IL21/IL21R: a novel target to promote islet graft tolerance

A. Petrelli¹, M. Carvello², E. Orsenigo², C. Staudacher², A. Secchi¹, J. Markmann³, P. Fiorina^4
1 San Raffaele Hospital, Transplant Medicine, Milan, Italy; ² San Raffaele Hospital, General Surgery, Milan, Italy; ³ Massachusetts General Hospital, Boston, MA, USA; ⁴ Children's Hospital, Boston, MA, USA

Background: Interleukin-21 (IL21) orchestrates immune response but its role in islet allograft rejection and tolerance is unknown. Therefore, we aim to study the role of IL21/IL21R pathway in a murine model of allo and auto anti-islet response.

Methods: IL21 levels and IL21R expression were determined in vitro and in vivo during allostimulation. We then targeted the IL21/IL21R pathway in C57BL/6 and NOD mice by using a murine IL21 receptor Fc fusion protein (mIL21R.Fc) alone or in combination with CTLA4-Ig or by genetical deletion of IL21R. mIL21R.Fc effect on the immune system were studied both in vivo and in vitro. In vitro Treg generation and potency were performed.

Results: IL21R is constitutively expressed on T and B cells both at baseline and in vitro/in vivo during allostimulation. IL21 levels in vivo significantly increase during acute rejection (IL21 baseline=20.712.9 vs. d14=149.931.5, pg/ml, p<0.0001). IL21/IL21R targeting with mIL21R.Fc resulted in a prolongation of islet allograft survival in C57BL/6 mice (MST: mIL21R.Fc=19 vs. untreated=14 days, p=0.01), while combo treatment with CTLA4-Ig led to graft tolerance in C57BL/6 mice(tolerance at 100 days: mIL21R.Fc+CTLA4-Ig 100 vs. CTLA4-Ig=60%, p=0.01) and to dramatic prolongation of graft survival in NOD mice. These data were confirmed in IL21R-/- mice. Mechanistic data suggest a reduced activation of CD4⁺ and CD8⁺ Teff cells and a dramatic increase of peripheral Tregs (CD4⁺CD25⁺FoxP3⁺:treated=15.00.9 vs. untreated=11.30.7, p=0.01)14 days after transplantation . Histology revealed a reduction of T cell infiltration and an increased number of FoxP3⁺ cells (FoxP3/CD3: mIL21R.Fc=0.60.1 vs. untreated=0.20.05, p=0.03). In vitro iTreg generation and potency (T eff. CFSE dilution: no drug=19.10.5, mrIL21=27.70.9 and mIL21R.Fc=16.10.3 %, no drug vs. all, p<0.05) were reduced in presence of mrIL21 while increased with the addition of mIL21R.Fc.

Conclusion: IL21 is an anti-tolerogenic cytokine and targeting IL21/IL21R pathway could be considered in islet transplantation.