Direct detection and quantitation of a distinct T-cell epitope derived from tumor-specific epithelial cell-associated mucin using human recombinant antibodies endowed with the antigen-specific, major histocompatibility complex-restricted specificity of T cells

The recent characterization of MHC-displayed tumor-associated antigens that recognize effector cells of the immune system has created new perspectives for cancer therapy. Antibodies that recognize these tumor-associated MHC-peptide complexes with the same specificity as the T-cell antigen receptor will therefore be valuable tools for immunotherapy, as well as for studying antigen presentation in human cancers. Most tumor-associated antigens are expressed in only one or a few tumor types; however, specific T-cell epitopes derived from the Mucin-1 tumor-associated antigen (MUC1) that are widely expressed in many cancers were identified and shown to be recognized by CTLs. We selected a large nonimmune repertoire of phage Fab antibodies on recombinant human class I HLA-A2 complexes displaying an antigenic T-cell epitope derived from MUC1. High frequency of anti-MHC-peptide binders was observed (84%), and surprisingly, a high percentage (80%) of antibodies was fully specific for the MUC1 epitope. We isolated a surprisingly large panel of 16 different high-affinity human recombinant Fab antibodies that exhibited peptide-specific, MHC-restricted binding characteristics of T cells. The analyzed Fabs not only recognize the cognate MHC-peptide complex in a recombinant soluble form but also the native complex as displayed on the surface of antigen-presenting cells and breast tumor cells. Therefore, these findings demonstrate the ability to transform the unique fine specificity but low intrinsic affinity of T-cell receptors on T cells into high-affinity soluble antibody molecules endowed with a T-cell antigen receptor-like specificity. These molecules may prove to be very important and widely applicable for monitoring the expression of specific MHC-peptide complexes on the surface of tumor and immune cells for structure-function studies of T-cell receptor-peptide-MHC interactions, as well as for developing new targeting agents for immunotherapy.