IL-21 Is an Antitolerogenic Cytokine of the Late-PhaseAlloimmune Response

OBJECTIVE—Interleukin-21 (IL-21) is a proinflammatory cytokine
that has been shown to affect Treg/Teff balance. However,
the mechanism by which IL-21 orchestrates alloimmune response
and interplays with Tregs is still unclear.
RESEARCH DESIGN AND METHODS—The interplay between
IL-21/IL-21R signaling, FoxP3 expression, and Treg survival
and function was evaluated in vitro in immunologically
relevant assays and in vivo in allogenic and autoimmune models
of islet transplantation.
RESULTS—IL-21R expression decreases on T cells and B cells
in vitro and increases in the graft in vivo, while IL-21 levels increase
in vitro and in vivo during anti-CD3/anti-CD28 stimulation/
allostimulation in the late phase of the alloimmune response.
In vitro, IL-21/IL-21R signaling (by using rmIL-21 or genetically
modified CD4+ T cells [IL-21 pOrf plasmid–treated or hIL-21-Tg
mice]) enhances the T-cell response during anti-CD3/anti-CD28
stimulation/allostimulation, prevents Treg generation, inhibits
Treg function, induces Treg apoptosis, and reduces FoxP3 and
FoxP3-dependent gene transcripts without affecting FoxP3 methylation
status. In vivo targeting of IL-21/IL-21R expands intragraft
and peripheral Tregs, promotes Treg neogenesis, and regulates
the antidonor immune response, whereas IL-21/IL-21R signaling
in Doxa-inducible ROSA-rtTA-IL-21-Tg mice expands Teffs and
FoxP32 cells. Treatment with a combination of mIL-21R.Fc and
CTLA4-Ig (an inhibitor of the early alloimmune response) leads to
robust graft tolerance in a purely alloimmune setting and prolonged
islet graft survival in NOD mice.
CONCLUSIONS—IL-21 interferes with different checkpoints of
the FoxP3 Treg chain in the late phase of alloimmune response
and, thus, acts as an antitolerogenic cytokine. Blockade of the
IL-21/IL-21R pathway could be a precondition for tolerogenic
protocols in transplantation. Diabetes 60:3223–3234, 2011