Abstract P2-20-06: Anti-B7-H3 Antibody (T-1A5) Blocks Immunomodulatory Function of B7-H3 and Enhances NK and T Cell–Mediated Cytotoxicity against Breast Cancer Cells

Abstract One of the main factors contributing to breast cancer (BC) initiation and metastasis is immune suppression by tumor cells. Immune checkpoint blockade has recently been shown to overcome tumor-induced immune suppression, but a significant proportion of patients do not respond, implying that more effective cancer immunotherapies are required. B7-H3 belongs to B7 family of immune checkpoint proteins that is overexpressed in various human malignancies. Here, we hypothesize that blocking B7-H3 using monoclonal antibodies (mAbs) enhances immune cell proliferation and function. To test our hypothesis, B7-H3 expression was determined using RNA-seq data from primary and metastatic breast tumors, and adjacent normal tissues, from the TCGA and METABRIC databases. To assess B7-H3 protein expression in BC, we performed immunohistochemistry (IHC) on frozen primary tumor tissues (n=50) and adjacent normal tissues (n=23) from TNBC patients. In addition, to investigate the immunomodulatory effect of B7-H3 in BC, we performed IHC for T-cell markers in subsets of patient tissues with high and low levels of B7-H3 expression. Next, we assessed B7-H3 expression in over 13 BC cell lines, including TNBC and ER+, PR+, and HER2+ cell lines, as well as TNBC PDX-derived cells. Moreover, to determine the effect of B7-H3 knockdown on NK- and T-cell activity, we co-cultured control and B7H3–KD BC cells in the presence and absence of NK and T cells and measured the induction of apoptosis in BC cells through IncuCyte live-cell imaging system. The effect of a novel B7H3-blocking mAb (clone T-1A5, isotype IgG1) on NK-cell and T-cell-mediated cytotoxicity in BC cell lines was examined using live-cell imaging. In the TCGA and METABRIC databases, B7-H3 was found to be significantly overexpressed (P< 0.0001) in tumor tissues than in adjacent normal tissues of BC patients. A survival analysis by the log-rank test indicated that patients with B7-H3high tumors had significantly lower progression-free (P=0.01) and relapse-free (P=0.0026) survival than patients with B7-H3low tumors. Moreover, B7-H3 is significantly upregulated in all the BC subtypes including basal, luminal A, luminal B and Her2-enriched with highest expression in basal type BC. Relative mRNA quantification and flow cytometry analysis demonstrated strong B7-H3 expression in most of the BC cell lines including TNBC and ER+, PR+, and HER2+ cell lines, as well as TNBC PDX-derived cells. Furthermore, IHC analysis revealed that compared to the matched normal tissue, B7-H3 expression was substantially higher in tumor tissue (N=16, P< 0.001). Also, patients with high B7-H3 expression had significantly lower numbers of CD3+, CD4+, and CD8+ T-cell, compared to patients with low B7-H3 expression (P< 0.001), indicating immunosuppressive role of B7-H3. Furthermore, NK cell and T cell mediated killing was significantly higher in B7H3-KD BC cell lines compared to control cells. We observed a significant increase in the killing of BC cells by NK cells and T cells in the presence of anti-B7H3 mAb T-1A5 in a concentration dependent manner (P < 0.001), suggesting that anti-B7H3 antibodies suppress the immunomodulatory function of B7-H3 and enhance NK and T cell–mediated ADCC in BC. To determine the antibody-dependent cell-mediated cytotoxicity, we developed a human-mouse chimera of T-1A5 (chT-1A5) and tested in combination with NK cells. Interestingly, we found a concentration and time dependent increase in ADCC in BC cells in the presence of chT-1A5 antibody. Our data suggests that B7-H3 is overexpressed in primary BC and inhibits immune-cell infiltration. Moreover, blocking the immunomodulatory functions of B7-H3 using anti-B7H3 antibody T-1A5 enhances NK and T cell–mediated killing of BC cells. Citation Format: Vivek Anand, Anudishi Tyagi, Venkata Lokesh Battula. Anti-B7-H3 Antibody (T-1A5) Blocks Immunomodulatory Function of B7-H3 and Enhances NK and T Cell–Mediated Cytotoxicity against Breast Cancer Cells [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-20-06.