Three Different Types of Focusing Events M a y Be Needed to Deliver the Suppressive Signal

We have described (1) a biologically active antigen-specific Ly-1-; 2 + T cell product that has the capacity to suppress the ability of Ly-2 cell-depleted spleen cells to make primary responses in Mishell-Dutton cultures. It has been presumed that this material, referred to as "Ly-2 suppressor factor from T cells (TsF), ''1 is a direct suppressor molecule because (a) it requires no other cells with a suppressor cell phenotype in the culture to work, and (b) its suppressive effect can be seen when the assay cultures are harvested early during the course of an in vitro immune response. Inducers of suppression require a longer latent period before their suppressive activity is expressed (2). An interesting feature of this Ly-2 TsF is that it does not express any I-J or other major histocompatibility complex (MHC)-controlled specificities that can be detected with the use of a large battery of antisera. Nonetheless, the biological activity of the molecule is restricted by the MHC polymorphisms expressed by the producer:acceptor cell partners. This latter finding might be interpreted to indicate that the Ly-2 TsF must react with an MHC-controlled gene product in the acceptor cell population to express its biological activity. We tested this proposition by treating normal Ly-1 T cells with an anti-I-J serum before adding these cells to T cell-depleted spleen cells (B cells) and determining whether the anti-I-J treatment of the normal Ly-1 cells had affected either (a) the ability of the Ly-1 cells to help the T cell-depleted spleen cells or (b) changed the ability of the Ly-2 TsF to suppress the cultures. We found that the anti-I-J treatment of the Ly-1 T cells did not affect their ability to help T cell-depleted spleen cell cultures but completely abrogated the ability of the Ly-2 TsF to suppress the helper activity in such cultures. We also found that the cultures that lack I-J + Ly-1 cells can be suppressed by the Ly-2 TsF when the eluate from an anti-I-J immunoabsorbent through which an I-J + Ly-1 T cell suppressor* Supported in part by grants Ca 29609, Ca 14216, and AI-07019 from the U. S. Public Health Service. Recipient of Postdoctoral Fellow training grant AI-07019 from the National Institutes of Health. § Department of Immunology, Faculty of Medicine, University of Tokyo, Japan. 1Abbreviations used in this paper: BSS, balanced salt solution; FCS, fetal calf serum; MHC, major histocompatibility complex; PFC, plaque-forming cells; SRBC, sheep erythrocytes; TSF, suppressor factor from T cells; TsiF, suppressor inducer-factor from T cells.