Adjuvant facilitates tolerance induction to factor VIII in hemophilic mice through a Foxp3-independent mechanism that relies on IL-10

Modulation of CD4 T cell function via CD6-targeting

In recent years molecules involved on the immune synapse became successful targets for therapeutic immune modulation. CD6 has been extensively studied, yet, results regarding CD6 biology have been controversial, in spite of the ubiquitous presence of this molecule on virtually all CD4 T cells. We investigated the outcome of murine and human antibodies targeting CD6 domain 1. We found that CD6-targeting had a major impact on the functional specialization of CD4 cells, both human and murine. Differentiation of CD4 T cells towards a Foxp3+ Treg fate was prevented with increasing doses of anti-CD6, while Th1 polarization was favoured. No impact was observed on Th2 or Th17 specialization. These in vitro results provided an explanation for the dose-dependent outcome of in vivo anti-CD6 administration where the anti-inflammatory action is lost at the highest doses. Our data show that therapeutic targeting of the immune synapse may lead to paradoxical dose-dependent effects due to modification of T cell fate.

Targeting Antigen-Specific B Cells Using Antigen-Expressing Transduced Regulatory T Cells

Controlling immune responses in autoimmunity and to biotherapeutics is an unmet need. In hemophilia, for example, up to one third of patients receiving therapeutic factor VIII (FVIII) infusions develop neutralizing Abs termed "inhibitors." To address this problem in a mouse model of hemophilia A, we used an Ag-specific regulatory T cell (Treg) approach in which we created a novel B cell-targeting chimeric receptor composed of an FVIII Ag domain linked with the CD28-CD3ζ transmembrane and signaling domains. We termed these "BAR" for B cell-targeting Ab receptors. CD4⁺CD25^hiCD127^low human Tregs were retrovirally transduced to express a BAR containing the immunodominant FVIII C2 or A2 domains (C2- and A2-BAR). Such BAR-Tregs specifically suppressed the recall Ab response of spleen cultures from FVIII-immunized mice in vitro and completely prevented anti-FVIII Ab development in response to FVIII immunization. Mechanistic studies with purified B cells and T cells from tolerized or control recipients demonstrated that the FVIII-specific B cells were directly suppressed or anergized, whereas the T cell response remained intact. Taken together, we report in this study a successful proof-of-principle strategy using Ag-expressing Tregs to directly target specific B cells, an approach which could be adapted to address other adverse immune responses as well.

Dexamethasone promotes durable factor VIII-specific tolerance in hemophilia A mice via thymic mechanisms

The development of inhibitory antibodies to factor VIII is the most serious complication of replacement therapy in hemophilia A. Activation of the innate immune system during exposure to this protein contributes to inhibitor development. However, avoidance of factor VIII exposure during innate immune system activation by external stimuli (e.g., vaccines) has not been consistently shown to prevent inhibitors. We hypothesized that dexamethasone, a drug with potent anti-inflammatory effects, could prevent inhibitors by promoting immunologic tolerance to factor VIII in hemophilia A mice. Transient dexamethasone treatment during ainitial factor VIII exposure reduced the incidence of anti-factor VIII immunoglobulin G in both a conventional hemophilia A mouse model (E16KO, 77% vs. 100%, P=0.048) and a hemophilia A mouse model with a humanized major histocompatibility complex type II transgene (E17KO/hMHC, 6% vs. 33%, P=0.0048). More importantly, among E17KO/hMHC mice that did not develop anti-factor VIII immunoglobulin G after initial exposure, dexamethasone-treated mice were less likely to develop a response after re-exposure six (7% vs. 52%, P=0.005) and 16 weeks later (7% vs. 50%, P=0.097). Similar results were obtained even when factor VIII re-exposure occurred in the context of lipopolysaccharide (30% vs. 100%, P=0.069). The ability of these mice to develop immunoglobulin G to human von Willebrand factor, a structurally unrelated antigen, remained unaffected by treatment. Transient dexamethasone administration therefore promotes antigen-specific immunologic tolerance to factor VIII. This effect is associated with an increase in the percentage of thymic regulatory T cells (12.06% vs. 4.73%, P<0.001) and changes in the thymic messenger ribonucleic acid transcription profile.

Route of Antigen Presentation Can Determine the Selection of Foxp3-Dependent or Foxp3-Independent Dominant Immune Tolerance

It has been shown that dominant tolerance, namely in transplantation, requires Foxp3⁺ regulatory T cells. Although most tolerance-inducing regimens rely on regulatory T cells, we found that induction of tolerance to proteins in aluminum hydroxide can be achieved in Foxp3-deficient mice using nondepleting anti-CD4 Abs. This type of tolerance is Ag specific, and tolerant mice retain immune competence to respond to unrelated Ags. We demonstrated with chicken OVA-specific TCR-transgenic mice that the same tolerizing protocol (CD4 blockade) and the same target Ag (OVA) achieves Foxp3-dependent transplantation tolerance to OVA-expressing skin grafts, but Foxp3-independent tolerance when the Ag is provided as OVA-aluminum hydroxide. In the latter case, we found that tolerance induction triggered recessive mechanisms leading to elimination of effector cells and, simultaneously, a dominant mechanism associated with the emergence of an anergic and regulatory CTLA-4⁺IL-2^lowFoxp3^- T cell population, where the tolerance state is IL-10 dependent. Such Foxp3-independent mechanisms can improve the efficacy of tolerance-inducing protocols.

The Induction and Maintenance of Transplant Tolerance Engages Both Regulatory and Anergic CD4+ T cells

Therapeutic tolerance to self-antigens or foreign antigens is thought to depend on constant vigilance by Foxp3⁺ regulatory T cells (Tregs). Previous work using a pancreatic islet allograft model and a short pulse of CD3 antibody therapy has shown that CD8⁺ T cells become anergic and use TGFβ and coinhibitory signaling as their contribution to the tolerance process. Here, we examine the role of CD4⁺ T cells in tolerization by CD3 antibodies. We show that both Foxp3⁺ Tregs and CD4⁺ T cell anergy play a role in the induction of tolerance and its maintenance. Foxp3⁺ Tregs resisted CD3 antibody-mediated depletion, unlike intragraft Th1 CD4⁺ lymphocytes coexpressing granzyme B and Tbx21, which were selectively eliminated. Tregs were mandatory for induction of tolerance as their depletion at the time of CD3 antibody therapy or for a short time thereafter, by an antibody to CD25 (PC61), led to graft rejection. Early treatment with CTLA-4 antibody gave the same outcome. In contrast, neither PC61 nor anti-CTLA-4 given late, at day 100 posttransplant, reversed tolerance once established. Ablation of Foxp3 T cells after diphtheria toxin injection in tolerant Foxp3^DTR recipient mice provided the same outcome. Alloreactive T cells had been rendered intrinsically unresponsive as total CD4⁺ or Treg-deprived CD4⁺ T cells from tolerant recipients were unable to mount donor-specific IFN-γ responses. In addition, intragraft Treg-deprived CD4⁺ T cells lacked proliferative capacities, expressed high levels of the inhibitory receptor PD-1, and exhibited a CD73^hiFR4^hi phenotype, thus reflecting a state of T cell anergy. We conclude that Tregs play a substantive and critical role in guiding the immune system toward tolerance of the allograft, when induced by CD3 antibody, but are less important for maintenance of the tolerant state, where T cell anergy appears sufficient.

Transplantation tolerance: context matters

Costimulation blockade has been one of the most studied strategies to achieve immune tolerance, particularly in transplantation. Yet, in spite of the robust nature of the tolerance-inducing potential of costimulation blockade, a comprehensive understanding of the molecular and cellular mechanisms underlying tolerance induction is still missing. Nevertheless, progress has been continuously made. In this issue of the European Journal of Immunology, Chai et al. [Eur. J. Immunol. 2015. 45: 2017-2027] show that transplantation tolerance induced with an anti-CD154 monoclonal antibody relies on the coexistence of several tolerogenic mechanisms rather than one simple regulatory mechanism. These observations highlight the importance of concerted actions involving multiple pathways, namely apoptosis, acquisition of regulatory cells, or inhibition of proliferation, all of which contribute to the induction and maintenance of robust immune tolerance. A better understanding of these distinct tolerogenic pathways may lead to the development of better tolerance-inducing therapeutics.

Allospecific CD4(+) T cells retain effector function and are actively regulated by Treg cells in the context of transplantation tolerance

Although donor-specific transfusion (DST) plus CD154 blockade represents a robust protocol for inducing transplantation tolerance, the underlying mechanisms are incompletely understood. In a murine T-cell adoptive transfer model, we have visualized alloantigen-specific, TCR-transgenic for H2-A(b) /H2-K(d) 54-68 epitope (TCR75) CD4(+) T cells with indirect allospecificity during the course of tolerance induction. Three main observations were made. First, although the majority of TCR75 CD4(+) T cells were deleted following DST plus CD154 blockade, the surviving TCR75 CD4(+) T cells were capable of making IL-2, upregulating CD44, and undergoing cell division, suggesting that they were functionally active. Indeed, residual TCR75 CD4(+) T cells reisolated from the primary recipients given DST plus CD154 blockade were fully capable of rejecting allografts upon secondary transfer. Second, in tolerant mice, TCR75 CD4(+) T cells were not induced to express Foxp3 in the graft-draining lymph node. TCR75 CD4(+) T cells were also absent in accepted graft tissues in which endogenous Treg cells were enriched. Finally, DST plus CD154 blockade resulted in an abortive expansion of TCR75 CD4(+) T cells, a process that required the presence of endogenous Treg cells. Collectively, surviving TCR75 CD4(+) T cells are immunocompetent but kept in check by an endogenous immunosuppressive network induced by DST plus CD154 blockade.

Non-depleting anti-CD4 monoclonal antibody induces immune tolerance to ERT in a murine model of Pompe disease

Approximately 35-40% of patients with classic infantile Pompe disease treated with enzyme replacement therapy (ERT) develop high, sustained antibody titers against the therapeutic enzyme alglucosidase alfa, which abrogates the treatment efficacy. Induction of antigen-specific immune tolerance would greatly enhance ERT for these patients. Here we show that a short-course treatment with non-depleting anti-CD4 monoclonal antibody successfully induced long-term ERT-specific immune tolerance in Pompe disease mice. Our data suggest an effective adjuvant therapy to ERT.

Suppression of inhibitor formation against FVIII in a murine model of hemophilia A by oral delivery of antigens bioencapsulated in plant cells

Hemophilia A is the X-linked bleeding disorder caused by deficiency of coagulation factor VIII (FVIII). To address serious complications of inhibitory antibody formation in current replacement therapy, we created tobacco transplastomic lines expressing FVIII antigens, heavy chain (HC) and C2, fused with the transmucosal carrier, cholera toxin B subunit. Cholera toxin B-HC and cholera toxin B-C2 fusion proteins expressed up to 80 or 370 µg/g in fresh leaves, assembled into pentameric forms, and bound to GM1 receptors. Protection of FVIII antigen through bioencapsulation in plant cells and oral delivery to the gut immune system was confirmed by immunostaining. Feeding of HC/C2 mixture substantially suppressed T helper cell responses and inhibitor formation against FVIII in mice of 2 different strain backgrounds with hemophilia A. Prolonged oral delivery was required to control inhibitor formation long-term. Substantial reduction of inhibitor titers in preimmune mice demonstrated that the protocol could also reverse inhibitor formation. Gene expression and flow cytometry analyses showed upregulation of immune suppressive cytokines (transforming growth factor β and interleukin 10). Adoptive transfer experiments confirmed an active suppression mechanism and revealed induction of CD4(+)CD25(+) and CD4(+)CD25(-) T cells that potently suppressed anti-FVIII formation. In sum, these data support plant cell-based oral tolerance for suppression of inhibitor formation against FVIII.

Harnessing FOXP3+ regulatory T cells for transplantation tolerance

Early demonstrations that mice could be tolerized to transplanted tissues with short courses of immunosuppressive therapy and that with regard to tolerance to self, CD4+FOXP3+ regulatory T cells (Tregs) appeared to play a critical role, have catalyzed strategies to harness FOXP3-dependent processes to control rejection in human transplantation. This review seeks to examine the scientific underpinning for this new approach to finesse immunosuppression.

Anti-CD4 treatment inhibits autoimmunity in scurfy mice through the attenuation of co-stimulatory signals

A major concept in autoimmunity is that disruption of Foxp3(+) regulatory T cells (Tregs) predisposes to breach of tolerance. This is exemplified by the Foxp3-linked disorder termed IPEX (immunodysregulation, polyendocrinopathy, enteropathy, X-linked) which affects newborn children. There has been considerable clinical interest in the role of non-depleting anti-CD4 antibodies as a means of upregulating the function of Foxp3(+) Tregs in order to control detrimental inflammatory responses such as transplant rejection. However, according to the paradigm of a Treg-dependent mechanism of action, the effectiveness of anti-CD4 antibodies as a therapy for human autoimmune diseases is unclear considering that Treg function might be intrinsically impaired. Specifically, anti-CD4 therapy is expected to fail in patients suffering from the IPEX syndrome due to the lack of functional Foxp3(+) Tregs. Taking advantage of natural Foxp3 mutant scurfy (sf) mice closely resembling the IPEX syndrome, and genetically engineered mice depleted of Foxp3(+) Tregs, we report here that anti-CD4 treatment induces tolerance independent of Foxp3(+) Tregs. This so far undefined mechanism is dependent on the recessive non-infectious tolerization of autoreactive T cells. Treg-independent tolerance alone is powerful enough to suppress both the onset and severity of autoimmunity and reduces clinically relevant autoantibody levels and liver fibrosis. Mechanistically, tolerance induction requires the concomitant activation of autoreactive T cells and is associated with the down-regulation of the co-stimulatory TNF-receptor superfamily members OX40 and CD30 sustaining CD4(+) T cell survival. In the light of ongoing clinical trials, our results highlight an unexpected potency of anti-CD4 antibodies for the treatment of autoimmune diseases. Particularly, CD4 blockade might represent a novel therapeutic option for the human IPEX syndrome.

CD4 blockade directly inhibits mouse and human CD4(+) T cell functions independent of Foxp3(+) Tregs

CD4(+) helper T cells orchestrate protective immunity against pathogens, yet can also induce undesired pathologies including allergies, transplant rejection and autoimmunity. Non-depleting CD4-specific antibodies such as clone YTS177.9 were found to promote long-lasting T cell tolerance in animal models. Thus, CD4 blockade could represent a promising therapeutic approach for human autoimmune diseases. However, the mechanisms underlying anti-CD4-induced tolerance are incompletely resolved. Particularly, multiple immune cells express CD4 including Foxp3(+) regulatory T cells (Tregs) and dendritic cells (DCs), both controlling the activation of CD4(+)Foxp3(-) helper T cells. Utilizing mixed leukocyte reactions (MLRs) reflecting physiological interactions between T cells and DCs, we report that anti-CD4 treatment inhibits CD4(+)Foxp3(-) T cell proliferation in an IL-2-independent fashion. Notably, YTS177.9 binding induces a rapid internalization of CD4 on both CD4(+)Foxp3(-) T cells and Foxp3(+) Tregs. However, no expansion or activation of immunosuppressive CD4(+)Foxp3(+) Tregs was observed following anti-CD4 treatment. Additionally, cytokine production, maturation and T cell priming capacity of DCs are not affected by anti-CD4 exposure. In line with these data, the selective ablation of Foxp3(+) Tregs from MLRs by the use of diphtheria toxin (DT)-treated bacterial artificial chromosome (BAC)-transgenic DEREG mice completely fails to abrogate the suppressive activity of multiple anti-CD4 antibodies. Instead, tolerization is associated with the defective expression of various co-stimulatory receptors including OX40 and CD30, suggesting altered signaling through the TCR complex. Consistent with our findings in mice, anti-CD4 treatment renders human CD4(+) T cells tolerant in the absence of Tregs. Thus, our results establish that anti-CD4 antibodies can directly tolerize pathogenic CD4(+)Foxp3(-) helper T cells. This has important implications for the treatment of human inflammatory diseases.