No recognition of MHC class II+ cells infected with a vaccinia virus encoding influenza type A nucleoprotein by class II-restricted T cells.

MHC class II is mostly charged by antigens derived from the outside of the antigen-presenting cell (APC), while class I presents endogenous antigens transported as peptides to the endoplasmic reticulum (ER) by specific transporters. Nevertheless, many antigens, especially glycoproteins, can be presented in vitro by class II even if endogenous. In order to investigate the class-II-restricted T-cell response to endogenously synthesized influenza nucleoprotein (NP) synthesized in infected cells as a model of non-glycosylated nuclear protein, class-II-restricted cytolytic T-cell (CTL) clones were established from BALB/c (H-2d) mice immunized with either influenza A/PR/8/34 (PR8) strain or with a vaccinia virus encoding the NP protein. Two of the clones were characterized in detail and turned out to be cytolytic, I-Ad-restricted and NP peptide 218-229 specific. Even though an in vivo class-II-restricted T-cell response was elicited in BALB/c mice immunized with a vaccinia virus encoding nucleoprotein (Vacc-NP), class II+ mouse lymphoma cells were not lysed by the class-II-restricted clones in vitro when they were infected with the same virus or with a vaccinia virus encoding a truncated form of NP with no karyophilic sequence, showing that the de novo synthesized protein targeted to the nucleus or remaining in the cytoplasm cannot charge class II through the same pathway as class I in murine APC. These results extend previous observations made on transfected cells to cells that express an antigen during viral infection.