The present invention relates to methods for expanding a population of alveolar macrophages in a long term culture. In particular, an object of the present invention relates to a method of expanding a population of alveolar macrophages in a long term culture comprising culturing the population of alveolar macrophages in a culture medium supplemented with an amount of GM-CSF. The population of alveolar macrophages as prepared by the method of the present invention is particularly suitable for the treatment of pulmonary diseases such as Pulmonary Alveolar Proteinosis (PAP) but also for treatment of various diseases such as autoimmune diseases, or transplantation.