Susceptibility and resistance to nematode infection: role of recruited vs. resident macrophages
Campbell, Sharon Mary
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Macrophages are phagocytic cells of the innate immune system, which have a central role in immune surveillance, tissue homeostasis and the immune response to bacterial, viral, protozoan and helminth parasites. It is now appreciated that many tissue resident macrophage (resMΦ) populations, including those in the peritoneal and pleural cavity, are derived prenatally prior to the establishment of definitive haematopoiesis in the bone marrow. Once seeded, these resMΦ populations are long-lived and capable of self-renewal via in situ proliferation driven by CSF-1. An inflammatory insult, such as bacterial infection, results in the recruitment of bone marrow derived macrophages (BMDMΦ) and the disappearance of the resMΦ population. BMDMΦ recruited to the site of infection become classically activated upon engagement of pathogen recognition receptors and subsequent STAT1 induction. Classically activated macrophages (CAMΦ) are highly bactericidal through the production of inflammatory cytokines, which direct the TH1 immune response, and upregulation of iNOS to generate high concentrations of intracellular nitric oxide. During resolution of acute inflammation resMΦ undergo a CSF-1 driven proliferative burst to repopulate the tissue. In contrast to bacterial infection, helminth parasites drive a TH2 immune response characterised by CD4+ T cell production of IL-4, which induces proliferation and alternative activation of the resMΦ population, thereby overcoming the need for an inflammatory influx of BMDMΦ. Alternatively activated macrophages (AAMΦ) are generated through signalling from the IL-4Rα subunit and subsequent expression of the molecules RELMα, YM1 and arginase-1. While both BMDMΦ and resMΦ upregulate RELMα, YM1 and Arg-1 in response to IL-4Rα stimulation, microarray analysis revealed an otherwise diverse transcriptional and cell surface phenotype between these populations. It was hypothesised that the diverse modes of macrophage accumulation enlisted by bacterial and helminth parasites, combined with the distinct alternative activation phenotypes employed by BMDMΦ and resMΦ populations would translate into important functional differences as regards anti-parasitic immunity. Chapter 1 and 2 of this thesis addresses the importance of macrophage origin during infection with the filarial nematode Litomosoides sigmodontis, taking advantage of the naturally occurring resistant C57BL/6 and susceptible BALB/c strains. A large disparity in MΦ accumulation was observed throughout the infection time course, with significantly larger numbers present within the pleural cavity of resistant C57BL/6 mice. This difference in MΦ number was a reflection of enhanced F4/80hi resMΦ accumulation. Through Ki67hi staining and the use of CCR2-/- and partial bone marrow chimeric mice, the expanded F4/80hi population in resistant C57BL/6 mice was shown to be a result of proliferation of the local F4/80hiGATA6+CD102+ resMΦ population. A high degree of BMDMΦ incorporation into the resMΦ pool through assumption of an F4/80hiGATA6+CD102+ phenotype was observed in both naïve and infected bone marrow chimeric animals, supporting a recent publication showing gradual incorporation of these cells into the resMΦ niche with age. Importantly, the degree of BMDMΦ incorporation into the F4/80hi population was equivalent between naïve and infected animals, despite a 27-fold difference in cell number, illustrating that expansion is a result of proliferation of local resMΦ, independent of origin. Susceptibility was marked by reduced resMΦ proliferation and enhanced recruitment of bone marrow derived F4/80lo MΦ and monocytes. These recruited BMDMΦ displaced the resMΦ population, failed to integrate the resMΦ niche and were highly positive for PD-L2, a marker specific to BMD AAMΦ. Prevention of monocyte influx and subsequent resMΦ displacement resulted in increased worm killing and a stronger TH2 immune response in susceptible BALB/c mice, thereby confirming a detrimental role for BMD AAMΦ in worm killing. Conversely, in order to confirm a protective role for the expanded resMΦ in resistant C57BL/6 mice we attempted to deplete the resMΦ population through intrapleural delivery of clodronate-loaded liposomes. Due to technical issues we were unable generate statistically significant results when depleting the resMΦ population, however a trend toward decreased worm killing in the absence of resMΦ is evident. Previously generated microarrays in the lab identified the complement cascade as being highly upregulated by AAMΦ induced in response to Brugia malayi infection compared to BMDMΦ (thioglycollate elicited). To investigate the role of complement in resistance to L. sigmodontis, Chapter 3 briefly phenotypes the macrophage compartment of C3-/- C57BL/6 mice during infection. No differences in worm burden or macrophage phenotype could be detected in C3-/- mice compared to WT controls, however this may be explained through differences in strain or MΦ origin. This chapter provides an important foundation for future studies on complement and its role in worm killing during L. sigmodontis infection. The final chapter of the thesis focuses on examining the bactericidal capabilities of BMD and resMΦ populations. An in vitro system was utilised to assess the interaction of bone marrow derived macrophages (thioglycollate elicited) and ResMΦ (from naïve mice) with Salmonella enterica serovar Typhimurium SL3261. I found that in vitro BMDMΦ are infected by/ingest SL3261 to a much greater degree than resMΦ. The resMΦ population is less efficient at both controlling the spread and killing intracellular SL3261 overtime, compared to the BMDMΦ population. In vivo however, there appears to be no difference in the ability of the monocyte derived F4/80lo MΦ and the F4/80hi resident MΦ to be infected by/ ingest SL3261, nor was a difference in bactericidal ability detected. Ultimately my work highlights that the anti-parasitic functions of MΦ populations are not dictated by origin but rather the activation phenotype upon infection and ability to respond to local stimuli.