Partial protection and intrathecal invasion of CD8(+) T cells in acute canine distemper virus infection

Initial non-inflammatory demyelination in canine distemper virus infection (CDV) develops against a background of severe immunosuppression and is ther efore, thought to be virus-induced. However, recently we found a marked inv asion of T cells throughout the central nervous system (CNS) in dogs with a cute distemper despite drastic damage to the immune system. In the present study, this apparent paradox was further investigated by immunophenotyping of lymphocytes, following experimental CDV challenge in vaccinated and non- vaccinated dogs. In contrast to CDV infected, unprotected dogs, vaccinated dogs did not become immunosuppressed and exhibited a strong antiviral immun e response following challenge with virulent CDV. In unprotected dogs rapid and drastic lymphopenia was initially due to depletion of T cells. In peri pheral blood, CD4(+) T cells were more sensitive and depleted earlier and f or a longer time than CD8(+) cells which recovered soon. In the cerebrospin al fluid (CSF) we could observe an increase in the T cell to B cell and CD8 (+) to CD4(+) ratios. Thus, partial protection of the CD8(+) cell populatio n could explain why part of the immune function in acute distemper is prese rved. As found earlier, T cells invaded the CNS parenchyma in these dogs bu t also in the protected challenged dogs, which did not develop any CNS dise ase at all. Since markers of T cell activation were upregulated in both gro ups of animals, this phenomenon could in part be related to non-specific pe netration of activated T cells through the blood brain barrier. However, in diseased animals much larger numbers of T cells were found in the CNS than in the protected dogs, suggesting that massive invasion of T cells in the brain requires CDV expression in the CNS. (C) 2001 Elsevier Science B.V. Al l rights reserved.