Superior tumor protection induced by a cellular vaccine carrying a tumor-specific T helper epitope by genetic exchange of the class II-associated invariant chain peptide.

Efficient loading of MHC class II molecules with a T helper epitope of choice can be achieved through genetic exchange of the MHC class II-associated invariant chain peptide (CLIP) sequence with a sequence encoding the helper peptide. We have now used this method to engineer a cellular vaccine that continuously expresses a tumor-specific helper epitope in a defined costimulatory context. We provide evidence (a) that this cellular vaccine induces peptide-specific helper T cells in vivo that are functional in protecting mice from challenge with a highly aggressive tumor, (b) that this vaccine can directly prime tumor-specific helper T cells in vivo, and (c) that this cellular vaccine is superior compared with similar cells loaded with synthetic T helper peptide in inducing tumor protection. In conclusion, cellular vaccines for activation of antigen-specific helper T cells can be greatly improved by the introduction of invariant chain constructs containing a T helper epitope by class II-associated invariant chain peptide exchange.