CD8(+) T cells are fundamental for immune-mediated clearance of
viral infections and contribute to immune pathology in
autoimmune diseases such as
type 1 diabetes. To execute these functions, CD8(+) T cells must differentiate into CTLs, a process that is precisely regulated by a variety of
cytokines, costimulatory molecules, and
transcription factors.
IL-21 is an
IL-2 family
cytokine and a
growth factor for multiple lymphocyte effector lineages, including cytotoxic CD8(+) T cells. Recent studies demonstrate that loss of
IL-21 signaling results in reduced viral clearance in models of lymphocytic choriomeningitis virus
infection, and also protection from
type 1 diabetes in the NOD model. This is most likely the result of impaired CD8(+) CTL function in the absence of
IL-21 signaling. Currently, the mechanisms by which
IL-21 promotes CTL differentiation in CD8(+) T cells remain unclear, particularly the identity of the relevant
transcription factor(s). We show that
IL-21 promotes CTL function in vitro and killing of pancreatic islets in vivo via the use of transgenic mice expressing
IL-21 in pancreatic β cells. We demonstrate that
IL-21 induces the expression of the
transcription factor T-bet in CD8(+) T cells, predominantly via STAT1, and that T-bet is required for the induction of cytolytic molecules, including
perforin and
granzyme B in response to
IL-21. Finally, we show that IL-21-induced CTL function is T-bet dependent, as T-bet deficiency results in defective IL-21-dependent cytotoxicity in CD8(+) T cells in vitro and in vivo. Thus,
IL-21 drives CD8(+) CTL differentiation via the actions of the
transcription factor T-bet.