Breaking of B cell tolerance toward a highly conserved self protein.

Self proteins are processed and presented by APCs in the same way as foreign proteins. Presentation of fragments derived from self proteins does not, however, lead to Th cell stimulation because of T cell tolerance. In this study, a novel approach was used to investigate whether B cell tolerance toward a self Ag could be due to the absence of this Th cell recognition. The highly conserved nonimmunogenic protein ubiquitin was used as a model protein. Two modified ubiquitin molecules were constructed with ubiquitin segments exchanged either with the T cell epitope, OVA(325-336), which binds to the mouse A(d) MHC class II molecule, or with the T cell epitope, hen egg lysozyme(50-61), which binds to the A(k) molecule. Mice were immunized with the resulting proteins. Both modified proteins elicited strong autoantibody responses toward soluble native ubiquitin, demonstrating that insertion of a single foreign T cell epitope can overcome the B cell nonresponsiveness. The T cell regulatory role of one of the inserted foreign T cell epitopes in ubiquitin was studied, and at least two different Th cell specificities were found to operate in the response. The T cells were directed against: 1) the inserted epitope, and 2) a combination of the inserted epitope and parts of the neighboring ubiquitin regions. Therefore, the absence of T cell help seems to be an important reason for B cell tolerance toward self proteins.