Purification and separation of subsets of human Ia molecules by papain digestion.

Papain digestion of human Ia(-like) molecules was performed under various conditions using 125I-labelled preparation of non-ionic-detergent-solubilized Ia antigens of Daudi cells. The products were examined for their allospecificities by a direct binding reaction with human Ia alloantisera. The Daudi Ia preparation is known to contain Ia molecules of DRw6 specificity, an HLA-DR specificity and also Ia molecules of DC1 specificity, a putative non-HLA-DR specificity. Limited papain digestion cleaved off the hydrophobic portion of human Ia molecules and gave smaller sized Ia products. The cleavage did not affect the Ia alloantigenic determinants and occurred much more readily with molecules of DC1 specificity than with molecules of DRw6 specificity. As a consequence, limited papain digestion of the Daudi Ia pool yielded an Ia preparation with DRw6 specificity but lacking DC1 specificity and another Ia preparation which was enriched in DC1 specificity. The limited papain digestion of the Daudi Ia pool followed by gel filtration and LcH affinity chromatography also produced Ia REPARATIONS OF HIGH PURITY. Extensive papain digestion damaged the Ia alloantigenic determinants but the DC1 determinant was much more resistant than the DRw6 determinant. Thus extensive papain digestion yielded an Ia preparation which was relatively rich in DC1 specificity and essentially devoid of DRw6 specificity.