DIFFERENTIAL EXPRESSION OF MHC CLASS-II MOLECULES BY MICROGLIA AND NEOPLASTIC ASTROGLIA - RELEVANCE FOR THE ESCAPE OF ASTROCYTOMA-CELLS FROM IMMUNE SURVEILLANCE

There is increasing evidence that microglia serve as antigen presenter s in the human CNS. Although the occurrence of MHC class II immunoreac tive cells has been reported in astrocytic gliomas, the relative contr ibution of microglia to this cell population has not been studied in d etail. Using computer-assisted image analysis, we have investigated th e expression of MHC class II molecules and of the microglia/macrophage markers Ki-M1P, RCA-1, KP1 and iba1, in 97 astrocytic gliomas compris ing all WHO grades to answer the question whether there is a correlati on between tumour grade and the number of MHC class II positive microg lia/macrophage profiles. Microglia expressing MHC class II were common in astrocytomas and anaplastic astrocytomas but rare in pilocytic tum ours although there was significant variation within each group. MHC c lass II immunoreactivity was reduced in highly cellular areas of gliob lastomas where large numbers of cells expressing macrophage markers we re still present. Thus, there was no simple relationship between tumou r grade and microglial/macrophage MHC class II expression. In addition , up to 55% of astrocytic gliomas contained MHC class II immunoreactiv e tumour cells. Microglia but not tumour cells were found to express t he BB1/B7 costimulator. We conclude that microglia in astrocytic gliom as are well equipped to function as antigen presenting cells. Yet, neo plastic astroglia appear to acquire the capacity to downregulate micro glial MHC class II expression and, at the same time, may induce T-cell clonal anergy through aberrant expression of MHC class II molecules.