IL-1 family member IL-37 limits innate inflammation in models of colitis and LPS-induced shock, but a role in adaptive immunity remains unknown. Here, we studied mice expressing human IL-37b isoform (IL-37tg) subjected to skin contact hypersensitivity (CHS) to dinitrofluorobenzene. CHS challenge to the hapten was significantly decreased in IL-37tg mice compared with wild-type (WT) mice (−61%; P < 0.001 at 48 h). Skin dendritic cells (DCs) were present and migrated to lymph nodes after antigen uptake in IL-37tg mice. When hapten-sensitized DCs were adoptively transferred to WT mice, antigen challenge was greatly impaired in mice receiving DCs from IL-37tg mice compared with those receiving DCs from WT mice (−60%; P < 0.01 at 48 h). In DCs isolated from IL-37tg mice, LPS-induced increase of MHC II and costimulatory molecule CD40 was reduced by 51 and 31%, respectively. In these DCs, release of IL-1β, IL-6, and IL-12 was reduced whereas IL-10 secretion increased (37%). Consistent with these findings, DCs from IL-37tg mice exhibited a lower ability to stimulate syngeneic and allogeneic naive T cells as well as antigen-specific T cells and displayed enhanced induction of T regulatory (Treg) cells (86%; P < 0.001) in vitro. Histological analysis of CHS skin in mice receiving hapten-sensitized DCs from IL-37tg mice revealed a marked reduction in CD8 + T cells (−74%) but an increase in Treg cells (2.6-fold). Together, these findings reveal that DCs expressing IL-37 are tolerogenic, thereby impairing activation of effector T-cell responses and inducing Treg cells. IL-37 thus emerges as an inhibitor of adaptive immunity.tolerance | skin inflammation | allergic contact dermatitis I nterleukin-37 (IL-37; formerly IL-1 family member 7) isoform b inhibits innate inflammation (1-3). In human peripheral blood from healthy subjects, low levels of steady-state IL-37 mRNA and protein are expressed in monocytes, dendritic cells (DCs), and plasma cells perhaps due to instability elements within the coding region (4, 5); however, stimulation with proinflammatory cytokines or Toll-like receptor (TLR) ligands induces IL-37 levels, which in turn suppress the proinflammatory cytokines IL-1α, IL-1β, IL-6, M-CSF, and GM-CSF but not anti-inflammatory cytokines IL-10 and IL-1Ra (4-7). Although an ORF for the murine homolog of IL37 is absent in various mouse databases, human IL-37 expression in a variety of human and murine cells inhibits innate immunity and suppresses production of proinflammatory cytokines and chemokines (2,3,8), indicating that human IL-37 is functional in murine cells. Compared with WT mice, mice expressing human IL-37b (IL-37tg mice) produce lower amounts of proinflammatory cytokines after lipopolysaccharide (LPS) administration and are protected from LPS-induced septic shock (3) and dextran sulfate sodium-induced colitis (8). Administration of recombinant human IL-37 in mice suppresses cytokine and chemokine production, neutrophil infiltration, and cell death, thereby ameliorating hepatic and myocardial ischemia/reperfu...