egress of HSPC from BM occurs as part of an immune response.