Lung Surfactant-associated Proteins and Type IV Collagen Share Common Epitopes: An Immunocytochemical Demonstration

Abstract

Among the surfactant-associated proteins (SP-A) characterized so far, there is a group of glycoproteins 26 to 34 kDa that features collagenlike sequences near their N-terminal end. We herein report the cross-reactivity of a rabbit polyclonal antibody to EHS tumor-derived type IV collagen towards rat SP-A. Rat lung tissues were processed for the localization of both type IV collagen and SP-A by high-resolution immunocytochemistry, applying the protein A-gold technique with specific antibodies. In addition to the various basal laminae of the pulmonary tissue, the antitype IV collagen antibody labeled the surfactant material found in alveolar spaces and macrophages, as well as in type II pneumocytes. The surfactant nature of the alveolar material labeled by the antiserum to type IV collagen was confirmed by the positive labeling obtained using an antibody to SP-A. This antibody labeled specifically the alveolar surfactant material, without binding any basal laminae. Several control experiments demonstrated the specificity of each labeling. These results were further supported by immunoblot experiments on nitrocellulose membrane. These findings thus provide further support to the existence of collagenlike sequences on SP-A, and further demonstrate that this structural similarity with collagens can lead to some cross-antigenicity.