IDENTIFICATION OF AN ASTROCYTE CELL-POPULATION FROM HUMAN BRAIN THAT EXPRESSES PERFORIN, A CYTOTOXIC PROTEIN IMPLICATED IN IMMUNE DEFENSE

The brain is an immunoprivileged organ isolated from the peripheral im mune system. However, it has been shown that resident cells, notably a strocytes and microglia, can express numerous innate immune molecules, providing the capacity to generate a local antipathogen system. Perfo rin is a cytolytic protein present in the granules of cytotoxic T lymp hocytes and natural killer cells. Expression in cells other than those of the hemopoetic lineage has not been described. We report here that fetal astrocytes in culture (passages 2 to 15), astrocytoma, and adul t astrocytes expressed perforin. Reverse transcriptase polymerase chai n reaction followed by Southern blot was carried out using multiple sp ecific primers and all cDNAs were cloned and sequenced. Human fetal as trocyte perforin cDNA sequence was similar to 100% identical to the re ported perforin cDNA cloned from T cells. Western blot analysis using monoclonal and polyclonal antiperforin peptide antibodies revealed a p rotein of 65 kD in both human fetal astrocyte and rat natural killer c ell lysates (n = 4). Immunostaining followed by FACS(R) and confocal a nd electron microscopy analysis revealed that perforin was expressed b y 40-50% of glial fibrillary acidic protein positive cells present in tile fetal brain culture (n = 11). Perforin was not localized to granu les in astrocytes but was present throughout the cytoplasm; probably i n association with the endoplasmic reticulum. Perforin was not detecte d in normal adult brain tissue but was present in and around areas of inflammation (white and grey matter) in multiple sclerosis and neurode generative brains. Perforin-positive cells were identified as reactive astrocytes. These findings demonstrate that perforin expression is no t unique to lymphoid cells and suggest that perforin produced by a sub population of astrocytes plays a role in inflammation in the brain.