*Purpose: Self-molecules containing damage-associated molecular patterns stimulate the pro-inflammatory response of infiltrating innate immune cells after their release during tissue injury. Yet, if local endogenous negative regulators are also present at the site of injury to control early innate immune responses is poorly understood.

*Methods: Wild type BALB/c, il33^+/+ Bm12, il33^-/- Bm12 HTx were transplanted heterotopically into WT or il33^-/- B6N recipient mice. In some groups, endogenous interleukin-33 (IL-33) was restored locally using WT or il33^– extracellular matrix (ECM) -bound nanovesicles (MBV) hydrogel at the time of transplantation. MBV were characterized by Cytokine Array and western blot. At days 3-7, or 100-120 post HTx, isolated grafts were stained with H+E or Trichrome or subjected to qRT-PCR, Western blot, and IF staining to define differences in cytokine expression. Leukocytes isolated from HTx and recipient spleens were compared through flow cytometry. Mitochondrial bioenergetics/function of bone marrow-derived dendritic cells (DC) was assessed using a Seahorse XF96e Analyzer. DC Mitochondrial mass, potential, and uptake of glucose and fatty acids were completed by flow cytometry.

*Results: We show that IL-33, a stromal-cell derived cytokine with emerging immunoregulatory properties is rapidly upregulated after heart transplantation and during graft rejection. We also establish that, in addition to its expected presence in the nucleus, IL-33 is also contained in MBV of the ECM. In a mouse heart transplant model, we show that transplants unable to express IL-33 displayed dramatically accelerated chronic rejection-associated fibrosis and vascular occlusion. This pathology was associated with augmented early graft infiltration by pro-inflammatory macrophages and monocyte-derived DC. Local delivery of IL-33⁺ MBV in an ECM-based hydrogel immediately after transplantation profoundly reduced the frequency of pro-inflammatory myeloid cells in IL-33 deficient-grafts. This treatment also reduced the subsequent development of chronic rejection. Related mechanistic studies demonstrate that IL-33 increases DC oxidative phosphorylation and fatty acid uptake.

*Conclusions: In total, our data establish that IL-33 is a powerful local regulatory factor that limits chronic rejection after heart transplantation by restraining the differentiation of infiltrating myeloid cells and causing a metabolic reprogramming that consistent with immunosuppressive DC and reparative macrophages. Our data also suggest the local delivery of IL-33 in ECM-based materials is a promising biologic for chronic rejection prophylaxis.

« Back to 2019 American Transplant Congress