Tissue Kallikrein 1 cleaves complement factor C3 and activates the alternative complement pathway

Hematopoietic stem cell transplant-associated thrombotic microangiopathy (HSCT-TMA), characterized by microvascular endothelial damage and severe renal injury, negatively affects HSCT survivorship with high mortality and long-term renal morbidity. After HSCT, innate immunity and inflammation are often dysregulated. The alternative complement pathway (AP) of the innate immune system is overactivated in HSCT-TMA, but the mechanisms of its initiation are poorly described. Complement component C3 of the AP can be cleaved by proteases outside of the AP. Because mRNA expression of tissue kallikrein 1 (KLK1), an inflammatory serine protease that produces kinins, has been found to be markedly elevated in the renal endothelium of inflamed mice, we hypothesized that increased KLK1 activity during inflammation contributes to AP overactivation and endothelial injury in HSCT-TMA. We assessed AP activation, KLK1 activity, endothelial injury, and renal function in HSCT-TMA experimental models and disease settings and investigated C3 cleavage and AP activation by KLK1. We found that patients with HSCT-TMA had significantly increased AP activation and decreased KLK1 inactivation at TMA diagnosis compared to pre-HSCT. Mice challenged with HSCT-TMA triggers cyclosporine A (CsA) and lipopolysaccharide (LPS) exhibited increased AP activation, renal endothelial injury, and impaired renal function in the setting of decreased KLK1 inactivation. We further demonstrated that KLK1 cleaved AP component C3 to C3b that functionally activated the AP. Our data indicate a noncanonical mechanism for AP activation by KLK1 in settings of HSCT-TMA. Key PointsO_LITissue kallikrein (KLK1) functionally cleaves complement factor C3. C_LIO_LIReduced KLK1 inhibition is associated with activation of the alternative pathway of complement. C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=111 SRC="FIGDIR/small/674260v1_ufig1.gif" ALT="Figure 1"> View larger version (13K): org.highwire.dtl.DTLVardef@8f260aorg.highwire.dtl.DTLVardef@a6511org.highwire.dtl.DTLVardef@7160f6org.highwire.dtl.DTLVardef@194d36d_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical Abstract.C_FLOATNO HSCT-TMA triggers (e.g. inflammation, immunosuppression) upregulate expression of the inflammatory serine protease KLK1, which cleaves C3 to initiate AP activation and amplification, leading to renal endothelial injury. Created with BioRender.com under its Academic License Terms with Baylor College of Medicine. C_FIG