In vitro response of lymphocytes to Micropolyspora faeni extract in cattle.

The in vitro stimulation of small lymphocytes to blast formation, measured by the rate of 3H-thymidine incorporation, was used to study the occurrence of cells sensitive to antigens of Micropolyspora faeni in cattle. M faeni extract induced a significant stimulation index in lymphocytes from the peripheral blood cells of cattle from an endemic area in autumn but rarely in spring. Blood lymphocytes from animals from a non-endemic area tested during the winter period rarely showed a positive reaction or only a relatively weak one. On the other hand, lymph node cells, particularly from bronchial lymph nodes, showed positive results in all investigated animals and even in those from non-endemic areas. In three-months-old calves, positive results were obtained mainly with cells from bronchial lymph nodes. It seems therfore that sensitisation to M faeni antigen is a widespread phenomenon but additional circumstances seem to be required for the clinical manifestation of farmer's lung disease in cattle. The most important factor is probably strong and repeated exposure to the M faeni organism. Whether or not existing reactive lymphocytes against M faeni antigen are directly involved in the pathogenesis of farmer's lung disease in cattle by producing a delayed type reaction remains to be clarified.