ST2 protein is a soluble splicing variant of ST2L protein, which is the receptor for interleukin-33 (IL-33). Previously, we reported that soluble ST2 suppressed the signal transduction of lipopolysaccharide (LPS) and cytokine production in monocytic cells. To investigate whether or not this inhibitory effect occurs in dendritic cells, which are the key players in innate and adaptive immunity, human monocyte-derived dendritic cells were pre-treated with soluble ST2 protein before LPS stimulation. Although soluble ST2 did not attenuate the LPS-induced maturation of dendritic cells, pre-treatment with soluble ST2 suppressed cytokine production and inhibited LPS signaling. Moreover, the proliferation of naive T cells was inhibited significantly by soluble ST2 pre-treatment. IL-33 had little effect on the cytokine production of immature monocyte-derived dendritic cells. Furthermore, soluble ST2 protein was internalized into dendritic cells, suggesting that soluble ST2 protein acts by a noncanonical mechanism other than the sequestration of IL-33. 1 The specific receptor for IL-33 is ST2L protein, which forms a receptor complex with IL-1 receptor accessory protein. Soluble ST2 protein (sST2) is a variant of ST2L arising from alternative splicing. Because it binds with IL-33 to sequestrate the effect of IL-33, sST2 is considered an antiinflammatory factor in cases such as asthma.2 In addition, we and other groups have reported that the serum level of sST2 rises in various diseases, including asthma, autoimmune diseases, cardiovascular diseases and sepsis, although the pathological relevance of these findings remains largely unknown.