Alternatively activated macrophages protect mice during induced intestinal inflammation

Inflammatory bowel disease (IBD) is an idiopathic disease characterized by chronic intestinal inflammation and ulceration. Canada has the highest incidence of IBD in the world with 1 in 150 people affected. While treatment options target symptoms and attempt to dampen down inflammation, an increasing population of patients is refractory to current therapeutic options. Macrophages are heterogeneous in their functions and while it is clear that inflammatory macrophages contribute to inflammation in IBD, multiple lines of evidence suggest that alternatively activated macrophages may offer protection during intestinal inflammation. In vivo SHIP deficient mouse macrophages are alternatively activated so SHIP deficient mice provide a unique genetic model of alternative macrophage activation. Using the dextran sodium sulfate (DSS)-induced model of colitis, I found that SHIP-/- mice are protected during induced intestinal inflammation, the protection is macrophage mediated, and can be conferred to a susceptible host. To determine how SHIP contributes to alternative activation of macrophages, I demonstrate that SHIP-deficient murine macrophages are more sensitive to IL-4-mediated skewing to an alternatively activated phenotype. Moreover, SHIP levels are reduced in alternatively activated macrophages and this is required for alternative activation because it is dependent on PI3K activity. Arginase (ArgI) induction is specifically dependent on the PI3Kp110δ isoform of class IA PI3K. As such, mice deficient in PI3Kp110δ catalytic subunit activity have increased clinical disease activity and histological damage during DSS-induced colitis. Colitis severity correlates with reduced numbers of ArgI+ M2 macrophages in the colon, increased nitric oxide production, and is macrophagedependent. Importantly, adoptive transfer of IL-4-treated macrophages from wild type mice, but not from PI3Kp110δ deficient mice, protects mice during DSS-induced colitis. Protection is lost when mice are treated with inhibitors that block arginase activity showing that ArgI activity is required for M2 macrophage-mediated protection from intestinal inflammation. These findings identify SHIP and the PI3K pathway as critical regulators of alternative macrophage activation and as potential targets for manipulation in IBD. In addition, adoptive transfer of alternatively activated macrophages to patients with IBD also offers a promising, new strategy for treatment that may be particularly useful in patients who are refractory to conventional therapies.