Antigen-presenting cells (APC) can be refaced with "protein paints" that change the appearance of their T cell-oriented trans signal arrays. Our group has developed three categories of protein paints suitable for this kind of APC engineering: artificial glycosylphosphatidylinositol (GPI) proteins, palmitated-protein A:Fc*1 fusion protein conjugates, and trans signal converter proteins. Protein paints have been devised with either immune enhancement or suppression in mind. Costimulator * GPI and palmitated-protein A costimulator * Fcgamma1 conjugates can be used to augment the immune-activating potential of tumor cells. Alternatively, protein paints can be designed to transform APC into artificial veto cells, in essence creating Trojan horses capable of inhibiting pathogenic T cells. Trans signal converter proteins (TSCP) have been devised for this purpose. Our first paradigmatic inhibitory TSCP, CTLA-4 * Fas ligand, binds to APC, and in so doing, simultaneously blocks B7 costimulation (via CTLA-4) and sends inhibitory trans signals (via Fas ligand) to T cells with dramatic efficacy. Protein transfer offers a number of advantages over gene transfer in facilitating quantitative and combinatorial protein expression and simplifying in vivo applications; the palette of protein paints with immunotherapeutic potential will undoubtedly continue to evolve.