*Purpose: IL-21 exhibits a wide range of immunomodulatory functions in the organism, regulating immune cells such as B cells and T cells. In this study, a mice transplantation model was constructed. B cells, Regulatory B cells (B10), CD4+ T cells, CD4+CD25+Foxp3+ Treg cells as well as related cytokines in different Th cell subgroups were observed dynamically. The regulation of IL-21 on Th cellular immunity was further elucidated, providing an updated basis for exploring the mechanism of acute rejection after renal transplantation.

*Methods: The kidney transplantation model was constructed using C57BL/6 mice (6-9 weeks, female) divided into 2 groups. 10 mice in each group were injected with 10ml 100 ng/ml IL-21 through caudal vein 1h before, 2d, 4d, and 6d after surgery (IL-21 group), and the control group received equivalent saline (control group).All peripheral blood and spleen tissue suspension were collected 7 days after operation and added to 1640 culture medium containing fetal bovine serum (FBS). The FACS Canto II flow cytometry was utilized to detect the number of B cells, B10 cells, CD4+ T cells and CD4+CD25+Foxp3+ Treg cells. The contents of IL-2, IFN-γ, TNF-α, IL-4, IL-10, IL-6, IL-17a and IL-21 in peripheral blood, transplanted kidney and spleen tissue suspension were detected by Elisa method. The allograft tissues of in each group were histopathological detected, based on the Banff grading system.

*Results: IL-21 induced proliferation and differentiation of various cell subgroups. In peripheral blood, CD4+T cells, Treg cells, B cells and proportion of B10 cells in B cells were higher in IL-21 group with a significant difference (9.05 ± 0.60 vs. 5.82 ± 0.52, 5.95 ± 0.69 vs. 2.14 ± 0.85, 36.98 ± 4.90 vs. 30.68 ± 3.89, 0.75±0.07 vs. 0.24±0.02, p<0.05). In the spleen tissue suspension, B cells and B10 cells were observed significantly lower (32.93 ± 1.56 vs. 42.60 ± 5.09, 1.58 ± 0.37 vs. 2.50 ± 0.59, p<0.05).

IL-21 promoted immune deviation of Th2 to Th1 cells. The cytokines related to Th cell subgroup in the IL-21 group demonstrated a trend of Th2 to Th1 immune deviation. Among them, Th1 specific cytokines IFN-γ and TNF-α in transplanted kidney significantly elevated in peripheral blood (120.89 ± 10.75 vs. 58.6 ± 7.01, 41.57 ± 3.63 vs. 29.58 ± 2.64, p<0.05). In the spleen, Th2 specific cytokines IL-10 and IL-6 decreased (18.23 ± 2.88 vs. 28.33 ± 2.61, 23.68 ± 2.33 vs. 29.54 ± 3.96, p<0.05) while IL-21 increased (4.42 ± 1.06 vs. 1.70 ± 0.48, p<0.05). TNF-α, IL-6 and IL-21 in peripheral blood elevated (45.25 ± 1.32 vs. 39.31 ± 2.20, 60.72 ± 10.53 vs. 39.83 ± 1.88, p<0.05) and IL-21 decreased (6.09 ± 0.54 vs. 10.05 ± 0.66, p<0.05).

IL-21 induced acute rejection of the transplanted kidney. Among control group that survived 7 days post operation (n=7), two (28.6%) showed no rejection reaction, one (14.3%) with mild interstitial inflammation, four (57.2%) with moderate or severe interstitial inflammation. One of them (14.3%) showed peripheral tubular capillary inflammation (PTC) of grade 1 and four (57.1%) of grade 2. Meanwhile, among the IL-21 group (n=9), all (100.0%) showed moderate to severe interstitial inflammation. Two (22.2%) with PTC grade 2, and three (33.3%) with PTC grade 3.

*Conclusions: IL-21 can promote differentiation and proliferation of spleen cells and induce immune deviation of Th2 to Th1 cells, aggravating acute rejection of the transplanted kidney.

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