(815) T-ALL Elicits Robust T Cell Responses Despite Low Tumor Mutational Burden in multiple tissues with distinct TMEs

Introduction/Rationale: T-cell acute lymphoblastic leukemia (T-ALL) is a devastating pediatric malignancy arising from immature T cells. Despite improved survival rates, treatment still relies on intensive, toxic chemotherapy that fails to cure all patients. More effective and targeted therapies are urgently needed. Most immunotherapy efforts in T-ALL have focused on engineered cells (e.g., CAR T cells), while strategies leveraging patients’ pre-existing anti-tumor T cells remain largely unexplored. This is partly due to the assumption that T-ALL’s low tumor mutational burden (TMB) limits immunogenicity. However, our data challenge this paradigm using both murine models and patient samples.

Methods: We used murine transplantation models in which spontaneous T-ALL from CD45.2+ LN3 or LMO2 mice were transferred into immune-competent CD45.1+ congenic hosts, allowing distinction between malignant and host T cells. OTI/II and NUR77-GFP mice were used to assess TCR-dependent activation.

Results: T-ALL-bearing mice showed high frequencies of TOX+PD-1+ T cells, consistent with chronic, antigen-specific activation. Ex vivo stimulation revealed that PD-1+ T cells retained function, with increased IFN-γ production. To confirm TCR specificity, we transplanted T-ALL into OT-I/II mice (OVA-specific), where exhausted phenotypes were not induced—supporting a TCR-dependent mechanism. NUR77-GFP mice further confirmed high TCR signal strength in T cells from T-ALL-bearing hosts. Finally, treatment with aPD-1 or aCD40 improved survival, with enhanced benefit in combination.

Conclusion: These findings provide proof-of-concept that robust, functional, leukemia-specific T cell responses are mounted against T-ALL despite low TMB. This challenges existing dogma and supports the development of immunotherapy strategies that activate endogenous T cells. Ongoing studies aim to define how divergent tumor microenvironments regulate these responses and identify targets for rational combination immunotherapy.