Rapid entry and downregulation of T Cells in the central nervous system During the reinduction of experimental autoimmune encephalomyelitis

We investigated the mechanisms whereby a previous attack of experimental autoimmune encephalomyelitis (EAE) modifies a subsequent attack in the Lewis rat. Active immunization with myelin basic protein (MBP) and complete Freund's adjuvant 28 days after the passive transfer of MBP-sensitized spleen cells induced a second episode of EAE, which occurred earlier than in naive control animals, but was less severe overall. The pattern of neurological signs was also different in rechallenged rats, which had less severe tail and hindlimb weakness but more severe forelimb weakness. In rechallenged rats, inflammation was more severe in the cervical spinal cord, cerebellum, brainstem and cerebrum, but less severe in the lumbar spinal cord, than in controls. The early onset of EAE in rechallenged rats was explained by a memory T cell response to MBP72-89 in the draining lymph node and spleen, and by the enhanced entry of T cells into the central nervous system (CNS). However, the number of alpha beta T cells in the spinal cord of rechallenged rats declined faster than in controls, especially in the lumbosacral cord, where the number of V beta 8.2+ T cells and the frequency of T cells reactive to MBP72-89 rapidly decreased, indicating rapid downregulation of the immune response in the previously inflamed spinal cord. Apoptosis of inflammatory cells in the CNS was increased in the rechallenged rats and is likely to contribute to this downregulation. Furthermore, during the disease course the generation of encephalitogenic T cells in the peripheral lymphoid organs was limited compared with controls. Thus, a previous attack of EAE modifies a subsequent attack through the interaction of the following processes: a memory T cell response to MBP; facilitated T cell entry into the CNS; downregulation of the immune response in the CNS, including increased apoptosis of inflammatory cells; and a limited generation of encephalitogenic T cells in the peripheral lymphoid organs