PSGL-1 is a novel macrophage checkpoint in immuno-oncology.

e15090 Background: Macrophages are both antigen presenting and effector cells of the innate immune system and play an important role in tissue homeostasis as well as in activation and modulation of the adaptive immune response in disease. They display phenotypic heterogeneity in different tissue environments but can be broadly subdivided into pro-inflammatory M1 macrophages, which promote immune response, and anti-inflammatory M2 macrophages, which are associated with immune suppression. Under steady-state conditions, the populations of immune-stimulatory and immune-regulatory macrophages are balanced, but disruption of this balance can result in disease. In the tumor microenvironment, tumor-associated macrophages (TAMs) acquire an M2-like phenotype and maintain suppression of the immune system and promotion of tumor progression. Methods: For functional screening of anti-PSGL-1 antibodies, monocytes were isolated from peripheral blood mononuclear cells (PBMC) obtained from healthy donors and differentiated into M2 macrophages in the presence or absence of anti-PSGL-1 antibodies. On day 8 of the assay, phenotypic profile of cells was analyzed by flow cytometry and chemokines / cytokines were measured by Luminex. Mixed-lymphocyte reaction assay, SEB assay and an ex vivo tumor model were used to further assess antibody functionality in a multi-cellular assay format. For T cell assays, purified T cells from PBMC were stimulated in the presence or absence of anti-PSGL-1 antibodies and responses were characterized by flow cytometry and Luminex. Results: We have identified PSGL-1 as a novel macrophage checkpoint. Our work demonstrates that targeting PSGL-1 via an antagonistic antibody re-polarizes human primary M2 macrophages to a more M1-like state both phenotypically and functionally. We observe this M2-to-M1 switch to have an impact on immune cell communication in complex multi-cellular assays. Moreover, targeting of PSGL-1 in an ex vivo tumor system demonstrates that anti-PSGL-1 treatment can increase pro-inflammatory cytokine and chemokine production known to be associated with beneficial clinical response. These effects can be predominantly attributed to macrophage modulation, as antibody targeting PSGL-1 on purified T cells has a very small, if any, impact on their activation and effector function. Conclusions: Our results demonstrate that treatments inducing macrophage phenotypic switches can promote an immune response in a tumor setting across multiple tumor types and provide support for targeting the novel macrophage checkpoint PSGL-1 across multiple cancer indications.