Recycling MHC class I molecules and endosomal peptide loading

Abstract

MHC class I molecules usually present peptides derived from endogenous antigens that are bound in the endoplasmic reticulum. Loading of exogenous antigens on class I molecules, e.g., in cross-priming, sometimes occurs, but the intracellular location where interaction between the antigenic fragment and class I takes place is unclear. Here we show that measles virus F protein can be presented by class I in transporters associated with antigen processing-independent, NH 4 Clsensitive manner, suggesting that class I molecules are able to interact and bind antigen in acidic compartments, like class II molecules. Studies on intracellular transport of green fluorescent protein-tagged class I molecules in living cells confirmed that a small fraction of class I molecules indeed enters classical MHC class II compartments (MIICs) and is transported in MIICs back to the plasma membrane. Fractionation studies show that class I complexes in MIICs contain peptides. The pH in MIIC (around 5.0) is such that efficient peptide exchange can occur. We thus present evidence for a pathway for class I loading that is shared with class II molecules.MHC molecules display antigenic peptides on the cell surface for surveillance by T lymphocytes. MHC class I molecules present peptides to CD8 ϩ cytotoxic T cells, whereas MHC class II molecules present peptides to CD4 ϩ Th cells. The current dogma is that antigens from the extracellular fluid enter the exogenous processing pathway by endocytosis and are partially degraded in acidic endosomal or lysosomal structures to yield peptides that bind MHC class II molecules. This type of processing is inhibited by reagents that prevent endosomal acidification (chloroquine, NH 4 Cl) (1). In the endogenous processing pathway intracellular proteins are degraded in the cytosol by the proteasome complex, generating peptides that are transported from the cytoplasm into the lumen of the endoplasmic reticulum (ER) by the transporters associated with antigen processing (TAP), where they bind to nascent MHC class I heavy chain-␤ 2 -microglobulin (␤ 2 m) heterodimers. Fully assembled class I͞peptide complexes exit the ER and are transported through the Golgi to the cell surface by the constitutive secretory route. This processing pathway can be blocked by proteasome inhibitors or Brefeldin A (BFA), an inhibitor of anterograde ER-Golgi transport, but not by lysosomotropic agents. Thus, in general endogenous antigens are presented by MHC class I molecules, and exogenous antigens are displayed at the cell surface by MHC class II molecules. However accumulating evidence has shown that this dichotomy in presentation of antigen from endogenous and exogenous origin is not absolute. It was demonstrated that cytotoxic T lymphocyte (CTL) responses can be primed in vitro and in vivo with exogenous antigen (reviewed in refs. 2 and 3). At least two fundamentally different pathways for presentation of exogenous antigens by MHC class I molecules in vitro have been described: one involving access of exogenous antigen to...

Alternative processing for MHC class I presentation by immature and CpG‐activated dendritic cells

“…A special compartment for processing of extracellular antigen for MHC class I expression in antigen-presenting cells, mostly phagocytic cells, was described to be endolysosomal [6][7][8][9]. The presence of internalized antigen together with MHC class I and class II molecules in a compartment similar to the classical low pH MHC class II processing compartment (MIIC) was demonstrated [25].…”

“…where this could occur have been suggested: transfer from uptake vesicles into the cytosol followed by the classical MHC class I pathway [5] and processing and peptide loading of MHC class I in endolysosomes or phagosomes [6][7][8][9][10]. The endolysosomal compartment can obtain MHC class I molecules either directly from the ER or by recycling from the cell surface [9,11] and can export, by regurgitation, antigenic material for the direct binding to MHC class I expressed on the cell surface [12].…”

“…The endolysosomal compartment can obtain MHC class I molecules either directly from the ER or by recycling from the cell surface [9,11] and can export, by regurgitation, antigenic material for the direct binding to MHC class I expressed on the cell surface [12]. The leakage pathway is unique to DC, whereas the endolysosomal pathway has been mostly described for other cells, including B cells, macrophages and neutrophils [5-9, 11, 12] but was also recently reported for DC [10,13,14].…”

Manipulation of MHC-I/TCR Interaction for Immune Therapy

“…The endosomal peptides generated TAP-independently are likely to bound to recycling MHC-I which are often found in the endosomes of DCs (33,34). Indeed, a unique endosome-to-cytosol transport pathway which permits selectively delivery of internalized antigens to the cytosol has been demonstrated (35).…”

“…The degron signal regulating degradation by the 26S protesome on the DRiPs pathway was introduced into the designation of epitope vaccines. Xiang et al designed an autologous oral DNA vaccine containing the murine ubiquitin gene fused to minigenes encoding epitopes gp100 (25)(26)(27)(28)(29)(30)(31)(32)(33) and TRP-2 (181-188), which could break peripheral T cell tolerance by effective antigen processing and presentation (109). When the degron signal linked with N terminal of protein galactosidase, the MHC-I presentation of the protein was enhanced and elevated T cell activation evoked (110).…”