Background: Soft tissue sarcoma (STS) is a heterogeneous malignancy including more than 50 molecularly distinct subtypes. Undifferentiated pleomorphic sarcoma (UPS) and leiomyosarcoma (LMS) are common subtypes with limited treatment options. LMS can be divided into two distinct subsets: uterine (uLMS) and retroperitoneal (rpLMS). STS subtypes have differing levels of immune infiltration and show variable response to checkpoint blockade. We hypothesize that immune infiltration of cytotoxic T lymphocytes (CTLs) and macrophages will differ among subtypes of STS. Methods: 30 patients with sarcoma, naïve to systemic treatment, were identified, 10 cases each: UPS, uLMS and rpLMS. 5µm slides were stained using qmIF protocol for DAPI, CD3 (T cells), CD8 (CTLs), CD68 (macrophages), vimentin (tumor marker), HLA-DR (activation) and PD-L1 (immune suppression). Tumor areas were chosen by a pathologist, multiplex images acquired using Vectra, then processed and analyzed using inForm software. Genomic analysis to be performed using nanoString. Results: All cases have been stained and are now being analyzed. Preliminary analysis was performed by qualitative grading of stains (Table 1). We find increased CTL infiltration and HLA-DR expression in UPS as compared to either LMS group. uLMS appears to exhibit higher CTL infiltration and HLA-DR expression than rpLMS. QmIF and genomic analysis is ongoing. Conclusions: Qualitative analysis of UPS, uLMS, and rpLMS immune TME finds that UPS appears to have higher levels of CTLs and PD-L1 expression, consistent with results from other studies using conventional immunohistochemistry. Subtypes of LMS harbor distinct immune TME features, as uLMS as higher CTL and HLA-DR expression than rpLMS. These findings could have therapeutic implications. This study is currently ongoing; multi-parameter immune phenotyping, spatial localization, and genomic analysis will be reported at the meeting.

- no cells, + few cells, ++ some cells, +++ many cells