Role of chain pairing and peptide occupancy for the production of functional soluble IA MHC class II molecules : Christopher Scottc, Christopher Garciac, Frank Carboneb, Ian Wilsonc, Luc Teytona, aP.R.I.R.W. Johnson, bMonash University, Australia, cDepartment of Molecular Biology, The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, CA 92037, USA