IL-21, produced by T-cells, binds to the common gamma chain family member, IL21R, expressed on immune and colonic epithelial cells. IL-21 signaling results in the maturation, activation and proliferation of T, B, NK-cells and DCs. IL-21 has been implicated in Th17 generation/amplification and also modulating Treg differentiation. However, the relevance of this in disease is unclear. Therefore, we wanted to study the effects of IL-21 depletion and the role of Th17 and Treg cells in the context of GVHD. A lethally irradiated, complete MHC disparate model (B6 to B10.BR) using donor bone marrow cells alone or with the addition of wild-type (wt) CD4+CD25- Effector T-cells and irrelevant or anti-IL-21 Ab from days 0 to 25 twice per week. The administration of anti-IL-21Ab lead to a significant delay of weight loss and mortality (P<0.0001). To determine whether IL-21 deficiency accelerates or inhibits GVHD, studies were performed comparing GVHD-inducing ability of wt or IL-21 knockout (IL21−/−) donor CD4+CD25- T cells. A striking survival advantage of recipients of IL-21−/− vs wt CD4+CD25- T cells was observed (100% vs 0% surviving, P < 0.0001). Subsequent studies were performed using whole T cells, a more clinically relevant donor T cell graft source. Whereas recipients of wt T-cells all died of GvHD within 50 days of GvHD, recipients of IL-21 −/− T-cells showed significantly less weight loss and superior long-term survival (P < 0.0001). Histopathological examination of recipients of CD25-depleted T-cells on d14 revealed significantly reduced GvHD scores of the colonic mucosa and a trend towards lower GvHD scores in the small intestine, liver and the spleen in recipients of IL-21−/− cells. Although flow cytometry analysis of mononuclear cells showed no changes in the frequency of IL-17 producing cells in spleen, liver and colon, the frequency of interferon gamma producing CD4+ T- (Th1) cells was significantly lower in the spleen and the colon of recipients of IL-21−/− vs. wt T-cells. Moreover, increased Treg frequencies were seen in the colon of IL-21−/− vs wt CD25-depleted T cells (mean values: 7.5% vs. 2.1%; P < 0.003). We conclude that IL-21 production by either CD25- depleted T-cells or CD4+CD25- T cells is sufficient to increase GVHD mortality and that Treg inhibition, rather than Th17 generation or amplification, are likely contributing to GVHD lethality acceleration. We also conclude that IL-21 neutralization represents a novel approach for GVHD inhibition that warrants further investigation.

Disclosures: Sivakumar:Zymogenetics Inc. WA: Employment, Equity Ownership.

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