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Folami Powell, Malita Jones, Ravirajsinh Jadeja, Menaka Thounaojam, Manuela Bartoli, Pamela M Martin; Hydroxycarboxylic acid receptor 2 (HCAR2/GPR109A) regulates CCR7-dependent retinal immunity and tolerance. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5224.
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© ARVO (1962-2015); The Authors (2016-present)
Chronic, subclinical inflammation is a hallmark of and detrimental consequence of diabetic retinopathy (DR). In the periphery, inflammatory stimuli upregulate the CCR7/CCL19/CCL21 signaling axis, promote increased recruitment and retention of peripheral blood leukocytes (PBLs) in affected tissues, and in turn accelerate the progression of inflammatory diseases. Whether and how this impacts endogenous immune cell activation and leukocyte recruitment/retention in the diabetic retina is not well understood. Here, we investigated the relevance of the anti-inflammatory HCAR2/GPR109A receptor in this phenomenon and therefore to the regulation and maintenance of retinal immunity and tolerance in normal and diabetic conditions.
Diabetes was induced in 4-wk old wild type C57BL/6J control (WT; Hcar2/Gpr109A+/+) and Hcar2/Gpr109A-/- (knockout; KO) mice using streptozotocin (STZ; 75 mg/kg). Beginning 16-weeks post-onset diabetes, the HCAR2/GPR109A cognate ligand β-hydroxybutyrate (β-HB) was administered daily (40 mg/mouse, i.p.) for 2 weeks. Age- and gender-matched non-diabetic (non-DB) WT and KO mice treated with/without β-HB served as controls. Peripheral blood was collected and used for hematology analysis. Retinal and PBL suspensions were subjected to FACS to differentiate leukocyte populations. The expression of CCR7, CCL19 and CCL21 was also analyzed using specific antibodies. Total RNA was also extracted and chemokine targets were analyzed by quantitative PCR (qPCR).
Basal levels of CCR7, CCL19 and CCL21 were higher both in retinal tissues and in PBL lysates obtained from KO compared to WT animals in the absence of diabetes. Additionally, significant numbers of pro-inflammatory immune cells were found in KO retinas and whole blood. CCR7/CCL19/CCL21 expression and immune cell infiltration into retina increased in all groups (WT and KO) in association with STZ diabetes. Treatment with β-HB abrogated this effect in WT but had little to no effect in KO-DB retinas.
Our study suggests that HCAR2/GPR109A functions as an essential regulator of retinal immunity and tolerance in part by controlling the CCR7/CCL19/CCL21 axis. Furthermore, agonism of HCAR2/GPR109A balances uncontrolled immune cell recruitment by blocking this single chemokine–receptor interaction as an ‘entry’ signal, and may open the way for promising, future pharmacological targeting in DR.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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