Plasmacytoid dendritic cells (pDCs) are unique bone-marrow-derived cells that produce large amounts of type I interferon in response to microbial stimulation. Furthermore, pDCs also promote T cell tolerance in sterile-inflammation conditions. However, the immunomodulatory role of aortic pDCs in atherosclerosis has been poorly understood. Here, we identified functional mouse and human pDCs in the aortic intima and showed that selective, inducible pDC depletion in mice exacerbates atherosclerosis. Aortic pDCs expressed CCR9 and indoleamine 2,3-dioxygenase 1 (IDO-1), an enzyme involved in driving the generation of regulatory T cells (Tregs). As a consequence, loss of pDCs resulted in decreased numbers of Tregs and reduced IL-10 levels in the aorta. Moreover, antigen presentation by pDCs expanded antigen-specific Tregs in the atherosclerotic aorta. Notably, Tregs ablation affected pDC homeostasis in diseased aorta. Accordingly, pDCs in human atherosclerotic aortas colocalized with Tregs. Collectively, we identified a mechanism of atheroprotection mediated by tolerogenic aortic pDCs.

Summary Arginase I (Arg1), an enzyme expressed by many cell types including myeloid cells, can regulate immune responses. Expression of Arg1 in myeloid cells is regulated by a number of cytokines and tissue factors that influence cell development and activation. Retinoic acid, produced from vitamin A, regulates the homing and differentiation of lymphocytes and plays important roles in the regulation of immunity and immune tolerance. We report here that optimal expression of Arg1 in dendritic cells requires retinoic acid. Induction of Arg1 by retinoic acid is directly mediated by retinoic acid-responsive elements in the 5′ non-coding region of the Arg1 gene. Arg1, produced by dendritic cells in response to retinoic acid, promotes the generation of FoxP3+ regulatory T cells. Importantly, blocking the retinoic acid receptor makes dendritic cells hypo-responsive to known inducers of Arg1 such as IL-4 and GM-CSF in Arg1 expression. We found that intestinal CD103+ dendritic cells that are known to produce retinoic acid highly express Arg1. Our results establish retinoic acid as a key signal in expression of Arg1 in dendritic cells.

No abstract