Membranous and nuclear staining of CLDN18 in HPV‐independent and HPV‐associated endocervical adenocarcinomas

OBJECTIVES: A classification system for endocervical adenocarcinoma (ECA) based on high‐risk human papillomavirus (HPV) status has been established; however, the immunohistochemical markers distinguishing HPV‐independent and HPV‐associated ECAs have not been fully described. Here, we aimed to characterize ECA immunopathological features. METHODS: We evaluated the immunohistochemical profile of CLDN18, CDX2, PAX8, p16, p53, and CEA in 60 ECAs comprising 10 HPV‐independent ECAs and 50 HPV‐associated ECAs. Both the membranous and nuclear expression levels of CLDN18 were analyzed. RESULTS: Membranous CLDN18 (CLDN18 [M]) was found to be expressed in the mucinous epithelium of all HPV‐independent ECAs, including eight gastric‐type ECAs (G‐ECAs), one endometrioid ECA, and one clear cell ECA, but no nuclear CLDN18 (CLDN18 [N]) expression was detected in HPV‐independent ECAs. Among HPV‐associated ECAs, CLDN18 (M) expression levels in intestinal‐type (I‐ECAs) and usual‐type ECAs (U‐ECAs) were significantly different from those in invasive stratified mucin‐producing (iSMILE) carcinomas (p = 0.036). Positive CLDN18 (M) staining was present in 55.6% (5/9) of intestinal‐type and 39.4% (13/33) of usual‐type ECAs and was not present in iSMILE ECAs. Silva pattern C cancers expressed higher levels of CLDN18 (M) than Silva pattern A and B cancers (p = 0.004), whereas the CLDN18 (N) expression levels in cancers showing Silva pattern A were significantly higher than those in cancers exhibiting Silva patterns B and C (p < 0.001). CONCLUSION: Membranous CLDN18 is expressed in ECAs and is particularly frequently expressed in HPV‐independent ECAs, and membranous CLDN18 expression has potential as a therapeutic target. Nuclear staining of CLDN18 is a new immunohistochemical marker for diagnosing Silva pattern A HPV‐associated ECAs and is associated with a good prognosis. Further studies should investigate the therapeutic and prognostic significance of membranous and nuclear CLDN18 expression and develop a related test that can be implemented in the clinical evaluation of ECAs.