Graft IL-33 regulates infiltrating macrophages to protect against chronic rejection.

Alarmins, sequestered self-molecules containing damage-associated molecular patterns, are released during tissue injury to drive innate immune cell pro-inflammatory responses. Whether endogenous negative regulators controlling early immune responses are also released at the site of injury is poorly understood. Herein, we establish that the stromal cell-derived alarmin interleukin-33 (IL-33) is a local factor that directly restricts the pro-inflammatory capacity of graft infiltrating macrophages early after transplantation. By assessing heart transplant recipient samples and using a mouse heart transplant model, we establish that IL-33 is upregulated in allografts to limit chronic rejection. Mouse cardiac transplants lacking IL-33 displayed dramatically accelerated vascular occlusion and subsequent fibrosis, which was not due to altered systemic immune responses. Instead, a lack of graft IL-33 caused local augmentation of pro-inflammatory iNOS+ macrophages that accelerated graft loss. IL-33 facilitated a metabolic program in macrophages associated with reparative and regulatory functions, and local delivery of IL-33 prevented the chronic rejection of IL-33-deficient cardiac transplants. Therefore, IL-33 represents a novel regulatory alarmin in transplantation that limits chronic rejection by restraining the local activation of pro-inflammatory macrophages. The local delivery of IL-33 in extracellular matrix based-based materials may be a promising biologic for chronic rejection prophylaxis.