CD4+CD28null T cells are not alloreactive unless stimulated by interleukin-15

Proinflammatory, cytotoxic CD4(+)CD28(null) T cells can be substantially expanded in patients with end-stage renal disease. These cells have been associated with the risk for rejection, but their alloreactive potential is unknown. CD4(+)CD28(null) T cells were stimulated with HLA-mismatched antigen presenting cells in the absence/presence of exogenous cytokines. Alloreactive potential was evaluated based on proliferation, degranulation, cytotoxicity, and cytokine production. Further, their suppressive capacity was assessed by measuring inhibition of proliferating alloreactive CD28(+) T cells. CD4(+)CD28(null) T cells contained alloreactive (CD137(+)) T cells but did not proliferate in response to allogeneic stimulation, unless interleukin (IL)-15 was added. However, they could proliferate on stimulation with cytomegalovirus antigen without exogenous cytokines. IL-15 increased the frequency of proliferating alloreactive CD4(+)CD28(null) T cells to 30.5% without inducing CD28 expression (P<.05). After allogeneic stimulation together with IL-15 and IL-21, frequency of degranulating CD107a(+)CD4(+)CD28(null) T cells increased significantly from 0.6% to 5.8% (P<.001). Granzyme B and perforin positivity remained similar, but production of interferon- and tumor necrosis factor- increased by the combination of IL-15 and IL-21 (P<.001 and P<.05, respectively). Finally, CD4(+)CD28(null) T cells did not show significant suppression. Thus, CD4(+)CD28(null) T cells represent a population with absent alloreactivity unless IL-15 is present. CD4+CD28null T cells can become alloreactive when the proper conditions are met, especially in the presence of IL-15, and IL-21 can enhance the cytotoxic potential of CD4+CD28null T cells.