(P 265) MULTI-SITE SINGLE CELL TRANSCRIPTOMICS REVEAL DISTINCT IMMUNE INFILTRATIVE PATTERNS IN DESMOPLASTIC SMALL ROUND CELL TUMOR

Objective: Desmoplastic small round cell tumor (DSRCT) is a rare soft-tissue sarcoma, most often affecting adolescent and young adult males, with poor overall survival. DSRCT cells exist within nests of stromal cells, with varying degrees of immune infiltration. The interplay of these cell types in the tumor microenvironment (TME) has not been thoroughly interrogated. To this end, we performed multi-site single cell RNA sequencing on tumor samples from patients with DSRCT to better define the TME within and among patients.

Methods: Tumor samples from 2-3 sites from 4 patients with DSRCT, who had prior cytoreductive chemotherapy, were collected from the operating room. Samples were processed and submitted for 10X Genomics single cell RNA sequencing. Analysis of the resultant data was performed, with DSRCT cells identified in a similar manner to Henon et. al., 2024.

Results: CD45+ immune cells had varying TME contributions, with 15-41% of total cells in patient 1, 34-80% in patient 2, and wider ranges in patients 3 and 4. Within the immune compartment, samples were either lymphocyte or neutrophil predominant; patient 1 had on average 70% lymphocytes with minimal neutrophils, with the inverse pattern in patients 2 and 4. This correlated with patients’ peripheral neutrophil-to-lymphocyte (NTL) ratio: 2.5 in patient 1, but 44 and 17 in patients 2 and 4, respectively. Interestingly, patient 3 had a neutrophil predominant pattern in sample 1 and a lymphocyte-predominant pattern in sample 2, with an NTL ratio of only 1.7. Finally, NK cells did not contribute significantly to any sample, despite low class I HLA transcripts in all DSRCT cells.

Conclusion: DSRCT remains a poorly understood and difficult to treat cancer. To better understand the DSRCT TME, we employed single cell transcriptomics on multi-site patient samples. We found that the intra-patient abundance of immune infiltrate remained consistent, with wide inter-patient variability. Samples had either lymphocyte or neutrophil predominance, correlating with peripheral blood NTL ratio, as seen in other cancers, though both patterns may be found within an individual patient. We also found no significant NK cell infiltration despite the low level of class I HLA transcript in all DSRCT cells, suggesting alternate mechanisms to allow surface expression. Analyses of the DSRCT transcriptome are underway to understand the mechanisms leading to these infiltration patterns, and to understand the functional status of infiltrating lymphocytes and neutrophils.