Increased microglia proliferation separates pilocytic astrocytomas from diffuse astrocytomas: a double labeling study

It is not known how many non-tumorous cells in gliomas contribute to the pr oliferation rate. We investigated the proliferative activity of microglia i n an immunohistochemical double-labeling study of pilocytic astrocytomas an d astrocytomas WHO grade II-IV using the antibodies MIB-1 (Ki67) as prolife ration-marker and Ki-M1P (CD68) as microglia marker. We found the highest i ndices of proliferating microglia in pilocytic astrocytomas with an average rate of 32% (+/-6.8) of all proliferating cells. In contrast, the prolifer ation indices of microglia were lowest in fibrillary astrocytomas with 8.6% (+/-2.5) of all proliferating cells. In anaplastic astrocytomas and gliobl astomas the percentage of proliferating microglia showed a slight increase to 8.8% (+/-3.6) and 13.4% (+/-8.7), respectively. We conclude that microgl ial cells in astrocytic brain tumors proliferate and show different prolife rative activities at different grades of malignancy with the highest rates of proliferating microglia especially in pilocytic astrocytomas. Thus, the proliferation rate does not solely reflect the proliferation of tumor cells , but also of non-tumorous cells. This should be considered in particular w hen proliferation rates are used as a criterion for prognosis and grading o f pilocytic astrocytomas.