Depletion of T Lymphocytes with Immunotoxin Retards the Progress of Experimental Allergic Encephalomyelitis in Rhesus Monkeys

FN18-CRM9 is an anti-rhesus anti-CD3 immunotoxin that can transiently deplete T cells to 1% of initial values in both the blood and lymph node compartments and can induce long-term tolerance to mismatched renal allografts. We have investigated the ability of this immunotoxin to interdict the course of an experimental rhesus T-cell-driven autoimmune disease, experimental allergic encephalomyelitis (EAE) induced by myelin basic protein. Monkeys showing CSF pleocytosis were then treated with FN18–CRM9 alone or in combination with cranial irradiation (325 or 650 cGy). EAE in nontreated control monkeys progressed rapidly. Paralysis occurred 4–6 days after CSF pleocytosis. Paralysis was either delayed or never occurred in treated monkeys, and histopathology revealed few inflammatory plaques that were notable for their low or absent T cell content. While T cells repopulate in the periphery posttreatment, they do not return to the CNS in large numbers, suggesting that the newly repopulated T cells have lost their previously acquired CNS homing capability. Anti-CD3 immunotoxin may be useful in treating clinical T-cell-driven autoimmune diseases such as rheumatoid arthritis and multiple sclerosis.