Human CD8 ⁺ T cells display a differential ability to undergo cytokine-driven bystander activation

A subset of CD44 ^hiCD8 ⁺ T cells in some, but not all mice, can be induced to rapidly secrete IFNγ during infection with Listeria monocytogenes. This response is dependent on the presence of both IL-12 and IL-18 and does not require engagement of the T cell receptor. In this study, we demonstrate that human CD8 ⁺ T cells also vary widely in their ability to secrete IFNγ within 15h of either Listeria infection or cytokine stimulation. The magnitude of the rapid IFNγ response correlated more closely with the intrinsic responsiveness of the T cells to cytokine stimulation rather than the amount of IL-12 produced. CD8 ⁺ T cells from 2 out of 16 blood donors (12.5%) failed to generate a significant IFNγ response. These results demonstrate that bystander activation of CD8 ⁺ T cells varies among individuals and validate further study of the differential responses observed using BALB/c vs. C57BL/6 mice.