The immunochemical analysis of Daudi Ia molecules by a variety of alloantisera led to the recognition of at least three molecular species carrying different antigenic determinants: DRw6, DC-1, and DC-2. Genetic as well as structural evidence indicates that DRw6 and DC-1 molecules are controlled by separate, HLA-linked loci, rather than by alleles at the same locus. The alloantigenic determinants appear to be expressed on the small Ia subunit. DC-1 and DC-2 determinants discussed had not been defined by serological analysis at the population level, but were demonstrated to be present by immunochemical analysis at the molecular level.

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ABSTRACT11,000-Dalton common portion fragments derived from HL-A antigen molecules were isolated and found to have a significantly high homology to p32-microglobulin in amino-acid composition. Common portion fragments are also very similar to 132-microglobulin with respect to molecular size, charge, and distribution in tissues. Moreover, both are found in the spent culture media of human cell lines and in human plasma and urine.Thus it appears that j62-microglobulin may well be the same substance as the common portion fragment of HL-A antigen molecules.We recently reported the isolation of a small fragment ofHIA antigens which appears to be a characteristic invariant portion of the structure of HLA antigens. It was isolated from papain-solubilized HL-A antigen molecules of various HL-A allotypes by mild degradation procedures (1, 2). The molecular size is about 11,000 daltons. The small fragment does carry an HI-A common antigenic activity which is a characteristic antigenic marker of HL-A antigens (2, 3) although the fragment is devoid of HL-A alloantigenic activity. This small fragment has been designated HLA common portion fragment. Cresswell et al. (4) recently reported the isolation of a similar I1,000-dalton fragment from papainsolubilized HL-A antigen. Similar 11,000-dalton fragments are present in human blood plasma (5) and urine and in spent culture media from cultured human cell lines (1). We recently purified the small fragments from spent culture media of cultured human lymphoid cell lines and found them to be identical to the HL-A common portion fragment that was obtained by acid cleavage of papain-solubilized HLA antigen molecules with respect to molecular size, electrophoretic mobility, and isoelectric point (unpublished). No antigenic differences were observed.It now appears to us that the above fragment is very closely related or identical to the fl2-microglobulin that was originally described by Berggard and Bearn (6) and that has a structural similarity to the constant parts of human immunoglobulin G (7,8).We have determined the amino-acid composition of the HL-A common portion fragment isolated from spent culture media of a human lymphoid cell line and compared it with the composition of the ,B2-microglobulin reported by Berggfrd and Bearn (6). Assessment of compositional relatedness between proteins, by the method described by Metzger et al.(9), indicated that ,32-microglobulin has a significantly high homology to HLA common portion fragment.The fragment subjected to amino-acid analysis was purified from spent culture media of RPMI 1788 cells by isolation 2863 procedures that involved differential ultrafiltration, gel filtration, and column electrophoresis. The HL-A common antigenic activity was followed by radioimmunoassay (3). The culture of RPMI 1788 cells with RPMI 1640 medium supplemented with 10% fetal-bovine serum and with penicillin and streptomycin was harvested at day 3. The spent culture medium was separated by centrifugation and filtered with an HA Millipore filter (Millipore Co...

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