The retinoic acid receptor agonist Am80 increases mucosal inflammation in an IL-6 dependent manner during Trichuris muris infection.

Purpose: Vitamin A metabolites, such as all-trans-retinoic acid (RA) that act through the nuclear receptor; retinoic acid receptor (RAR), have been shown to polarise T cells towards Th2, and to be important in resistance to helminth infections. Co-incidentally, people harbouring intestinal parasites are often supplemented with vitamin A, as both vitamin A deficiency and parasite infections often occur in the same regions of the globe. However, the impact of vitamin A supplementation on gut inflammation caused by intestinal parasites is not yet completely understood.

Methods: Here, we use Trichuris muris, a helminth parasite that buries into the large intestine of mice causing mucosal inflammation, as a model of both human trichuriasis and IBD, treat with an RARα/β agonist (Am80) and quantify the ensuing pathological changes in the gut.

Results: Critically, we show, for the first time, that rather than playing an anti-inflammatory role, Am80 actually exacerbates helminth-driven inflammation, demonstrated by an increased colonic crypt length and a significant CD4(+) T cell infiltrate. Further, we established that the Am80-driven crypt hyperplasia and CD4(+) T cell infiltrate were dependent on IL-6, as both were absent in Am80-treated IL-6 knock-out mice.

Conclusions: This study presents novel data showing a pro-inflammatory role of RAR ligands in T. muris infection, and implies an undesirable effect for the administration of vitamin A during chronic helminth infection.