RNA expression profiling reveals PRAME, a potential immunotherapy target, is frequently expressed in solitary fibrous tumors.

Solitary fibrous tumors are a type of translocation-associated sarcoma with up to 30% rates of metastasis and poor response to conventional chemotherapy. Other translocation-associated sarcomas have been shown to display elevated expression of various cancer-testis antigens which may render them susceptible to immunotherapy strategies such as cancer vaccines and adoptive T-cell therapy. After an RNA sequencing assay brought the cancer-testis antigen Preferentially Expressed Antigen In Melanoma (PRAME) to our attention as possibly being upregulated in aggressive TERT promoter-mutated solitary fibrous tumors, we used tissue microarrays to asses PRAME expression in a large series of previously characterized solitary fibrous tumors, with correlation to various clinicopathologic features, as well as with tumor-infiltrating macrophages and the associated signal regulatory protein α (SIRPα)-CD47 regulatory checkpoint. We found that PRAME was expressed in 165/180 solitary fibrous tumors, with high expression seen in 58%, irrespective of TERT promoter status. Elevated PRAME expression was more frequent in primary intrathoracic solitary fibrous tumors and correlated with older age at primary diagnosis. Elevated PRAME was also associated with features suggestive of immune evasion, including lower numbers of antigen-presenting CD163+ and CD68+ macrophages, and expression of the "don't eat me" receptor CD47 on tumor cells. Taken together, these features suggest that strategies targeting PRAME with or without concomitant SIRPα-CD47 axis inhibition may represent a potential future therapeutic option in aggressive solitary fibrous tumor.