We have developed a system for probing protein/protein interactions which makes use of the bacterial flagellum to display random peptide libraries on the surface of E. coli. In developing the system the entire coding sequence of E. coli thioredoxin (trxA) was inserted into a dispensable region of the gene for flagellin (fliC), the major structural component of the E. coli flagellum. The resulting fusion protein (FLITRX) was efficiently exported and assembled into partially functional flagella on the bacterial cell surface. A diverse library of random dodecapeptides were displayed in FLITRX on the exterior of E. coli as conformationally constrained insertions into the thioredoxin active-site loop, a location known to be a highly permissive site for the insertion of exogenous peptide sequences into native thioredoxin. To demonstrate that members of this library could be bound and selected via specific protein/protein interactions to a target protein, a method was devised to enable efficient isolation of those bacteria displaying peptides with affinity to immobilized antibodies. We have unambiguously mapped three different antibody epitopes using this method. Peptides selected as FLITRX active-site fusions retain their binding specificity when made as native thioredoxin active-site loop fusions. This will facilitate future structural characterizations and broaden the general utility of the system for exploring other classes of protein-protein interactions.

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IL-12 is a potent inducer of NK and cytolytic T cell activity, IFN-gamma production, and T cell proliferation, and is necessary for differentiation of naive T cells to the Th1 subset. We have previously shown that IL-12 promotes a primary Th1 response and suppresses a primary Th2 response in lymph nodes of mice primed with a model hapten-protein conjugate, 2,4,6-trinitrophenyl (TNP)-keyhole limpet hemocyanin (KLH). We have now extended these studies to determine the Th phenotype of the recall response following immunization with soluble Ag and IL-12. For these experiments, mice were primed with TNP-KLH with or without treatment with IL-12, allowed to progress beyond the primary immune response, and challenged by i.p. injection of TNP-KLH. The phenotype of the recall response was monitored by measuring ex vivo production of IFN-gamma and IL-4 in Ag-stimulated lymph node and spleen cell cultures. Titer and isotype of TNP-specific serum Abs were also evaluated. Mice primed with Ag+IL-12 developed a Th1 recall response, as detected by KLH-specific IFN-gamma production from cultured spleen cells and the presence of TNP-specific IgG2a Ab in serum. However, they also developed an Ag-specific Th2 recall response, as characterized by Ag-induced IL-4 production from spleen cells and the presence of high titers of anti-TNP IgG1 in the serum. Studies of the cytokine profile during the primary response revealed that IL-12 induced in spleen cells the capacity to express both IL-4 and IFN-gamma. CD4+ T cells are necessary for production of IL-4 in the spleens of IL-12-treated mice, and most likely account for the Th2 recall response detected in mice primed with Ag+IL-12. These results indicate that the Th1 phenotype induced by immunization with IL-12 and Ag is maintained so that a Th1 recall response is expressed upon subsequent challenge with Ag. However, immunization with IL-12 also supports the development of a Th2 recall response, indicating that the Th1-inducing effect of IL-12 in vivo is not accompanied by a long lasting suppression of Th2 development.