Common antigenic structures of HL-A antigens. II. Small fragments derived from papain-solubilized HL-A antigen molecules.

A distinctive fragment of the HL-A antigen molecule which retains characteristic structural features of the intact molecule has been obtained from soluble HL-A antigens by mild degradation procedures. When molecular fragments of 48,000 Daltons which are derived from cultured human lymphoid cells and carry HL-A alloantigenic activity are exposed to a pH of 2.4 in glycine buffer for 15 minutes at 0–4°, they split and a fragment of 11,000 Daltons can be isolated by gel filtration. This fragment does not carry HL-A alloantigenic activity but does carry HL-A common activity. This common activity had previously been found on papain-solubilized HL-A antigens as well as on intact lymphoid cells. It is immunogenic in rabbits and reacts with antiserum raised in rabbits against lymphoid cell membrane fractions. These small fragments appear to be characteristic, common structural components of HL-A antigen molecules of 48,000 Daltons and hence are designated `HL-A common-portion fragments'. Similar molecular fragments are found in solutions of the HL-A molecular fragments of 48,000 Daltons that have been stored in buffer at pH 7.8 at 4° for some time.