B-cell activation in vitro by helper T cells specific to region alpha 146-162 of Torpedo californica nicotinic acetylcholine receptor.

We have previously mapped the T and B cell epitopes on the alpha-subunit of acetylcholine receptor (AChR) in human myasthenia gravis (MG) and in experimental autoimmune MG-susceptible (C57BL/6 (B6)) and nonsusceptible mouse strains. In addition to regions recognized by both T and B cells, the AChR alpha-subunit has regions that are recognized solely by T cells. An exclusive T cell epitope within residues alpha 146-162 of Torpedo californica (t), tAChR, plays an important role in experimental autoimmune MG pathogenesis in B6 mice. To study its function, we established, from tAChR-primed B6 mice, two t alpha 146-162-specific T cell lines (P14Th) which comprised Th2-type cells because they secreted IL-4 but not IL-2. P14Th did not recognize the corresponding region on mouse (m) AChR (m alpha 146-162). They caused in vitro differentiation of tAChR-primed B cells into plasma cells that secreted anti-AChR Abs directed, in decreasing order, against the following tAChR alpha regions: t alpha 122-138 > t alpha 134-150 > t alpha 45-60 > t alpha 170-186 > t alpha 56-71. Little or no Ab response could be detected against peptides t alpha 182-198 or t alpha 146-162 itself. The major enhancement was in the Abs against region t alpha 122-150 (spanning the t alpha 122-138/t alpha 134-150 overlap) that is involved in ACh binding. These Abs cross-reacted completely with m alpha 122-150, the corresponding region on mAChR. Therefore, t alpha 146-162-specific T cells, although unable to recognize m alpha 146-162, are nevertheless pathogenic because they help B cells responding to a tAChR region that is conserved in mAChR and involved in ACh binding. These Abs cross-react with the corresponding effector-binding region of mAChR, thereby disrupting the normal physiologic function of the mouse receptor.