Molecular identifications of HLA-DR4, MT3, and TB21 antigens on HLA-DR4 homozygous B cell lines.

The HLA-DR locus in the human major histocompatibility complex determines a series of polymorphic cell surface antigens; these antigens are composed of two noncovalently associated glycoproteins of 34,000 daltons and 29,000 daltons and are predominantly expressed on B cells, monocytes, and activated T cells (1). To date, 10 HLA-DR specificities have been well characterized by using alloantisera obtained from multiparous women (2). In addition to these DR antigens, two other B cell alloantigen systems (MT and MB) were proposed on the basis of the serologic data (3,4); the MB1 or MT1 specificities are found in association with DR1, DR2, and DRw6, whereas MT2 is associated with DR3, DR5, DRw6, and DRw8; MT3 is associated with DR4, DR7, and DRw9, and MB3 or MT4 is associated with DR4, DR5, and DRw8, respectively (3, 4). We have also described that the TB21 antigen is expressed in association with DR4, DR5, and DRw9 (5). The HLA-DR, MB, MT, or TB antigens are collectively referred to as class II antigens. The number of human class II antigens has been of primary concern because of their similarity to murine Ia antigens and especially because of their close relationship to certain diseases. With the aid of immunochemical means and alloantisera, it has been demonstrated that at least two loci control human class II molecules (6-10). The data presented in this report show the existence of three class II molecules carrying well-known allospecificities, DR4, MT3, and TB21, on HLA-DR4 homozygous cell lines.