Distinct roles for CD28 and cytotoxic T lymphocyte-associated molecule- 4 receptors during T cell activation?

C Ytotoxic T lymphocyte-assodated molecule-4 (CTLA-4) is a lymphocyte cell surface receptor originally discovered in a search for molecules having a role in T cell cytotoxicity (1). This molecule is a member of the immunoglobulin superfamily and is homologous to another T cell surface receptor, CD28. Both CD28 and CTLA-4 bind the same counter-receptors, members of the B7 family on APCs. While the role of CD28 during T cell responses to antigen has been well studied, much less is known about the role of CTLA-4. Recent studies on the function of this molecule have been controversial and have yielded seemingly conflicting results. In this issue, a new study (2) provides further evidence on the function of this molecule and lends support to growing evidence that CTLA-4 is a receptor having unique function during an immune response. What follows in this article is an account of the development of this story, an analysis of the significance of the new data, and a discussion of potential fruitful areas of future research on CTLA-4. Role of B7 Molecules in T Cell Costimulation. The interactions of T lymphocytes with APC are key to the generation of an immune response to foreign pathogens, transplanted organs, or to self tissue during autoimmune disease. The specificity of these T cell-APC interactions is provided by the recognition of antigenic peptide-MHC complexes by clonotypic TCR. However, TCR engagement alone generally does not lead to full T cell activation, but may instead lead to T cell clonal anergy. A successful immune response requires additional interactions between the T cell and the APC. These costimulatory interactions thus determine the outcome of TCR engagement, i.e., whether this engagement activates or inactivates subsequent immune responses (3). While the molecular nature of these costimulatory interactions is not fully understood, it is now clear that a key T cell costimulatory signal is provided by interaction of CD28 receptors on T cells with B7 counter-receptors on APC (4). Engagement of the CD28 receptor by B7 molecules triggers a signaling pathway that regulates T cell cytokine production, particularly I1,-2 (4). Two B7 molecules are known, B7-1 (CD80) and B70/B7-2 (CD86), each of which binds CD28 with similar avidity and elicits similar functional effects (5). CTLA-4. CTLA-4 is a second high avidity receptor for B7 molecules that was cloned from a subtracted cytolytic T cell cDNA library (1, 6). CTLA-4 transcripts were detected in activated lymphocytes and were coinduced with T cell cytoxicity. Human and mouse genes encoding CTLA-4 map to the same chromosomal band as CD28 (7, 8), and human CTLA-4 and CD28 have been linked at the molecular level on a yeast artificial chromosome clone (9). Recombinant soluble CTLA-4 binds CD80 and CD86 with higher avidity than recombinant soluble CD28 (5, 10). Although originally identified as a cytolytic T cell-associated molecule, CTLA-4 transcripts have been detected in both CD4 + and CD8 + T cell clones (11). Cell surface expression of CTLA-4 on activated T cells has been detected using specific mAbs. In human T cells (12), CTLA-4 is expressed equally on CD4 + and CD8 + T subsets of activated T cells, whereas with murine cells, CTLA-4 expression is higher on the CD8 + subset (13). Expression of CTLA-4 is highly activation dependent; CTLA-4 is not detected on resting cells but is induced during T cell activation (1, 11-13). Expression is regulated in part by levels of mRNA (1, 11). With activated human T cells, maximal CTLA-4 protein expression was "~2-3% of CD28 (12). CTLA-4 is therefore a high avidity, low abundance receptor for B7 molecules. Functions of CTLA-4 during T Cell Activation. Relatively little is known of the role of CTLA-4 during T cell activation, and the few experiments reported are largely contradictory. Complete conservation was noted in the amino acid sequences of the cytoplasmic tails of murine and human homologues of CTLA-4 (14). Since the cytoplasmic tail of CTLA-4 presumably mediates signal transduction, this sequence conservation suggested that CTLA-4 has an important conserved signaling function. The cytoplasmic tails of CTLA-4 and CD28 show more limited sequence homology (7), so it was therefore difficult to predict from sequence comparisons alone whether CTLA-4 would have similar or different functions than CD28. Initial experiments (12) showed that combinations of CTLA-4 mAb and anti-CD28 mAb or Fab fragments were synergistic in blocking adhesion of activated CD4 + lymphocytes to CDS0-transfected C H O cells, and in blocking T cell proliferation during a mixed lymphocyte reaction (MLR). Anti-CTLA-4 mAb had weak costimulatory activity together with anti-TCR mAb on previously primed CD4 § T cells, but the effects were less than those observed with anti-CD28 mAb plus anti-TCR mAb. Combinations of suboptimal amounts of anti-TCR mAb, anti-CD28 mAb plus antiCTLA-4 mAb were synergistic in their costimulatory ability (12). Another study from this group (15) extended the original findings and showed that the cooperative costimulatory effects of anti-CTLA-4 and anti-CD28 mAbs were greatest at low concentrations of anti-CD28 mAb (and, hence, low occupancy