Immunofluorescent evaluation of the lower respiratory tract of healthy horses and of horses with chronic bronchiolitis.

Pulmonary parenchymal tissues from 6 healthy horses and from 9 horses with chronic bronchiolitis were evaluated by use of an indirect immunofluorescent technique. In horses of both groups, the diffuse interstitial immunofluorescence was most intense for immunoglobulin (Ig)G, moderate for IgA, and minimal for IgM. Intensity of fluorescence was increased markedly in diseased lungs exposed to anti-IgA and anti-IgG. Around small bronchi and large bronchioles of healthy horses, IgA-containing cells generally were more numerous than were IgG-Fc fragment (Fc)-containing cells; in small bronchioles, however, IgA-containing cells and IgG (Fc)-containing cells were identified with approximately equal frequency. Immunoglobulin-containing cell types usually were located in the lamina propria and submucosa of airways, with occasional Ig-containing cells seen interepithelially and intraluminally; IgG (T)- and IgM-containing plasma cells were observed infrequently in the same regions. The number of Ig-containing cells became fewer as the diameter of the airway decreased; therefore, at the level of the terminal bronchiole, Ig-containing cells only were observed occasionally. This pattern of Ig-containing cell distribution was similar in horses with chronic bronchiolitis; however, horses with chronic bronchiolitis had more IgA-containing cells forming perivascular and peribronchiolar cuffs than did healthy horses. The number of IgG (Fc)-containing lymphocytes and non-Ig-stainable lymphocytes also were higher in horses with chronic bronchiolitis than in healthy horses, but the number did not approach the magnitude of that observed for IgA-containing cells. One horse had deposits of complement C3 and IgG (Fc) within alveolar septa.