Role of the hydroxycarboxylic acid receptor 2 in the interplay between intestinal microbiota and retinal immunity in diabetes

Abstract

Purpose : Altered gut microbiota composition has been linked to impaired immune functionality in diabetic patients. Gut microbiota can both suppress and contribute to colonic and extra-intestinal inflammatory diseases by altering T regulatory (Treg) cell differentiation. Our prior published collaborative work coupled with that of others demonstrate that absence of the anti-inflammatory receptor, hydroxycarboxylic acid receptor 2 (HCAR2), facilitates microbiota-induced colonic inflammation. Here we evaluated the impact of microbiota-induced inflammation, with and without insulin deficiency, on the retina.

Methods : HCAR2 knockout (KO; Hcar2^-/-) mice were bred with rag1-deficient mice that do not produce mature T cells, to generate double knockout (DKO; Hcar2^-/-/Rag1^-/-). Mice were made diabetic using streptozotocin (STZ; 75 mg/kg). Oral, broad-spectrum antibiotics were administered daily to a subset of dKO mice for 16 weeks. Hematoxylin and eosin-stained sections were used for morphological comparisons of HCAR KO colonic and retinal specimen to WT and Rag1^-/- control specimen. qPCR was performed to evaluate mRNA expression of inflammatory markers in neuroretina, RPE and colonic lamina propria (LP). Specific immune cell populations (Tregs, macrophages) were stained and quantified by flow cytometry in respective tissues.

Results : Chamber flare, phthisis bulbi, and spontaneous uveitis were apparent in 73.6% of diabetic DKO mice. Inflammatory markers were significantly upregulated in the LP and particularly in the RPE and corresponded to loss of colonic Tregs. Additionally, an extremely high number of macrophages were detected in retinal samples. These effects were partially ameliorated with delivery of antibiotics.

Conclusions : The opposing, dual nature of the gut microbiome is regulated by the expression of HCAR2. In the colon, the receptor is activated by short chain fatty acids produced by microbiota in the colon and potentiates anti-inflammatory responses by promoting T regulatory (Treg) cell differentiation. With the loss of HCAR2 expression, opportunistic pathogens mount a negative inflammatory response in the colon and in peripheral tissues including retina, a tissue that is by tradition considered to be “immune privileged”. These data suggest that targeting commensal gut bacteria in diabetic patients may be an effective avenue to prevent and treat diabetic retinopathy.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.