Epithelial lymphocyte and macrophage distribution in the adult and fetal equine lung

Leucocytes in the lung epithelium play an important role in the ability of an animal to respond appropriately to inhaled pathogens. The distribution o f lymphoid and myeloid cells associated with the lung epithelium was examin ed immunohistochemically throughout the respiratory tract of four horses, c omprising two adults from an abattoir, one pregnant mare, and her fetus (in the final stage of gestation). Cross and tangential cryosections were labe lled with monoclonal antibodies against T-cell, B-cell, macrophage/dendriti c myeloid cell, and major histocompatibility Class (MHC) II surface antigen s. Cell numbers were determined by microscopy. In the three adult horses, e pithelial CD3(+) T-cell numbers decreased progressively from the upper to t he lower respiratory tract, but in the fetus there were low numbers of T ce lls (at most, 10% of those seen in the adult airways) and little variation in different parts of the respiratory tract. MHC Class II was expressed on the airway epithelium of the two abattoir horses, but not that of the mare and her fetus. In these two animals occasional large, mostly irregularly-sh aped, Class II-positive cells were seen. Very few epithelium-associated cel ls in any animal were labelled by anti-CD21 antibody, which identifies B ce lls, or anti-myeloid cell antibodies; an anti-rat macrophage antibody (ED2) was shown, for the first time, to identify mature equine alveolar macropha ges. Despite the small number of animals, the results suggest that in norma l adult horses the greatest numbers of epithelial T cells are found where t here is greatest contact with airborne antigens, and that there is constitu tive epithelial MHC Class II expression. The low level of MHC Class II expr ession in the fetus, together with the reduced numbers of T cells, was cons istent with the suggestion that the fetal immune system requires exposure t o airborne stimuli for full development. The low level of MHC Class II expr ession in the mare may have reflected the immunosuppression that accompanie s pregnancy. (C) 1999 W.B. Saunders Company Limited.