Distinct Costimulation Dependent and Independent Autoreactive T-Cell Clones in Primary Biliary Cirrhosis

Background & Aims: Previous work has suggested that CD4⁺ CD28^- or costimulation-independent T cells are increased in autoimmune diseases. In this study, we compared frequency and qualitative characteristics of autoreactive costimulation-independent or CD4⁺ CD28^- T cells in primary biliary cirrhosis (PBC) by taking advantage of the well-defined immunodominant autoepitope of the E2 component of pyruvate dehydrogenase (PDC-E2). Methods: We determined the frequency of costimulation-independent autoreactive T cells that respond to PDC-E2 163-176 and the frequency of CD4⁺ CD28^- T cells. Finally, we determined the role of biliary epithelial cells (BEC) as both an antigen-presenting cell or, alternatively, as a target cell for T-cell-mediated cytotoxicity. Results: The precursor frequency of costimulation-independent CD4⁺ T cells that respond to PDC-E2 163-176 and the frequency of CD4⁺ CD28^- T cells were dramatically elevated in PBC. Furthermore, 2 types of T-cell clones that respond to PDC-E2 163-176 emerged from this study. One type was costimulation dependent and the other costimulation independent. Both types of clones lyse BEC in a similar effector target (E/T) ratio distribution. However, BEC did not help the proliferation of any T-cell clones. Furthermore, costimulation-independent T-cell clones do not become anergic by BEC. Conclusions: In PBC, costimulation-independent autoreactive T cells, which do not become anergic, increase and maintain the autoimmune response. In controls, although autoantigens are expressed on BEC and autoantigen-reactive T cells exist around BEC, autoantigen-reactive T cells are costimulation dependent and will become anergic and maintain peripheral tolerance.