Abstract A050: Characterizing the importance of the interaction between complement component C1q in the tumor microenvironment and colorectal cancer

The current study aimed to understand the role of the complement (C) system in colorectal cancer. This included the interaction of both membrane-localized and secreted C1q and C1qRs in the tumor microenvironment, as well as potential differences in these factors between African American (AA) and Caucasian American (CA) patients. Cell lines HCT116 and HT29 (CAs) and SB501 and SB521 (AAs) were used as in vitro models for colorectal cancer. Using ELISA, cell culture medium from each cell line was collected and analyzed for C1q and gC1qR secretion: 24, 48, and 72 hours. Cells from each line were treated with C1q, gC1qR, cC1qr, and the antibodies (α) to each. An MTT assay was used to study cell viability across various concentrations: 48 and 72 hours. For each cell line, Western Blot analysis was used to detect both whole-cell and surface expressions of C1q, gC1qR, and cC1qR. All four cell lines expressed both C1qRs at a whole-cell and membrane level but did not express C1q itself. As such, no secretion of C1q was detected in any of the four cell lines; however, gC1qR was found to be secreted into the extracellular environment by all four cell lines. Colorimetric analysis showed that in the AA cell lines, the concentration of secreted gC1qR increased significantly between 24, 48, and 72 hours. 48-hour treatments of HCT116 with exogenous C1q led to increased cell proliferation at concentrations ⩾ 10 μg/mL; 20 μg/mL in HT29. 72-hour treatments of HCT116 and HT29 with exogenous C1q at concentrations ⩾ 2.5 μg/mL led to increased cell proliferation. 48-hour treatment of SB521 with C1q led to a significant decrease in viability at 20 μg/mL of C1q, and after 72 hours, a significant increase in proliferation at 15 μg/mL. SB501 showed no response to C1q. 72-hour treatment of HCT116 with α-C1q caused significant decreases in viability at concentrations ⩾10 μg/mL. After both 48 and 72 hours, similar responses were observed for SB501 at concentrations ⩾ 2.5 μg/mL and SB521 at 20 μg/mL. HT29 showed no response to α-C1q. After 48 hours, treatment of HCT116 and HT29 with α-cC1qR resulted in significant and progressive decreases in cell viability at concentrations ⩾ 5 μg/mL; treatment with pure cC1qR showed significant effects only with increased proliferation in HT29 at 2.5 and 5 μg/mL. The results of the present study suggest that, within the tumor microenvironment, the presence of increased levels of C1q promotes tumor cell proliferation in colorectal cancer, likely favoring the overexpression of C1q by other cell types within the microenvironment for growth. This is further supported by the drop in viability attributed to the inhibition of the receptor cC1qR. Cell lines from CAs produced greater pro-proliferative responses in the presence of C1q. AA cell lines demonstrated greater sensitivity to inhibition of C1q than to treatment with it. Secretion of gC1qR into the tumor microenvironment may compete with membrane-localized gC1qR for the binding of antibodies, potentially playing a protective role against immune-cell-expressed C1q in the microenvironment. Citation Format: Jianna R. Carter, Serena Benjamin, Ping Ji, Anne Savitt, Berhane Ghebrehiwet, Jennie Williams. Characterizing the importance of the interaction between complement component C1q in the tumor microenvironment and colorectal cancer [abstract]. In: Proceedings of the 18th AACR Conference on the Science of Cancer Health Disparities; 2025 Sep 18-21; Baltimore, MD. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2025;34(9 Suppl):Abstract nr A050.