Impaired mobilization of hematopoietic stem/progenitor cells in C5-deficient mice supports the pivotal involvement of innate immunity in this process and reveals novel promobilization effects of granulocytes

Abstract

We reported that complement cascade (CC) becomes activated in bone marrow (BM) during granulocyte colony stimulating factor (G-CSF) mobilization of hematopoietic stem/progenitor cells (HSPCs) and demonstrated that while the third CC component (C3)-deficient mice are easy mobilizers, the fifth CC component (C5)-deficient mice mobilize very poorly. To explain this, we postulated that activation/cleavage of CC releases C3a and C5a anaphylatoxins that differently regulate mobilization. Accordingly, C3a, by enhancing responsiveness of HSPCs to decreasing concentrations of stromal-derived growth factor-1 (SDF-1) in BM, prevents mobilization and promotes their BM-retention. As such, we focused on the mobilization-enhancing role of C5a herein. We found that C5a receptor (C5aR) is not expressed on the surface of HSPCs, and C5a-mediated pro-mobilization effects are mediated by stimulation of granulocytes. Overall, our data support a following model. First C5aR+ granulocytes are chemoattracted by plasma C5 cleavage fragments, being the first wave of cells leaving BM. This facilitates subsequent egress of HSPCs. In the next step, after leaving the BM, granulocytes undergo degranulation in response to plasma C5a and secrete some cationic peptides (cathelicidin, β-defensin) that as demonstrated here for a first time highly enhance responsiveness of HSPCs to plasma SDF-1 gradient. In conclusion, our data reveal the underappreciated central role of innate immunity in mobilization where C5 cleavage fragments via granulocytes orchestrate this process.

Innate immunity as orchestrator of stem cell mobilization

“…19 Data from our laboratory indicate that CC cleavage fragments affect the retention/mobilization process differently. 21,19,22,41,77 That is, although C3 (C3a or desArg C3a) cleavage fragments increase retention of HSPCs in BM, C5 (C5a or desArg C5a) cleavage fragments enhance their egress into PB. This was evidenced in mobilization studies performed in C3-and C5-deficient animals, which revealed that C3-deficient mice are easy mobilizers 41,44,77 and C5-deficient mice are poor mobilizers.…”

“…This was evidenced in mobilization studies performed in C3-and C5-deficient animals, which revealed that C3-deficient mice are easy mobilizers 41,44,77 and C5-deficient mice are poor mobilizers. 22 To explain why C3 mice are easy mobilizers, we demonstrated that C3a and desArg C3a increase responsiveness of HSPCs to the BM SDF-1 gradient, 41 which seems to be crucial for…”

“…[71][72][73][74][75][76] Overall, the activation of CC during HSPC mobilization was confirmed by (i) Enzyme-linked immunosorbent assay to detect C3a and C5a clevage fragments in plasma, 19 (ii) immunofluorescence showing deposition of iC3b on BM stroma and endothelial cells [41][42][43] and (iii) Enzyme-linked immunosorbent assay and histochemical detection of membrane attack complex (MAC) in BM tissue. 21 Work from our laboratory demonstrates that CC activation in BM is triggered by several mobilizing agents, including G-CSF, mobilizing polysaccharides, such as zymosan, 22 as well as AMD3100. 19 During CC activation, several cleavage fragments are released that modulate (for example, C3a, desArg C3a, C5a or desArg C5a anaphylatoxins) 19,42,43,68,77 or are crucial for the execution of this process (for example, MAC (C5b-C9)).…”

“…19,21,22 In this review we will present evidence that during stress situations, innate immunity and its major arms, (i) activation of the complement cascade (CC), (ii) granulocytes, (iii) naturally occurring antibodies (NA-Ig) and (iv) the network of Toll-like receptors (TLRs), orchestrate egress of HSPCs into the PB. Thus, mobilization of stem cells is an evolutionarily conserved mechanism that is part of the immune response/ reaction to inflammation and organ/tissue damage.…”

Mechanisms of G-CSF-mediated hematopoietic stem and progenitor mobilization

“…94 This is not a direct effect of C5 on HSPCs, as they do not express C5a receptor, but it may be related to its effect on neutrophil activation. 95 In addition, formation of the membrane attack complex is believed to lead to the lysis of red blood cells, which releases the lipid sphingosine-1-phosphate, which is a potent chemoattractant for HSPCs. 96 How complement is activated during G-CSF is not entirely clear, but evidence suggests that it may be via antibody activation of complement.…”