Abstract Body: Kawasaki Disease (KD) is an acute febrile systemic vasculitis of unknown etiology and the leading cause of acquired heart disease among children. The complement system is a major component of innate immunity with both soluble factors and cell surface receptors that respond to invading pathogens. The three arms of the complement pathway (classical, alternative, and mannose-binding lectin) converge on C3 and C5 activation. C3 is an opsonizing factor that labels microbial targets, including viruses, for clearance. Activation of the complement system has been implicated in the development of KD vasculitis, however, its contribution to disease development remains unknown. We hypothesized that blocking the activity of the complement system would exacerbate vascular inflammation by impairing the clearance of microbial components via opsonization in a murine model mimicking KD vasculitis. We used the Lactobacillus casei cell wall extract (LCWE) murine model of KD and assessed the disease severity by histopathological examination. We performed immunofluorescence staining of the complement receptors C3aR1 and C5aR1, and assessed mRNA expression of different complement products in serum. We further analyzed publicly available datasets to determine the expression of complement fragments in serum of KD patients. Next, we performed a binding assay to show deposition of C3 to LCWE. Finally, we injected mice with fluorescently labeled LCWE to assess its deposition and clearance from cardiovascular tissues. LCWE-injected mice exhibited higher complement products in the serum, indicative of the activation of the complement pathway in this model. In addition, we observed a significant increase in mRNA and protein levels of C3aR1 and C5aR1 in vascular tissues after LCWE injection. Moreover, C3-deficient mice or WT mice treated with the C5aR1 antagonist had more severe LCWE-induced KD vasculitis. Furthermore, LCWE binds significantly less to heat-inactivated or C3-deficient serum and we observed more deposition of fluorescently labeled LCWE in the livers of C3-deficient mice, indicating that absence of C3 hinders LCWE clearance. Our data indicate that blocking the complement system exacerbates LCWE-induced KD vasculitis, potentially by impairing C3-mediated clearance of bacterial components. Our studies reveal the contribution of the complement system in regulating LCWE-induced KD vasculitis and suggests complement pathway as a beneficial novel marker.