(P 368) DECODING THE IMMUNOGENOMIC DETERMINANTS OF RADIOTHERAPY RESPONSE IN SOFT TISSUE SARCOMAS

Objective: Myxofibrosarcoma and undifferentiated soft tissue sarcoma (USTS) are two subtypes of genetically complex sarcomas with distinct morphological features. The mainstay of treatment is surgery, potentially in combination with neoadjuvant chemo- or radiotherapy. In recent years, patients with USTS or myxofibrosarcomas have experienced benefit from T cell immune checkpoint blockade that produced responses in about 25% of patients, mainly in USTSs. To improve our understanding of the immunobiology of these soft tissue sarcomas, we aimed to characterize the immune microenvironment through transcriptomic and immunophenotypic profiling.

Methods: In our retrospective study, we included 29 patients with high grade tumors (13 myxofibrosarcomas and 16 USTSs). Immune cell phenotypes and spatial interactions were mapped with a 40-marker imaging mass cytometry panel on tissues including pre-treatment biopsies, treatment-naïve resections and post-radiation resection. In parallel, immune-related gene signatures were obtained for treatment-naïve samples that underwent RNA sequencing, including seven myxofibrosarcomas and 13 USTSs. Histological response to radiotherapy was assessed by a bone and soft tissue pathologist and clinical follow-up data was collected for survival analysis.

Results: In comparison with other genetically complex sarcomas, both USTSs and myxofibrosarcomas presented an enrichment for hallmarks of ongoing inflammatory responses, as assessed through RNA sequencing analysis of immune-related genes. The immunologic constant of rejection signature in combination with the microenvironment cell population counter deconvolution algorithm revealed a subgroup of highly infiltrated soft tissue sarcomas as well as a general enrichment for T cell-related signaling. The 33 immunophenotypes discovered with imaging mass cytometry provided important additional information regarding the actual levels of infiltration. In general, USTSs comprise higher densities of immune cells than myxofibrosarcomas, which might explain the reported differences in response to checkpoint blockade between both subtypes. Moreover, myxofibrosarcomas appeared to be relatively enriched for lymphoid immune cells like innate lymphoid cells and T cells, whereas USTS were more enriched for myeloid immune cells such as macrophages and monocytes. Intriguingly, we observed that both subtypes respond differently to radiotherapy, specifically that USTSs appear more radiosensitive than myxofibrosarcomas. By comparing the treatment-naïve microenvironment of responders with non-responders, we found that various types of T cells, as well as B cells and dendritic cells were more abundant in patients who responded to radiotherapy before treatment. These findings suggest a link between pre-existing anti-tumor immunity and responsiveness to radiotherapy.

Conclusion: In conclusion, high grade myxofibrosarcoma and USTS are generally considered immunogenic as assessed by both transcriptomic and immunophenotypic profiling. USTSs are more often immunogenic than myxofibrosarcomas and appear more sensitive to radiotherapy. Enrichment for pro-inflammatory hallmarks before treatment could potentially be used as a predictive biomarker for response to radiotherapy in USTS and for guiding combination treatment strategies.