PD-L1 expression in the Merkel cell carcinoma microenvironment: association with inflammation, Merkel cell polyomavirus and overall survival

Abstract

Merkel cell carcinoma (MCC) is a lethal, virus-associated cancer that lacks effective therapies for advanced disease. Agents blocking the PD-1/PD-L1 pathway have demonstrated objective, durable tumor regressions in patients with advanced solid malignancies and efficacy has been linked to PD-L1 expression in the tumor microenvironment. To investigate whether MCC might be a target for PD-1/PD-L1 blockade, we examined MCC PD-L1 expression, its association with tumor-infiltrating lymphocytes (TILs), Merkel cell polyomavirus (MCPyV), and overall survival. Sixty-seven MCC specimens from 49 patients were assessed with immunohistochemistry for PD-L1 expression by tumor cells and TILs, and immune infiltrates were characterized phenotypically. Tumor cell and TIL PD-L1 expression were observed in 49% and 55% of patients, respectively. In specimens with PD-L1(+) tumor cells, 97% (28/29) demonstrated a geographic association with immune infiltrates. Among specimens with moderate-severe TIL intensities, 100% (29/29) demonstrated PD-L1 expression by tumor cells. Significant associations were also observed between the presence of MCPyV DNA, a brisk inflammatory response, and tumor cell PD-L1 expression: MCPyV(-) tumor cells were uniformly PD-L1(-). Taken together, these findings suggest that a local tumor-specific and potentially MCPyV-specific immune response drives tumor PD-L1 expression, similar to previous observations in melanoma and head and neck squamous cell carcinomas. In multivariate analyses, PD-L1(-) MCCs were independently associated with worse overall survival (hazard ratio 3.12; 95% CI, 1.28-7.61; p=0.012). These findings suggest that an endogenous immune response promotes PD-L1 expression in the MCC microenvironment when MCPyV is present, and provide a rationale for investigating therapies blocking PD-1/PD-L1 for patients with MCC.

Keywords: B7-H1; Merkel cell carcinoma; Merkel cell polyomavirus; PD-L1; immunotherapy.

Figure 1

Geographic co-localization of PD-L1+ tumor cells with infiltrating immune cells in MCC. The predominant pattern of tumor cell PD-L1 expression is at the interface with infiltrating immune cells. At this interface, MCC cells (CK20 immunostain demonstrating a characteristic perinuclear dot-like staining pattern), CD3+ T cells, and associated CD68+ macrophages all demonstrated PD-L1 expression. This pattern was most often observed in geographic foci with a high density of CD8+ T cells. Green arrows show lymphocytes expressing PD-L1, and the yellow arrow demonstrates a PD-L1+ tumor cell cluster immediately adjacent to the immune infiltrate. Original magnification 400x, all panels.

Figure 2

Constitutive PD-L1 expression in MCC. Only one among 29 PD-L1+ MCC specimens demonstrated diffuse membranous expression by tumor cells that was out of proportion to the scant number of infiltrating CD3+ T lymphocytes. Orange arrows point to CD3+ T lymphocytes. Original magnification 400x, all panels.

Figure 3

Correlation of PD-L1 expression with overall survival in 49 patients with MCC. (A) Kaplan-Meier survival curve showing improved survival for MCC patients whose tumor cells demonstrated membranous PD-L1 expression. (B) TIL and associated histiocyte PD-L1 expression was not significantly associated with improved outcomes.