CD99 immunoreactivity in ependymoma.

The expression of CD99 in normal ependymal cells and ependymoma has been reported. However, only limited numbers of tumors have been studied, and the pattern of CD99 expression has not been described. The authors' purpose was to investigate CD99 immunoreactivity in ependymoma and its use for differential diagnosis. Twenty-five ependymomas were immunostained with antibody directed at CD99. The result of immunostaining of ependymomas was compared with 63 nonependymal tumors that histologically resemble ependymal neoplasms. The nonependymal tumors included 19 astrocytic tumors, 6 oligodendroglial tumors, 8 choroid plexus neoplasms, 2 central neurocytomas, 5 medulloblastomas, 10 primitive neuroectodermal tumors (PNET), and 13 pituitary adenomas. All ependymomas showed strong expression of CD99 in membranous pattern with intracytoplasmic or intercellular dots (ICDs). The expression pattern of CD99 was not correlated with histologic type or grade of ependymomas. Among 63 nonependymal tumors, 11 (17.5%) showed incomplete membrane staining for CD99; diffuse in 4 PNETs and focal in 5 choroid plexus neoplasms (3 papillomas and 2 carcinomas) and one each of pituitary adenoma and oligodendroglioma. The ICD was not found in nonependymal tumors except a case of choroid plexus papilloma. However, membrane staining or ICD for CD99 was not distinctive in nonependymal tumors. In conclusion, the characteristic pattern of anti-CD99 antibody, i.e., diffuse strong membranous immunostaining with ICDs, is useful in distinguishing ependymomas from the central nervous system tumors that histologically mimic ependymoma.