Direct transport of newly synthesized HLA-DR from the trans-Golgi network to major histocompatibility complex class II containing compartments (MIICS) demonstrated using a novel tyrosine-sulfated chimera

Binding of antigenic peptides to major histocompatibility complex (MHC) cla ss II glycoproteins occurs in specialized endocytic compartments of antigen -presenting cells, which in man are termed MIICs. Newly synthesized MHC cla ss II molecules are transported from the trans-Golgi network to MIICs, but previous studies of this important step in antigen processing have failed t o conclusively determine whether most immature MHC class II complexes are t ransported directly to the processing compartments or are first transiently exposed at the cell surface. To attempt to resolve this question, I constr ucted a chimeric HLA-DR alpha chain containing two optimal tyrosine sulfati on motifs. When expressed in a human B lymphoblastoid cell line lacking fun ctional DR alpha chains, the chimera was correctly incorporated into comple xes containing endogenous beta and invariant chains, transported to the tra ns-Golgi network, and efficiently sulfated. Pulse-chase experiments showed that the sulfated complexes were rapidly transported to processing compartm ents with kinetics consistent with direct transport from the trans-Golgi ne twork. The rate of maturation was not significantly altered in cells expres sing a temperature-sensitive mutant of dynamin under conditions where the e ndocytosis of transferrin was inhibited by 95%, confirming that endocytosis was not required for delivery to MIICs. Maturation of MHC class II-contain ing complexes was inhibited by aluminum fluoride and brefeldin A, indicatin g the involvement of heterotrimeric G-proteins and ADP-ribosylation factor in the transport event(s). The procedure described provides a unique mechan ism to study critical events in antigen processing and presentation.