Endogenously expressed matrix protein M1 and nucleoprotein of influenza A are efficiently presented by class I and class II major histocompatibility complexes.

Current influenza vaccines containing primarily hypervariable haemagglutinin and neuraminidase proteins must be prepared against frequent new antigenic variants. Therefore, there is an ongoing effort to develop influenza vaccines that also elicit strong and sustained cytotoxic responses against highly conserved determinants such as the matrix (M1) protein and nucleoprotein (NP). However, their antigenic presentation properties in humans are less defined. Accordingly, we analysed MHC class I and class II presentation of endogenously processed M1 and NP in human antigen presenting cells and observed expansion of both CD8(+)- and CD4(+)-specific effector T lymphocytes secreting gamma interferon and tumour necrosis factor. Further enhancement of basal MHC-II antigenic presentation did not improve CD4(+) or CD8(+) T-cell quality based on cytokine production upon challenge, suggesting that endogenous M1 and NP MHC-II presentation is sufficient. These new insights about T-lymphocyte expansion following endogenous M1 and NP MHC-I and -II presentation will be important to design complementary heterosubtypic vaccination strategies.