Loss of Autotaxin inhibits Toll‐like receptor4‐mediated immune mechanisms and promote spontaneous colitis in Il10‐ko mice

Abstract

Autotaxin (Atx) is a secreted enzyme converting lysophosphatidylcholine (LPC) and sphingosyl‐phosphorylcholine into lysophosphatidic acid and sphingosine 1‐phosphate, respectively. Given the high affinity of LPC to cholesterol and the enrichment of cholesterol and sphingolipids in lipid rafts wherein LPS sensor Toll‐like receptor 4 (TLR4) and its co‐receptor CD14 reside, we hypothesized that Atx deficiency inhibits TLR4‐mediated innate and adaptive immunity; thereby, accelerating the susceptibility to microbe‐induced intestinal inflammation.To study this hypothesis, we generated myeloid cell lineage‐restricted Atx‐knockout (ko) mice (Atx‐dME/dME) to investigate TLR4‐mediated immune and inflammatory responses and examined LPS‐receptor complex formation through fluorescence resonance energy transfer technique, confocal microscopy, and flow cytometry. Phagocytic activity of macrophages was assessed. Lamina propria CD4+ T cells and bacterial load in the intestinal mucosa were examined. An impact of Atx deficiency in inflammatory bowel diseases (IBD) was investigated using Atx‐dME/dME;Il10‐/‐ mice that have both Atx‐deletion in myeloid cells and a global Il10 deletion. We examined the serum samples from IBD patients.With peritoneal macrophages from Atx‐dME/dME mice, we identified that Atx‐ko disrupted the integrity of lipid rafts at the plasma membrane, resulting in the inhibition of TLR4 complex formation. Accordingly, the recruitment of adaptor molecules to TLR4 was suppressed, and TLR4‐mediated responses were substantially reduced in Atx‐ko macrophages. TLR4‐induced innate immunity such as phagocytosis was attenuated in Atx‐ko macrophages. The activation of CD4+ effector T cells and regulatory T cells was diminished in the lamina propria lymphocytes of Atx‐dME/dME mice compared to that of Atx‐wt littermates. Consequently, Atx‐dME/dME mice had a higher bacterial prevalence in the intestinal mucosa compared to controls. Just like the notion that commensal microbes translocated from the lumen into the intestinal mucosa can elicit spontaneous colitis in Il10‐/‐ mice, combining Atx‐dME/dME with Il10‐/‐ mice (Atx‐dME/dME;Il10‐/‐) did accelerate spontaneous colitis development. Atx‐dME/dME;Il10‐/‐ mice had gross inflammation occurring throughout the colon, massive neutrophil infiltration and necrosis in the colonic mucosa, and increased mortality (Log‐rank P=0.0046), while Atx‐wt;Il10‐/‐ littermates are normal. Notably, ATX serum protein level was lower in UC (n=26) and CD (n=34) patients compared to the level in normal subjects (n=26) (P<0.001), indicating an association of reduced ATX levels with intestinal inflammation.Collectively, we found that Atx deficiency suppresses TLR4‐mediated innate and adaptive immune mechanisms; thereby accelerating the susceptibility to microbe‐induced gut inflammation.