Studies On The Bronchial Tree And The Characteristics Of Mucosal Immune Barrier In Adult Bactrian Camels

Bronchial tree is an important passageway for gas exchange with the outside environment and maintains basic lung morphology.Meanwhile,bronchus-associated lymphoid tissue(BALT)distributed in the bronchial mucosa,can be induced to mainly produce IgA~+and IgG~+cells,when the immunoglobulins secreted by these cells and transported by pIgR and FcRn into airway lumen,SIgA and IgG formed.SIgA and IgG are important mucosal immune effector molecules,when two of them adhere to the mucin layer principally secreted by epithelial goblet cells,they will act as the first and second mucosal immune barrier.To explore the structural characteristics of the bronchial tree and mucosal immune barrier in Bactrian camels,twelve healthy adult Bactrian camels were divided into two groups(6 camels in each group,three males and three females),and through anatomical,histological,immunohistochemical,micro-image analysis and statistical methods,the study was divided into following four aspects:First,the structural characteristics of lung and bronchial tree of Bactrian camels were observed systematically,meanwhile,the bronchial luminal areas and the goblet cells density were analysed by statistical methods.The results showed that,Bactrian camel lungs were constituted by the left cranial lobe,left caudal lobe,right cranial lobe,right caudal lobe and accessory lobe,but lacked the middle lobe.The cranial lobe was narrow and small,the caudal lobe was extremely developed(almost 4 times the cranial lobe in size),and the accessory lobe was smaller than the cranial lobe.The bronchial tree,an unequal dichotomy with a tracheobronchial branch,was composed of dorsal,ventral,lateral and medial bronchiole systems.The trachea was covered by dense pseudostratified ciliated columnar epithelium embedded with many goblet cells,however,with the bronchial branches continuous extend,the relative bronchial luminal areas significantly decreased(p<0.05),and the goblet cells density also significantly declined(p<0.05).Second,the structural and distributive characteristics of BALT in each bronchial branch and lung respiratory portion of Bactrian camels were carefully observed.The results showed that isolated lymphoid follicles(the chief type)and aggregates of lymphoid follicles revealed two types of BALT,and germinal centres,follicle-associated epithelium and high endothelial venules could be observed in some well-developed BALT.Additionally,BALT was scattered along the bronchial tree in the entire lung,and the density increased from the trachea to the lower graded branches(densest in the bronchioles)and then decreased,with the occasional location around respiratory bronchioles or among the pulmonary mesenchyme.In the conducting portion,BALT was primarily located in the mucosa lamina propria but was also found in the submucosa,under the muscular layer,and around the submucosal glands and cartilage.Third,the distributive characteristics of IgA~+and IgG~+cells in bronchial LP of Bactrian camels were systemically observed.The results showed that IgA~+and IgG~+cells were mainly scattered in each bronchial LP.More important,with the bronchial branches continuous extend,IgA~+and IgG~+cells density of associated bronchial LP significantly decreased(p<0.05).Finally,pIgR and FcRn expression characteristics in each bronchial branch and lung respiratory portion of Bactrian camels were systemically observed.The results showed that pIgR was mainly expressed in the bronchial ciliated epithelial cells,and was also expressed in the serous cells of bronchial glands,Clara cells and alveolar type 2(AT2)cells in Bactrian camels.But pIgR was not expressed in the epithelial basal cells,goblet cells,alveolar type 1(AT1)cells and mucous cells of bronchial glands.FcRn was mainly expressed in the bronchial ciliated epithelial cells and basal cells,and was also expressed in AT1 cells,the serous cells of bronchial glands,Clara cells,vascular endothelial cells,smooth muscle cells and dust cells in Bactrian camels.But FcRn was not expressed in goblet cells,AT2 cells and mucous cells of bronchial glands.However,when the bronchial branches continually extended,pIgR and FcRn expression level in the relative bronchial ciliated cells all significantly declined(p<0.05),and in the different branches at the same grade of the same lobar bronchi,the areas of the larger lumens were significantly higher than that of the smaller lumens(p<0.05).This study firstly demonstrated that the structural characteristics of bronchial tree and mucosal immune barrier in Bactrian camels as follows:(1)The lung morphology of Bactrian camels was similar to that of horses,but the bronchial branches were more closely related to those of ruminants.(2)BALT was mainly scattered in the conducting portion,and bronchioles,as the final“checkpoint”in the surveillance,capture and recognition of antigens before pulmonary exchange,were the pivotal locational position of BALT.(3)Bronchial LP was the major distribution position of IgA~+and IgG~+cells,this was identical with the distributive characteristics of IgA~+and IgG~+cells in mucosal immune system.(4)Bronchial ciliated cells were the major expression sites of pIgR and FcRn,and pIgR and FcRn were also expressed in the serous cells of bronchial glands,Clara cells,alveolar type 2 cells(only pIgR)and alveolar type 1 cells(only FcRn).These characteristics facilitated secretory immunoglobulin A(SIgA)and IgG transmembrane transport through the bronchial ciliated cells,and formed an intact protective barrier on the entire bronchial mucosa.(5)when the bronchial branches continually extended,IgA~+and IgG~+cells density in bronchial LP,pIgR and FcRn expression level in the bronchial ciliated cells,bronchial epithelial goblet cells density were all positively correlated with the bronchial luminal areas.Our study provided morphological evidence for further insight into mucosal immune barrier of the lower respiratory tract in Bactrian camels.