Two processing pathways for the MHC class II-restricted presentation of exogenous influenza virus antigen

The natural Ag influenza virus A was used to test the requirements for the HLA-DR1-restricted presentation of the epitopes 18-29 in the matrix protein and 307-318 in the hemagglutinin protein. CD4+ cytotoxic T cell clones of similar efficiency were used to detect presentation of these two epitopes. Presentation of the matrix epitope by APC pulsed with either inactivated virus particles or purified soluble protein followed the classical pathway in that 1) it required invariant chain expression, 2) it was blocked by inhibition of protein synthesis, and 3) it was dependent on a function(s) encoded in the MHC class II region. These characteristics suggest that peptides corresponding to the matrix epitope can only load onto newly synthesized class II molecules that were targeted to a processing compartment by the invariant chain. In contrast, presentation of the hemagglutinin epitope processed from virus particles followed a different pathway. First, presentation of hemagglutinin was independent of invariant chain expression. Second, a human B lymphoblastoid cell line in which protein synthesis was inhibited for 9 h was still able to present hemagglutinin even at very low doses of Ag. Third, a DR1-transfected mutant B cell line missing the MHC class II region was able to present hemagglutinin. Thus, mature class II alpha beta molecules can acquire immunogenic peptides derived from intact natural Ags for presentation to CD4+ T cells. This pathway may be useful for the binding of peptides derived from Ags that are rapidly degraded upon uptake into APC.