Targeting a Polyepitope Protein Incorporating Multiple Class II-Restricted Viral Epitopes to the Secretory/Endocytic Pathway Facilitates Immune Recognition by CD4(+) Cytotoxic T Lymphocytes: a Novel Approach to Vaccine Design

The role of CD4(+) and CD8(+) cells in the generation of an effective immune response against viral infections is well established. Moreover, there is an increasing realization that subunit vaccines which include both CD4(+)- and CD8(+)-T-cell epitopes are highly effective in controlling viral infections, as opposed to those which are designed to activate a CD8(+)- or CD4(+)-T-cell response alone. One of the major limitations of epitope-based vaccines designed to stimulate virus-specific CD4(+) T cells is that endogenously expressed class II-restricted minimal cytotoxic-T-lymphocyte (CTL) epitopes are poorly recognized by CD4(+) CTLs. In the present study we attempted to enhance the efficiency of class II-restricted endogenous presentation of minimal class II-restricted CTL epitopes by specifically targeting a polyepitope protein to class II processing compartments through the endosomal and/or lysosomal pathway. A significantly enhanced stimulation of virus-specific CD4(+)-T-cell clones by antigen-presenting cells (APC) expressing the recombinant polyepitope protein targeted to the endocytic/secretory pathway was readily demonstrated in cytotoxicity assays. In addition, in vitro activation of Epstein-Barr virus- and influenza virus-specific CD4(+) memory CTLs by the recombinant constructs encoding the polyepitope protein, specifically targeted to the lysosomal compartment, was also demonstrated. The enhanced stimulatory capacity of APC expressing a lysosome-targeted polyepitope protein has important implications for vaccine design.