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Supplemental Table 1. Summary of the clinical and histopathologic characteristics of Merkel cell carcinoma patients. Supplemental Table 2. Summary of antibodies used in this study. Supplemental Table 3. Correlation between tumor associated immune infiltrate and co-localized expression of CD31 and B7-H3 in Primary MCCs.

Supplemental Figure 1. Dynamic range of co-localized expression of CD31 and B7-H3 in primary Merkel cell carcinoma. Each CD31+ and B7-H3+ cell centroid was tabulated as a series of (X, Y) coordinates (cell centroids). The frequency of their respective overlap for each of the 52 primary MCCs was calculated within a given distance as indicated: blue boxes show G-function colocalization index applying a 1 pixel (1.57 �m) minimum distance of overlapped expression as our cutoff, and green boxes show G-function colocalization index applying a 2 pixel (3.14 �m) minimum distance of overlapped expression as our cutoff.

Purpose:Merkel cell carcinoma (MCC) is an aggressive cutaneous malignancy whose pathogenesis and prognosis are related to the integrity of the host immune system. Despite promising clinical responses to immune-checkpoint blockade, response and resistance remain unpredictable, underscoring a critical need to delineate novel prognostic biomarkers and/or therapeutic targets for this disease.Experimental Design: Expression of immune-regulatory markers (PD-L2, B7-H3, B7-H4, IDO-1, ICOS, TIM3, LAG3, VISTA, and OX-40) was assessed using singlet chromogenic IHC in 10 primary MCCs. Multiplex immunofluorescence quantified CD31 and B7-H3 expression in 52 primary and 25 metastatic MCCs. B7-H3 and CD31 expressions were tabulated as a series of independent (X,Y) cell centroids. A spatial G-function, calculated based on the distribution of distances of B7-H3+ (X,Y) cell centroids around the CD31+ (X,Y) cell centroids, was used to estimate a colocalization index equivalent to the percentage of CD31-positive cell centroids that overlap with a B7-H3–positive cell centroid.Results:Primary and metastatic MCCs exhibit a dynamic range of colocalized CD31 and B7-H3 expression. Increasing colocalized expression of B7-H3 with CD31 significantly associated with increased tumor size (P = 0.0060), greater depth of invasion (P = 0.0110), presence of lymphovascular invasion (P = 0.0453), and invasion beyond skin (P = 0.0428) in primary MCC. Consistent with these findings, increasing colocalized expression of B7-H3 and CD31 correlated with increasing vascular density in primary MCC, but not metastatic MCC.Conclusions:Our results demonstrate that colocalized expression of B7-H3/CD31 is a poor prognostic indicator and suggest therapies targeting B7-H3 may represent an effective approach to augmenting immune-activating therapies for MCC.

Supplemental Figure 2. Correlation between B7-H3/CD31 G-function co-localization index in MCPyV-positive MCCs. Plots showing the correlation B7-H3/CD31 G-function co-localization index for each primary MCPyV-positive MCC plotted against tumor size (A), depth of invasion (B), invasion beyond skin (C) and metastasis beyond SLN (D). G-function calculated according to a 1 pixel (1.57 �m) radius.

Supplemental Figure 3. Correlation between B7-H3/CD31 G-function co-localization index in MCPyV-negative MCCs. Plots showing the correlation B7-H3/CD31 G-function co-localization index for each primary MCPyV-negative MCC plotted against the frequency of associated malignancies (A) and lymphovascular invasion (B). G-function calculated according to a 1 pixel (1.57 �m) radius.

Supplemental Figure 4. Correlation between B7-H3/CD31 G-function co-localization index and vascular density. Plots showing the correlation B7-H3/CD31 G-function co-localization index plotted against vascular density for the whole cohort of primary MCC (vascular density tabulated as the average of the regions of interest in each of the 52 tumors).