Tumoral PD-L1 expression in desmoplastic melanoma is associated with depth of invasion, tumor-infiltrating CD8 cytotoxic lymphocytes and the mixed cytomorphological variant

Recently, patients with metastatic desmoplastic melanoma (DM) have been shown to respond more favorably to anti-PD1/PD-L1 therapy than other melanoma subtypes. Given this, we evaluated PD-L1/2 expression in primary DM samples and correlated these with subtype, CD8+ lymphocyte status, histopathological prognosticators, and select genetic alterations. Eighty-six (36 mixed DM, 50 pure DM) archival annotated samples met inclusion criteria and were immunohistochemically semiquantitatively evaluated. Per established criteria, for PD-L1/L2, cases with >= 5% tumoral expression, and for CD8, cases with a predominantly peri/intratumoral CD8+ infiltrate were scored positive. Univariate analysis (chi-square and Wilcoxon) identified potential confounders and a nested case-control study was accomplished using multiple logistic regression. For PD-L1, 49% of cases were positive and 71% of cases with thickness 44 mm were positive; PD-L1 expression differed by median depth (3.29 mm, interquartile range = 3.58 mm for PD-L1 positives vs 1.75 mm, interquartile range = 2.04 mm for PD-L1 negatives, P = 0.0002) and was linearly associated with increasing depth of invasion (P = 0.0003). PD-L1-positive cases were more likely to display CD8+ lymphocytes (60 vs 28% P = 0.0047). The presence of CD8+ lymphocytes correlated significantly with depth of invasion > 1mm (P = 0.022). On multivariate analysis, PD-L1 was 6.14 x more likely to be expressed in mixed DM than pure DM (P = 0.0131), CD8+ staining was 6.22 x more likely in PD-L1 positive cases than in PD-L1 negative (P = 0.0118), and tumor depth was associated with greater odds of PD-L1 expression (OR = 1.61, P = 0.0181). PD-L2 expression was observed in 48% of cases but did not correlate with any variables. Correlation of tumoral PD-L1 with increased depth and CD8+ lymphocytes implicates the tumoral immune microenvironment with advancing disease in DM. Enhanced tumoral PD-L1 expression in the mixed cytomorphological variant provides an insight into the differential pathogenesis of the subtypes and suggests that these patients are likely better candidates for anti-PD/PD-L1 therapy.