Specific killing of lymphocytes that cause experimental autoimmune myasthenia gravis by ricin toxin-acetylcholine receptor conjugates.

Experimental autoimmune myasthenia gravis (EAMG) is an autoimmune disease in which antibodies to acetylcholine receptor (AChR) cause loss of AChR from muscle, thereby impairing neuromuscular transmission. Here we report the use of a hybrid molecule that contains ricin toxin, irreversibly coupled to AChR to specifically suppress the immune response to AChR in vitro. Lymph node cell cultures from rats with EAMG pretreated with ricin toxin-AChR conjugates exhibited suppressed T helper cell proliferation and B cell antibody synthesis in response to the subsequent addition of AChR. Nonspecific toxicity of the conjugates was measured by suppression of the T cell proliferative response to the mitogen concanavalin A and the antigen keyhole limpet hemocyanin (KLH), and B cell antibody production to KLH. We have evaluated different pretreatment conditions and ricin toxin covalently coupled to AChR in different molar ratios to optimize specific immunosuppression. By varying the number of ricin molecules covalently bound to AChR in the immunotoxin, we were able to minimize the nonspecific toxicity while still maintaining specific killing of AChR-reactive lymphocytes. Furthermore, B cells were more susceptible to specific killing than were the T cells. The specific immunosuppression was potentiated by performing the pretreatment with immunotoxin in the presence of chloroquine. Chloroquine raises lysosomal pH and probably delays the degradation of immunotoxin in the cell. It should be noted that ricin toxin was covalently coupled to AChR by using a novel, non-reducible reaction. These in vitro results suggest that it may be feasible to use immunotoxin molecules to specifically suppress this autoimmune response in vivo.