Neutrophil S100A9 supports M2 macrophage niche formation in granulomas.

Mycobacterium infection gives rise to granulomas predominantly composed of inflammatory M1-like macrophages, with bacteria-permissive M2 macrophages also detected in deep granulomas. Our histological analysis of Mycobacterium bovis bacillus Calmette-Guerin-elicited granulomas in guinea pigs revealed that S100A9-expressing neutrophils bordered a unique M2 niche within the inner circle of concentrically multilayered granulomas. We evaluated the effect of S100A9 on macrophage M2 polarization based on guinea pig studies. S100A9-deficient mouse neutrophils abrogated M2 polarization, which was critically dependent on COX-2 signaling in neutrophils. Mechanistic evidence suggested that nuclear S100A9 interacts with C/EBPß, which cooperatively activates the Cox-2 promoter and amplifies prostaglandin E2 production, followed by M2 polarization in proximal macrophages. Because the M2 populations in guinea pig granulomas were abolished via treatment with celecoxib, a selective COX-2 inhibitor, we propose the S100A9/Cox-2 axis as a major pathway driving M2 niche formation in granulomas.

Neutrophils and the S100A9 protein critically regulate granuloma formation.

Macrophages have the potential to undergo cellular transformation into epithelioid cells, and their concentric accumulation in tissues results in the development of granulomas. Although epithelioid cells are an essential and dominant component of granulomas, other cell types have also been detected, which may contribute to the establishment of well-organized granulomas, as observed in human granulomatous diseases. We herein demonstrated that neutrophils may mediate these functions. By taking advantage of the guinea pig pulmonary granuloma model, we obtained a rat monoclonal antibody with unique reactivity to granuloma cells. This antibody, termed G213, reacted with clusters of neutrophils located in the central area of granulomas, and a biochemical analysis identified the G213-reactive antigen as S100A9, a calcium-binding protein of the S100 family, which was expressed abundantly in neutrophils. Consistent with the multifaceted functions attributed to S100A9, including its role in neutrophil extravasation and macrophage activation, the blockade of S100A9 functions with the specific inhibitor, tasquinimod, impaired the formation of organized granulomas with neutrophil cores. These results demonstrate the critical role of neutrophils and the S100A9 protein in granuloma formation. Because intragranuloma S100A9+ neutrophils were also detected in humans, these results indicate the potential of tasquinimod, a new anticancer drug candidate, for manipulating human granulomatous diseases.