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 proliferation rate. We investigated the proliferative activity of microglia in an immunohistochemical double-labeling study of pilocytic astrocytomas and astrocytomas WHO grade II-IV using the antibodies MIB-1 (Ki67) as proliferation-marker and Ki-M1P (CD68) as microglia marker. We found the highest indices of proliferating microglia in pilocytic astrocytomas with an average rate of 32% (+/- 6.8) of all proliferating cells. In contrast, the proliferation indices of microglia were lowest in fibrillary astrocytomas with 8.6% (+/- 2.5) of all proliferating cells. In anaplastic astrocytomas and glioblastomas the percentage of proliferating microglia showed a slight increase to 8.8% (+/- 3.6) and 13.4% (+/- 8.7), respectively. We conclude that microglial cells in astrocytic brain tumors proliferate and show different proliferative 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 when proliferation rates are used as a criterion for prognosis and grading of pilocytic astrocytomas.