Generation of an MHC class II-restricted T cell epitope by extracellular processing of hepatitis delta antigen.

Hepatitis delta virus is a human pathogen that is responsible for a severe form of hepatitis affecting hepatitis B envelope Ag carriers. We have previously identified a series of hepatitis delta Ag (HDAg) epitopes that are recognized by CD4+ T cell clones isolated from infected subjects. Herein, we show that the presentation of soluble HDAg to CD4+ T cell clones that are specific for the HDAg(106-121) epitope was exceptionally unaffected by the inhibition of the APC-processing machinery when APCs were fixed with glutaraldehyde before Ag pulsing or treated with chloroquine or brefeldin A. Interestingly, 5 h of pulsing was strictly required for the efficient presentation of the HDAg(106-21) epitope by fixed APCs, suggesting that some form of extracellular processing had occurred. Indeed, fixed APCs were able to present HDAg after only 1 h of pulsing when HDAg was preincubated with serum for 5 h. More important, presentation was completely abrogated when Ag was previously incubated in medium containing serum in the presence of a potent inhibitor of trypsin activity such as the secretory leukoprotease inhibitor. These results show that HDAg may undergo extracellular processing and suggest that the generation of immunogenic epitopes directly by serum proteases could play a role in the immune response against hepatitis delta virus during infection.