J. Vet. Med. A 46, 75–85 (1999) © 1999 Blackwell Wissenschafts-Verlag, Berlin ISSN 0931–184X Institute of Animal Neurology, Faculty of Veterinary Medicine, University of Bern, La ¨nggass-strasse 122, CH-3012 Bern, Switzerland Lymphocyte Subset Distribution in Steroid Responsive Meningitis-Arteriitis in Comparison to Different Canine Encephalitides A. TIPOLD 1,2,4 , P. MOORE 3 , A. ZURBRIGGEN 1 and M. VANDEVELDE 1 Institute of Animal Neurology 1 and Veterinary Virology 2 , University of Bern, Switzerland; 3 VM Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, CA, USA; 4 Corresponding author: Andrea Tipold, Institute of Animal Neurology and Veterinary Virology, University of Bern, La ¨nggass-strasse 122, CH-3012 Bern, Switzerland With 5 figures and 1 table Received for publication October 15, 1998 Summary Steroid responsive meningitis-arteriitis (SRMA) is a well-known disease in dogs, but the aetiology and pathogenesis are not yet understood. In the peripheral blood an overrepresentation of B cells was found. In the present study we therefore evaluated the distribution of lymphocyte subsets in SRMA in paraffin-embedded tissue sections directly at the lesion sites and compared the results to different canine encephalitides. An intriguing finding was that the B cell/T cell distribution varied depending on the aetiology of the disease: in viral encephalitides, T cells were the predominant cell population in perivascular cuffs, whereas in protozoal and bacterial diseases B cells prevailed. In SRMA an overrepresentation of B cells occurred in meningeal lesions, as already found in the peripheral blood. The distribution of lymphocyte subsets was similar to bacterial and protozoal diseases and was not a unique phenomenon for this specific inflammatory lesion in the canine central nervous system (CNS). Multiple mechanisms seem to be responsible for recruitment and activation of different leukocyte subsets after alteration of the CNS tissue by an environmental factor. A specific finding in SRMA was that the distribution of T and B cells depended also on the lesion site. In contrast to meningeal lesions, in inflamed arteries T cells were the only lymphocyte population found. In these vessels, diffuse infiltration with immunoglobulins was revealed. Inactivated or resting lymphocytes and large granular lymphocytes occurred in each of the diseases examined. These similarities between SRMA and infectious CNS diseases of the dog support earlier suggestions that the disease is somehow triggered by a hitherto unknown environmental factor which leads to the dysregulation of the immune system. Introduction Steroid responsive meningitis-arteriitis (SRMA) in the dog is a very painful disease, which occurs worldwide in companion dogs and has become a well-known disease in small animal practices (MERIC et al., 1985, 1986; PRESTHUS, 1991; TIPOLD and JAGGY, 1994). In addition, SRMA is a recognized animal model for human diseases, since the lesions are reminiscent of a variety of immune-mediated vasculitides in humans (BURNS et al., 1991; FELSBURG et al., 1992). The disease is similar to a condition known in laboratory dog colonies as the ‘Beagle pain syndrome’ (HARCOURT, 1978; STEJSKAL et al., 1982; BROOKS, 1984; HAYES et al., 1989; SCOTT MONCRIEFF et al., 1992). The most important clinical signs of SRMA include fever, cervical rigidity and pain in the acute form of the disease. Additional neurological deficits consistent with a spinal cord or multifocal lesion as an expression of secondary damage to the U.S. Copyright Clearance Center Code Statement: 0931-184X/99/4602-0075 $14.00/0