Many tumor-associated antigens are nonmutated, poorly immunogenic tissue differentiation antigens. Their weak immunogenicity may be due to “self”-tolerance. To induce autoreactive T cells, we studied immune responses to gp100/pmel 17, an antigen naturally expressed by both normal melanocytes and melanoma cells. Although a recombinant vaccinia virus (rVV) encoding the mouse homologue of gp100 was nonimmunogenic, immunization of normal C57BL/6 mice with the rVV encoding the human gp100 elicited a specific CD8+ T cell response. These lymphocytes were cross-reactive with mgp100 in vitro and treated established B16 melanoma upon adoptive transfer. To understand the mechanism of the greater immunogenicity of the human version of gp100, we characterized a 9-amino acid (AA) epitope, restricted by H-2Db, that was recognized by the T cells. The ability to induce specific T cells with human but not mouse gp100 resulted from differences within the major histocompatibility complex (MHC) class I–restricted epitope and not from differences elsewhere in the molecule, as was evidenced by experiments in which mice were immunized with rVV containing minigenes encoding these epitopes. Although the human (hgp10025–33) and mouse (mgp10025–33) epitopes were homologous, differences in the three NH2-terminal AAs resulted in a 2-log increase in the ability of the human peptide to stabilize “empty” Db on RMA-S cells and a 3-log increase in its ability to trigger interferon γ release by T cells. Thus, the fortuitous existence of a peptide homologue with significantly greater avidity for MHC class I resulted in the generation of self-reactive T cells. High-affinity, altered peptide ligands might be useful in the rational design of recombinant and synthetic vaccines that target tissue differentiation antigens expressed by tumors.

Background: Aggregated biotherapeutics have the potential to induce an immune response.Results: Aggregates can enhance innate and adaptive immune responses of PBMC.Conclusion: The response depends on aggregate type, immunogenicity of the monomer, donor immune status, and high particle numbers in the in vitro assay.Significance: This is the first study showing the impact of aggregate characteristics on the potential immune response of PBMC.

Recombinant poxviruses encoding tumorassociated antigens (TAA) are attractive as candidate cancer vaccines. Their effectiveness, however, will depend upon expression of the TAA in appropriate antigen-presenting cells. We have used a murine model in which the TAA is ␤-galactosidase (␤-gal) and a panel of recombinant vaccinia viruses (rVV) in which ␤-gal was expressed under early or late promoters at levels that varied over 500-fold during productive infections in tissue culture cells. Remarkably, only those rVV employing early promoters were capable of prolonging the survival of mice bearing established tumors expressing the model TAA. Late promoters were ineffective regardless of their determined promoter strength. The best results were obtained when ␤-gal was regulated by a strong early promoter coupled to a strong late promoter. When a variety of cell types were infected with the panel of viruses in vitro, dendritic cells were found to express ␤-gal only under the control of the early promoters even though late promoters were intrinsically more active in other cell types. Furthermore, in a functional assay, dendritic cells infected in vitro with rVV encoding ␤-gal regulated by an early promoter activated ␤-gal-specific cytotoxic T lymphocytes, whereas similar rVV with a late promoter-regulated gene did not. These data indicate that promoter strength per se is not the most critical quality of a recombinant poxvirus-based tumor vaccine and that the use of promoters capable of driving the production of TAA in ''professional'' antigen presenting cells may be crucial.