Density and subtypes of mast cells in the morphologically intact lower respiratory tract of subadult and adult cattle and in lungs of calves with pneumonia after experimental infection with Mycoplasma bovis

In vertebrates, mast cells (MC) are widely distributed throughout the vascularised tissues, particularly near surfaces exposed to the environment, including skin, airways and gastrointestinal tract. MC are capable of recognising and reacting to a wide range of bacteria. It has been postulated that MC represent one of the most important means of defence in the lung in both innate and acquired immune responses. MC can be visualised in tissues applying metachromatic stains such as kresylecht-violet or toluidine blue staining or using enzyme-histochemical or immunohistochemical techniques for demonstration of tryptase and chymase. In contrast to the extended investigations of MC in humans and rodents, few studies exist in cattle in which the distribution of MC and their numbers and subtypes in the respiratory tract or other tissues have been described. Those studies revealed that in bovines, as in humans and other species, three different MC subtypes can be distinguished: MC that contain either tryptase (MCT) or chymase (MCC) alone or combined in one cell (MCTC). It has been postulated that MC play an important role as modulator of host defence in the lung. In calves infected with bovine respiratory syncytial virus (BRSV) evidence for the involvement of MC degranulation and MC mediators in the pathogenesis of BRSV-induced lung lesions was reported. Furthermore, studies in calves with experimental respiratory infection with Mannheimia haemolytica have shown that MC contribute to the acute inflammatory response. Mycoplasma (M.) bovis is the most pathogenic mycoplasma for bovines. It is an important cause of respiratory disease and arthritis in feedlot cattle, young dairy and veal calves. M. bovis is certainly capable of causing acute respiratory disease in the respiratory tract of cattle, although it is more accepted as a cause of chronic bronchopneumonia with caseous and sometimes coagulative necrosis characterised by a persistent infection. To date, investigations in which the subtypes and number of MC in the lung of cattle infected with M. bovis were characterised are not available. In the present study, a metachromatic staining method (kresylecht-violet, KEV) and a combined enzyme- and immunohistochemical double-labelling technique for tryptase-positive MC (MCT), chymase-positive MC (MCC), and both proteases-bearing MC (MCTC) were performed to determine the density and variety of KEV-stained MC and of different MC subtypes (MCT, MCC, MCTC) in formalin-fixed and paraffin-embedded sections of tracheae and lungs from cattle of three different age groups (calves, 6-7 months-old animals, adult animals) without morphological alterations of the respiratory tract and in lungs of calves after experimental infection with M. bovis. In this study, no statistically significant differences were found concerning the density of tracheal KEV-positive MC between the different age groups of animals without morphological alterations of the respiratory tract (group 1-3). Adult cattle (group 3) had significantly more KEV-positive MC in their cranial lung lobes than the younger animals of groups 1 and 2. Both in tracheae and lungs of cattle without morphological alterations (groups 1-3) a higher MC density was detected with the double-labelling technique than with the metachromatic staining method (KEV staining). In tracheae and lungs of animals of groups 1-3, mainly MCC and MCT were detected with the double-labelling technique, while only single MCTC were seen in few animals. In the trachea statistically significant (p > 0.05) more MCC were found in animals of group 2 than in those of group 1 and 3. The low MCT density in trachea and lungs of all animals without morphological alterations (group 1-3) contrasts with the reports of other investigators. In comparison with the results for lungs of animals without morphological alterations, a statistically significant lower number of KEV-stained MC was found in the lungs of M. bovis-infected calves of all three examined groups (groups 4-6). In the infected calves of all 3 groups significantly higher numbers especially of MCC were present than in lungsof animals without histological alterations. The calves of group 5 (cranial lung lobes) and calves of group 6 (both cranial and caudal lung lobes) had a significantly higher number of MCT. In the lungs of calves of group 4, however, a significantly lower number of MCT was found.