Mast cell density and substance P-like immunoreactivity during the initiation and progression of lung lesions in ovine Mannheimia (Pasteurella) haemolytica pneumonia

To determine the density of mast cells (MCs) and the extent of substance P (SP) immunoreactivity during initiation and progression of pneumonic pasteu rellosis (PP), 18 lambs were inoculated intrabronchially with Mannheimia (P asteurella) haemolytica or saline, and lung tissue was collected at 1, 15 a nd 45 days post-inoculation (n=3, each group). Additionally, the left (non- inoculated) contralateral lungs in bacteria-inoculated animals were collect ed as controls. At 1 day after bacterial inoculation the lungs had typical M. haemolytica lesions. These pneumonic lesions had fewer numbers of MCs an d reduced histamine content. Macrophages infiltrating some of the inflamed areas were strongly immunoreactive for SP. At 15 days, MCs remained scarce at sites where lung damage persisted, i.e. pyogranulomatous foci, but were increased in number in areas of interstitial damage. Pulmonary ganglion neu rons were strongly immunoreactive for SP. By 45 days the fibrosing changes became more defined as pleural fibrosis, fibrosing alveolitis, alveolar epi thelial hyperplasia and bronchiolitis obliterans. These lungs had increased numbers of MCs, but histamine content was not different from saline- and n on-inoculated left lungs. Substance P immunoreactivity occurred only in ner ves and was scarce and mild. This work demonstrates that MC density decreas es initially with PP, but increases with progression of PP. SP fibres tend to be decreased during the initiation and at 45 days of PP, but other cells , such as macrophages and neuronal ganglion cells, produce substance P duri ng progression of PP and thereby constitute an additional source of substan ce P. (C) 2001 Academic Press.