Abstract 3845: Elucidating the role and mechanisms by which siglec-15 promotes immune dysregulation in lymphoma
Authors: Francis Dailia B. , Dougan Jodi , Pillsbury Claire , Park Sunita , Liu Linda , Porter Christopher 
Published: June 2022

Abstract
Despite important advances in therapeutic options accounting for improved outcomes in childhood cancers, there remains a significant subset of patients with hematologic malignancies for whom outcomes remain dismal. Notably among this group are patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL). DLBCL accounts for approximately 10-20% of pediatric Non-Hodgkin’s Lymphoma (NHL) cases and with intensive multiagent chemotherapy the 5-year event free survival is ~90%. However, for those with progressive or relapsed disease, survival is dismal at &lt;30% and remains a clinical challenge even with use of intensive salvage regimens. As such, identifying novel agents that will increase survival in this group highlights an unmet need. Siglec-15, a member of the sialic acid binding immunoglobulin-like lectin family of proteins, has recently been identified as a critical immune suppressor that is highly expressed in human cancers and intra-tumoral myeloid cells. Importantly, inhibiting Siglec-15, either through genetic knockout or knockdown, had a restorative effect on local anti-tumor immune responses and abrogated tumor progression. While reported in solid malignancies, a role for Siglec-15 in promoting disease progression in hematologic malignancies has not yet been described.
We have evaluated Siglec-15 expression in primary human lymphoma patient samples as well as various lymphoma (human and mouse) cell lines using western blot, quantitative PCR as well immunohistochemistry and immunofluorescence methods. We are evaluating the effect of inhibiting expression of Siglec-15 through genetic downregulation in human lymphoma cell lines as well as the well-established murine lymphoma cell line A20 on T cell function and proliferation using methods such as flow cytometry and ELISA.
Analyses of public RNA-seq data sets demonstrates higher Sig15 expression in DLBCL cells compared to normal B cells. Western blot shows higher Sig15 expression in lymphoma cell lines compared to PBMC. IHC of a tumor microarray and validation samples from children shows high Sig15 expression in NHL samples with distinct staining patterns based on subtype. Specifically, Sig15 appears to be highly expressed and associated with the cell membrane in most DLBCL and Burkitt’s lymphoma, with more variable expression in anaplastic large cell lymphoma, primarily in the cytoplasm at low levels and/or in cells with morphology consistent with macrophages. In addition, preliminary data suggest more than one isoform of Sig15, raising the possibility of alternative functions. Lastly, knockdown of Sig15 in A20 cells abrogates disease progression in immune competent but not immune deficient recipients, consistent with a role for Sig15 in immune evasion in lymphoma. Together, these data implicate Sig15 as an immune checkpoint that may be inhibited therapeutically to promote an immune response to lymphoma cells.
Citation Format: Dailia B. Francis, Jodi Dougan, Claire Pillsbury, Sunita Park, Linda Liu, Christopher Porter. Elucidating the role and mechanisms by which siglec-15 promotes immune dysregulation in lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3845.