Suppression of acute and chronic experimental allergic encephalomyelitis in Strain 13 guinea pigs. A clinical and pathological study.

Adult inbred Strain 13 guinea pigs develop an acute, fatal form of experimental allergic encephalomyelitis (EAE) about 2 weeks after a single injection of isologous spinal cord in complete Freund's adjuvant (CFA), but similarly injected juveniles develop a delayed, rarely fatal chronic form. Thirty-seven sensitised adult Strain 13 animals were separated into 2 groups. One group was permitted to develop acute EAE. The other group was injected intramuscularly with 1 mg of guinea pig or bovine myelin basic protein (MBP) in incomplete Freund's adjuvant (IFA) on day 2, 7 or 10 post-inoculation (PI) followed by 0.2 mg in IFA every third day for a total of 10 doses. Animals in the unsuppressed group succumbed to acute EAE 13-16 days post-sensitisation. No animal in the suppressed group died during this period. Animals treated with MBP beginning 2 days PI showed no clinical signs, but mild clinical manifestations occurred in animals suppressed from days 7 and 10 PI. These signs remitted by 21 days post-sensitisation. One suppressed animal (out of 21) died during the fourth week postsensitisation. The other 20 suppressed animals appeared clinically normal towards the end of the course of MBP injections and remained so for the 6 months of study. Morphological examination revealed that CNS lesions occurred in all animals. In animals suppressed with MBP beginning on day 2 PI, lesions consisted only of a few meningeal inflammatory cells. Animals given MBP beginning on day 7 or 10 PI and sampled 1-2 weeks later, had lesions which could not be distinguished from those occurring in the non-suppressed acute EAE group. In time, the suppressed animals developed lesions which were typical of chronic EAE with remyelination as a predominant feature. Preliminary experiments on the suppression of chronic EAE in 5 juvenile Strain 13 guinea pigs have revealed that 3 MBP-injected animals failed to develop clinical disease over a 28-week period of study although lesions typical of chronic EAE were present. Simultaneously, 2 non-suppressed juvenile animals developed clinical signs by 12 weeks. These were associated with both acute inflammation and demyelination superimposed upon regions of chronic demyelinative activity.