PD-L1, RB1 and mismatch repair protein immunohistochemical expression in neuroendocrine carcinoma, small cell type, of the uterine cervix.

AIMS: Neuroendocrine carcinoma, small cell type, of the uterine cervix (SmCC-Cx) is a rare human papilloma virus (HPV) related tumour with limited therapeutic options. Merkel cell carcinoma, another virus-associated neuroendocrine malignancy, has significant programmed death ligand 1 (PD-L1) expression rates. PD-L1 expression has been reported in other malignancies of the cervix. We aimed to determine the prevalence of PD-L1 in the context of mismatch repair protein (MMR) and RB1 expression status in SmCC-Cx.

METHODS AND RESULTS: Ten cases of SmCC-Cx were tested by immunohistochemistry for expression of PD-L1, MLH1, MSH2, MSH6, PMS2, RB1, CD3, CD20 and for HPV by in-situ hybridisation (ISH). PD-L1 expression was scored quantitatively (H-score) in tumour cells and lymphocytes (tumoral/peritumoral). PD-L1 positivity was seen in seven cases, focal in most (H-score range 3-140). Three of nine cases showed MMR deficiency. PD-L1 expression levels correlated with MMR expression status: all three MLH1/PMS2-deficient cases had a ≥5% PD-L1 staining and an H-score ≥10 (P = 0.01). RB1 was lost in four of nine cases, all PD-L1 positive, but this correlation was not statistically significant. Seven of nine tumours were positive for HPV-ISH; two of these had MLH1/PMS2 loss. Of the two HPV-ISH negative tumours, one had MLS1/PMS2 loss.

CONCLUSIONS: PD-L1 expression, predominantly focal, is seen in 70% of SmCC-Cx, while loss of MMR expression is seen in 33% of SmCC-Cx in our cohort. PD-L1 expression in more than 10% of tumour cells is seen in a subset of tumours in association with loss of MMR expression. These patients may be amenable to immune checkpoint inhibitor therapy as a promising alternative for this aggressive disease.