A HUMAN T-CELL RECEPTOR RECOGNIZES O-LINKED SUGARS FROM THE HINGE REGION OF HUMAN IGA1 AND IGD

A receptor which binds secretory IgA (sIgA) is expressed on human T ce lls from patients with systemic lupus erythematosus, rheumatoid arthri tis, Behcet's syndrome and IgA nephropathy and on normal T cells follo wing phytohaemagglutinin (PHA) stimulation. The specificity of this re ceptor was initially probed with a panel of normal serum immunoglobuli ns in competitive inhibition assays with sIgA using two-colour immunof luorescence. While the receptor showed the strongest affinity for IgA1 (IC(50)10(-6)M), IgD which has a similarly glycosylated hinge region to IgA1, also bound to the receptor (IC50 10(-5)M). IgA2, which lacks the 'O'-glycosylated hinge region, did not significantly inhibit the b inding at these concentrations suggesting that the IgA determinants fo r this receptor might be the oligosaccharides present in the hinge reg ion of IgA1. IgA1 has up to 10 'O'-linked oligosaccharides and four N- linked oligosaccharides per molecule. In order to probe the role of th e 'O'-linked hinge sugars in the binding event, a sugar library was pr epared from IgA1 by a procedure designed to release 'O'-linked oligosa ccharides preferentially, and to retain them in the natural closed rin g formation. The sugars were released by hydrazinolysis at 65 degrees and the resulting oligosaccharide library analysed by high voltage pap er electrophoresis (HVE) and P4 gel permeation chromatography. Competi tive inhibition studies demonstrated that both the library and the ind ividual 'O'-linked sugars associated with IgA1 were implicated in the binding of IgA1 to this receptor (IC50 between 1 x 10(-5)M and 6 x 10( -5)M). Within this range the individual sugars showed small difference s in their affinity for the receptor in the following order: Gal beta 3GalNAc = NeuNAc2 alpha 3(6)Gal beta 3GalNAc > NeuNAc2 alpha 3(6)Gal b eta 3[NeuNAc2 alpha 6]GalNAc greater than or equal to GalNAc.