Detection and isolation of an Ia glycoprotein antigen from mouse serum

Abstract

Ia antigenic activity is present in mouse serum and exists predominantly (>90 percent) as a dialyzable oligosaccharide which can be purified readily. This paper describes experiments which indicate, however, that a small proportion (<10 percent) of the Ia antigenic activity in serum, particularly from mitogen-injected mice, exists in a high molecular-weight form. Furthermore, this high molecular-weight Ia antigen, like the Ia oligosaccharide, appears to be secreted by T lymphocytes, particularly by T cells which have been activated recently by mitogens. The high molecular-weight Ia antigen was isolated from mouse serum by a procedure which utilized salt fractionation, gel filtration, density separation, and sucrose gradient sedimentation. This procedure gave a 190–260-fold increase in specific Ia antigenic activity, and up to 80 or 90 percent of high molecular-weight Ia antigenic activity detected in the original serum was recovered in the purified fraction. On the basis of sucrose gradient sedimentation studies, the high molecular-weight Ia antigen was estimated to have a molecular weight of approximately 500,000 daltons, if it is assumed that it exists as a globular protein. In addition, the protein has anα2-macroglobulin-like electrophoretic mobility, contains little or no lipid, and, based on its ability to bind to Concanavalin A columns, is a glycoprotein. The relationship between this Ia glycoprotein antigen and other molecular forms of Ia antigen is discussed.