Innate Immunology and Microbial Invasion in a Spontaneous Model of Colitis

IBD is comprised of ulcerative colitis (UC) and Crohn’s disease (CD) and is characterized by excessive chronic inflammation in the intestinal lining. Although the exact mechanisms underlying IBD are not fully known, the consensus stipulates it is driven by the interplay of genetic mutations, the immune system, and the microbial community within an organism. TNFAIP3 is an essential modulator of pro-inflammatory signaling and is implicated in many autoimmune diseases (AD), including inflammatory bowel disease (IBD). The overall objective of this research proposal is to define the role of TNFAIP3 expression in IEC and innate immunity in IBD.

TNFAIP3 is an anti-inflammatory enzyme inhibiting NF-κB through a negative feedback loop. In intestinal epithelium cells (IEC) it prevents IEC apoptosis and increases barrier function, thus TNFAIP3 in IEC is implicated as a key player in colitis. To assess the role of TNFAIP3 on innate immune system driven colitis, we crossed RAG-1-/- mice, immunocompromised mice lacking T and B cells, and villin-TNFAIP3 transgenic mice, which overexpress TNFAIP3 in IEC, these mice hereafter are referred to as TRAG mice. Neither RAG-1-/- nor v-TNFAIP3 mice spontaneously develop colitis, but all v-TNFAIP3 x RAG-1-/- mice exhibited early onset spontaneous colitis around 6-8 weeks of age. TRAG mice display upregulation of proinflammatory cytokines, inflammatory monocytes, neutrophils, and innate lymphoid cells (ILCs). We found the inhibition of ILCs ameliorated colitis, both prophylactically and therapeutically. Furthermore, targeting JAK pathways essential for inducing and secreting ILCs cytokines also showed ameliorating effects on colitis, however, targeting inflammatory cytokines has no effect. Thus, ILCs are the main drivers of inflammation in our model.

These mice also presented altered anti-microbial factor expression and displayed an inner mucus (IM) layer infiltrated with microbes. 16s sequencing analysis reveled the TRAG mice have an upregulation of pathogenic bacteria and a downregulation of commensal bacteria in the inner mucus layer compared to the RAG mice. Thus, we believe that early onset spontaneous colitis in this model is attributed to both the immunocompromised state and microbes invading the IM. The microbial invasion caused the alteration of antimicrobial factors by the over expression of TNFAIP3 allows opportunistic bacteria to infiltrate the IM layer and come into contact with the intestinal epithelium. This triggers inflammatory cytokine production by antigen presenting cells, and the upregulation of ILCs. The ILCs further propagate the inflammation with secreting on inflammatory cytokines and increasing inflammatory cells, thus, leading to the spontaneous chronic inflammation displayed by our model.