Abstract 5135: Tertiary lymphoid structures and T-cell aggregates signal a robust anti-tumor immune response in a subset of leiomyosarcoma patients

Abstract Cancer immunotherapy has ushered in a new era for treatment of many difficult to treat cancers such as Leiomyosarcoma (LMS), one of the most common subtypes of sarcomas. There are few biomarkers which can be used to identify patients who might benefit from immunotherapy. There is an urgent need to discover novel biomarkers which might be prognostic of a strong anti-tumor response. We thus sought to comprehensively profile the microenvironment of LMS tumors to identify features which might be suggestive of an active immune response and molecular features prognostic of response to immunotherapy. We began our analysis by classifying the LMS tumors from The Cancer Genome Atlas (TCGA) into 4 subtypes based on previously identified molecular features and comparing the enrichment of various immune gene signatures. Two of the subtypes had a high expression of various immune gene signatures including B Cells, M2 macrophages and Cytotoxic T cells. Visual inspection of the Hematoxylin and Eosin-stained tissue slides available with TCGA pointed to a presence of lymphoid aggregates or Tertiary lymphoid structures (TLS) in 10-15% of the tumors - predominantly belonging to one of the two subtypes. This subtype displayed features of smooth muscle differentiation and was also associated with improved survival. To validate our findings, we performed immunohistochemistry (IHC) and bulk RNA sequencing of LMS specimens (n=66) obtained from surgical resections from the Johns Hopkins Hospital. We identified mature CD21+ TLS in 15% of these cases and non-germinal center T cell aggregates present in more than 30% of the cases and these features were associated with improved survival. We digitally merged the different IHC sections to study if the markers were spatially correlated. Interestingly, spatial analysis showed that these CD8 T cells aggregates co-localized with neighborhoods of high density of PD-L1+ cells, hinting at the possibility of immune mediated upregulation of PD-L1 as means of immune suppression. The deconvolution of the transcriptome of these samples showed a marked presence of cytotoxic CD8 T cells and plasma B cells in this subset of patients, an enrichment of pathways related to T cell trafficking, activation and proliferation. Single cell RNA sequencing of tumor infiltrating immune cells showed an oligoclonal expansion of B and T cells. Taken together, this study shows that a subset of LMS patients have a strong anti-tumor immune response which is characterized by the presence of Tertiary Lymphoid Structures and patches of high density of PD-L1 expressing cells. These features might serve as biomarkers in screening LMS patients which might benefit from immunotherapy. Citation Format: Aditya Suru, Alexandre Maalouf, Lingling Chen, Ada Tam, William Villasi, Kaushal Sharma, Lindy Zhang, Christian Meyer, Jonathan Greer, Fabian Johnston, Carol Morris, Sophia Strike, Adam Levin, Daniel Rhee, Brian Ladle, Kornel Schuebel, Yan Zhang, Rulin Wang, Srinivasan Yegnasubramanian, John Gross, Robert Anders, Andrew Pardoll, Nicolas Llosa. Tertiary lymphoid structures and T-cell aggregates signal a robust anti-tumor immune response in a subset of leiomyosarcoma patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5135.