Difference In Reactivities Between Immature Dendritic Cells And Differentiated Dendritic Cells Induced By CpG ODN

Difference in reactivities between immature dendritic cells and differentiated dendritic cells induced by CpG ODNDendritic cells (DC) are the most potent antigen-presenting cells (APC), with a unique ability to induce primary immune responses against microbial infection. Immature DC (imDC) that residue in peripheral tissues effectively capture antigen (Ag) and then migrate to regional lymph nodes, where they develop into mature DC (mDC) characterized by production of cytokines, up-regulation of co-stimulatory molecules, and increased ability to activate T cells. In vitro studies demonstrated that mature DC are terminally differentiated and succumb to activation-induced apoptosis. However, our previous study demonstrated that mature DC can further differentiate into a regulatory subset under the influence of the lymphoid microenvironment both in vitro and in vivo. Compared with mature DC, these differentiated DC (diffDC) express lower levels of major histocompatibility complex (MHC) class II, CD86, CD54, and CD11c, but higher levels of co-stimulatory molecules such as CD80, CD40, CD106 and CD11b, and secrete more interleukin 10 (IL-10 ) and nitric oxide (NO) but less interleukin 12 (IL-12) and transforming growth factor-β (TGF-β). Furthermore, diffDCs strongly inhibit the proliferation response of naive CD4⁺T cells to antigen presented by mDC.Microbial molecules activate immature DC to become mature DC through Toll-like receptors (TLRs). Although the exact gene numbers may differ between species, it is likely that most mammalian species have 10 to 15 TLRs, whichrecognizes different pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS) and CpG containing oligodeoxynucleotides (CpG ODN), conferring specificity onto the host innate immune system. TLR9 has been shown to recognize bacteria-derived CpG DNA, which is supported by the results that TLR9 deficient mice show defective responses to CpG ODN. CpG ODN containing unmethlated CpG dinucleotides can mimic the activity of microbial DNA to activate mitogenactivated protein kinases (MAPKs) and nuclear factor-κB (NF-κB) through MyD88/IRAK/TRAF6 kinases cascade and thus to activate immune cells, including B cells, NK cells, macrophages and dendritic cells, to produce NO and a variety of cytokines and chemokines. CpG ODN also induces maturation of dendritic cells and facilitates activation of Th1 immune response.Immune response is strictly controlled in mammals, and must be restored to a resting state once responsiveness is no longer required. Mouse immature DC are sensitive to CpG ODN stimulation, however, little is known regarding the responsiveness of diffDC to CpG stimulation. In the present study, we compared difference in the reactivities to CpG ODN between imDC and diffDC. After CpG ODN stimulation, diffDC produced more IL-10 and NO but less IL-12 compared with mDC. However, diffDC expressed similar level of CD40, CD80, CD86 and MHC class II, and displayed comparable capability to promote proliferation and activation of T cells with mDC. DiffDC expressed equal level of TLR9 mRNA but higher levels of suppressor of cytokine signaling (SOCS) mRNA. Compared with imDC, CpG ODN-treated diffDC showed enhanced activation of extracellular signal-regulatedkinase (ERKl/2) and c-Jun N-terminal kinase (JNK), but lower phosphorylation level of p38-MAPK, as well as signal transducer and activator of transcription (STAT) 1/3. SB203580, a p38 inhibitor, U0126, a mitogen-activated protein kinase kinase (MEK) inhibitor, and SP600125, a c-Jun NH2-terminal kinase (JNK) inhibitor, suppressed CpG-induced diffDC IL-10 production. The less IL-12p70 production by diffDC following CpG stimulation is not due to the increased IL-10 production. Taken together, these studies suggest that diffDC and imDC subsets may play distinct roles in regulating immune response to sustain immune homeostasis.