Pur i fca t ion and Biological Characterization of an Antigen - specific Suppressive Protein Synthesized by Cloned T Cells * By MANUEL FRESNO , LAILA McVAY - BOUDREAU

Antigen-specific T cell function is not understood in molecular terms. There is evidence that inducer T lymphocytes (Ly-1 cells) recognize antigen in association with products encoded by the I region of the major histocompatibility complex (1-3). By contrast, suppressive (Ly-2 +) lymphocytes and material extracted or derived from these ceils can bind free antigen (4). It is likely that a portion of cell-free material responsible for antigen binding by T-suppressor (Ts) 1 cells is closely related to the cells' membrane-bound receptors (5), as is the case for B cells (6). Studies of extracts or supernates of Ts cells have indicated two main types of antigen-specific moieties. One is likely to be a single chain of ~70,000 mol wt that does not possess I-region or immunoglobulin determinants (5, 7, 8), whereas a second group of smaller proteins bears I-region determinants (9-13). Both types of factors may bear determinants carried by the variable portion of Ig heavy chains (5, 9, 13, 14). In the previous report, we have analyzed the properties of a cloned T cell population that expresses the surface phenotype and function associated with Ts in heterogeneous lymphoid populations (15). These cloned cells synthesized antigen-specific molecules (70,000 mol wt) that specifically preempted the antibody response to sheep erythrocytes. This analysis was based on antigen-binding properties of supernatant peptides and did not define the biologic properties of a distinct protein responsible for specific immunologic suppression. Although these data indicated that partially purified peptides completely inhibited the in vitro primary response to a complex antigen (15), this suppression might reflect the combined biologic activities of many different 70mol wt polypeptides or polypeptides associated with 70-mol wt material by noncovalent interactions. To obtain definitive information on the structural basis of antigenspecific suppression by T cells, we have purified the relevant molecule to virtual homogeneity and analyzed the biologic activity of this purified protein.