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.
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.