Using the thioglycollate-elicited murine peritonitis model for investigation of macrophage behaviour and its relationship with Irg1 expression

Macrophages play a crucial role in both innate and adaptive immunity. With a wide tissue distribution, macrophages are found in almost all tissues throughout the body. Upon activation, macrophages orchestrate the initiation and resolution of inflammation, promote wound repair and maintain homeostasis in a tissue-specific manner. This study investigates the behaviour of macrophages during sterile inflammation in the peritoneal cavity. By using a thioglycollate-induced murine model of peritonitis inflammation, the phenotypic and functional alterations of peritoneal immune cells was investigated and analysed over time. A particular emphasis was set on the dynamics of the macrophage lineage cells. In addition, the expression of Irg1 as a marker for macrophage activation during inflammation was investigated. Finally, the effectiveness of the sterile inflammation model in the context of drug discovery was tested by using the atypical antipsychotic drug clozapine. Phenotypic analysis showed that the number of resident macrophages was decreased after thioglycollate treatment. This effect was accompanied by a strong increase in monocyte-derived macrophages over time. Furthermore, macrophage colony-stimulating factor (CSF-1) appeared to be one of the key factors in recruiting circulating monocytes and promoting the differentiation of recruited monocytes to macrophages in this inflammation model. In addition, the analysis at different timepoints showed that a population of monocytes (Ly6C-) was present in the peritoneal cavity two days after thioglycollate treatment. For the activation signal of macrophages, the production of the two cytokines IL-10 and TNFα was assessed at two time points by intracellular cytokine staining. Although all investigated cell types produced both cytokines, there was little difference between different treatments. Furthermore, Irg1 expression was found decreased in macrophages stimulated with thioglycollate compared to unstimulated macrophages. Finally, the results from this study suggest that circulating monocytes are likely to be the key cell type responding to clozapine treatment based on the cell population dynamics and cytokine production. Taken together these results indicate that there is a dynamic change during the thioglycollate-elicited peritonitis involving a massive influx of infiltrating circulating leukocytes and proliferation of peritoneal macrophages. Moreover, application of this system for drug discovery may uncover the effects of a test drug in the context of sterile inflammation thus facilitating the development of effective treatments in the future.