Type II Activation of Macrophages and Microglia: Roles in T cell biasing and experimental autoimmune encephalomyelitis

Multiple Sclerosis (MS) is an immune-mediated disease of the central nervous system (CNS) which causes demyelination and damage to the neuronal axons. MS is a significant health problem in New Zealand, affecting 1 in 1400 people. One of the major cell types involved in MS and its animal model, experimental autoimmune encephalomyelitis (EAE), are the proinflammatory subsets of T helper (Th) cells. However, many other cell types are also involved, including macrophages (MΦ) and microglia (MG). MΦ and MG are considered to be important drivers of inflammation during MS; however there is also evidence that indicates these cells can also play a protective role. In the periphery, MΦ can be induced into several activation states. One of these activation states, type II activation, is a regulatory phenotype, producing increased IL-10 and decreased IL-12 compared to classical activation. In vitro type II activation is induced by ligating FcγR with immune complexes (IC) with concurrent stimulation with lipopolysaccharide. Previous research has shown that type II activated MΦ and type II-inducing treatments are protective in EAE.  MΦ activation state affects the way the Th response develops, with classically activated MΦ promoting a Th1 response and type II MΦ promoting a Th2 response. In the current study, the role of MΦ in T cell biasing is investigated further. Type II activated MΦ altered the Th1/Th2 dichotomy away from a Th1 response and towards a Th2 response, as demonstrated by decreased production of IFN-γ and increased levels of CD124. Also, in a novel finding, type II activated MΦ also increased the production of IL-17A from T cells. This study also aimed to elucidate the pathways involved in biasing of the T cell response by classical and type II MΦ by blocking or enhancing specific pathways. It was found that, while the level of IFN-γ (the prototypical Th1 cytokine) was largely dependent on the levels of IL-10 and IL-12, IL-17A and CD124 expression appeared to be independent of these two cytokines. Type II MΦ have decreased expression of CD40 and PD-L1, it was found that these pathways are not strongly involved in T cell biasing by type II MΦ.  While it is acknowledged that MG can be pathogenic and protective, the direct effect type II activating treatments have on MG is unknown. In order to investigate whether MG can also be type II activated, MG were isolated from the CNS of adult mice their phenotype under different activating condition was assessed. Under type II activating conditions MG produced less IL-12 and more IL-10, suggesting type II activation is occurring. In addition, in MG:T cell co-cultures, T cells cultured with classical or type II activated MG have similar, but not identical, profiles to T cells cultured with MΦ. Type II activated MΦ induced increases in CD124 and IL-17A from T cells, however, all MG activation states induced similar levels of IFN-γ.  To determine the effect of type II activating treatments in vivo bone marrow chimeric mice were created using mice congenic for CD45, which allow MG to be distinguished from invading cells. Treatment of mice with the IC before and during EAE results in decrease the incidence and severity of disease. IC treatments altered both the peripheral and the CNS immune environments. Despite inducing protection, IC treatments induced an increase in IL-17A in the peripheral immune system. In the brain, a population of resident cells that are CD45(int)CD11b positive, and are likely to be CNS associated MΦexpress decreased levels of MHC class II, suggesting a decreased ability to interact with T cells. Furthermore, parenchymal MG from the brains of IC treated animals have different levels of Iba1 compared to those from untreated animals, suggesting differential activation. Overall the data suggests that IC treatment induces a change in the activation state of CNS resident immune cells, that is likely to be protective in EAE.