Recognition of microbial components by APCs and their activation through Toll-like receptors (TLR) leads to the induction of adaptive immune responses. In this study, we show that activation of TLR2 by its synthetic ligand Pam3Cys, in contrast to activation of TLR9 by immunostimulatory DNA (ISS-ODN), induces a prominent Th2-biased immune response. Activation of APCs by Pam3Cys resulted in the induction of Th2-associated effector molecules like IL-13, and IL-1β, GM-CSF and up-regulation of B7RP-1, but low levels of Th1-associated cytokines (IL-12, IFNα, IL-18, IL-27). Accordingly, TLR2 ligands aggravated experimental asthma. These data indicate that the type of TLR stimulation during the initial phase of immune activation determines the polarization of the adaptive immune response and may play a role in the initiation of Th2-mediated immune disorders, such as asthma.

Dendritic cells (DCs) are capable of cross-presenting exogenous Ag to CD8+ CTLs. Detection of microbial products by Toll-like receptors (TLRs) leads to activation of DCs and subsequent orchestration of an adaptive immune response. We hypothesized that microbial TLR ligands could activate DCs to cross-present Ag to CTLs. Using DCs and CTLs in an in vitro cross-presentation system, we show that a subset of microbial TLR ligands, namely ligands of TLR3 (poly(inosinic-cytidylic) acid) and TLR9 (immunostimulatory CpG DNA), induces cross-presentation. In contrast to presentation of Ag to CD4+ T cells by immature DCs, TLR-induced cross-presentation is mediated by mature DCs, is independent of endosomal acidification, and relies on cytosolic Ag processing machinery.

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Immunostimulatory sequence (ISS) DNA containing unmethylated CpG dinucleotides stimulate NK and APC to secrete proinflammatory cytokines, including IFN-αβ and -γ, TNF-α, and IL-6 and -12, and to express costimulatory surface molecules such as CD40, B7-1, and B7-2. Although ISS DNA has little direct effect on T cells by these criteria, immunization of wild-type mice with ISS DNA and OVA results in Ag-specific CTL and Th1-type T helper activity. This investigation examines the mechanisms by which ISS DNA primes CD8+ and CD4+ lymphocyte activities. In this report we demonstrate that ISS DNA regulates the expression of costimulatory molecules and TAP via a novel autocrine or paracrine IFN-αβ pathway. Coordinated regulation of B7 costimulation and TAP-dependent cross-presentation results in priming of Ag-specific CD8+ CTL, whereas CD40, B7, and IL-12 costimulation is required for priming of CD4+ Th cells by ISS-based vaccines.