Abstract 1548: T-cell acute lymphoblastic leukemia (T-ALL) drives robust T cell responses that can be harnessed with aCD40 and aPD1 combination therapy

Abstract T-cell acute lymphoblastic leukemia (T-ALL) is a malignancy of immature T cells characterized by a low mutational burden and limited neoantigens. Despite improved survival rates, the standard of care has not progressed from an intensive chemotherapy regimen, which is highly toxic with a high relapse rate. Therefore, there is a need for more targeted therapies. Immunotherapy, like checkpoint blockade, that aims to harness the patient’s pre-existing anti-tumor T cells, is a viable alternative. However, whether or not T-ALL is responsive to this approach is unknown due to low tumor mutational burden. Using murine models transplanted with CD45.2+ T-ALL cells from spontaneously leukemic mice into immune-competent CD45.1+ hosts, we investigated host (CD45.1+) T-cell responses over time. Employing NUR77, RAG knock-out, OT-I, OT-II mouse models and in vitro models, we confirmed leukemia-specific, TCR-driven activation, while single-cell RNA sequencing identified expanded antitumor effector subsets.We found that T-ALL elicits leukemia-specific T cells despite its low mutational burden. Immune profiling revealed elevated effector-memory T cells expressing PD-1, TOX, and FoxP3+ Tregs, consistent with chronic exhaustion and an ongoing immune response. Ex vivo cytokine assays demonstrated reduced IL-2 but increased IFN-γ production, further indicating partial exhaustion. To confirm antigen specificity, we transplanted T-ALL into OT-I and OT-II mice, whose fixed TCRs recognize OVA (absent in our leukemia model). These mice lacked PD-1+TOX+ T cells, confirming that exhaustion observed in wild-type mice was leukemia-specific. NUR77 transgenic analyses verified that TCR signaling drove this activation. Myeloid cells exhibited upregulated PD-L1 and enrichment of suppressive subsets, suggesting inhibitory crosstalk restraining leukemia-specific T cells. Based on this, we evaluated combined immunotherapy using αCD40 to activate myeloid antigen-presenting cells and αPD-1 to reverse T-cell exhaustion. While either monotherapy modestly prolonged survival, dual treatment induced robust, durable responses with ∼50% of mice achieving complete remission. Re-challenge experiments confirmed long-term immune memory. Collectively, our findings demonstrate that despite low antigenicity, T-ALL can generate leukemia-specific T-cell responses that can be therapeutically harnessed. Co-targeting myeloid activation and PD-1 inhibition represents a promising immunotherapeutic avenue for T-ALL. Citation Format: Faizah Alabi, Alex Somma, Todd Triplett. T-cell acute lymphoblastic leukemia (T-ALL) drives robust T cell responses that can be harnessed with aCD40 and aPD1 combination therapy abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1548.