Abstract 2174: Diacylglycerol kinase inhibitors in MHC-I-induced ICB-resistant triple negative breast cancer

Abstract This study is designed to assess how diacylglycerol kinase alpha/zeta inhibitors (DGKis) can be used to improve response to immune checkpoint blockade (ICB) in patients with downregulated MHC-I expression. Antigen presentation by MHC-I is required for CD8 T cell-mediated tumor death and response to ICB, e.g. a-PD-1/a-PD-L1 therapy. Triple negative breast cancers (TNBC) downregulate MHC-I to suppress anti-tumor immunity and patients with decreased antigen presentation have worse outcomes to ICB. Thus, loss of MHC-I expression may be a candidate biomarker for lack of ICB response, and therapeutic strategies to overcome MHC-I downregulation are an area of high translational relevance and interest. DGKis increase T cell activation by increasing concentrations of diacylglycerol (DAG) in T cells via inhibition of the conversion of DAG into phosphatidic acid and enhancing downstream DAG signaling. This putatively amplifies T cell responses to decreased levels of antigen presentation and low-affinity stimulation in vitro and enhances response to ICB in semi-responsive models. We hypothesized that DGKis will improve response to ICB in TNBC with low MHC-I expression by improving CD8 T cell response to decreased antigen presentation. Modeling low MHC-I expression, as opposed to CRISPR-mediated total KO, presents a technical challenge. Thus, we created a novel model of MHC-I post-transcriptional downregulation, leveraging miRNA binding element mutations to titrate MHC-I expression - miSFITs. We used isogenic miSFIT cell lines in the orthotopic EMT6 model of TNBC. We observed trends of increasingly aggressive tumor growth with decreased MHC-I expression. Coordinately, when treated with a-PD-L1, there was decreasing benefit of ICB treatment with decreasing MHC-I expression (IgG v aPDL1, day 14 tumor volume, n = 10 mice/group; 100% expression p = 0.002, 60% p = 0.02, 30% p = 0.02, 10% p = 0.19, 0% p = 0.2). In the moderately a-PD-L1 responsive parental EMT6 cell line, DGKi/a-PD-L1 dual therapy further improved response through decreased tumor growth (IgG v. DGKi/a-PD-L1, p=0.0023, n=20) and doubled survival rates. However, in non-ICB responsive models of TNBC (4T1 and E0771) the same benefit of combination therapy was not observed. Treatment of the MHC-I downregulated miSFIT cell lines with DGKi + a-PD-L1 revealed a trending benefit of single agent DGKi therapy which was only slightly enhanced by the addition of a-PD-L1 therapy and did not reach significance. This is in contrast with recent in vitro findings in which decreased T cell activation in response to low levels of antigen presentation was overcome with DGKi treatment, implying further nuance in vivo that remains to be understood. Thus far, our data shows that DGKis are beneficial in deepening responses to ICB in already response-prone patients but may not extend benefit to patients who are not responsive to ICB due to highly decreased tumor-specific MHC-I levels. Citation Format: Jacey L. Marshall, Justin M. Balko. Diacylglycerol kinase inhibitors in MHC-I-induced ICB-resistant triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2174.