In rheumatoid arthritis (RA) and several other chronic inflammatory diseases, the fraction of plasma IgG molecules that lack terminal galactose residues [Gal(0)] is elevated. These changes are not seen in many other infectious and autoimmune diseases, raising the interesting possibility that increased Gal(0) IgG is a marker for a specific set of changes in the immune system. Moreover, the absence of Gal residues significantly alters some properties of IgG, adding further importance to an exploration of the altered galactosylation of these molecules. The increased Gal(0) IgG in RA is at least partially due to altered IgG biosynthesis because isolated B cells from RA patients secrete increased amounts of Gal(0) IgG. However, the molecular and cellular basis of the defective galactosylation is not well understood. We will explore three aspects of this problem by comparing B cells from normal individuals and patients with RA and SLE, a second autoimmune disease that may be associated with elevated Gal(0) IgG. 1. The molecular mechanism of the galactosylation defect will be addressed. The amounts and kinetic properties of Golgi galactosyltransferases will be examined and a second Golgi activity that is required for galactosylation, the transporter that carries the Gal donor, UDP-Gal, from its site of synthesis in the cytoplasm into the Golgi lumen, will also be studied. CDNA clones encoding this protein will be isolated and used to generate probes to measure levels of transporter MRNA and protein. These studies will allow us to test athe hypothesis that dysregulation f B cell galactosyltransferases and/or the UDP-Gal transporter contribute to the increased Gal(0) igg in RA. 2. The extent of the galactosylation defect will be determined by studying class I and II MHC molecules in B cells. This will allow us to determine whether defective galactosylation is restricted to IgG or whether it is also found in other B cell proteins. A more pervasive galactosylation defect in RA could affect the properties of surface proteins that play crucial roles in B cell function. 3. The cellular basis of decreased galactosylation will be studied by comparing freshly isolated cells to cells grown in culture after viral transformation. This will allow us to decide whether the galactosylation defect in RA is caused by changes in effectors that act on B cells or by alterations in the cells themselves. These experiments will also allow us to test the hypothesis that reduced galactosylation in RA is due to the expansion of a subpopulation of B cells that normally has altered properties. In a growing number of instances, the glycosylation of plasma proteins has been found to be altered in disease and inflammation. These modulations may have significant functional consequences because they affect glycoprotein properties. Consequently, investigation of these adaptive and defensive responses will provide important insights into homeostatic and regulatory mechanisms.
Showing the most recent 10 out of 92 publications
