Alternative complement pathway is activated in the brains of scrapie-infected rodents

Activation of complement system in central nervous system (CNS) of the patients suffering from prion diseases or animal models infected with prion agents experimentally is reported repeatedly, but which pathways are involved in the complement system during prion infection is not well documented. Here, we evaluated the level of complement factor B (CFB), which is the key factor that triggers alterative pathway (AP) of complement in the brain tissues of scrapie-infected mice with various methodologies. We found that the levels of mRNA and protein of CFB significantly increased in the brain tissues of scrapie-infected mice. Morphologically, the increased CFB-specific signal overlapped with the elevated C3 signal in brain sections of scrapie-infected mice, meanwhile overlapped with damaged neurons and activated microglia, but not with the proliferative astrocytes. Additionally, the level of complement factor P (CFP), the key positive regulator of AP, also increased remarkably in the brain tissues of infected mice. The transcriptional levels of CD55 and CD46, two negative regulators of AP, decreased without significance in brain tissues of scrapie-infected mice at the terminal stage. However, the mRNA and protein levels of CFH, another negative regulator of AP, increased. Through the dynamic analyses of the expressions of CFB, CFP, and CFH in brain sections of 139A-infected mice, which were collected at different time-points during incubation period, illustrated time-dependent increase levels of each factor during the incubation period of scrapie infection. Taken together, our data here demonstrate that the AP of complement cascade is activated in the CNS microenvironment during prion infection.