Recycling MHC class I molecules and endosomal peptide loading

Recycling MHC class I molecules and endosomal peptide loading

Departments of ^†Tumor Biology and ^§Cell Biology, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands; ^‡Department of Virology, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands; and ^¶Josephine Nefkens Institute, P.O. Box 2408, 2301 CA, Leiden, The Netherlands

Communicated by Johannes van Rood, Leiden University, Leiden, The Netherlands (received for review March 18, 1999)

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, NH₄Cl-sensitive 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.

Footnotes

↵ ‖ To whom reprint requests should be addressed. E-mail: JNeefjes{at}nki.nl.
ABBREVIATIONS:

ER,

endoplasmic reticulum;

TAP,

transporters associated with antigen processing;

β2m,

β2-microglobulin;

BFA,

Brefeldin A;

CTL,

cytotoxic T lymphocyte;

GFP,

green fluorescent protein;

MIIC,

MHC class II compartment;

EBV,

Epstein–Barr virus;

MV,

measles virus;

DGE,

density gradient electrophoresis;

HRP,

horseradish peroxidase