Immunodeficient Mice Are Poor Mobilizers -Novel Evidence That Demonstrates a Pivotal Role of Complement in Triggering Mobilization of HSPC.

Abstract

Complement (C) is activated by immunoglobulin-dependent classical and immunoglobulin-independent alternative pathways, and we recently reported that the activation of C enhances the responsiveness of hematopoietic stem/progenitor cells (HSPC) to an SDF-1 gradient cascade by releasing C3a and desArgC3a cleavage fragments (Blood 2003; 101:3784; Blood 2004; 103: 2071). We also found that bone marrow (BM) concentration of C3a and desArgC3a increases during mobilization with G-CSF or cyclophosphamide (CY). To explain this phenomenon we envisioned that these mobilizing agents expose a neo-antigen in BM tissue by turning BM into a highly proteolytic microenvironment. As a consequence of this, the newly exposed neo-epitope becomes bound by natural IgM antibodies leading to the activation of the classical C cascade. In our studies to elucidate the role of C activation in triggering the mobilization of HSPC, we found that C is effectively activated in the BM of G-CSF- or CY- mobilized wild-type (wt) but not IgM-deficient (RAG2null) mice. More importantly we found that several immunodeficient murine strains (RAG2null, SCID and Xid) displayed severely reduced G-CSF-induced mobilization of HSPC which we believe is a result of lack of B lymphocytes and complement-activating immunoglobulins. Supporting this, we found T cell depletion in wt mice did not affect mobilization. Moreover, G-CSF-induced mobilization in RAG2null, SCID and Xid animals was restored after infusion of murine inmmunoglobulins. Furthermore, since mobilization by zymosan or sulfated glycans activates C via the alternative immunoglobulin-independent pathway, we focused on the role of C activation during zymosan-induced mobilization. As expected, zymosan-induced immunoglobulin-independent C activation and mobilization of HSPC were unaffected in immunodeficient (RAG2null, SCID and Xid) mice which have a normal serum level of C3. However, these processes were severely reduced in C3-deficient animals. Thus our data strongly support the notion that C and innate immunity is an important trigger of mobilization of HSPC. While G-CSF and cyclophosphamide activate C by a classical IgM-dependent pathway, zymosan and sulfated polyglycans activate it by employing an alternative pathway. In conclusion, mobilization of HSPC could be envisioned as part of a more global immune response that is triggered/mediated by C3 activation/cleavage.