Antigen presentation of hen egg-white lysozyme but not of ribonuclease A is augmented by the major histocompatibility complex class II-associated invariant chain

The influence of the class II-associated invariant chain (Ii) on the presentation of the protein antigens hen egg-white lysozyme (HEL) and ribonuclease A (RNase) was investigated. For this purpose the Ii- rat-2 fibroblasts were transfected with I-Ak genes with or without Ii. Transfectants expressing Ii were superior in the presentation of the complete HEL protein to a panel of I-Ak-restricted T hybridomas characterized by distinct specificities for different HEL peptides and by different sensitivities to antigen concentration. There appeared to be a correlation between the antigen-presenting capacity and the amount of Ii, in that transfectants expressing large amounts of Ii were the best antigen presentors. The presentation of synthetic HEL peptides was not influenced by Ii. In contrast to the findings with HEL, the presentation of RNase by the same set of transfectants was clearly independent of Ii. Both antigens, HEL and RNase, required processing in the chloroquine-sensitive compartment. However, only the presentation of HEL but not of RNase could be efficiently blocked by brefeldin A. These data confirm that presentation of HEL depends on de novo synthesized class II molecules, whereas the presentation of RNase seems to be predominantly mediated by a pool of pre-existing class II molecules whose interaction with endocytosed antigen does not depend on Ii. These results suggest different mechanisms for the presentation of HEL and RNase and they raise the possibility that different antigens intersect the class II pathway at distinct intracellular locations.