Acute inflammatory response is caused by the rapid recruitment of leukocytes, mainly neutrophils and monocytes, from blood to the tissue site. Diverse agents, including invading pathogens, injured or dead cells, and other irritants, may stimulate this response. In the ensuing inflammatory response, the recruited leukocytes and their secreted molecules help in eliminating or containing the injurious agents and promoting tissue regeneration. But often this response is imprecise and can lead to bystander tissue damage. Unchecked neutrophil activation is implicated in the pathology of many inflammatory conditions. An in-depth understanding of the pathways regulating this response, therefore, becomes critical in identifying therapeutic targets for these diseases. In this study, we investigate the role of intestinal commensal bacteria in regulating the acute inflammatory response. Furthermore, we examine the mechanism by which Interleukin-1 (IL-1) controls the inflammatory response to sterile agents.

Inflammatory responses have been studied in the context of host defense against pathogens. However, we report that the innate immune system needs to be primed by intestinal flora to enable neutrophil recruitment to diverse microbial or sterile inflammatory signals. This priming requires myeloid differentiation primary response gene (88) (MyD88) signaling. In antibiotic-treated mice, which have depleted intestinal flora, we show that neutrophils get released into the blood from the bone marrow, but have a specific defect in migration into the inflammed tissue. This deficiency can be restored by pre-stimulating the mice with a purified MyD88 ligand. Despite having reduced number of infiltrating neutrophils, antibiotic-treated mice make higher levels of pro-inflammatory cytokines in the tissue, after inflammatory challenge. This suggests that antibiotic-treated mice produce some anti-inflammatory molecule(s) that counteract the effect of the pro-inflammatory cytokines. However, this effect is not due to the overproduction of the anti-inflammatory cytokine, Interleukin-10 (IL-10). In summary, our findings highlight the role of commensals in the development of acute inflammatory responses to microbial and sterile particles.

The inflammatory response to sterile dead cells has been shown to be critically dependent upon IL-1. However, several key aspects of the IL-1 signaling cascade including the source of IL-1 and the cellular target of IL-1 were unresolved. We find that in most cases, the injured cells are not a major contributor of IL-1 that is required to propagate the inflammatory signal. On the contrary, we demonstrate that both the isoforms of IL-1, IL-1α/IL-1β are generated by bone marrow-derived, tissue-resident responding cells, upon sensing the injury. We also sought to determine the identity of the cellular target of IL-1 signaling. Previous studies have shown that for cell death-induced neutrophil recruitment, interleukin-1 receptor (IL-1R) expression is required on parenchymal cells. To identify this parenchymal cell, we are currently in the process of making the conditional knockout mouse of IL-1R. The latter would facilitate the parenchymal tissue-specific deletion of IL-1R. In summary, this study reports our progress in unraveling key aspects of IL-1 signaling during sterile inflammation.

Taken together, we have identified key modulators of the acute inflammatory response and their mechanisms of regulation. These findings would facilitate the development of new therapies for inflammatory diseases triggered by both microbe and sterile agents.