Abstract PS4-03-20: Spatial and biomarker associations of CD83, a promising CAR-T target for breast cancer

Abstract Background: We previously reported that CD83, a member of the immunoglobulin super family, is a putative target for CAR-T therapy in breast cancer based on preclinical activity against breast cancer cell lines and expression by IHC in a breast cancer tissue microarray. Here we use high-plex tissue imaging (cyclic immunofluorescence) to describe the single-cell and sample based expression of CD83 in tissue compartments and measure its association with canonical breast cancer biomarkers, estrogen receptor, HER2 receptor. Methods: Tissue microarrays underwent dewaxing and antigen retrieval using a BOND auto stainer in the Advanced Tissue Imaging Shared Resource (ATISR). Tissues then underwent bleaching followed by iterative staining of unconjugated or conjugated antibodies. We completed six cycles of staining for all tissues, comprising 18 markers per tissue. Images were acquired at 20x on Leica Thunder Tissue Imager (ATISR). Images were processed using the MCMICRO pipeline and downstream analysis performed in R and Scimap. Results: After filtering for cellularity and technical artifacts, we analyzed 81 TMA cores (41 patients) of which 48 cores were HER2+ (IHC 3+) and 33 were HER2-low (30 HER2 2+ FISH negative, 3 HER2 IHC 1+). 42 cores were ER+ while 28 were ER neg by IHC with 11 not classified. We analyzed a total of 5.0 x105 cells of which 34% were tumor cells (pan-cytokeratin+), 37% were immune cells (CD45+), with the remainder being stromal cells(alpha-smooth muscle actin+). We found that single cell CD83 expression was positively correlated with pan-CK and CD45, consistent with CD83 expression (fluorescence) in both tumor and immune cells. Median CD83 single cell expression was negatively correlated with tumor ER expression (r -0.34, p= 0.0002) but not correlated with median tumor HER2 expression (r=0.03, p=0.7) by CyCIF or IHC (1+, 2+, 3+). Mean CD83 tumor cell expression was higher in ER neg (1% by IHC) vs ER pos tumors (0.18 +/- 0.1 vs 0.13 +/- 0.7, p 0.001) while CD83 immune cell expression was higher in ER+ tumors (0.24 vs 0.21, p0.001). Conclusions: Together, these data suggest that tumors that are ER negative may express highest CD83 in the tumor compartment. Given our previous work showing activity of CD83 CAR-T against LM2 TNBC murine tumor model, these data suggest that ER negative tumors may be more likely to respond to CD83-targeting CAR-T cells. Further spatial analysis and correlation of CD83 with outcome will be presented at the conference. Citation Format: M. Homsi, M. DaVila, R. Brentjens, B. Betts, S. Kabraji. Spatial and biomarker associations of CD83, a promising CAR-T target for breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-03-20.