Lymphocyte Activation Gene 3 Expression, γδ T-Cell/Major Histocompatibility Complex Class 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 major histocompatibility complex class I (MHC-I). Anti-PD-1/programmed death ligand 1 checkpoint inhibitors have revolutionized treatment for MCC, producing objective responses in approximately 50% of patients, and are now the standard of care; however, a substantial proportion of patients either fail to respond or develop resistance to checkpoint inhibitors. Given these recent successes, identification of other targetable immune checkpoints in the MCC tumor microenvironment is of great interest. Additionally, γδ T cells may play critical roles in response to MHC-I-deficient cancers; therefore, evaluating γδ T cells as a prognostic biomarker is warranted. We characterized the expression of programmed death ligand 1, PD-1, CD3, CD8, lymphocyte activation gene 3 (LAG-3), MHC-I, and γδ T cells by immunohistochemistry in a preimmunotherapy retrospective cohort of 54 cases of MCC and quantified expression levels and marker density using HALO software. The increased density of LAG-3 and γδ T cells correlated with other markers of an inflamed tumor microenvironment, with significant positive associations across all 6 markers (P < .002). Reflective of their putative role in the response to MHC-I-suppressed cancers, cases with low human leukocyte antigen I density showed a trend toward a higher ratio of γδ T cells:CD3+ T cells (Spearman r = -0.1582; P = .21). Importantly, high CD3 density (hazard ratio [HR], 0.23; P = .002), LAG-3 density (HR, 0.47; P = .037), γδ T-cell density (HR, 0.26; P = .02), and CD8 density (HR, 0.27; P = .03) showed associations with improved progression-free survival. Conditional tree analysis demonstrated that high CD8 and TCRδ expression were nonsignificant predictors of improved progression-free survival and overall survival. Overall, LAG-3 is expressed in MCC infiltrates and is prognostic in preimmunotherapy 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.