Abstract 4151: Siglec-15 is a novel immunomodulatory effector of adaptive immune evasion in acute lymphoblastic leukemia

Abstract

Abstract Despite advances in therapy for acute lymphoblastic leukemia (ALL), it remains a leading cause of illness-related death in children and has poor long term disease-free survival and prognosis in recurrence in adults. Recent developments in immune-based therapies, such as CAR T cells, have shown great promise, but immune evasion mechanisms of leukemia remain poorly understood, and alternative strategies to engage the immune system are needed. Our group has characterized a role for calcineurin (CN) as a driver of a potent immune escape mechanism in an aggressive murine model of BCR-ABL+ ALL. We performed RNA-seq analysis of control and CN-deficient ALL cells to identify downstream mediators of this effect. Siglec-15 emerged as one of the most differentially expressed genes, being preferentially downregulated in the more immunogenic CN-deficient cells. The role of this Siglec-family immunomodulator is yet unstudied in the context of leukemia and could represent a novel mechanism for immune evasion. We first validated lower expression of Siglec-15 in CN-deficient leukemia cells by RT-qPCR. We next mined publicly available databases, which revealed higher expression of Siglec-15 in primary B-ALL and AML leukemias as compared to normal PBMCs, as well as in post-chemotherapy MRD samples as compared to at-diagnosis samples. Western blot analysis of human leukemia cell lines demonstrated upregulation of Siglec-15 expression in B-ALL, some AML, as well as DLBCL, as compared to normal PBMCs. To determine if Siglec-15 mediates immune evasion in vivo, we knocked down Siglec-15 in the murine model of BCR-ABL+ ALL, which resulted in rapid suppression of the leukemia following engraftment in immunocompetent mice, an effect lost in immunocompromised counterparts. Preliminary studies with cells in which Siglec-15 was knocked-out using CRISPR/Cas9 revealed similar results. Importantly, Siglec-15-deficient cells do not have any changes in proliferation or susceptibility to DNA damage-induced apoptosis. To address the capacity for therapeutic targeting of Siglec-15, mice with parental leukemia were treated with a monoclonal blocking antibody against Siglec-15, yielding an overall reduction in leukemic burden and prolonged survival. Taken together, these results suggest that Siglec-15 is a novel immunomodulatory effector and has a therapeutically exploitable role in immune evasion of ALL. Citation Format: Claire Pillsbury, Jairo A. Fonseca, Jodi Dougan, Lori Gardner, Jennifer Rabe, Christy Gearheart, Chris C. Porter. Siglec-15 is a novel immunomodulatory effector of adaptive immune evasion in acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4151.