IL-13 signaling via IL-13Rα2 triggers TGF-β1-dependent allograft fibrosis.

Background: Allograft fibrosis still remains a critical problem in transplantation, including heart transplantation. The IL-13/TGF-β1 interaction has previously been identified as a key pathway orchestrating fibrosis in different inflammatory immune disorders. Here we investigate if this pathway is also responsible for allograft fibrosis and if interference with the IL-13/TGF-β1 interaction prevents allograft fibrosis.

Methods: FVB or control DBA/1 donor hearts were transplanted heterotopically into DBA/1 recipient mice and hearts were explanted at day 60 and 100 post-transplantation. Cardiac tissue was examined by Masson's trichrome staining and immunohistochemistry for CD4, CD8, CD11b, IL-13, Fas ligand, matrix metalloproteinase (MMP)-1, MMP-13, β2-microglobulin, and Gremlin-1. Graft-infiltrating cells were isolated and analyzed by flow cytometry. IL-13 and TGF-β1 levels were determined by enzyme-linked immunosorbent assay (ELISA) and the amount of collagen was quantified using a Sircol assay; IL-13Rα2 expression was detected by Western blotting. In some experiments IL-13/ TGF-β1 signaling was blocked with specific IL-13Rα2 siRNA. Additionally, a PCR array of RNA isolated from the allografts was performed to analyze expression of multiple genes involved in fibrosis.

Results: Both groups survived long-term (>100 days). The allogeneic grafts were infiltrated by significantly increased numbers of CD4+ (P <0.0001), CD8+ (P <0.0001), and CD11b+ cells (P = 0.0065) by day 100. Furthermore, elevated IL-13 levels (P = 0.0003) and numbers of infiltrating IL-13+ cells (P = 0.0037), together with an expression of IL-13Rα2, were detected only within allografts. The expression of IL-13 and IL-13Rα2 resulted in significantly increased TGF-β1 levels (P <0.0001), higher numbers of CD11bhighGr1intermediateTGF-β1+ cells, and elevated cardiac collagen deposition (P = 0.0094). The allograft fibrosis found in these experiments was accompanied by upregulation of multiple profibrotic genes, which was confirmed by immunohistochemical stainings of allograft tissue. Blockage of the IL-13/TGF-β1 interaction by IL-13Rα2 siRNA led to lower numbers of CD11bhighGr1intermediateTGF-β1+, CD4+, CD8+, and CD11b+ cells, and prevented collagen deposition (P = 0.0018) within these allografts.

Conclusions: IL-13 signaling via IL-13Rα2 induces TGF-β1 and causes allograft fibrosis in a murine model of chronic transplant rejection. Blockage of this IL-13/TGF-β1 interaction by IL-13Rα2 siRNA prevents cardiac allograft fibrosis. Thus, IL-13Rα2 may be exploitable as a future target to reduce allograft fibrosis in organ transplantation.