Immunoglobulin G is associated with surfactant protein A aggregate isolated from patients with pulmonary alveolar...

We have previously shown that the purified preparation of surfactant protein A (SP-A) isolated from patients with pulmonary alveolar proteinosis (PAP) contains a very small amount of immunoglobulin G (IgG). We have recently found that there exists an abnormal multimerized form (alveolar proteinosis protein-I, APP-I) in SP-As isolated from patients with PAP in addition to normal-sized octadecameric APP-II. We examined which of the populations of APP that IgG is associated with. The APP was purified by mannose-affinity column followed by gelfiltration over Bio Gel A5m after the delipidation with 1-butanol. Analysis by gel filtration over Bio Gel A15m showed two elution peaks of APP-I and APP-II. When the fractions eluted from the Bio Gel A15m column were coated onto microtiter wells and reacted with HRP-labeled antihuman IgG, the elution peak of IgG was superimposed on that of APP-I but not on that of APP-II. The immunoblotting analysis also revealed that a very small amount of IgG, which could not be detected by staining with Coomassie blue or amido black, was associated with APP-I but not with APP-II or normal SP-A. APP-I bound to nonimmune IgG coated onto microtiter wells in a concentration-dependent manner, whereas APP-II, normal human SP-A, and rat SP-A exhibited almost no binding to IgG. The results indicate an unusual property of SP-A during the diseased state.