Cytokine transcripts expressed by microglia in vitro are not expressed by ameboid microglia of the developing rat central nervous system

Because of morphological similarities between ameboid microglia in the deve loping central nervous system (CNS), brain macrophages in the injured CNS, and cultured microglia in vitro, it is thought that these cell types are fu nctionally equivalent. To investigate the validity of this assumption, we h ave compared mRNA levels of interleukin-1a and -1b (IL-1a and IL-1b), tumor necrosis factor-a and -b (TNF-a and TNF-b), transforming growth factor-b1 (TGF-b1), and macrophage colony-stimulating factor (M-CSF) in the postnatal day 4 (P4) supraventricular corpus callosum (SVCC) with those in unstimula ted cultured microglia. Control tissues included spleen, cortex, hippocampu s, and cerebellum. Our analyses have shown that while IL-1a, IL-1b, TNF-a, TNF-b, and TGF-b1 transcripts are abundantly expressed by cultured microgli a, they are very low to virtually undetectable in the SVCC. These data stro ngly suggest that ameboid microglia, which are concentrated in the SVCC, ar e unlikely to be a significant source of these cytokines. Our study, which shows clear differences in the functional status of cultured microglia vs. ameboid microglia in vivo, stresses the importance of using caution when in terpreting in vitro findings in terms of the in vivo functions of microglia . (C) 1999 Wiley-Liss, Inc.