Abstract B012: Reviving exhausted T cells in pediatric B-cell acute lymphoblastic leukemia

Abstract Despite significant treatment advances, the poor long-term prognosis for children with relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL) persists. The application of immune cell-based therapies, such as chimeric antigen receptor-expressing T cells and bispecific T-cell engagers, has improved progression-free survival for many B-ALL patients. However, many of these patients still experience disease relapse. While tumours are widely recognized to influence T cell function, this relationship has not been extensively studied in B-ALL. A clinically silent pre-leukemic phase usually precedes childhood B-ALL. Since only a small percentage of children subsequently develop disease, the impact of pre-leukemia on T cell function remains elusive. To address this knowledge gap, we investigated the development of T cell dysfunction in B-ALL and explored ways to restore it using in situ and transplanted leukemia models Using the Emu-Ret transgenic mouse model of hyperdiploid B-ALL, we investigated the induction of exhaustion-associated marker expression (PD-1, CTLA-4 and Tim-3) on T cells. Longitudinal analysis of marker expression revealed that an upregulation of multiple immune checkpoints such as PD-1 and Tim-3, on T cells occurred only in the presence of leukemia cells, not during the extended preleukemia phase. This suggests that T cell exhaustion occurs exclusively during the established malignancy phase, highlighting the evolving nature of T cell responses as disease progresses. Next, we measured exhaustion-associated marker expression in a syngeneic model of transplanted B-ALL. While expression of PD-1 and Tim-3 on T cells correlated with disease progression, a significant increase in these markers was already detectable at low leukemia burden. This indicates that T cell exhaustion is not merely a reflection of high disease burden and highlights the differing impact exerted by preleukemic and leukemic cells on T cells. Phenotypically exhausted T cells from both mouse models showed reduced proliferation, activation, and cytokine secretion. Previously, we have demonstrated the ability of toll-like receptor (TLR) ligands to induce immune control over B-ALL cells. Thus, we investigated the impact of CpG oligodeoxynucleotides (CpG ODN), a ligand for TLR-9, to overcome T cell dysfunction. The treatment of leukemia bearing mice with CpG ODN led to a decrease in PD-1 and Tim-3 expression on T cells, which was also associated with a reduction in leukemia burden. Lastly, the administration of a checkpoint blockade antibodies, with or without TLR agonists as adjuvant therapy led to increased survival of leukemia bearing mice. Overall, this study demonstrates the functional impact of interactions between B-ALL cells and the immune system. The findings from this study may inform therapeutic approaches to overcome B-ALL immune evasion mechanisms and enhance patient outcomes. Citation Format: Tanmaya Atre, Gregor S.D Reid. Reviving exhausted T cells in pediatric B-cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2024 Oct 18-21; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2024;12(10 Suppl):Abstract nr B012.