The complement cascade not only provides protection from infection but can also mediate destructive inflammation. Complement is also involved in elimination of neuronal synapses which is essential for proper development, but can be detrimental during aging and disease. C1q, required for several of these complement-mediated activities, is present in the neuropil, microglia, and a subset of interneurons in the brain.

To identify the source(s) of C1q in the brain, the C1qa gene was selectively inactivated in the microglia or Thy-1⁺ neurons in both wild type mice and a mouse model of Alzheimer’s disease (AD), and C1q synthesis assessed by immunohistochemistry, QPCR, and western blot analysis.

While C1q expression in the brain was unaffected after inactivation of C1qa in Thy-1⁺ neurons, the brains of C1qa ^FL/FL :Cx3cr1 ^CreERT2 mice in which C1qa was ablated in microglia were devoid of C1q with the exception of limited C1q in subsets of interneurons. Surprisingly, this loss of C1q occurred even in the absence of tamoxifen by 1 month of age, demonstrating that Cre activity is tamoxifen-independent in microglia in Cx3cr1 ^{CreERT2/WganJ} mice. C1q expression in C1qa ^FL/FL : Cx3cr1 ^{CreERT2/WganJ} mice continued to decline and remained almost completely absent through aging and in AD model mice. No difference in C1q was detected in the liver or kidney from C1qa ^FL/FL : Cx3cr1 ^{CreERT2/WganJ} mice relative to controls, and C1qa ^FL/FL : Cx3cr1 ^{CreERT2/WganJ} mice had minimal, if any, reduction in plasma C1q.

Thus, microglia, but not neurons or peripheral sources, are the dominant source of C1q in the brain. While demonstrating that the Cx3cr1 ^{CreERT2/WganJ} deleter cannot be used for adult-induced deletion of genes in microglia, the model described here enables further investigation of physiological roles of C1q in the brain and identification of therapeutic targets for the selective control of complement-mediated activities contributing to neurodegenerative disorders.