Lymphocyte subset distribution in steroid responsive meningitis-arteriitisin comparison to different canine encephalitides

Steroid responsive meningitis-arteriitis (SRMA) is a well-known disease in dogs, but the aetiology and pathogenesis are not yet understood. In the per ipheral blood an overrepresentation of B cells was found. In the present st udy we therefore evaluated the distribution of lymphocyte subsets in SRMA i n paraffin-embedded tissue sections directly at the lesion sites and compar ed the results to different canine encephalitides. An intriguing finding wa s that the B cell/T cell distribution varied depending on the aetiology of the disease: in viral encephalitides, T cells were the predominant cell pop ulation in perivascular cuffs, whereas in protozoal and bacterial diseases B cells prevailed. In SRMA an overrepresentation of B cells occurred in men ingeal lesions, as already found in the peripheral brood. The distribution of lymphocyte subsets was similar to bacterial and protozoal diseases and w as not a unique phenomenon for this specific inflammatory lesion in the can ine central nervous system (CNS). Multiple mechanisms seem to be responsibl e for recruitment: and activation of different leukocyte subsets after alte ration 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 infiltratio n with immunoglobulins was revealed. Inactivated or resting lymphocytes and large granular lymphocytes occurred in each of the diseases examined. Thes e similarities between SRMA and infectious CNS diseases of the dog support earlier suggestions that the disease is somehow triggered by a hitherto unk nown environmental factor which leads to the dysregulation of the immune sy stem.