B cell-targeted therapies in autoimmunity: rationale and progress

B cells are recognized as main actors in the autoimmune process. Autoreactive B cells can arise in the bone marrow or in the periphery and, if not properly inhibited or eliminated, can lead to autoimmune diseases through several mechanisms: autoantibody production and immune complex formation, cytokine and chemokine synthesis, antigen presentation, T cell activation, and ectopic lymphogenesis. The availability of agents capable of depleting B cells (that is, anti-CD20 and anti-CD22 monoclonal antibodies) or targeting B cell survival factors (atacicept and belimumab) opens new perspectives in the treatment of diseases such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, and multiple sclerosis

T-cell co-stimulatory blockade in kidney transplantation: back to the bench

It is believed that blocking positive T-cell co-stimulatory pathways should lead to long-term graft acceptance. Despite the exciting initial achievements in experimental animal models, targeting co-stimulatory pathways has shown to be much more complex in the clinic. In addition to multiple binding partners, some co-stimulatory interactions have been found to be inhibitory in nature, whereas others were demonstrated to be important in the development of regulatory T cells. Moreover, memory T cells have been shown to be resistant to co-stimulation blockade. Herein we focus on the B7:CD28 pathway and describe the evolution of targeting this pathway with cytotoxic T-lymphocyte antigen-4-Ig from bench to clinic. We also attempt to address possible causes for the unexpected high rejection rate observed in the phase III clinical trials with belatacept, using experimental data obtained from basic science research

Critical Role of the Programmed Death-1 (PD-1) Pathway in Regulation of Experimental Autoimmune Encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is mediated by autoantigen-specific T cells dependent on critical costimulatory signals for their full activation and regulation. We report that the programmed death-1 (PD-1) costimulatory pathway plays a critical role in regulating peripheral tolerance in murine EAE and appears to be a major contributor to the resistance of disease induction in CD28-deficient mice. After immunization with myelin oligodendrocyte glycoprotein (MOG) there was a progressive increase in expression of PD-1 and its ligand PD-L1 but not PD-L2 within the central nervous system (CNS) of mice with EAE, peaking after 3 wk. In both wild-type (WT) and CD28-deficient mice, PD-1 blockade resulted in accelerated and more severe disease with increased CNS lymphocyte infiltration. Worsening of disease after PD-1 blockade was associated with a heightened autoimmune response to MOG, manifested by increased frequency of interferon γ–producing T cells, increased delayed-type hypersensitivity responses, and higher serum levels of anti-MOG antibody. In vivo blockade of PD-1 resulted in increased antigen-specific T cell expansion, activation, and cytokine production. Interestingly, PD-L2 but not PD-L1 blockade in WT animals also resulted in disease augmentation. Our data are the first demonstration that the PD-1 pathway plays a critical role in regulating EAE

Insulin-induced remission in new-onset NOD mice is maintained by the PD-1–PD-L1 pathway

The past decade has seen a significant increase in the number of potentially tolerogenic therapies for treatment of new-onset diabetes. However, most treatments are antigen nonspecific, and the mechanism for the maintenance of long-term tolerance remains unclear. In this study, we developed an antigen-specific therapy, insulin-coupled antigen-presenting cells, to treat diabetes in nonobese diabetic mice after disease onset. Using this approach, we demonstrate disease remission, inhibition of pathogenic T cell proliferation, decreased cytokine production, and induction of anergy. Moreover, we show that robust long-term tolerance depends on the programmed death 1 (PD-1)–programmed death ligand (PD-L)1 pathway, not the distinct cytotoxic T lymphocyte–associated antigen 4 pathway. Anti–PD-1 and anti–PD-L1, but not anti–PD-L2, reversed tolerance weeks after tolerogenic therapy by promoting antigen-specific T cell proliferation and inflammatory cytokine production directly in infiltrated tissues. PD-1–PD-L1 blockade did not limit T regulatory cell activity, suggesting direct effects on pathogenic T cells. Finally, we describe a critical role for PD-1–PD-L1 in another powerful immunotherapy model using anti-CD3, suggesting that PD-1–PD-L1 interactions form part of a common pathway to selectively maintain tolerance within the target tissues

Tissue expression of PD-L1 mediates peripheral T cell tolerance

Programmed death 1 (PD-1), an inhibitory receptor expressed on activated lymphocytes, regulates tolerance and autoimmunity. PD-1 has two ligands: PD-1 ligand 1 (PD-L1), which is expressed broadly on hematopoietic and parenchymal cells, including pancreatic islet cells; and PD-L2, which is restricted to macrophages and dendritic cells. To investigate whether PD-L1 and PD-L2 have synergistic or unique roles in regulating T cell activation and tolerance, we generated mice lacking PD-L1 and PD-L2 (PD-L1/PD-L2−/− mice) and compared them to mice lacking either PD-L. PD-L1 and PD-L2 have overlapping functions in inhibiting interleukin-2 and interferon-γ production during T cell activation. However, PD-L1 has a unique and critical role in controlling self-reactive T cells in the pancreas. Our studies with bone marrow chimeras demonstrate that PD-L1/PD-L2 expression only on antigen-presenting cells is insufficient to prevent the early onset diabetes that develops in PD-L1/PD-L2−/− non-obese diabetic mice. PD-L1 expression in islets protects against immunopathology after transplantation of syngeneic islets into diabetic recipients. PD-L1 inhibits pathogenic self-reactive CD4+ T cell–mediated tissue destruction and effector cytokine production. These data provide evidence that PD-L1 expression on parenchymal cells rather than hematopoietic cells protects against autoimmune diabetes and point to a novel role for PD-1–PD-L1 interactions in mediating tissue tolerance

A novel role of CD4 Th17 cells in mediating cardiac allograft rejection and vasculopathy

T-bet plays a crucial role in Th1 development. We investigated the role of T-bet in the development of allograft rejection in an established MHC class II–mismatched (bm12 into B6) model of chronic allograft vasculopathy (CAV). Intriguingly, and in contrast to IFN-γ−/− mice that are protected from CAV, T-bet−/− recipients develop markedly accelerated allograft rejection accompanied by early severe vascular inflammation and vasculopathy, and infiltration by predominantly IL-17–producing CD4 T cells. Concurrently, T-bet−/− mice exhibit a T helper type 1 (Th1)–deficient environment characterized by profound IFN-γ deficiency, a Th2 switch characterized by increased production of interleukin (IL) 4, IL-5, IL-10, and IL-13 cytokines, as well as increased production of the proinflammatory cytokines IL-6, IL-12p40, and IL-17. Neutralization of IL-17 inhibits accelerated allograft rejection and vasculopathy in T-bet−/− mice. Interestingly, CD4 but not CD8 T cell deficiency in T-bet−/− mice affords dramatic protection from vasculopathy and facilitates long-term graft acceptance. This is the first study establishing that in the absence of Th1-mediated alloimmune responses, CD4 Th17 cells mediate an aggressive proinflammatory response culminating in severe accelerated allograft rejection and vasculopathy. These results have important implications for the development of novel therapies to target this intractable problem in clinical solid organ transplantation

Blockade of the Programmed Death-1 (PD1) Pathway Undermines Potent Genetic Protection from Type 1 Diabetes

Aims/Hypothesis Inhibition of PD1-PDL1 signaling in NOD mice accelerates onset of type 1 diabetes implicating this pathway in suppressing the emergence of pancreatic beta cell reactive T-cells. However, the molecular mechanism by which PD1 signaling protects from type 1 diabetes is not clear. We hypothesized that differential susceptibility of Idd mouse strains to type 1 diabetes when challenged with anti PDL1 will identify genomic loci that collaborate with PD1 signaling in suppressing type 1 diabetes. Methods: Anti PDL1 was administered to NOD and various Idd mouse strains at 10 weeks of age and onset of disease was monitored by measuring blood glucose levels. Additionally, histological evaluation of the pancreas was performed to determine degree of insulitis. Statistical analysis of the data was performed using Log-Rank and Student's t-test. Results: Blockade of PDL1 rapidly precipitated type 1 diabetes in nearly all NOD Idd congenic strains tested, despite the fact that all are moderately (Idd5, Idd3 and Idd10/18) or highly (Idd3/10/18 and Idd9) protected from spontaneous type 1 diabetes by virtue of their protective Idd genes. Only the Idd3/5 strain, which is nearly 100% protected from spontaneous disease, remained normoglycemic following PDL1 blockade. Conclusions: These results indicate that multiple Idd loci collaborate with PD1 signaling. Anti PDL1 treatment undermines a large portion of the genetic protection mediated by Idd genes in the NOD model of type 1 diabetes. Basal insulitis correlated with higher susceptibility to type 1 diabetes. These findings have important implications since the PD1 pathway is a target for immunotherapy

The Limits of Linked Suppression for Regulatory T Cells

Background: We have previously found that CD4+CD25+ regulatory T cells (Tregs) can adoptively transfer tolerance after its induction with costimulatory blockade in a mouse model of murine cardiac allograft transplantation. In these experiments, we tested an hypothesis with three components: (1) the Tregs that transfer tolerance have the capacity for linked suppression, (2) the determinants that stimulate the Tregs are expressed by the indirect pathway, and (3) the donor peptides contributing to these indirect determinants are derived from donor major histocompatibility complex (MHC) antigens (Ags). Methods: First heart transplants were performed from the indicated donor strain to B10.D2 recipients along with costimulatory blockade treatment (250 μg i.p. injection of MR1 on day 0 and 250 μg i.p. injection of CTLA-4 Ig on day 2). At least 8 weeks later, a second heart transplant was performed to a new B10.D2 recipient who had been irradiated with 450 cGy. This recipient was given 40 × 106 naive B10.D2 spleen cells + 40 × 106 B10.D2 spleen cells from the first (tolerant) recipient. We performed three different types of heart transplants using various donors. Results: (1) Tregs suppress the graft rejection in an Ag-specific manner. (2) Tregs generated in the face of MHC disparities suppress the rejection of grafts expressing third party MHC along with tolerant MHC. Conclusion: The limits of linkage appear to be quantitative and not universally determined by either the indirect pathway or by peptides of donor MHC Ags