Plasticity of macrophages from helminth infection

The prime function of classically activated macrophages (CAM; activated by Thi -type (or 'type 1') signals such as IFN-? ) is microbial destruction. Alternatively activated macrophages (AAM; activated by Th2 (or 'type 2') cytokines, such as IL-4) play important roles in allergy and responses to parasite infection. A murine model for filarial infection has been used as an in vivo source of AAM, which we have termed nematodeelicited Macrophages (NeM). Mice are surgically implanted into the peritoneal cavity with adult Brugia malayi. By one week post-infection, the PEC population is dominated by macrophages that display IL-4 dependent features such as the expression of Arginase 1, RELM-a and Ymi. In light of the increasing evidence that macrophages show functional adaptivity, it was decided to study the NeM response to pro-inflammatory Thi activating signals as a model to investigate whether the switch between alternative and classical activation can occur in macrophages differentiated in an in vivo infection setting. Despite the long-term exposure to Th2 cytokines and anti-inflammatory signals in vivo, we found that NeM were not terminally differentiated but could switch from alternative activation to a more classically activated phenotype in response to LPS/IFN-y. This was reflected by a switch in the enzymatic pathway for arginine metabolism from arginase to iNOS and the reduced expression of RELM-a and I'm]. To ask whether these AAM could be induced to become antimicrobial, we also carried out infections with "type 1 "-inducing pathogens, It was found that LPS/IFN-y, treated NeMp were able to control infection with Leishmania mexicana as effectively as LPS/IFN-y activated thioglycollate-elicited macrophages (ThioM) and parasite killing was mediated by nitric oxide production. NeM were also infected with the mycobacterium Mycobacterium bovis BCG. It was found that NeMO responded to low doses of BCG by controlling it for the entire timeframe of the study, i.e. 6 days. NeM responded to high doses of BCG infection with early control of infection and high levels of apoptosis, and that this phenotype is independent of IL-4. Next we asked whether the macrophage phenotype would alter during co-infection with both a 'type 1' and 'type 2'inducing pathogen. Ym1 and RELM-cx were looked at in the lungs of mice co-infected with malaria (Plasmodium chabaudi; 'type 1' pathogen) and the helminth worm Nippostrongylus brasilisensis ('type 2' pathogen). After 7 days of infection it was found that there was less Ym1 and RELM-a present in co-infected, as compared with Nippostrongylus-only infected mice, which suggested that the immune system could switch from an alternative to a more classically activated state in order to deal with a type-i infection or that the malarial infection was causing a delay, or decrease in the Th2 response. Lastly, we wanted to look at what innate signals determine the AAM4 phenotype and its ability to switch towards classical activation. It is currently unknown whether signaling through toll-like receptors (TLRs) plays any role in the alternative activation of macrophages. In light of this, it was decided to investigate, using MyD88-/animals, whether alternative activated macrophages could be recruited in the absence of the important adaptor molecule, MyD88, and thus determine whether TLR signals impact negatively or positively on the AAIvh. Of the NeM present in the peritoneal cell populations (PEC) of both wild type (WT) and MyD88-deficient mice we found no difference in terms of arginase production or the expression of RELM-a or YM 1. Therefore, lack of MyD88 had no apparent effect on the NeM phenotype.