Lymphocyte surface membrane immunoglobulin in myeloma. II. T cells with IgA-Fc receptors are markedly increased in mice with IgA plasmacytomas.

BALB/c mice with established subcutaneous IgA plasmacytomas (MOPC-315, MOPC-167, McPC-603, or TEPC-15) develop large numbers of circulating theta-bearing lymphocytes that have surface membrane receptors for IgA. The extraordinary expansion of T alpha cells in mice with IgA plasmacytomas accounts for the large number of lymphocytes with surface membrane myeloma protein that are found in these mice. The IgA myeloma protein that was originally bound to the T cell receptor in vivo was competitively displaced in vitro by other purified IgA myeloma proteins but not by their F(ab)' fragments. After overnight incubation in vitro, or brief exposure to pronase, IgA was released from the T cell surface, rendering available a surface membrane receptor for IgA. In vitro binding of purified IgA to the T alpha cell receptor was not inhibited by purified IgG1 or IgM myeloma proteins. T alpha cells were not increased in mice with three variant plasmacytomas that did not secrete large amounts of IgA. These observations: i) establish the generality of T alpha cell expansion in mice with IgA plasmacytomas, ii) establish an association between elevated serum IgA levels and T alpha cell expansion, and iii) identify a source of large numbers of T alpha cells that can be specifically purified for structural and functional studies.