In work performed by a number of laboratories, it has become quite clear that the oral administration of autoantigens exerts a profoundly suppressive effect on the development and long-term clinical course of autoimmune disease. Specific peptide sequences derived from the autoantigens are similarly suppressive. An interesting sidelight to emerge from specificity studies is that oral administration of a self-protein or peptide sequence (i.e., rat MBP peptide administered to a rat) is markedly less tolerogenic than oral administration of a non-self or even closely related sequence (guinea pig MBP peptide administered to a rat). The dose of oral antigen is now known to play a critical role in determination of the mechanism of oral tolerance, with low doses of antigen causing active suppression with concomitant release of TGFbeta1. Studies outlined here suggest that oral administration of higher antigen doses (e.g., 20 mg MBP to rats or mice) results in deletion of specific antigen-reactive T lymphocytes. This conclusion stems from the fact that injections of IL-2 could not reverse high-dose tolerance while reversing low-dose oral tolerance. Moreover, feeding MBP to MBP-TCR transgenic mice caused trafficking of transgenic cells to the intestine followed by a profound depletion of transgene-positive cells and reduction in proliferative function in all peripheral lymphoid organs. Oral tolerance has proven to be of therapeutic benefit in other animal models of autoimmune disease as well, including uveitis, collagen-induced arthritis, adjuvant arthritis, thyroiditis, myasthenia gravis, and diabetes. Initial human trials in multiple sclerosis, rheumatoid arthritis, and uveitis show promising results.

We characterized antigen-specific CD4 ؉ T cells in six patients with treatment-resistant Lyme arthritis, using an HLA-DRB1*0401 major histocompatibility complex (MHC) class II tetramer covalently loaded with OspA164-175, an immunodominant epitope of Borrelia burgdorferi. Direct analysis of OspA-tetramer binding CD4 ؉ cells in patients expressing the HLA-DRB1*0401 allele revealed frequencies of between <0.005 and 0.1% in peripheral blood (n ‫؍‬ 6), and between <0.005 and 3.1% in synovial fluid (n ‫؍‬ 3). OspAtetramer ؉ CD4 ؉ cells were directly cloned at 1 cell per well and expanded by mitogen and IL-2 on allogeneic feeder cells. As measured by [ 3 H]thymidine incorporation, 95% of 168 T cell clones from synovial fluid binding the OspA-tetramer were antigenreactive. Clones generated from peripheral blood revealed a different pattern of responsiveness when compared with clones generated from synovial fluid, as measured by proliferation, IFN-␥, and IL-13 secretion. These clones, selected on the basis of their peptide binding, also responded to whole protein, but with a different cytokine profile. Our studies demonstrate that MHC class II tetramers can be used in humans to directly identify, isolate, and characterize antigen-reactive T cells from an inflammatory compartment.