Studies of the role of growth factor secretion by lung macrophages in a rabbit model of pulmonary fibrosis

Deposition of collagen in the alveolar structures of the lung is a central feature of all forms of pulmonary fibrosis. This is preceded by or associated with an influx of inflammatory cells, and in turn there is an increase in fibroblast numbers. This thesis examines the role of growth factor secretion by lung macrophages in the control of fibroblast replication, A rabbit model of pulmonary fibrosis, induced by intratracheal bleomycin, was used. Inflammatory cells were lavaged from the lungs of saline and bleomycin treated animals, cultured for 24 hours and the level of growth factor secretion measured on two fibroblast cell lines. This showed that the administration of bleomycin lead to a rapid and marked influx of inflammatory cells, of which macrophages remained the predominant cell. These cells secreted growth factors, although the secretion rate per cell was not higher in the bleomycin treated animals. Nonetheless the increased numbers of inflammatory cells in the alveoli resulted in an increased alveolar burden of growth factor which may account for the increase in lung collagen seen in this animal model. As part of this work three subsidiary problems were addressed. The effect of culture of alveolar macrophages on their state of activation was quantitated by the measurement of protein synthesis rates, in vivo and in vitro. The rate was fivefold higher in vitro. A colorimetric method for estimating fibroblast numbers in 96 well plate cultures was developed and validated. Using this method the ability of alveolar macrophages from normal rabbits to secrete growth factor was confirmed and some of the physicochemical properties of the growth factors were determined