Ligation of the functional domain of complement receptor type 2 (CR2, CD21) is relevant for complex formation in T cell lines.
We investigated the potential of CD21, the complement receptor type 2, to form receptor complexes with other membrane molecules on T cell lines. CD21 from T cell lines transformed with human T cell leukemia virus type I (MT2, HUT-102, C5.MJ, Mondi, and C91.PL) and T cell lines that were not virus transformed was analyzed by coprecipitation following cell lysis with digitonin. mAbs binding to functional and nonfunctional epitopes of CD21 and a polyclonal antiserum against its intracellular portion precipitated CD21, which was indistinguishable from CD21 on B cell lines. In contrast to B cells, where CD21 is complexed with CD19 and CD81 (target of anti-proliferative Ab 1) or, alternatively, with CD35 (CR1), no surface molecules could be coprecipitated with three of four mAbs from these T cell lines. Therefore, we assume that CD21 is not part of a preformed complex in T cell lines. OKB7, the only mAb directed against the functional C3d binding site, coprecipitated two proteins of 105 and 55 Mr with CD21 from MT2 and Mondi cells and from the T cell lines Jurkat E6-1 and SupT1. These bands were also recovered with CD21 precipitated from MT2 cells with C3d bound to Sepharose via the internal thioester, but were absent in CD21-expressing B cell lines. As C3d and OKB7 are functional ligands for B cells, we propose that upon ligation on T cells, CD21 associates with molecules of 105/55 Mr in the plasma membrane. Whether this is the first event of a signal delivered to the T cell is under current investigation.