Abstract A28: Mechanisms of adaptive immune evasion by pre-leukemic DNMT3A-mutated blood cell clones

Hematopoietic stem cells (HSC) frequently acquire gene mutations with age. An expansion of blood cells derived from a single HSC is termed clonal hematopoiesis (CH), a condition that often precedes acute myeloid leukemia (AML). CH is common in elderly individuals: by age 60, approximately 10% of individuals have a clonally expanded blood cell population with a leukemia-associated mutation. The most common mutations in CH (50% of all cases) inactivate the DNA methyltransferase DNMT3A, which regulates gene expression via methylation of CpG rich regions. These mutations provide a selective advantage over healthy HSCs during aging and inflammation. Our goal is to discover mechanisms that allow for clonal expansion of DNMT3A-driven CH. A better understanding of these mechanisms will inspire new therapeutic strategies to limit clonal expansion and increase survival. To evaluate the transcriptional consequences of DNMT3A mutations in myeloid cells, we analyzed our previously published single-cell RNA-sequencing data of DNMT3A-mutated AML patient samples. We found a significant enrichment of antigen presentation signatures in DNMT3A mutated cells along the myeloid differentiation trajectory. CD8+ T cells from these same patients had significantly increased signatures of T cell activation and T cell exhaustion. T cell exhaustion occurs after an activated T cell has been continuously exposed to antigens. Exhausted T cells lack most effector functions and the ability to properly surveil (i.e. recognize mutant peptides presented on the surface of mutated cells and kill the target upon recognition) and restrain mutant cells from expanding. We hypothesize that DNMT3A-driven CH leads to escape of immune surveillance via chronic antigen stimulation and eventual promotion of T cell exhaustion. This would allow for further expansion of the mutated cells and the transformation of CH into AML. To functionally test this hypothesis, we evaluated whether a mouse model of Dnmt3a-CH recapitulated our human data. Indeed, scRNA-seq data of mouse Dnmt3a-CH cells showed significantly increased antigen presentation signatures across the myeloid differentiation trajectory. Additionally, CD8+ T cells had increased signatures of activation. We next functionally tested whether Dnmt3a-mutated CH cells had enhanced antigen presentation by pulsing bone marrow cells derived from Dnmt3a-CH mice with DQ-Ovalbumin, a self-quenched exogenous protein that becomes fluorescent when endocytosed and digested in the phagolysosome. Dnmt3a-mutated HSCs and LSKs had significantly increased DQ-OVA uptake and processing. In conclusion, we found that DNMT3A-CH cells have increased antigen presentation and processing which corresponds with enhanced immunogenicity. These findings support a model in which T cells constrain the expansion of DNMT3A-mutated HSCs, until T cell exhaustion occurs, leading to further expansion of the mutated clone and an increased chance of transformation into AML. Citation Format: Kyle A Romine, Daniel Ssozi, Tina Mujica, Yoke Lee, Jennifer Trowbridge, Peter van Galen. Mechanisms of adaptive immune evasion by pre-leukemic DNMT3A-mutated blood cell clones [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr A28.