Developmental regulation of MHC II expression and transport in human plasmacytoid-derived dendritic cells

Abstract

Plasmacytoid predendritic cells (pDCs) play a key role in antiviral immunity through their capacity to produce large amounts of type I interferons in response to Toll-like receptor triggering, and to differentiate into dendritic cells (DCs). However, their antigen processing and presentation pathways remain poorly characterized. In this study, we analyzed major histocompatibility complex class II (MHC II) synthesis and transport in primary human pDCs. We show that stimulation of pDCs with influenza virus leads to a sustained neosynthesis of MHC II molecules, which rapidly accumulate in antigen loading compartments organized around the microtubule organization center. MHC II endocytosis as well as antigen internalization remain active during the entire process of pDC differentiation into DCs, suggesting a capacity to constantly renew surface peptide-MHC II complexes. IntroductionHuman peripheral blood contains 2 main populations of dendritic cells (DCs): the "conventional" DCs (cDCs), and the plasmacytoid predendritic cells (pDCs). 1 These 2 subsets express different sets of Toll-like receptors (TLR) and differentially respond to TLR ligands, indicating that they display nonredundant functions. 2 pDCs play a central role in the immune system by linking the innate with the adaptive immune responses. 3 First, pDCs rapidly produce large amounts of type I interferons (IFNs) after microbial stimulation. 4,5 pDC activation occurs through the recognition of viral nucleic acids (single-stranded RNA) by TLR7, and viral and bacterial DNA by TLR9. 6 Second, depending on the stimuli, pDCs can differentiate into cells with a dendritic phenotype. 3,7 This differentiation is accompanied by a strong up-regulation of the surface expression of major histocompatibility complex class I (MHC I) and class II (MHC II), as well as T-cell costimulatory molecules, suggesting that they become potent antigen-presenting cells. 3,7,8 However, the capacity of pDCs to act as antigenpresenting cells remains controversial. Depending on the stimuli, 3,9,10 the source of pDCs, 11,12 the way the antigen is internalized, 13,14 and the dose of antigen, 12 pDCs are able or not to activate naive CD4 T cells. It has also been proposed that pDCs are endowed with tolerogenic capacities by inducing regulatory T cells producing interleukin-10 (IL-10) both in vitro [15][16][17][18][19][20] and in vivo. 21,22 It is well established that antigen presentation by MHC II heavily depends on their intracellular traffic. The biosynthetic route followed by nascent MHC II has been well characterized in various antigen-presenting cells. 23 MHC II are synthesized in the endoplasmic reticulum (ER) where they associate with the chaperone invariant chain (Ii) to form ␣␤Ii complexes. Such complexes exit from the ER and traffic through the Golgi apparatus to most probably reach the plasma membrane where they are rapidly internalized into endosomes. There, Ii is degraded, allowing MHC II to acquire their peptide load. MHC II peptide-loaded complexes are then exported to the c...

Small molecule Toll-like receptor 7 agonists localize to the MHC class II loading compartment of human plasmacytoid dendritic cells

“…Examination of pDCs by confocal microscopy showed that all compounds colocalized with CpG-B in LAMP-1 ϩ CD63 ϩ HLA-DR ϩ endosomes, independently of their chemotype or labeling fluorophore. Our observations are in line with an elegant study by Sadaka et al 38 showing that MHC II molecules converge with LAMP-1 and CD63 (but not with TfR and EEA1 markers) in human pDCs treated with IL-3. LAMP-1 and CD63 are markers of late endosomes and lysosomes, whereas HLA-DR identifies MHC II molecules.…”

“…Therefore, the vesicles where small molecule TLR7 agonists accumulate appear to belong to the MIIC. 20,38,39 Indeed, in pDCs, like in other antigen-presenting cells, the endolysosomal compartment is specialized for MHC II antigen presentation and is considered as a lysosome-related organelle. 39 The function of MHC II molecules is to access the endolysosomal network, bind peptides derived from the antigens, and display them on the cell surface to CD4 ϩ T cells.…”

“…39 The function of MHC II molecules is to access the endolysosomal network, bind peptides derived from the antigens, and display them on the cell surface to CD4 ϩ T cells. Notably, the regulation of MHC II expression and transport is completely different in pDCs compared with conventional DCs (cDCs) from both humans 38 and mice. 40,41 In particular, cDCs halt the synthesis and degradation of MHC II after activation, whereas pDCs maintain synthesis and turnover of MHC II.…”

Positive Regulatory Domain I (PRDM1) and IRF8/PU.1 Counter-regulate MHC Class II Transactivator (CIITA) Expression during Dendritic Cell Maturation

“…Bcl6 has been shown to be down-regulated upon maturation of monocyte-derived DC (68); however, the functional interplay between these two transcriptional repressors in DCs has not been characterized. Finally, pDCs differ markedly from cDCs as they continually transcribe MHC-II genes even after antigen encounter (69,70). CIITA expression in this subset is driven primarily by CIITApIII.…”

Spatiotemporal trafficking of HIV in human plasmacytoid dendritic cells defines a persistently IFN-α–producing and partially matured phenotype

“…In contrast, CpGA stimulation of pDCs does not lead to the formation of MHC II intracellular clusters (51). CpGB-stimulated pDCs efficiently process and present CMV antigens and are capable of stimulating CMV-specific effector memory T helper cells.…”