Quinolinate immunoreactivity in experimental rat brain tumors is present in macrophages but not in astrocytes

Experimental tumors of the central nervous system were investigated with antibodies to quinolinate to assess the cellular distribution of this endogenous neurotoxin. In advanced F98 and RG-2 glioblastomas and E367 neuroblastomas in the striatum of rats, variable numbers of quinolinate immunoreactive cells were observed in and around the tumors, with the majority being present within tumors, rather than brain parenchyma. The stained cells were morphologically variable, including round, complex, rod-shaped, and sparsely dendritic cells. Neuroblastoma and glioma cells were unstained, as were neurons, astrocytes, oligodendrocytes, ependymal cells, endothelial cells, and cells of the choroid plexus and leptomeninges. Glial fibrillary acidic protein immunoreactivity was strongly elevated in astrocytes surrounding the tumors. Dual labeling immunohistochemistry with antibodies to quinolinate and glial fibrillary acidic protein demonstrated that astrocytes and the cells containing quinolinate immunoreactivity were morphologically disparate and preferentially distributed external and internal to the tumors, respectively, and no dual labeled cells were observed. Lectin histochemistry with Griffonia simplicifolia B4 isolectin and Lycopersicon esculentum lectin demonstrated numerous phagocytic macrophages and reactive microglia in and around the tumors whose distribution was similar to that of quinolinate immunoreactive cells, albeit much more numerous. Dual labeling studies with antibodies to quinolinate and the lectins demonstrated partial codistribution of these markers, with most double-labeled cells having the morphology of phagocytes. The present findings suggest the possibility that quinolinate may serve a functional role in a select population of inflammatory cell infiltrates during the immune response to brain neoplasms.