Exogenous antigen enters the major histocompatibility complex (MHC) class I pathway via multiple endocytic compartments with distinct molecular and proteolytic requirements (78.31)

Abstract Peptides derived from exogenous proteins are displayed by MHC class I via a pathway termed cross-presentation. Despite extensive study, the endocytic compartment(s) that mediate cross-presentation are poorly understood. Immunization of mice with hen egg-white lysozyme (HEL) protein cross-primes CD8 T cells recognizing HEL 23-31 presented by Db. Here we used liposome-encapsulated HEL to examine the cross-presentation capacity of specific subcellular compartments. Liposome-encapsulated HEL targeted to early endosomes, but not late endosomes/lysosomes, entered the MHC class I pathway. Cross-presentation of early endosomal HEL liposomes apparently required the transporter associated with antigen processing (TAP). However, additional experiments indicated no requirement for proteasome-dependent processing or nascent Db. To test the possibility that early endosomal HEL liposomes were presented by endosomal Db independently of TAP, we incubated TAP-deficient antigen presenting cells (APCs) with early endosomal HEL liposomes and a peptide to promote Db surface-stabilization and eventual entry into the endosomal pathway. Addition of Db surface-stabilizing peptide rescued cross-presentation of early endosomal HEL liposomes by TAP-deficient APCs, implying that Db mis-localization accounted for the cross-presentation defect observed previously. Cross-presentation of early endosomal HEL liposomes was inhibited when endolysosomal acidification was blocked by chloroquine. Surprisingly, chloroquine treatment facilitated cross-presentation of both late endosomal/lysosomal HEL liposomes and soluble HEL. Altogether, these data indicate that exogenous protein antigen can enter the MHC class I pathway through multiple routes within the APC.