187P - Local and systemic anti-tumor response during tumor development in an immune privileged site: the case of uveal melanoma

Background

Uveal melanoma (UM), an uncommon but aggressive eye cancer, exhibits genetic diversity, classifying into M3 (poor prognosis) and D3 (better outcome) subsets. Despite low mutation rates, metastatic UM patients have detectable CD4 and CD8 T cells, suggesting an immune response. We recently identified public neo-antigens generated by SF3B1 mutations in D3 tumors, triggering CD8+ T cell responses. In M3 tumors with BAP1 inactivation, immune infiltration increases, but CD8 T cell specificity remains unclear.

Methods

We collected blood and tumor samples from uveal melanoma patients at Institut Curie. Specific peptide-loaded HLA-A2 tetramers were labeled and used for staining blood and tumor cells. Single-cell RNA-Seq experiments were performed on TILs.

Results

In our analysis of 24 enucleated patient samples, we observed an enrichment of CD8+ T cells over CD4+ T cells within the tumor microenvironment. A notable fraction of CD8+ T cells exhibited exhaustion markers like PD-1 and CD39, suggesting the presence of an Ag-specific immune response. Among these exhausted CD8+ T cells, 1-10% were specific for HLA-A2: Melan-A. By using scRNA-seq and VDJ TCR-seq we explored T cells response within 3 primary HLA-A2+ M3 tumors. Our analysis identified nine distinct clusters, notably Cluster 0, which displayed upregulation of genes such as PDCD1 (PD-1), ENTPD1 (CD39), HAVCR2 (TIM3), LAG3, GZMB, and GZMK. This gene expression profile suggests chronic activation due to exposure to tumor antigens. Melan-A tetramer-positive cells predominantly localized to Cluster C0, and the largest clonotype was associated with this cluster. We also assessed systemic recirculation by sequencing TCRs in PBMCs, revealing varying recirculation levels among different T-cell clusters. While chronically activated (C0) T cells displayed limited recirculation, T cells with a memory phenotype (C2), cytotoxic functions (C4), and effector T-cell responses (C6) were more present in the peripheral blood.

Conclusions

These findings challenge the notion of the eye as an immune-privileged site, as we consistently observed immune responses within UM tumors. The mechanism by which tumor antigens prime naïve T cells in this site remains mysterious.

Legal entity responsible for the study

F. Lucibello.

Funding

Ligue Contre le Cancer.

Disclosure

All authors have declared no conflicts of interest.