The mechanisms that allow antigen-presenting cells (APCs) to selectively present extracellular antigen to CD8+ effector T cells (cross-presentation) or to CD4+ T helper cells are not fully resolved. We demonstrated that APCs use distinct endocytosis mechanisms to simultaneously introduce soluble antigen into separate intracellular compartments, which were dedicated to presentation to CD8+ or CD4+ T cells. Specifically, the mannose receptor supplied an early endosomal compartment distinct from lysosomes, which was committed to cross-presentation. These findings imply that antigen does not require intracellular diversion to access the cross-presentation pathway, because it can enter the pathway already during endocytosis.

Antiviral or antitumor immunity requires activation of cytotoxic CD8+ T cells by dendritic cells, which present viral or tumor antigens on major histocompatibility complex (MHC) class I molecules. The intracellular mechanisms facilitating MHC class I-restricted presentation of extracellular antigens ('cross-presentation') are unclear. Here we demonstrate that cross-presentation of soluble antigen occurred in an early endosomal compartment distinct from the endoplasmic reticulum where endogenous antigen is loaded onto MHC class I. Efficient cross-presentation required endotoxin-induced, Toll-like receptor 4- and signaling molecule MyD88-dependent relocation of the transporter associated with antigen processing, essential for loading of MHC class I, to early endosomes. Transport of cross-presented antigen from endosomes to the cell surface was inhibited by primaquine, which blocks endosomal trafficking. Thus, cross-presentation is spatially and mechanistically separated from endogenous MHC class I-restricted antigen presentation and is biased toward antigens containing microbial molecular patterns.

T helper cells secreting interleukin (IL)-17 (Th17 cells) play a crucial role in autoimmune diseases like multiple sclerosis (MS). Th17 differentiation, which is induced by a combination of transforming growth factor (TGF)-β/IL-6 or IL-21, requires expression of the transcription factor retinoic acid receptor–related orphan receptor γt (RORγt). We identify the nuclear receptor peroxisome proliferator–activated receptor γ (PPARγ) as a key negative regulator of human and mouse Th17 differentiation. PPARγ activation in CD4+ T cells selectively suppressed Th17 differentiation, but not differentiation into Th1, Th2, or regulatory T cells. Control of Th17 differentiation by PPARγ involved inhibition of TGF-β/IL-6–induced expression of RORγt in T cells. Pharmacologic activation of PPARγ prevented removal of the silencing mediator for retinoid and thyroid hormone receptors corepressor from the RORγt promoter in T cells, thus interfering with RORγt transcription. Both T cell–specific PPARγ knockout and endogenous ligand activation revealed the physiological role of PPARγ for continuous T cell–intrinsic control of Th17 differentiation and development of autoimmunity. Importantly, human CD4+ T cells from healthy controls and MS patients were strongly susceptible to PPARγ-mediated suppression of Th17 differentiation. In summary, we report a PPARγ-mediated T cell–intrinsic molecular mechanism that selectively controls Th17 differentiation in mice and in humans and that is amenable to pharmacologic modulation. We therefore propose that PPARγ represents a promising molecular target for specific immunointervention in Th17-mediated autoimmune diseases such as MS.