IL21 and neoplasm: In this regard, the accelerated appearance of thymus-selected RTEs triggered by exogenous IL-21 administration would not only compete with peripheral alloreactive T cells to access secondary lymphoid organs, homeostatic cytokines, and self-MHC ligands impeding therefore alloreactive T cell expansion/activation, but it will also ensure efficient immune surveillance and tumor rejection as we have observed with the P815 challenge experiment conducted in IL-21-treated animals (Fig. 6).