MHC CLASS II-ASSOCIATED INVARIANT CHAIN-INDUCED ENLARGED ENDOSOMAL STRUCTURES - A MORPHOLOGICAL-STUDY

The major histocompatibility complex class II-associated invariant cha in is believed to direct newly synthesized class II to endocytic compa rtments. Invariant chain synthesized at high levels in transiently tra nsfected cells induces formation of large vesicular structures. We hav e examined the effect of stable expression of invariant chain in human fibroblasts by light and electron microscopy. Invariant chain express ion dramatically modified endocytic compartments and induced the forma tion of greatly enlarged structures. These modifications were not leth al. Ultrastructurally, at least three morphologically distinct enlarge d compartments could be discerned in the cells. These three compartmen ts may represent early and late endosomes and lysosomes. Internalizati on of anti-invariant chain antibodies shows that invariant chain may r each the large endosomes via rapid internalization from the plasma mem brane. Internalized protein remained in the enlarged vesicles for 4-6 h, indicating an invariant chain-induced delay in the pathway to lysos omes. Although the large invariant chain-induced vesicles have not yet been seen in professional antigen-presenting cells, the invariant cha in-induced effects may play a role in regulating the endocytic path wa y, creating a special environment for MHC class II to bind antigen. (C ) 1997 Academic Press.