p75NTR regulates brain mononuclear cell function and neuronal structure in Toxoplasma infection-induced neuroinflammation

Neurotrophins mediate neuronal growth, differentiation, and survival via tropomyosin receptor kinase (Trk) or p75 neurotrophin receptor (p75(NTR)) signaling. The p75(NTR) is not exclusively expressed by neurons but also by certain immune cells, implying a role for neurotrophin signaling in the immune system. In this study, we investigated the effect of p75(NTR) on innate immune cell behavior and on neuronal morphology upon chronic Toxoplasma gondii (T. gondii) infection-induced neuroinflammation. Characterization of the immune cells in the periphery and central nervous system (CNS) revealed that innate immune cell subsets in the brain upregulated p75(NTR) upon infection in wild-type mice. Although cell recruitment and phagocytic capacity of p75(NTRexonIV) knockout (p75(-/-)) mice were not impaired, the activation status of resident microglia and recruited myeloid cell subsets was altered. Importantly, recruited mononuclear cells in brains of infected p75(-/-) mice upregulated the production of the cytokines interleukin (IL)-10, IL-6 as well as IL-1 alpha. Protein levels of proBDNF, known to negatively influence neuronal morphology by binding p75(NTR), were highly increased upon chronic infection in the brain of wild-type and p75(-/-) mice. Moreover, upon infection the activated immune cells contributed to the proBDNF release. Notably, the neuroinflammation-induced changes in spine density were rescued in the p75(-/-) mice. In conclusion, these findings indicate that neurotrophin signaling via the p75(NTR) affects innate immune cell behavior, thus, influencing the structural plasticity of neurons under inflammatory conditions.