Outbred ICR/CD1 mice display more severe neuroinflammation mediated by microglial TLR4/CD14 activation than inbred C57Bl/6 mice.

Neuroinflammation mediated by microglia is a pathological hallmark of many CNS disorders. Cell lines derived from inbred C57Bl/6 and outbred ICR/CD1 mice (BV-2 and N9 respectively), are often used to study microglial inflammatory activities. Although many studies demonstrate different responses of these cell lines to the same stimulus, no comparisons have been done in vivo. Because inbreeding reduces resistance to pathogens and parasites, we hypothesized that microglia from outbred ICR/CD1 mice would have a stronger response to centrally administered LPS than microglia from inbred C57Bl/6 mice. The evaluation of gene expression in freshly isolated CD11b+ cells from brain revealed that microglia from ICR/CD1 mice were more pro-inflammatory than those from C57Bl/6 mice, although these differences did not appear to result from alterations in the expression levels of the LPS receptors TLR4 or CD14. Notably, the timing of inflammatory gene expression did not correlate with CD11b+ cell proliferation/infiltration. The highest expression of TNFα, IL-6 and iNOS occurred 3 h after LPS injection when the number of CD11b+ cells was not changed. Whereas the expression of these pro-inflammatory genes had returned to basal by 48 h when the highest number of CD11b+ cells in the brain was found, the expression of the anti-inflammatory cytokine IL-10 was still significantly up-regulated. This is important because the increased presence of CD11b+ cells in the CNS is often used as an indicator of neuroinflammation. While LPS did not affect the expression of the growth factors VEGF or BDNF, we observed that mechanical injury (caused by intraparenchymal injection) induced distinct patterns of microglial activation characterized by increased expression of VEGF and down-regulation of BDNF. It remains to be determined which type of microglia is more beneficial/detrimental to the CNS, but our data suggest that genetic traits determining microglial properties may have profound effect on many CNS pathologies.