Surfactant protein A and lamellar bodies: a homologous secretory function of peritoneum, synovium, and lung

Abstract

Objective: To review recent discoveries on the existence of lamellar body secreting cells in extrapulmonary sites with respect to their constitution as a previously unrecognized coherent biological system and to evaluate the role of mesothelial research in peritoneal dialysis in initiating this new field of medical research.

Data sources: Studies in the literature on the production and metabolism of pulmonary surfactant with respect to lamellar bodies and surfactant protein A (SP-A). Published investigations on the identification of lamellar bodies and SP-A in extrapulmonary sites and their possible role in systemic expression of autoimmune disorders.

Results: It is now established that lamellar bodies of identical periodicity and ultrastructural geometry are present in lung (type II pneumocytes), serosal mesothelium (peritoneum, pleura, and pericardium), and joints (type A and type B synoviocytes). Not only pulmonary lamellar bodies but those at extrapulmonary sites are found in association with SP-A, while SP-A or SP-A-like proteins are present in measurable quantities in normal serosal fluids. These findings have disclosed totally new avenues of research into multisystem disorders, where for the first time an organelle and associated protein provide a link between diverse tissues affected by rheumatoid disease. Evidence of shared epitopes between SP-A and mycobacterial 65-kD heat shock protein indicates a possible etiologic mechanism.

Conclusions: A hitherto ultrastructurally hidden system of lamellar body secretion is now being revealed as a major biological system which appears to subserve surfactant, lubricant, surface protection, and sealant functions in body surfaces and tissue interfaces. This new frontier was born out of investigations into the hitherto neglected biology of peritoneal mesothelium, research which was vital for advancing the therapy of peritoneal dialysis and for preservation of the dialyzing membrane.