CD40 ligation triggers IL-12 production by dendritic cells (DC) in vitro. Here, we demonstrate that CD40 cross-linking alone is not sufficient to induce IL-12 production by DC in vivo. Indeed, resting DC make neither the IL-12 p35 nor IL-12 p40 subunits and express only low levels of CD40. Nevertheless, after DC activation by microbial stimuli that primarily upregulate IL-12 p40 and augment CD40 expression, CD40 ligation induces a significant increase in IL-12 p35 and IL-12 p70 heterodimer production. Similarly, IL-12 p70 is produced during T cell activation in the presence but not in the absence of microbial stimuli. Thus, production of bioactive IL-12 by DC can be amplified by T cell-derived signals but must be initiated by innate signals.

Dendritic cells (DC) can produce Th-polarizing cytokines and direct the class of the adaptive immune response. Microbial stimuli, cytokines, chemokines, and T cell-derived signals all have been shown to trigger cytokine synthesis by DC, but it remains unclear whether these signals are functionally equivalent and whether they determine the nature of the cytokine produced or simply initiate a preprogrammed pattern of cytokine production, which may be DC subtype specific. Here, we demonstrate that microbial and T cell-derived stimuli can synergize to induce production of high levels of IL-12 p70 or IL-10 by individual murine DC subsets but that the choice of cytokine is dictated by the microbial pattern recognition receptor engaged. We show that bacterial components such as CpG-containing DNA or extracts from Mycobacterium tuberculosis predispose CD8α+ and CD8α−CD4− DC to make IL-12 p70. In contrast, exposure of CD8α+, CD4+ and CD8α−CD4− DC to heat-killed yeasts leads to production of IL-10. In both cases, secretion of high levels of cytokine requires a second signal from T cells, which can be replaced by CD40 ligand. Consistent with their differential effects on cytokine production, extracts from M. tuberculosis promote IL-12 production primarily via Toll-like receptor 2 and an MyD88-dependent pathway, whereas heat-killed yeasts activate DC via a Toll-like receptor 2-, MyD88-, and Toll/IL-1R domain containing protein-independent pathway. These results show that T cell feedback amplifies innate signals for cytokine production by DC and suggest that pattern recognition rather than ontogeny determines the production of cytokines by individual DC subsets.

Dendritic cells (DC) initiate T cell responses and direct the class of T cell immunity through the production of Th‐polarizing cytokines. In the mouse, immunization with CD8α+ DC has led to Th1 priming whereas immunization with CD8α– DC has been associated with Th2 induction. Here, we use a direct T cell priming assay in vitro to re‐examine the Th‐directing potential of total DC or purified CD4+ DC, CD8α+ DC or CD4– CD8α– (double‐negative; DN) DC subsets from mouse spleen. We show that the default Th effector phenotype induced by priming with DC depends on the protocol used for T cell purification, the T cell:antigen‐presenting cell ratio and the antigen dose but is only marginally affected by DC subtype. All DC subsets can direct increased Th1 development in response to microbial stimuli known to elicit IL‐12 production. Similarly, all subsets can suppress Th1 development and allow Th2 cellsto expand upon exposure to IL‐10‐inducing microbial agents. The flexibility of DC in directing Th development in function of microbial signals argues against the notion of pre‐determined "DC1" and "DC2" subsets and suggests that multiple DC subtypes can direct an appropriate Th response to different classes of infectious agents.

B cells and dendritic cells (DC) internalize and degrade exogenous Ags and present them as peptides bound to MHC class II molecules for scrutiny by CD4+ T cells. Here we use an Ab specific for a processed form of the model Ag, hen egg lysozyme (HEL), to demonstrate that this protein is not efficiently presented by lymph node DC following s.c. immunization. HEL presentation by the DC can be dramatically enhanced upon coinjection of a microbial adjuvant, which appears to act by enhancing peptide loading onto MHC class II. CD40 cross-linking or the presence of a high frequency of T cells specific for HEL can similarly improve presentation by DC in vivo. For any of these activating stimuli, CD8α+ DC consistently display the highest proportion of HEL-loaded MHC class II molecules. These data indicate that exogenous Ags can be displayed to T cells in lymphoid tissues by a large cohort of resident DC whose presentation is regulated by innate and adaptive stimuli. Our data further reveal the existence of a feedback mechanism that augments Ag presentation during cognate APC-T cell interactions.

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