ABSTRACT
Ag presentation is a key step in the initiation of adaptive immune responses that depends on the expression of MHC Ags and costimulatory molecules. Immune-enhancing CpG and non-CPG oligodeoxynucleotides (ODNs) stimulate Ag presentation by stimulating the expression of these molecules and by promoting dendritic cell maturation. In this report, we identify immunoregulatory orthophosphorothioate non-CpG molecules, referred to as regulatory ODNs (rODNs), by their ability to inhibit allogeneic monocyte-stimulated T cell responses and down-regulate HLA-DR in human primary monocytes. The rODNs promoted the survival of macrophages and were able to activate IL-8 secretion through a chloroquine-resistant pathway. Messenger RNAs for HLA-DR alpha and beta and the MHC CIITA were reduced by rODNs but not by stimulatory CpG ODN2006 and non-CpG ODN2006a. CIITA transcription in monocytes was controlled primarily by promoter III and not by promoter I or IV. rODNs blocked promoter III-directed transcription of CIITA in these cells. Under conditions that induced dendritic cell differentiation, rODNs also reduced HLA-DR expression. The activity of rODNs is phosphorothioate chemistry and G stretch dependent but TLR9 independent. G tetrads were detected by circular dichroism in active rODNs and associated with high m.w. multimers on nondenaturing gels. Heat treatment of rODNs disrupted G tetrads, the high m.w. aggregates, and the HLA-DR inhibitory activity of the ODNs. The inhibition of immune responses by regulatory oligodeoxynucleotides may be useful for the treatment of immune-mediated disorders including autoimmune diseases and graft rejection.