TLR4-SNP Mice Reveal the Role of M2a Macrophages in Resolution of Chemically-Induced Colitis

Abstract

Toll-like receptor 4 (TLR4) is an innate immune receptor responsive to Gram negative lipopolysaccharide (LPS). Single nucleotide polymorphisms (SNPs) in human TLR4 that encode an A896G transition at SNP rs4986790 (D299G) and a C1196T transition at SNP rs4986791 (T399I) render individuals hyporesponsive to LPS. In humans, these SNPs have also been associated with increased susceptibility to inflammatory bowel disease (IBD). IBD, including Crohn’s Disease (CD) and Ulcerative Colitis (UC), impacts millions of individuals worldwide and severely impairs quality of life for these patients. While multiple treatments are available for IBD, there are several problems: (1) not all patients respond, (2) responses may diminish over time, and (3) treatments often have undesirable side effects. Some patients with IBD express these SNPs and, using knock-in mice engineered to express the murine homologues of these human TLR4 mutations (“TLR4-SNP” mice), we have shown that TLR4-SNP mice develop significantly more severe colitis induced by dextran sodium sulfate (DSS) than wild-type (WT) mice. Previous studies have provided indirect evidence for a role of “tissue repair” M2 macrophages (Mφ) in the resolution of colitis. Signaling through the shared IL-4/IL-13 receptor, IL-4Rα, leading to activation of the transcription factor peroxisome proliferator-activated receptor (PPARγ) has been shown to be required for induction of M2a Mφ and our data provide direct evidence for the involvement of both in repair of DSS-induced colonic damage. In response to DSS, colons of TLR4-SNP mice produced reduced levels of M2a Mφ marker mRNA and protein. Additionally, PPARγ protein levels were reduced in colons from DSS-treated TLR4-SNP mice and therapeutic administration of the PPARγ agonist ligand, rosiglitazone, ameliorated colitis in TLR4-SNP mice. Together, these data indicate that the failure of TLR4-SNP mice to resolve DSS-induced colitis may be secondary to their failure to induce “tissue repair” M2a Mφ. Our findings provide insight into the potential development of novel therapies targeting Mφ signaling pathways that aim to alleviate the debilitating symptoms experienced by individuals with IBD.