HLA class II molecules were partially purified from cells of an HLA deletion mutant cell line, LCL721.82, that lost DR and DQ expression but retained DPw2 specificity and labeled with radioactive 125I. The radioiodinated preparation bound to DP- specific monoclonal antibody B7/21 as well as rabbit anti-HLA class II antiserum. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the component involved in these bindings gave, unlike known HLA class II molecules, a sharp and dense band of ~60 kDa under nonreducing conditions and a single but diffuse band of ~30 kDa under reducing conditions. By screening 401 anti-HLA class II alloantisera, including those distributed in the 9th International Histocompatibility Workshop and also those locally available, eight were found to possess significant binding activity. Specificity analysis of these eight binding- positive antisera on a panel of DP-pretyped HLA homozygous typing cells revealed the presence of two clusters, one corresponding to an allodeterminant associated with DPw1, 2 and 3 and the other to that associated with DPw2 and 4. These two determinants were shown by the sequential binding test to reside on the B7/21-defined HLA class II molecules. Thus, two major conclusions were drawn: (a) two distinct allodeterminants are carried by a single DP molecule; and (b) these serologically detected DP allodeterminants are supertypic to cellularly defined DP allospecificities.
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