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Published online before print March 21, 2006
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Department of Viral Diseases and Immunology, National Public Health Institute, Helsinki, Finland
1Correspondence: Department of Viral Diseases and Immunology, National Public Health Institute, Mannerheimintie 166, FIN-00300 Helsinki, Finland. E-mail: mari.strengell{at}ktl.fi
Dendritic cells (DCs) play an important role in innate and adaptive immune responses. In addition to their phagocytic activity, DCs present foreign antigens to naïve T cells and regulate the development of adaptive immune responses. Upon contact with DCs, activated T cells produce large quantities of cytokines such as interferon-
(IFN-) and interleukin (IL)-21, which have important immunoregulatory functions. Here, we have analyzed the effect of IL-21 and IFN- on lipopolysaccharide (LPS)-induced maturation and cytokine production of human monocyte-derived DCs. IL-21 and IFN- receptor genes were expressed in high levels in immature DCs. Pretreatment of immature DCs with IL-21 inhibited LPS-stimulated DC maturation and expression of CD86 and human leukocyte antigen class II (HLAII). IL-21 pretreatment also dramatically reduced LPS-stimulated production of tumor necrosis factor 
, IL-12, CC chemokine ligand 5 (CCL5), and CXC chemokine ligand 10 (CXCL10) but not that of CXCL8. In contrast, IFN- had a positive feedback effect on immature DCs, and it enhanced LPS-induced DC maturation and the production of cytokines. IL-21 weakly induced the expression Toll-like receptor 4 (TLR4) and translation initiation region (TIR) domain-containing adaptor protein (TIRAP) genes, whereas the expression of TIR domain-containing adaptor-inducing IFN-ß (TRIF), myeloid differentiation (MyD88) 88 factor, or TRIF-related adaptor molecule (TRAM) genes remained unchanged. However, IL-21 strongly stimulated the expression of suppressor of cytokine signaling (SOCS)-1 and SOCS-3 genes. SOCS are known to suppress DC functions and interfere with TLR4 signaling. Our results demonstrate that IL-21, a cytokine produced by activated T cells, can directly inhibit the activation and cytokine production of myeloid DCs, providing a negative feedback loop between DCs and T lymphocytes.
Key Words: human • cell activation • cytokines
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