Increased number of microglia in the brain of severe combined immunodeficient (SCID) mice.

To assess the in vivo influence of the systemic immune system upon microglia, six defined brain regions of adult SCID mice (n = 10) lacking functional T- and B-lymphocytes have been analyzed by NDPase histochemistry, morphometry and immunohistochemistry. Despite absence of neuropathology and lack of microglial activation, microglial numerical density was significantly increased in SCID mice. Elevation was most marked in the cerebellar granular layer (by 32.6%; 95% confidence interval: 9.9-58.7%), followed by the fimbria hippocampi and the molecular layer of hippocampal CA1/CA3 region. These data need to be taken into account when using SCID mice as a model for microglial reaction in immunodeficient mice.