IL-33 Is Up-Regulated During Heart Allograft Rejection.

Abstract

Background: IL-33 has been reported to be expressed in cell types that are continuously exposed to the environment or line the vasculature, such as dendritic cell, macrophages, epithelial and endothelial cells. Extracellular IL-33 functions as a cytokine by binding its receptor ST2. Depending on its receptor complex composition, IL-33/ST2 signaling culminates in activation of nuclear factor (NF)-κB, mitogen-activated protein (MAP) kinases or activator protein-1. One of the few studies on intranuclear IL-33 showed that IL-33 sequesters intranuclear NF-κB and subsequently dampens the expression of pro inflammatory cytokines regulated by NF-κB. There are a few studies investigated the protective role exogenous IL-33 in murine heart transplant model. However, the endogenous IL-33 expression modulation is yet to be studied. Methods: For acute rejection study, we transplanted BALB/c (H2<sup style=“font-family: Cambria;”>d[/sup]) hearts into C57BL/6 (H2<sup style=“font-family: Cambria;”>b[/sup], B6) recipients and explanted the heart allografts 7 days post transplant. For chronic rejection study, CB6.C-H2bm12/KhEgJ (H2bm12, Bm12) hearts were transplanted into B6 recipients and the transplants were harvested 90 days post transplant. All transplants were fixed, snap-frozen and sectioned for immunofluorescence staining. IL-33 and cardiac troponin I (cTnI), CD31, CD11b, CD45, vimentin, desmin, c-Kit, mesothelin, α-smooth muscle actin (α-SMA) and/or pholloidin were stained. DAPI was used for nuclear counterstain. Stained slides were studied with Olympus Fluoview 1000-2 confocal microscope. Results: IL-33 was always found in nuclei. Unexpectedly, in acutely rejected BALB/c heart allograft, CD11b+, CD45+ and CD31+ cells rarely expressed IL-33. cTnI+, c-Kit+ cells were not found to express IL-33. In contrast, almost all IL-33+ cells expressed vimentin. 50% percent mesothelin+ cells and <5% α-SMA+desmin+ cells expressed IL-33. Similar with acute rejection, almost all IL-33+ cells expressed vimentin but not CD45 in chronically rejected Bm12 heart allografts. Conclusion: In both acute and chronic murine heart allograft rejection models, IL-33 is mainly expressed by vimentin+ mesenchymal stromal cell and mesothelial precursor cells and a small portion of vascular smooth muscle cells but not myocardial cells, endothelial cells, c-Kit+cardiac stem cells or leukocytes.