Abstract

Background. Several mechanisms have been postulated for orchestrating the mobilization of hematopoietic stem/progenitor cells (HSPCs), and it has been proposed that activation of the complement cascade plays a crucial role in the initiation and execution of the egress of HSPCs from bone marrow (BM) into peripheral blood (PB). In support of this notion, we demonstrated that mice deficient in the mannan-binding lectin (MBL) pathway, which activates the proximal part of the complement cascade (Leukemia. 2017, 31, 262-265), as well as mice deficient in the fifth component of the complement cascade (C5), which is the central molecule of the distal part of the complement cascade, are poor mobilizers (Leukemia. 2014, 28, 2148-2154). While enhancing mobilization is of clinical importance for poor mobilizers, inhibition of increased motility of HSPCs could be of therapeutic importance in patients suffering from paroxysmal nocturnal hemoglobinuria (PNH) (J Cell Mol Med. 2015, 19, 2193-201) . It is well known that in PNH patients pathological activation of the complement cascade leads to accelerated lysis of erythrocytes due to formation of C5b-C9 (also known as the membrane attack complex, MAC). As a consequence of this unwanted activation, the bioactive lipid sphingosine-1-phosphate (S1P) is released from lysed erythrocytes, and this major chemoattractant of HSPCs preferentially increases motility of PNH-affected HSPCs in the bone marrow (BM) microenvironment (Leukemia 2010;24:976-85). This increased motility promotes their expansion by enabling them to outcompete normal HSPCs for BM niches. To inhibit the complement cascade activation that initiates this sequence of events, monoclonal antibodies are currently employed against C5 (eculizumab). Nevertheless, recent clinical data prompt for developing other more potent inhibitors of complement cascade activation (Clin Immunol 2017, 180, 25-32). Aim of the study. To better address the involvement of the distal part of complement cascade activation in regulating the motility of HSPCs, we performed experiments in mice that do not express the receptor C5aR, which binds the C5 cleavage fragments, C5a and desArgC5a. We also tested in an in vivo model the plasma-stable, mixed L-RNA/L-DNA-based aptamer, AON-D21, that is directed against C5a to inhibit the in vivo activity of the C5aR-C5a axis. Hypothesis. We hypothesized that inhibition of the C5aR -C5a/desArgC5a axis could be an efficient strategy to inhibit the uncontrolled motility of HSPCs during hemolytic episodes . Results. We present evidence that C5aR-deficient mice are poor mobilizers of HSPCs, which, together with the fact that motility of these cells is impaired after administration of the anti-C5a aptamer AON-D21, confirms the critical role of the C5a/desArgC5a-C5aR axis in enhancing the motility of HSPCs. Conclusions. We propose that novel drugs that inhibit the C5a-C5aR axis, such as the newly developed L-aptamer AON-D21 or C5aR antagonists, can find practical application in preventing the enhanced motility of HSPCs that would otherwise lead to the expansion of PNH clones in the BM of PNH-affected patients. Since eculizumab as recently reported may not be able to efficiently block C5 cleavage by serine proteases such as thrombin (Clin Immunol 2017, 180, 25-32) and close bidirectional interactions between the coagulation cascade and the complement system are well described, AON-D21 could play a supportive role. Moreover, inhibitors of the C5a-C5aR axis could also find application in other situations when complement cascade is hyperactivated such as for example in Atypical Hemolytic Uremic Syndrome.

Disclosures

Hoehlig: Aptarion Biotech AG: Employment, Patents & Royalties. Vater: Aptarion Biotech AG: Employment, Patents & Royalties.

Author notes

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Asterisk with author names denotes non-ASH members.