A Novel and Pivotal Role of the Mannose-Binding Lectin (MBL) Pathway of Complement Cascade (ComC) Activation in Triggering Mobilization of Hematopoietic Stem/Progenitor Cells (HSPCs)

Background. The complement cascade (ComC) is part of the innate immunity system, which is not adaptable and does not change over the course of an individual's lifetime; however, it can be recruited and brought into action by the adaptive immune system. The ComC has several pleiotropic effects, and, as we have previously demonstrated, it is required for mobilization of HSPCs during infection or tissue/organ injuries and responding to pharmacological mobilizing agents (Blood 2004, 103, 2071-2078). The ComC is activated by three pathways: the classical, alternative, and mannose-binding lectin (MBL) pathways. While a requirement for ComC activation and, in particular, the pivotal role of the distal part of complement activation and generation of C5 cleavage fragments was previously demonstrated by us (Leukemia 2009, 23, 2052–2062), mice with mutations to components of the classical and alternative pathways in which the distal pathway of C5 activation remained intact did not show impairment of HSPC mobilization (Leukemia 2010, 24, 1667–1675). However, no studies so far have been performed to address the role of the MBL pathway of ComC activation in triggering the mobilization of HSPCs. The MBL pathway is homologous to the classical pathway, but contains opsonin, MBL, and ficolins instead of C1q. MBL functions by pattern recognition, as opsonin binds to mannose residues on the surface of pathogens and certain cells, and activates the MBL-associated serine proteases, MASP-1, and MASP-2, which can then split C4 (into C4a and C4b) and C2 (into C2a and C2b) to form the classical C3-convertase, as in the classical pathway. Interestingly, it is known that ~10% of the population has defective activation of the MBL pathway.

Hypothesis. We hypothesized for first time that the MBL ComC-activation pathway is involved in triggering ComC-mediated mobilization of HSPCs and that MBL deficiency results in poor mobilization.

Materials and Methods. In our experiments, 2-month-old, MBL-deficient mice (MBL^–/–) and normal wild type (WT) littermates were mobilized for 6 days with G-CSF or AMD3100. Following mobilization, we measured in peripheral blood (PB) i) the total number of white blood cells (WBC), ii) the number of circulating clonogenic colony-forming unit granulocyte/macrophage (CFU-GM) progenitors, and iii) the number of Sca-1⁺c-kit⁺lineage^– (SKL) cells. In parallel, we evaluated activation of the MBL pathway in WT animals after administration of G-CSF and AMD3100.

Results. We found that pattern recognition by the MBL ComC activation pathway is involved in pharmacological G-CSF- and AMD3100-induced mobilization of HSPCs, and activation of the MBL pathway was confirmed by ELISA in WT animals. As predicted, MBL KO mice were found to be poor mobilizers.

Conclusions. We identified a previously unrecognized role of the MBL pathway in triggering ComC activation in the process of HSPC mobilization. This finding explains the pivotal role of the MBL pathway in triggering activation of the proximal part of the ComC and explains why, even with a deficiency in activation of classical and alternative pathway components, mobilization of HSPCs proceeds normally as long as the MBL pathway is intact. On the other hand, if the MBL pathway of the ComC is defective, neither classical nor alternative pathways can trigger optimal mobilization of HSPCs. Taking into consideration that ~10% of normal people are poor activators of the MBL pathway, we are currently investigating whether MBL deficiency correlates with poor mobilization in these patients.