Dietary DHA Induces LXA₄ Producing PMN in Lymph Nodes and is Protective in Immune-driven Dry Eye Disease

Abstract

Purpose: The protective actions of DHA have been attributed to formation of specialized proresolving mediators (SPM) whose biosynthesis and mechanism of action is analogous to the eicosanoid LXA₄. The endogenous role of SPM in dry eye pathogenesis has not been explored even though epidemiological studies indicate protective effect of dietary fish oil in dry eye syndrome. We have reported that a population of tissue PMN controls immune-driven dry eye disease (DED) in females by producing LXA₄, these regulatory PMN are a potential target for limiting DED. We set out to investigate how dietary DHA impacts DED and immune regulatory PMN.

Methods: Mice were placed on a DHA or w-3 PUFA-deficient diet for 4 weeks before DED induction. DED was induced using a standard desiccating stress model. Severity of DED was evaluated by Schirmer’s test and clinical fluorescence scoring system. PMN cell population was confirmed and quantified by myeloperoxidase assay and fluorescence confocal microscopy. Sub-conjunctiva and intravitreal injection of DHA were performed in DHA-deficient mice. PMN were negatively isolated by using a neutrophil isolation kit. Lipid formation was quantified by LC/MS/MS-based lipidomics, and gene expression by QPCR

Results: Mice on DHA-deficient diet exhibited amplified dry eye disease. Regardless of diet, a basal population of tissue PMN resides in the corneal limbus, lacrimal gland and draining lymph nodes. DHA diet significantly increased the PMN content in all the tissues. Unexpectedly, increased tissue levels of PMN in the DHA diet group directly correlated with amplified formation of LXA₄ in lymph nodes, lacrimal glands and cornea. Consistent with our hypothesis, isolated draining lymph node PMN from DHA-rich mice had a significantly higher capacity (3.6 fold) to generate LXA₄ than lymph node PMN from DHA-deficient mice. In a rescue experiment, local DHA treatment of DHA-deficient mice significantly triggered recruitment of regulatory PMN to the limbus, lacrimal gland and draining lymph nodes and reduced dry eye disease.

Conclusions: These findings provide evidence for a novel mechanism of action for dietary DHA and local DHA treatment in Dry Eye Disease pathogenesis, namely that it drives the recruitment of a unique population of tissue PMN that generate immune-regulatory LXA₄.