Modulating Macrophage Phenotypes During Infection In Zebrafish

Macrophages are important innate immune cells that are highly phagocytic, responding to infections by upregulation of antimicrobial factors (termed proinflammatory/M1 macrophage polarisation). After the initial response, macrophage populations change behaviours to more anti-inflammatory phenotypes to participate in healing and restoration of homeostasis (termed anti-inflammatory/M2 macrophage polarisation). A hallmark of macrophage polarisation is their antimicrobial nitric oxide (NO) output, with proinflammatory macrophages having upregulated inducible Nitric Oxide Synthase (iNOS) leading to high NO levels and anti-inflammatory macrophages having upregulated Arginase, an enzyme that competes with iNOS for a shared substrate, leading to decreased NO (Figure A). iNOS and Arginase are therefore important markers of macrophage polarisation (1). Despite a deep understanding of macrophage phenotypes in vitro, how diverse macrophage behaviours manifest in infected tissues is not well understood and represents an opportunity for therapeutic intervention during infections.
Bacterial infections are becoming increasingly resistant to antibiotics, leading to a global crisis and an urgent need for new medicines. Our approach is to better understand the innate immune response to infections to boost white blood cell bacterial killing to clear our bodies of infection before drug resistance can develop. Tuberculosis (TB) is a global crisis and treatments require lengthy regimens of failing antibiotics. TB is characterised by parasitisation of macrophages by the bacterium, Mycobacterium tuberculosis (Mtb), promoting spread across the body from the infected lung and driving hallmark granuloma formation that provides a niche in which Mtb can remain protected from the immune system for many years. The formation of the granuloma niche is driven by exploitation of macrophage polarisation by Mtb, via reduction of proinflammatory killing towards a more anti-inflammatory phenotype (2), but the mechanisms behind these processes are not fully understood and require an in vivo setting to model the complex host-pathogen interactions occurring. We propose that therapeutic strategies that target the host macrophages, alongside antibiotics, would be more efficacious than antibiotics alone and would combat the increasing prevalence of antimicrobial resistance. Therapeutic manipulation of proinflammatory/anti-inflammatory macrophage polarisation may promote early microbial killing before infection can take hold and improve subsequent recovery in TB. Here we propose to develop a zebrafish larval model of anti-inflammatory macrophages that will allow examination of macrophage polarisation in TB in vivo. This will replace mammalian models of invasive infection and reduce the overall number of animals used.