Tolerance induced by IL-6 deficient donor heart is significantly involved in myeloid-derived suppressor cells (MDSCs)
Introduction
Our previous studies demonstrated that cytokine IL-6 is closely involved in not only graft injury [1] but transplant tolerance induction [2]. The transplant organs undergoing ischemia-reperfusion injury become inflamed, causing an apparent production of inflammatory cytokines including IL-6, IL-1beta, and TLR liagands [3]. And IL-6 is capable of orchestrating cellular differentiation [1]. Enhanced production of IL-6 can promote the proliferation of effector T cells and inhibit the function of regulatory T cells [4], [5]. Furthermore, combined with TGF-beta, IL-6 can potentiate the commitment of naive human or murine CD4 + T cells into the highly cytopathic Th17 phenotype [6]. In turn, Th17 cells can stimulate antigen-presenting cells to express IL-6 [7]. Therefore, the presence of IL-6 in the transplant settings could render allograft to be tolerance abrogation, causing tissue destruction [4], [5].
IL-6 can be formed in local cytokine milieu and produced by various tissues particularly pro-inflammatory cell types [2]. In heart, not only cardiac fibroblast but cardiac myocytes can produce massive IL-6 [8], [9]. It is well-accepted that IL-6 as a pleiotropic cytokine plays an important role in adaptive immune responses [5], [10]. Nevertheless, the role of IL-6 in innate immune responses is not well understood. Our previous study exhibited that donor IL-6 deficiency could significantly affect innate immune responses in a pre-sensitized transplant model, as evidenced by an evident infiltration of myeloid-derived suppressor cells (MDSCs) including CD11b + Gr1− low and CD11b + Gr1− int subsets [2]. We firstly revealed that CD11b + Gr1− low have strong immunosuppression activity in the pre-sensitized transplant settings [2]. However, it is undefined whether tolerance induced by IL-6 deficient donor heart is significantly involved in myeloid-derived suppressor cells (MDSCs). Our present study manifested that IL-6 deficiency could result in a remarkable increase of the frequency of splenic CD11b + Gr1− low, implying that apart from Tregs, innate immunoregulatory cells also contributed to the tolerance induction by donor IL-6 deficiency in the non-sensitized cardiac transplantation. Past study displayed that an increase of graft-infiltrating MDSCs is accompanied by an enhancement of IL-10 expression in transplantation [11]. In addition, IL-6 in combination with IL-1beta or TGF-beta is capable of inducing IL-17 expression [6], [12]. However our findings did not show any effect of Th17 cells in the process, as evidenced by no prolongation of allograft survival of RORgama T-deficient recipient (unpublished data). This would further advance the understanding of the novel mechanism of transplant tolerance induction.
Section snippets
Experimental animals
Eight-twelve week male BALB/c (B/c; H-2d) and C57BL/6J (B6; H-2b) mice were obtained from the Model Animal Research Center of Nanjing University. B6:129S2-IL-6tm1Kopf/J (IL-6 KO; H-2b) was purchased from the Jackson Laboratories (JAX, Bar Harbor, ME). All animals were housed on a light/dark (12/12 h) cycle at 24 °C free access to food and water ad libitum. Animal studies were approved by the Institutional Animal Care and Use Committee (IACUC) at Anhui University. Allogeneic heterotopic heart
Prolonged survival of IL-6 deficient cardiac allografts
In the past a series of studies consistently showed that donor IL-6 and recipient's high IL-6 level could affect allograft survival [2], [5], [15]. Our present research data further supported that a significant prolonged survival of IL-6 deficient heart allografts was observed in comparison to wild-type donors (Mantel–Cox Test, p = 0.001; Gehan–Breslow–Wilcoxon Test, p = 0.0016). Utilization of IL-6 −/− heart grafts caused a significant increase of the mean survival time (MST) (19.5 ± 4.08 days vs. 8.33
Discussion
Dr. Perkins DL et al. reported that intragraft IL-6 level was dramatically increased at 6 h after transplantation (wild-type C57BL/6→BABL/c), whereas the level of IL-6 was not detectable in the IL-6 deficient graft [5]. It implied that graft heart producing IL-6 is the main source for orchestrating intragraft immune responses. After organ transplantation, the injury of donor graft can ignite proinflammatory gene expression such as IL-6 cytokine, subsequently promoting allogeneic immunoresponses
Disclosure
There is no any conflict of interest.
Acknowledgment
The work was supported by the Research Fund of Health Bureau of Zhejiang Province (Grant No. 2011KYA070), the Department of Education of Zhejiang Province (Grant Nos. Y201226017 and Y201328155), the National Natural Science Foundation of China (No. 81270323, No. 81470527), the Qianjiang Talents Project of Zhejiang Province (Grant No.2012R10022), and the Zhejiang Provincial Outstanding Youth Foundation (Grant No. LR13H020001).
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