CD4⁺ CD28^null 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-γ and tumor necrosis factor-α 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.