MHC class II distribution in dendritic cells and B cells is determined by ubiquitin chain length

Abstract

Dendritic cells (DCs) and B cells present antigen-derived peptides bound to MHC class II (MHC II) molecules for recognition by CD4-positive T lymphocytes. DCs control the intracellular traffic of peptide-MHC II complexes by regulating the ubiquitination of MHC II. In resting or "immature" DCs, ubiquitinated MHC II molecules are targeted to lysosomes, but upon pathogen-induced "maturation," ubiquitination is down-regulated and MHC II can accumulate on the plasma membrane of mature DCs. Although B cells constitutively ubiquitinate their MHC II, it unexpectedly remains at the surface. We find that DCs and B cells differ in MHC II-conjugated ubiquitin (Ub) chain length: four to six Ub in immature DCs vs. two to three in B cells. In both cell types, experimentally increasing Ub chain length led to efficient lysosomal transport of MHC II, whereas MHC II with fewer than two Ubs did not reach lysosomes. Thus, Ub chain length plays a crucial role in regulating the intracellular fate and function of MHC II in DCs and B cells.D endritic cells (DCs) and B lymphocytes are professional antigen-presenting cells (APCs) capable of stimulating efficient T-cell responses (1, 2). However, their approaches to antigen presentation differ in important respects. Whereas DCs are highly endocytic and internalize a wide variety of antigens, B cells take up and process only the single antigen recognized by their B-cell receptor. DCs are also distinguished by their ability to regulate antigen processing and presentation by "maturation" (3, 4). Immature DCs, found in peripheral tissues, are adept at endocytic uptake of antigen but do not efficiently generate peptide-MHC class II (MHC II) complexes or express them stably on the cell surface. In part, this is because MHC II in immature DCs is ubiquitinated on a single conserved lysine in the cytoplasmic domain of the β-chain (5, 6) by E3 ligases of the membrane-associated RING-CH (MARCH) family (7,8). Like other ubiquitinated membrane proteins (9), ubiquitinated MHC II molecules are targeted to and sequestered in multivesicular late endosomes and lysosomes. Upon receiving a maturation stimulus (e.g., Toll-like receptor agonist), however, ubiquitination ceases (5, 6) and peptide-MHC II complexes are translocated to and accumulate at the plasma membrane (10-13). In B cells, MHC II surface expression is always high despite also being ubiquitinated by MARCH ligases in naïve B cells (8).Internalization and down-regulation of receptor tyrosine kinases by ubiquitination is well known. Ligand binding activates the kinase, resulting in autophosophorylation and subsequent recruitment of soluble E3 ligases (e.g., Cbl) that ubiquitinate one or more acceptor lysines. The ubiquitin (Ub) moieties are recognized by Ub-interaction motif (UIM)-containing adapter molecules (e.g., epsins, eps15) that associate with clathrin-coated pits, leading to receptor internalization (14-18). Upon delivery to early endosomes, Ub is recognized by members of the endosomal sorting complex required for transport (ESCRT) complexe...

Ubiquitination by March-I prevents MHC class II recycling and promotes MHC class II turnover in antigen-presenting cells

“…When normalized for total expression of MHC-II in each cell type, pMHC-II in freshly isolated spleen B cells and DCs was ubiquitinated to a much greater extent than pMHC-II in BMDCs. As was observed previously (19), pMHC-II isolated from spleen B cells has a short ubiquitin chain length (generally one and two ubiquitin moieties per molecule), whereas ubiquitin chain length of pMHC-II in BMDCs was longer (usually four and five ubiquitin moieties per molecule). We did not observe dramatic differences in ubiquitin chain length in pMHC-II isolated from spleen B cells and spleen DCs.…”

MHC Class II Antigen Presentation by Dendritic Cells Regulated through Endosomal Sorting

“…Although MARCH1 seems to play a more prominent role in controlling pMHCII traffic in DCs , it cannot be excluded that the two ligases act at distinct sites, for example, at the plasma membrane and at the endosomal membrane, leaving dissimilar sorting signatures. In this regard, it was recently shown that sorting of MHCII is highly dependent on the length of the conjugated ubiquitin chain (Ma et al 2012). Specificities of MARCH ligases may differ for distinct MHCII isotypes (Jahnke et al 2012).…”

The use of signal peptide domains as vaccine candidates

“…3 In parallel, CD4 C T-cell activation is important for the induction of specific CD8 C T-cell and humoral responses. 4,5 To be presented to B-cells 6 and CD4…”