Minocycline ameliorates CNS autoimmunity through restraint of CD4⁺CD11bc⁺ cells

Minocycline is a tetracycline antibiotic with profound anti-inflammatory activity directed primarily against microglial cells and macrophages. Activation of nuclear factor erythroid 2-related factor 2 (Nrf2) is considered as an important target of minocycline. This drug has been considered a promising therapeutic for multiple sclerosis, an autoimmune disorder of the central nervous system (CNS). Still, its effect on CNS autoimmunity is not entirely understood. The aim of this study was to investigate the potency of short-term minocycline administration in fully developed CNS inflammation. To this end, an animal model for multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), was used and minocycline was applied at the peak of the disease. The treatment led to immediate reduction of EAE clinical signs, and also to reduction of overall EAE clinical course severity. The effect of minocycline at the peak of EAE was paralleled with inhibited NO and reactive oxygen species (ROS) production by immune cells in CNS. Interestingly, the reduction was specific for CD4⁺CD11bc⁺ cells, as it was not observed in CD8⁺CD11bc⁺ cells or neutrophils. Minocycline efficiently reduced NO and ROS generation in macrophages in vitro. Surprisingly, this effect was not abolished in macrophages obtained from Nrf2 knockout mice. Further, beneficial effects of minocycline in EAE were also observed in Nrf2 knockout mice, thus corroborating the notion that Nrf2 activation is not the decisive way of anti-encephalitogenic effects of the drug. Taken together, these data indicate that beneficial effects of minocycline in EAE is primarily based on its ability to limit the generation of ROS and NO in CNS CD4⁺CD11bc⁺ cells and that its overall effect on EAE is Nrf2-independent. Our results substantiate the idea that minocycline is a valid candidate for further multiple sclerosis-related research and point to CD4⁺CD11bc⁺ cells as an important disease causative myeloid cell population in the CNS autoimmunity.