Milestones In PAP Research

History and Scientific Background

Acquired PAP has only been recognized in the literature since 1958 when an article published in The New England Journal of Medicine and authored by doctors Rosen, Castleman, and Liebow, commented on the abnormal yet distinctive histological appearance of lung tissue specimens marked by accumulation of lipoproteinaceous material. In 1965, Larson and Gordinier first suggested that this material was an abnormal accumulation of surfactant.

For many years, the cause of the accumulation of lipoproteinaceous material in the alveoli of the lungs was debated. Was it due to abnormalities in the production, break-down, or actual structure of surfactant? It wasn’t until1994 that the discovery of a pulmonary disorder in knock-out mice (mice in which a gene has been silenced or made non-functional) played a key role in understanding the true nature of the abnormalities. These mice were knock-outs for a gene that encoded for a growth factor called GM-CSF, thought to be responsible for stimulating the growth and differentiation of blood cells derived from bone marrow. The mice, deficient in GM-CSF, developed a condition that mirrored the alveolar lipoproteinosis in humans.

Genetically Altered Mice Deficient In GM-CSF Develop PAP

Researchers, seeking to determine if the GM-CSF deficiency was in fact the cause of alveolar proteinosis in the knock-out mice, tried replacing GM-CSF by multiple methods and consistently were able to reverse the lung disorder, indicating the key role that this growth factor plays in the development of the disease. With this new information, research soon turned to examining why lack of GM-CSF caused alveolar proteinosis in mice.

Researchers first investigated to see if GM-CSF deficiency was somehow causing an overproduction of surfactant in the lungs of the knock-out mice, but studies of gene expression in the mice indicated that there was no increased production of surfactant proteins or phospholipids as compared to normal mice. However, in other studies severe impairments in the clearance of proteins and phospholipids were observed. The scientists soon began to suspect that perhaps some sort of defect in the lungs’ alveolar macrophages was resulting in inadequate clearance of surfactant from the lungs of the GM-CSF knock-out mice. Supporting this idea was the observation of abnormal, enlarged alveolar macrophages with a foamy appearance that were obtained from the lungs of the knock-out mice.

Follow up studies examining alveolar macrophages demonstrated that in the absence of GM-CSF, the alveolar macrophages are capable of engulfing (ingesting) surfactant, but are unable to properly break it down, possibly explaining the enlarged, foamy appearance of the alveolar macrophages and the accumulation of lipoproteinaceous material in the lungs. This also supported the previously hypothesized idea that inadequate clearance of surfactant from the lungs of GM-CSF knock-out mice was due to impairment of alveolar macrophage function. Subsequent studies showed that GM-CSF stimulated a key growth factor for macrophages and indicated that lack of GM-CSF leads to defective maturation of the alveolar macrophages and, consequently, malfunction.

The Role of GM-CSF In Primary PAP In Humans

One key difference that makes the alveolar lipoproteinosis in mice different from in the acquired human disease is that in the human version, there is no deficiency in GM-CSF production. What then accounts for the disorder in humans?

In September of 1999, a ground-breaking paper by Kitamura et al. was published in The Journal of Experimental Medicine announcing the discovery that acquired PAP is an autoimmune disease. An autoimmune disease is a disorder that occurs when the body mistakenly mounts an immune response against its own self-made substances, cells, or tissues. Typically in an autoimmune disease, the body produces antibodies against whatever it has misidentified as foreign. In the case of acquired PAP, the researchers discovered the presence of an antibody against GM-CSF in both the blood and in Bronchoalveolar Lavage Fluid (BALF, fluid that is obtained when the lungs are rinsed with saline). This "neutralizing" antibody was not found in healthy patients.

More recently, investigations into the effects of the autoantibody obtained from acquired PAP patients demonstrated that the presence of the antibody inhibits the growth and development of alveolar macrophages, further supporting the concept that GM-CSF plays an important role in the maturation of these immune cells. In addition, studies have also demonstrated that the autoantibody is present in high enough quantities to neutralize the activity of GM-CSF in the lungs, indicating that this autoantibody is likely key in the development of acquired pulmonary alveolar proteinosis.