Expression of RCAS1 protein in microglia/macrophages accompanying brain tumours. An immunofluorescence study

The expression of protein RCAS1 (receptor-binding cancer antigen expressed on SiSo cells), possibly involved in the mechanisms of evasion of immune surveillance by tumours, has been studied in brain astrocytomas grade III and IV and in metastatic carcinomas to the brain by means of double immunofluorescence with antibodies against RCAS1 and respectively anti-GFAP (astroglia) or CD68 or CD74 (macrophages/microglia). Expression of RCAS1 has been reported in many types of carcinomas and in some normal cells, including bone marrow macrophages. Nakabayashi and coworkers recently reported expression of RCAS1 in gliomas. So far no attention has been paid to expression of RCAS1 in non-neoplastic cellular elements of tumours such as macrophages and to the expression of RCAS1 in metastatic carcinomas. We found expression of RCAS1 co-localizing with GFAP+ cells of gliomas and with CD68 and CD74 in large macrophages infiltrating metastatic and primary tumours and sometimes in cells which had morphological characteristics of microglia. Moreover, sometimes strong RCAS1 positivity has been found in metastatic carcinomas. Whether expression of RCAS1 in macrophages accompanying brain tumours is of any importance it is not possible to ascertain at present. However, when elucidating the possible role of RCAS1 in tumour biology, it seems to be necessary to include its presence not only in neoplastic cells.