Radiation-mediated complement cascade activation within the lung tumor microenvironment diminishes the efficacy of radiation therapy

Non-small cell lung cancer (NSCLC) is the primary cause of cancer-related mortality worldwide, necessitating novel interventions to improve survival rates. Current treatment modalities for locally advanced NSCLC involve cytotoxic chemotherapy with concurrent external beam radiation therapy (XRT), followed by immunotherapy. However, the impact of XRT on the complement system, a crucial component of innate immunity, remains unclear. The complement regulatory proteins (CRPs), including CD46, CD55, and CD59, tightly control complement activation mediated by the three pathways. In this study, we investigated the surface expression of CRPs in NSCLC cell lines in vitro and within the tumor microenvironment (TME) in vivo . We assessed the activation of the three complement pathways in the TME and monitored tumor growth and survival in C57BL/6 wild-type, C3 -/- , and C3AR -/- mice following XRT. Following XRT, NSCLC cell lines A549 and H460 exhibited a significant increase in the cell surface expression of CD46, CD59, and CD55 compared to controls (p<0.05), whereas normal lung epithelial cells (MRC5) did not display such upregulation. Irradiated Lewis lung carcinoma (LLC) tumors in wild-type mice demonstrated elevated levels of C3 and C5 mRNA and protein (p=0.0013). Also, C3AR and C5AR were significantly upregulated in the irradiated LLC tumor as compared to the sham. Additionally, the alternative complement pathway components, factor B and factor D, were significantly upregulated (p<0.001) in the LLC TME following XRT. However, the membrane attack complex (MAC) did not form in XRT-treated LLC tumors compared to sham. Deficiency of the central C3 and C3AR complement components in C3 -/ - and C3AR -/- mice delays LLC tumor growth in combination with XRT. Our findings suggest that XRT increases the expression of CRPs in NSCLC, potentially facilitating immune evasion by tumor cells. We also found activation of the alternative pathway of the complement system. Targeting C3 and C3AR effectively delayed tumor growth, highlighting the potential of complement pathway modulation to enhance the efficacy of XRT in NSCLC treatment. These results underscore the importance of elucidating the intricate interplay between radiation-induced complement activation and tumor survival for devising novel therapeutic strategies in NSCLC.