Methods and compositions for generating novel nucleic acid molecules through targeted spliceosomal mediated trans-splicing that result in expression of an immunogenic polypeptide. The invention includes pre-trans-splicing molecules (PTMs) designed to interact with a target precursor messenger RNA molecule (target pre-mRNA) and mediate a trans-splicing reaction resulting in the generation of a novel chimeric RNA molecule (chimeric RNA) capable of encoding the immunogenic polypeptide, recombinant vector systems capable of expressing the PTMs of the invention, and cells expressing said PTMs. The target pre-mRNA are those encoding proteins that function in antigen uptake, antigen presentation and chaperoning. The methods of the invention encompass contacting the PTMs of the invention with a target pre-mRNA, under conditions in which a portion of the PTM is trans-spliced to a portion of the target pre-mRNA to form a chimeric mRNA molecule capable of encoding an immunogenic polypeptide.