A relapsing form of allergic encephalomyelitis in rats due to enhanced absorption of inoculum.

Lewis rats were prepared by injection of a mild chemical irritant to produce a sterile peritonitis. This procedure has been shown recently to enhance the lymphatic absorption of oily and particulate materials inoculated one week later. Rats pretreated in this manner responded to intraperitoneal injection of spinal cord tissue suspended in mineral oil with the expected clinical signs of experimental allergic encephalomyelitis (EAE). The clinical signs remitted but were soon followed by a recurrence of equal or greater severity in most of the rats. Most rats survived the relapse and had a second remission although persistent demyelination was often found histologically. Rats inoculated without a prior peritonitis had almost no relapses. Emulsifying agents and mycobacteria reduced or eliminated the occurrence of relapses. The occurrence of relapses correlated with enhanced absorption of inoculum into the draining lymph nodes. The weight and histological appearance of the thymuses supported the hypothesis that the occurrence of relapses and remissions in this model was due to endocrine regulation of the immune response with increased endogenous immunosuppressive corticosteroids playing a crucial role.