LAG-3 Expression, γδ-T cell/MHC-I Interactions and Prognosis in Merkel Cell Carcinoma.

Merkel Cell Carcinoma (MCC) is an aggressive cutaneous malignancy with a poor prognosis. One of the major mechanisms of immune evasion in MCC involves downregulation of MHC-I. Anti-PD-1/PD-L1 checkpoint inhibitors (CKIs) have revolutionized treatment for MCC, producing objective responses in ∼50% of patients, and are now standard of care; however, a substantial proportion of patients either fail to respond or develop resistance to CKIs. Given these recent successes, identification of other targetable immune checkpoints in the MCC tumor microenvironment (TME) is of great interest. Additionally, γ-delta (γδ) T cells may play critical roles in the response to MHC-I deficient cancers; therefore, evaluating γδ-T cells as a prognostic biomarker is warranted. We characterized the expression of PD-L1, PD-1, CD3, CD8, LAG-3, MHC-I, and γδ-T cells by IHC in a pre-immunotherapy retrospective cohort of 54 cases of MCC, and quantified expression levels and marker density via HALO. The increased density of LAG-3 and γδ-T cells correlated with other markers of an inflamed TME, with significant positive associations across all six markers (p<.002). Reflective of their putative role in the response to MHC-I suppressed cancers, cases with low HLA-I density showed a trend towards a higher ratio of γδ-T cells:CD3+ T cells (Spearman's r=-0.1582, p=0.21). Importantly, high CD3 density (HR=0.23, p=0.002), LAG-3 density (HR=0.47, p=0.037), γδ-T cell density (HR=0.26, p=0.02), and CD8 density (HR=0.27, p=0.03) showed associations with improved progression-free survival. Conditional tree analysis demonstrated that high CD8 and TCRδ expression were non-significant predictors of improved PFS and OS. Overall, LAG-3 is expressed in MCC infiltrates and is prognostic in pre-immunotherapy MCC, suggesting a potential role for LAG-3 inhibition in MCC. Additionally, CD8 and γδ-T cells may play a critical role in the response to MCC, and γδ-T cell density may represent a novel biomarker in MCC.