Histological and immunohistochemical characterization of mucosa‐associated lymphoid tissue and antigen‐presenting cells in trachea and lung of cattle

Abstract

The presence of bronchus‐associated lymphoid tissue (BALT) and its structural components has been described in different healthy animal species and in animals with diseases of the respiratory tract. In contrast to normal mammals, BALT is absent in healthy human adult lungs, but has been found in the lungs of children. The histological characteristics of organized mucosa‐associated lymphoid tissue (MALT), its subsets of immune cells and their in situ distribution in the lung of healthy subadult and adult cattle shows close similarities with BALT in humans and other animal species such as sheep, horses and pigs. This study clearly demonstrates that organized MALT also occurs in the tracheal mucosa of cattle. The absence of tracheal MALT and BALT in calves suggest that these structures are not constitutive. In the mucosa of bovine trachea, bronchi and bronchioli, MHC II+ and CD11c+ dendritic cells (DCs) are located in the epithelium and in the lamina propria mucosae. These DCs are already present in calves soon after birth. Examination of tangential epithelial sheets shows that in the bovine tracheal epithelium, like in man and rat, a dense network of MHC II+ and CD11c+ DCs exists and that their number is considerably higher than in conventional transverse sections. In the bovine tracheal and bronchial epithelium, MHC II+ DCs are extending their dendrites towards the lumen indicating that these DCs possibly are involved in sampling of luminal antigens. The presence of significantly higher numbers of MHC II+ DCs in the tracheal and bronchial/bronchiolar mucosa of older cattle in than in calves possibly results from local stimulation with exogenous antigens during postnatal life. Detection of DCs expressing the costimulatory molecules CD80 and CD86 in calves and cattle suggests maturation of DCs, which is most likely induced by stimulation with exogenous antigens.