Background: The complement system is an important arm of the innate immune system, and complement proteins are produced by both cancer cells and cells of the TME. A primary function of the complement system is to remove pathogens and injured cells, and it had been assumed that complement activation would result in elimination of cancer cells. However, recent work has revealed new biologic effects of the complement activation fragments C3a and C5a. For example, C3a and C5a directly stimulate the growth of some tissues. Furthermore, inflammatory cells as well as CD4 and CD8 T cells express receptors for both of these activation fragments (C3aR and C5aR), and a recent study reported that C5a reduces the anti-tumor function of CD8+ T cells. Nevertheless, the role of complement activation in the growth and spread of non-small cell lung cancer is incompletely understood. While complement activation can proceed by multiple pathways, a central event is the deposition of C3 on the cell surface. The goal of this study was to examine the role of complement in lung cancer progression employing an immunocompetent orthotopic model which recapitulates many of the features of human lung cancer.
Methods: Two murine lung cancer cell lines derived from C57BL/6 mice were used in an orthotopic model of tumor progression: Lewis Lung Carcinoma (LLC) cells, which are a de-differentiated cell line in which driver mutations have not been identified; and CMT167 cells, which are an epithelial lung cancer cell line that express oncogenic K-Ras. Tumor cells were implanted in the left lungs of syngeneic mice WT C57BL/6 mice, or mice which were globally knocked out for C3 (C3-KO). Primary tumor growth and formation of liver metastases were quantitated in the two groups of mice. Complement activation in the plasma was determined by ELISA. Single cell suspensions of the left lung of tumor-bearing mice were prepared and populations of myeloid cells and immune cells quantitated by flow cytometry.
Results: Implantation of cancer lead to systemic complement activation as assessed by ELISA in plasma. Primary tumors and liver metastases were decreased in both cell lines implanted into C3-KO mice compared to controls. However, the degree of inhibition was much greater in CMT167 tumors. LLC cells expressed C3 in vitro, whereas CMT167 cells did not, suggesting that autonomous C3 production by cancer cells may contribute to tumor progression. Recruitment of inflammatory myeloid cells was not altered in tumors growing in C3-KO mice. At early time points, we observed equal increases in T cells (CD4, CD8) in the two groups of mice. However, at later time points (3 weeks), the number of T cells had decreased in WT mice, but remained elevated in C3-KO mice.
Conclusions: Complement activation occurs in the setting of tumors. C3 plays a critical role in tumor progression, with strong inhibition observed in a subset of cancer cells. The differential response to complement inhibition may depend on the ability of the cancer cells to produce C3. Effects of complement activation may be mediated by inhibiting cytotoxic T cells. These data suggest that complement inhibitors may represent novel immunotherapeutic targets for lung cancer, and suggest interactions between the complement system and checkpoint inhibitors.
Citation Format: Jeff Kwak, Howard Li, Jennifer Laskowski, Mary Weiser-Evans, Joshua Thurman, Raphael A. Nemenoff. Complement activation is critical for tumor progression in an orthotopic immunocompetent model of lung cancer. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A121.
- ©2016 American Association for Cancer Research.