The present invention relates to polypeptides comprising a fragment of a teleost invariant chain optionally fused to one or more antigens or a teleost invariant chain fused to one or more antigens or antigenic fragments thereof, a polynucleotide encoding such polypeptides, vectors comprising such polynucleotides, collection of vectors comprising such polynucleotides and use of such polypeptides, polynucleotides, vectors for treating or preventing diseases, in particular tumor diseases. The teleost invariant chain polypeptides or fragments thereof act as T cell enhancer converting non-immunogenic antigenic sequences into immunogenic T cell antigens.