Loss rather than downregulation of CD4 T cells as a mechanism for remission from experimental allergic encephalomyelitis

SJL/J mice recover from clinical signs of experimental allergic encephalomyelitis (EAE) 2 to 3 days following the onset of the initial attack. The immunoregulatory events that induce clinical recovery are not well understood. In this paper we have compared the activation state of the T cells infiltrating the central nervous system (CNS) in symptomatic and remitted mice. We isolated mononuclear cells from the CNS at various time points during the course of EAE and used flow cytometry to describe the kinetics of CNS infiltration by CD45⁺, CD2⁺, CD3⁺, TCRαβ⁺, CD4⁺ cells. There was a 30-fold reduction in the number of CNS CD4⁺ T cells in remitted mice 10 days following the initial attack. More than 60% of CNS CD4⁺ cells were of a CD44^high, CD45RB^low memory/effector phenotype both in active EAE, peak EAE and in remission, contrast to lymph nodes where this phenotype never constituted more than 17%. The proportion of CD8⁺ T cells was not increased in remitted mice, and we detected no TCRγδ⁺ cells within the CNS. Our findings demonstrate an overt loss of CD4⁺ T cells from the CNS and the maintenance of an activated state by T cells within the CNS and during remission from EAE. This argues against downregulation of T cell function as a mechanism for remission.