Deborah J. Lenschow

B7/CD28 Costimulation Is Essential for the Homeostasis of the CD4+CD25+ Immunoregulatory T Cells that Control Autoimmune Diabetes

CD28/B7 costimulation has been implicated in the induction and progression of autoimmune diseases. Experimentally induced models of autoimmunity have been shown to be prevented or reduced in intensity in mice rendered deficient for CD28 costimulation. In sharp contrast, spontaneous diabetes is exacerbated in both B7-1/B7-2-deficient and CD28-deficient NOD mice. These mice present a profound decrease of the immunoregulatory CD4+CD25+ T cells, which control diabetes in prediabetic NOD mice. These cells are absent from both CD28KO and B7-1/B7-2KO mice, and the transfer of this regulatory T cell subset from control NOD animals into CD28-deficient animals can delay/prevent diabetes. The results suggest that the CD28/ B7 costimulatory pathway is essential for the development and homeostasis of regulatory T cells that control spontaneous autoimmune diseases.

CTLA-4 can function as a negative regulator of T cell activation

CD28 and CTLA-4 are related glycoproteins found on T cells. Ligation of CD28 following antigen receptor engagement provides a costimulatory signal required for T cell activation. Anti-CTLA-4 antibodies were generated to examine the role of the CTLA-4 receptor on murine T cells. Expression of CTLA-4 as a homodimer is up-regulated 2-3 days following T cell activation. Anti-CTLA-4 antibodies and Fab fragments augmented T cell proliferation in an allogeneic MLR. However, when optimal costimulation and Fc cross-linking were present, anti-CTLA-4 Mabs inhibited T cell proliferation. Together, these results suggest that the MAb may obstruct the interaction of CTLA-4 with its natural ligand and block a negative signal, or directly signal T cells to down-regulate immune function.

CD28/B7 Regulation of Th1 and Th2 Subsets in the Development of Autoimmune Diabetes

CD28 ligation delivers a costimulatory signal important in T cell activation. This study demonstrates that the disruption of the CD28/B7 pathway early in the nonobese diabetic mouse strain, using CD28-/- and CTLA41g transgenic mice, promoted the development and progression of spontaneous autoimmune diabetes. Functional analyses of T cells isolated from CD28-deficient mice demonstrated that the GAD-specific T cells produced enhanced Th1-type cytokines (IL-2 and IFN gamma) and diminished Th2-type cytokine, IL-4. Moreover, there was a significant decrease in serum levels of anti-GAD antibodies of the IgG1 isotype consistent with a profound suppression of Th2-type responses in these animals. Thus, the early differentiation of naive diabetogenic T cells into the Th2 subset is dependent upon CD28 signaling and extends our understanding of the importance of Th1/Th2 balance in the regulation of this spontaneous autoimmune disease.

Blockade of CD28/B7-1 Interaction Prevents Epitope Spreading and Clinical Relapses of Murine EAE

Relapsing experimental autoimmune encephalomyelitis (R-EAE) induced with the immunodominant epitope from proteolipid protein, PLP139-151, is characterized by the development of recurrent relapses with recruitment of T cells reactive to additional myelin peptides, including PLP178-191 (epitope spreading). In this study, we have determined that the CD28/B7 costimulatory pathway is involved in this process. We found preferential up-regulation of B7-1 during the course of R-EAE and a selective increase in its functional costimulatory activity, relative to B7-2. Anti B7-1 F(ab) fragment therapy, but not anti B7-2 MAb therapy, blocked clinical relapses, ameliorated CNS pathology, and blocked epitope spreading. These results suggest that the maintenance of autoimmune reactivity in EAE depends on CD28/B7-1-dependent costimulation of newly recruited T cells responsible for epitope spreading. These studies have important implications for the role of epitope spreading in disease progression and the clinical application of costimulatory antagonists in autoimmune diseases.

Inhibition Of Transplant Rejection Following Treatment With Anti-b7-2 And Anti-b7-1 Antibodies

Antigen-specific T cell activation depends initially on the interaction of the T cell receptor (TCR) with peptide/MHC. In addition, a costimulatory signal, mediated by distinct cell surface accessory molecules, is required for complete T cell activation leading to lymphokine production and proliferation. CD28 has been implicated as the major receptor on T cells responsible for delivering the costimulatory signal. Although two distinct ligands for CD28, B7-1 and B7-2, have been identified on antigen-presenting cells (APC), the co-stimulatory role of each molecule during a physiological immune response remains unresolved. In the present study, the relative roles of B7-1 and B7-2 interactions were evaluated in an allogeneic pancreatic islet transplant setting. In isolation, anti-B7-2 mAbs and, to a much lesser degree, anti-B7-1 mAbs suppressed T cell proliferative responses to allogeneic islets or splenic APC in vitro. Maximal inhibition of the allogeneic response was observed using a combination of the anti-B7-1 and anti-B7-2 mAbs. Administration of anti-B7-2 but not anti-B7-1 mAbs prolonged C3H allograft survival in B6 recipients, with a combination of both mAbs significantly prolonging rejection beyond either mAb alone. The immunosuppressive effects of the in vivo mAb treatment were not manifested in in vitro analyses as T cells isolated from suppressed mice responded normally to allogeneic stimuli in terms of both proliferation and lymphokine production. However, combined mAb therapy in vivo selectively delayed CD4+ T lymphocyte infiltration into the graft. These data suggest that both B7-1 and B7-2 costimulatory molecules are active in vivo, although B7-2 plays a clearly dominant role in this allograft model. The mechanism of immune suppression in vivo remains unresolved but may occur at sites distinct from the allograft.

T cell co-stimulation and in vivo tolerance.

Previous studies have shown that effective T-cell activation requires the engagement of the T-cell receptor complex with MHC-peptide, in parallel with co-stimulation via cell surface adhesion molecules. Blocking these co-stimulatory interactions, in particular the signal transduction via the CD28 molecule, inhibits T-cell activation in vitro and induces T-cell clones into a state of unresponsiveness, termed T-cell anergy. Recent studies have examined the therapeutic effects of treating mice with CD28-B7 antagonists and highlighted the complexity of the CD28 co-stimulatory pathway, as illustrated by the finding that multiple cross-binding ligands for the CD28 and B7 molecules exist that may differentially regulate immune responses.

Treatment with intact anti-B7-1 mAb during disease remission enhances epitope spreading and exacerbates relapses in R-EAE

PLP139-151-induced experimental autoimmune encephalomyelitis in the SJL mouse is a Th1-mediated inflammatory demyelinating disease characterized by a relapsing-remitting clinical course (R-EAE). Clinical relapses are mediated by T cells specific for a non-cross reactive secondary PLP epitope (PLP178-191) induced by epitope spreading. We have previously shown that B7-1 expression is upregulated in SJL mice undergoing R-EAE and in vivo treatment during remission with F(ab) fragments of anti-B7-1 mAb, blocked epitope spreading and disease progression. In contrast, the present study shows that treatment with intact anti-B7-1 mAb exacerbated clinical disease relapses and enhanced CNS demyelination. Anti-B7-1-treated mice showed enhanced in vivo delayed-type hypersensitivity (DTH) to the relapse-associated PLP178-191 epitope and responses to the immunodominant MBP84-104 epitope which are absent in the controls. Thus, ligation of B7-1 by intact mAbs has effects opposite to those of anti-B7-1 F(ab) fragments suggesting that the mAb is directly signaling through B7-1 expressed on T cells and/or APCs.

Immunomodulation of transplant rejection using monoclonal antibodies and soluble receptors

The main objective of our studies has been to optimize the effects of monoclonal antibodies (MAbs) and other immunosuppressive reagents to enhance organ graft survival. One such agent is OKT3, a MAb that is directed against the CD3 component of the human T-cell receptor (TCR) complex. Treatment of a rejection episode with OKT3 results in a rapid and efficient clearing of circulating T cells and reversal of most rejection episodes. Its wider use in transplantation and in the treatment of immune-mediated disease is limited by adverse reactions that follow the initial dose, the production of neutralizing Abs, and the transient nature of the immunosuppression. We have engineered CDR-grafted “humanized” anti-CD3 MAbs that lack Fc-receptor binding activity through mutagenesis of amino acids in the Fc portion of the MAb. This results in an immunosuppressive anti-CD3 MAb that is less antigenic and one that does not induce the first-dose side effects. In addition, we have pursued a goal of developing a therapy that will induce donor-specific tolerance while maintaining overall recipient immune competency. Because antigen-specific T-cell activation depends not only on TCR-ligand interaction, but also on additional costimulatory signals mediated by accessory molecules such as CD28, blocking the binding of CD28 on T cells to its ligand B7, during TCR engagement, might modulate transplantation responses. Using a soluble fusion protein of human CTLA4, CTLA4-Ig, that binds B7 with high affinity, inhibition of human pancreatic islet rejection that occurs, at least in part, by affecting T-cell recognition of human B7+ antigen-presenting cells has been demonstrated. In addition, CTLA4-Ig induces long-term, donor-specific unresponsiveness.