The CD28 and CTLA-4 signaling pathways are important participants in a very complex group of regulatory events that maintain immunologic homeostasis. Experimental manipulation of these pathways is being employed with increasing frequency to optimize immune responses to various diseases.
Activation of a resting T cell requires two complementing signals. Engagement of the T cell receptor must be accompanied by a second signal that results from the ligation of receptors on the T cell with either soluble factors, such as IL-2, or cell-surface molecules on the antigen-presenting cell.
|Figure 1. CD28 signaling promotes T cell activation and upregulation of CTLA-4. Blockade of CD28 signal with CTLA-4 Ig or B7 antibodies inhibits T cell activation. CTLA-4 signaling down-regulates T cell responses. Inhibition of this signal with anti-CTLA-4 maintains an active immune response. Resting T cells express TCR and CD28; activated T cells express CD28, CTLA-4, and TCR; and APCs express MHC plus peptide and B7.|
CD28 and CTLA-4 are receptors on T cells that play critical roles in the initial activation and subsequent control of cellular immunity.1-3 CD28 is expressed constitutively on T cells. It provides a major costimulatory signal upon binding to target ligands on antigen-presenting cells. CD28-deficient mice have impaired proliferative responses to antigen and anti-CD3 monoclonal antibody activation.4
In contrast, CTLA-4 is transiently expressed following T cell activation. The signal delivered via CTLA-4 down-regulates T cell function and inhibits excessive expansion of activated T cells.1,2 CTLA-4 knockout mice exhibit massive lymphoproliferative disorder and spontaneous autoimmune disease that is uniformly fatal by 4-5 weeks.5
Both receptors bind two well-characterized ligands, B7-1 and B7-2,1 although other candidate ligands have also been reported.6,7 The B7 proteins are expressed on various types of antigen-presenting cells and differ from one another in their expression characteristics and in their affinities and kinetics of binding to CD28 and CTLA-4.1 These differences provide for distinct functional roles for B7-1 and B7-2 that are not yet completely defined.
Elucidation of the functional roles of CD28, CTLA-4 and their ligands has been aided by monoclonal antibodies against CTLA-4 and CD28 that trigger signal delivery and antibodies against B7-1 and B7-2 that block signaling. CTLA-4 Ig, a fusion protein comprised of the extra cellular domain of CTLA-4 and the Fc portion of IgG, has served as a particularly effective blocking reagent that competes with the cell-surface receptors for B7 binding sites.
Blocking of CD28-dependent T cell activation has been used to modulate deleterious immune responses in experimental models of transplant rejection and autoimmunity.1 In numerous animal transplant models, treatment with CTLA-4 Ig, either alone or in combination with other immunosuppressive agents, prolonged graft survival, reduced the lethality of graft-vs.-host disease, and in some cases resulted in donor-specific tolerance. In animal autoimmunity models of diabetes, lupus, and encephalomyelitis, interruption of the CD28 pathway with CTLA-4 Ig or anti-B7-1 antibody prevented disease onset and also improved outcomes in established disease.
Efforts to enhance anti-tumor immunity have targeted the CD28 and CTLA-4 signaling pathways.1,8 Although tumors express antigens that are recognizable by T cells, many tumors are weakly immunogenic because they lack the costimulatory molecules necessary to activate T cells. In several model systems, tumor cell transfectants that expressed B7 molecules on their cell surface induce potent responses against both transfected tumor cells and unmodified parent cell lines. In mouse models of colon carcinoma and fibrosarcoma, anti-tumor immunity is also enhanced by blocking the CTLA-4 signal which normally limits T cell responsiveness. Mice immunized with a non-stimulatory antibody against CTLA-4 display a dramatic resistance to tumor challenge and, in some cases, are able to clear established tumors.8
Future efforts will further define the details of the functional roles of CD28, CTLA-4, and the B7 molecules and may identify additional ligands involved in these signaling pathways. A thorough understanding of these pathways combined with refined therapeutic approaches will have powerful clinical applications.