MHC class I-restricted presentation of maleylated protein binding to scavenger receptors

Pathways for loading exogenous protein-derived peptides on MHC class I are thought to he present mainly in monocyte-lineage cells and to involve phago cytosis- or macropinocytosis-mediated antigenic leakage into either cytosol or extracellular milieu to give peptide access to MHC class I. We show tha t maleylation of OVA enhanced its presentation to an OVA-specific MHC class I-restricted T cell line by both macrophages and B cells. This enhanced pr esentation involved uptake through receptors of scavenger receptor (SR)-lik e ligand specificity, was TAP-l-independent. and was inhibited by low level s (2 mM) of ammonium chloride. To peptide loading or bystander APCs by male ylated (maleyl) OVA-pulsed macrophages was detected. Demaleylated maleyl-OV A showed enhanced MHC class I-restricted presentation through receptor-medi ated uptake and remained highly sensitive to 2 mM ammonium chloride, Howeve r, if receptor binding of maleyl-OVA was inhibited by maleylated BSA, the r esidual presentation aas relatively resistant to 2 mM ammonium chloride. Ma leyl-OVA directly? introduced into the cytosol ria osmotic lysis of pinosom es was poorly presented, confirming that receptor-mediated presentation of exogenous malegl-OVA was unlikely to involve a cytosolic pathway, Demaleyla ted maleyl-OVA was well presented as a cytosolic Ag, consistent with the de pendence of cytosolic processing on protein ubiquitination. Thus, receptor- specific delivery of exogenous protein Ags to APCs can result in enhanced M HC class I-restricted presentation, suggesting that the exogenous pathway o f peptide loading for MHC class I may be a constitutive property dependent mainly on the quantity of Ag taken up by APCs.