Regulatory T cells inhibit protein kinase C theta recruitment to the immune synapse of naive T cells with the same antigen specificity

Protein Kinase C at the Crossroad of Mutations, Cancer, Targeted Therapy and Immune Response

The frequent PKC dysregulations observed in many tumors have made these enzymes natural targets for anticancer applications. Nevertheless, this considerable interest in the development of PKC modulators has not led to the expected therapeutic benefits, likely due to the complex biological activities regulated by PKC isoenzymes, often playing ambiguous and protective functions, further driven by the occurrence of mutations. The structure, regulation and functions of PKCs have been extensively covered in other publications. Herein, we focused on PKC alterations mostly associated with complete functional loss. We also addressed the modest yet encouraging results obtained targeting PKC in selected malignancies and the more frequent negative clinical outcomes. The reported observations advocate the need for more selective molecules and a better understanding of the involved pathways. Furthermore, we underlined the most relevant immune mechanisms controlled by PKC isoforms potentially impacting the immune checkpoint inhibitor blockade-mediated immune recovery. We believe that a comprehensive examination of the molecular features of the tumor microenvironment might improve clinical outcomes by tailoring PKC modulation. This approach can be further supported by the identification of potential response biomarkers, which may indicate patients who may benefit from the manipulation of distinctive PKC isoforms.

Cell-Penetrating Anti-Protein Kinase C Theta Antibodies Act Intracellularly to Generate Stable, Highly Suppressive Regulatory T Cells

Regulatory T cells maintain immunological tolerance and dampen inflammatory responses. Administering regulatory T cells can prevent the immune-mediated tissue destruction of graft-versus-host disease, which frequently accompanies hematopoietic stem cell transfer. Neutralizing the T cell-specific kinase, protein kinase C theta, which promotes T cell effector functions and represses regulatory T cell differentiation, augments regulatory T cell immunosuppression and stability. We used a synthetic, cell-penetrating peptide mimic to deliver antibodies recognizing protein kinase C theta into primary human CD4 T cells. When differentiated ex vivo into induced regulatory T cells, treated cells expressed elevated levels of the regulatory T cell transcriptional regulator forkhead box P3, the surface-bound immune checkpoint receptor programmed death receptor-1, and pro-inflammatory interferon gamma, previously ascribed to a specific population of stable, highly suppressive human induced regulatory T cells. The in vitro suppressive capacity of these induced regulatory T cells was 10-fold greater than that of T cells differentiated without antibody delivery. When administered at the time of graft-versus-host disease induction, using a humanized mouse model, antibody-treated regulatory T cells were superior to non-treated T cells in attenuating lethal outcomes. This antibody delivery approach may overcome obstacles currently encountered using patient-derived regulatory T cells as a cell-based therapy for immune modulation.

Cell Type-Specific Regulation of Immunological Synapse Dynamics by B7 Ligand Recognition

B7 proteins CD80 (B7-1) and CD86 (B7-2) are expressed on most antigen-presenting cells and provide critical co-stimulatory or inhibitory input to T cells via their T-cell-expressed receptors: CD28 and CTLA-4. CD28 is expressed on effector T cells and regulatory T cells (Tregs), and CD28-dependent signals are required for optimum activation of effector T cell functions. CD28 ligation on effector T cells leads to formation of distinct molecular patterns and induction of cytoskeletal rearrangements at the immunological synapse (IS). CD28 plays a critical role in recruitment of protein kinase C (PKC)-θ to the effector T cell IS. CTLA-4 is constitutively expressed on the surface of Tregs, but it is expressed on effector T cells only after activation. As CTLA-4 binds to B7 proteins with significantly higher affinity than CD28, B7 ligand recognition by cells expressing both receptors leads to displacement of CD28 and PKC-θ from the IS. In Tregs, B7 ligand recognition leads to recruitment of CTLA-4 and PKC-η to the IS. CTLA-4 plays a role in regulation of T effector and Treg IS stability and cell motility. Due to their important roles in regulating T-cell-mediated responses, B7 receptors are emerging as important drug targets in oncology. In this review, we present an integrated summary of current knowledge about the role of B7 family receptor–ligand interactions in the regulation of spatial and temporal IS dynamics in effector and Tregs.

The Role of PKC-θ in CD4+ T Cells and HIV Infection: To the Nucleus and Back Again

Protein kinase C (PKC)-θ is the only member of the PKC family that has the ability to translocate to the immunological synapse between T cells and antigen-presenting cells upon T cell receptor and MHC-II recognition. PKC-θ interacts functionally and physically with other downstream effector molecules to mediate T cell activation, differentiation, and migration. It plays a critical role in the generation of Th2 and Th17 responses and is less important in Th1 and CTL responses. PKC-θ has been recently shown to play a role in the nucleus, where it mediates inducible gene expression in the development of memory CD4+ T cells. This novel PKC (nPKC) can up-regulate HIV-1 transcription and PKC-θ activators such as Prostratin have been used in early HIV-1 reservoir eradication studies. The exact manner of the activation of virus by these compounds and the role of PKC-θ, particularly its nuclear form and its association with NF-κB in both the cytoplasmic and nuclear compartments, needs further precise elucidation especially given the very important role of NF-κB in regulating transcription from the integrated retrovirus. Continued studies of this nPKC isoform will give further insight into the complexity of T cell signaling kinases.

The immunological synapse: the gateway to the HIV reservoir

A major challenge in the development of a cure for human immunodeficiency virus (HIV) has been the incomplete understanding of the basic mechanisms underlying HIV persistence during antiretroviral therapy. It is now realized that the establishment of a latently infected reservoir refractory to immune system recognition has thus far hindered eradication efforts. Recent investigation into the innate immune response has shed light on signaling pathways downstream of the immunological synapse critical for T-cell activation and establishment of T-cell memory. This has led to the understanding that the cell-to-cell contacts observed in an immunological synapse that involve the CD4⁺ T cell and antigen-presenting cell or T-cell–T-cell interactions enhance efficient viral spread and facilitate the induction and maintenance of latency in HIV-infected memory T cells. This review focuses on recent work characterizing the immunological synapse and the signaling pathways involved in T-cell activation and gene regulation in the context of HIV persistence.

Potent CD4+ T-cell epitope P30 enhances HER2/neu-engineered dendritic cell-induced immunity against Tg1-1 breast cancer in transgenic FVBneuN mice by enhanced CD4+ T-cell-stimulated CTL responses

One of the major obstacles in human epidermal growth factor receptor (HER)-2/neu-specific trastuzumab immunotherapy of HER2/neu-positive breast cancer is the development of trastuzumab resistance, warranting the search for other therapeutic strategies. Although dendritic cell (DC) vaccines have been extensively applied in clinical trials for cancer treatment, the vaccination efficacy is still limited, mostly because DC vaccines are not sufficient to break tumor-associated antigen-specific self-immune tolerance in cancer patients. P30 (FNNFTVSFWLRVPKVSASHLE) derived from tetanus toxin is a universally potent CD4(+) T helper epitope capable of enhancing CD8(+) cytotoxic T-lymphocyte (CTL) responses. In this study, we constructed two recombinant adenoviral vectors (AdVs), AdVOVA-P30 and AdVHER2/neu-P30, expressing ovalbumin (OVA)-P30 and HER2/neu-P30. In order to enhance DC vaccine efficacy, we transfected mouse bone marrow (BM)-derived DCs with AdVOVA-P30 and AdVHER2/neu-P30 to generate engineered DCOVA-P30 and DCHER2/neu-P30 vaccines, respectively. We, then, compared CD4(+) and CD8(+) T-cell responses and antitumor immunity derived from DCOVA-P30 and DCHER2/neu-P30 vaccination in wild-type C57BL/6 and transgenic FVBneuN mice, respectively. We demonstrate that engineered DCOVA-P30 vaccine stimulates more efficient CD4(+) and CD8(+) T-cell responses than DCOVA in C57BL/6 mice. Interestingly, the increased DCOVA-P30-induced CTL responses are mainly contributed by enhanced CD4(+) T-cell-stimulated CTL proliferation. We show that DCOVA-P30 vaccine also stimulates more efficient therapeutic immunity against OVA-expressing BL6-10OVA melanoma than DCOVA in C57BL/6 mice. In addition, we demonstrate that DCHER2/neu-P30 vaccine stimulates more efficient CD4(+) and CD8(+) T-cell responses and protective immunity against HER2/neu-expressing Tg1-1 breast cancer than DCHER2/neu in transgenic FVBneuN mice with HER2/neu-specific self-immune tolerance. Therefore, the engineered DCHER2/neu-P30 vaccine may provide a new immunotherapy alternative for women with HER2/neu(+) breast cancer, especially for trastuzumab-resistant HER2/neu(+) breast cancer patients.

Molecular mechanisms of treg-mediated T cell suppression

CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) can suppress other immune cells and, thus, are critical mediators of peripheral self-tolerance. On the one hand, Tregs avert autoimmune disease and allergies. On the other hand, Tregs can prevent immune reactions against tumors and pathogens. Despite the importance of Tregs, the molecular mechanisms of suppression remain incompletely understood and controversial. Proliferation and cytokine production of CD4(+)CD25(-) conventional T cells (Tcons) can be inhibited directly by Tregs. In addition, Tregs can indirectly suppress Tcon activation via inhibition of the stimulatory capacity of antigen presenting cells. Direct suppression of Tcons by Tregs can involve immunosuppressive soluble factors or cell contact. Different mechanisms of suppression have been described, so far with no consensus on one universal mechanism. Controversies might be explained by the fact that different mechanisms may operate depending on the site of the immune reaction, on the type and activation state of the suppressed target cell as well as on the Treg activation status. Further, inhibition of T cell effector function can occur independently of suppression of proliferation. In this review, we summarize the described molecular mechanisms of suppression with a particular focus on suppression of Tcons and rapid suppression of T cell receptor-induced calcium (Ca(2+)), NFAT, and NF-κB signaling in Tcons by Tregs.

Biological functions of regulatory T cells

The subpopulation of CD4(+) T lymphocytes that co-express the transcription factor Foxp3 plays a unique role as regulatory T lymphocytes (Tregs) that modulate many aspects of the immune response. Multiple mechanisms have been proposed for the suppressor function of CD4(+)Foxp3(+) T cells based on in vitro studies, but much less is known about how Tregs suppress immune responses in vivo. Both polyclonal Tregs and antigen-specific Tregs are capable of exerting potent suppressive effects in vivo, and it is likely that they mediate their biologic functions using different mechanisms. Antigen-specific Tregs primarily target dendritic cells and inhibit dendritic cell functions including the expression of costimulatory molecules and the presentation of antigen early during the generation of the immune response. The end result is a complete inhibition of both the expansion and the differentiation of T effector cells. Polyclonal Tregs also act on dendritic cells, but at a later phase, and do not inhibit expansion of T effector cells, but appear to modulate differentiation and cell trafficking. The cell surface molecules involved in the interaction of Tregs with dendritic cells, as well as the biochemical pathways modified by this interaction remain to be fully elucidated. A complete understand of the biological functions of Tregs in vivo should facilitate the development of pharmacologic and biologic agents that can be used to modulate Treg function in a therapeutic setting.

APC, T cells, and the immune synapse

CD4(+) T cells engage different activating cells during their generation in the bone marrow and thymus and during their homeostasis and activation in the periphery. During these processes, T cells or their precursors establish a molecular platform for communication in the interface between the two cells that is called immune synapse (IS). Here we review the current knowledge about those different IS. Apart from looking at the structure and signalling of the IS from the T cell region, we will also focus on the area of the IS partner, mostly antigen-presenting cells (APC). We will discuss the features of different APC and their role played in the control of the resulting activated or differentiated T cell. We will also demonstrate that despite 10 years of research into the subject, large areas of this field are yet to be explored. This will keep us busy for the years to come - new exciting results lie ahead of us.

Plasticity of immunological synapses

TCR engagement with peptide/MHC complexes displayed on the surface of the antigen-presenting cells is the crucial event in developing an adaptive immune response and occurs within specialized signaling areas named immunological synapses. Immunological synapses are diverse both in structure and function and exhibit a strikingly dynamic molecular organization. In this review, we focus on the diversity of immunological synapses and on their plasticity in response to stimulation. We discuss how the study of the adaptable features of immunological synapses can be instrumental to a better understanding of the complex regulation of adaptive immunity.

Effects of the novel protein kinase C inhibitor AEB071 (Sotrastaurin) on rat cardiac allograft survival using single agent treatment or combination therapy with cyclosporine, everolimus or FTY720

NVP-AEB071 (AEB, sotrastaurin), an oral inhibitor of protein kinase C (PKC), effectively blocks T-cell activation. The immunosuppressive effects of oral AEB were demonstrated in a rat local graft versus host (GvH) reaction and rat cardiac transplantation models. T-cell activation was suppressed by 95% in blood from AEB-treated rats, with a positive correlation between T-cell inhibition and AEB blood concentration. In GvH studies, AEB inhibited lymph node swelling dose-dependently (3-30 mg/kg). BN and DA cardiac allografts were acutely rejected within 6-10 days post-transplantation in untreated LEW rats. AEB at 10 and 30 mg/kg b.i.d. prolonged BN graft survival to a mean survival time of 15 and >28 days, and DA grafts to 6.5 and 17.5 days, respectively. In the DA to LEW model, combining a nonefficacious dose of AEB (10 mg/kg b.i.d.) with a nonefficacious dose of cyclosporine, everolimus or FTY720 led to prolonged median survival times (26 days, >68 days and >68 days, respectively). Pharmacokinetic monitoring excluded drug-drug interactions, suggesting synergy. In conclusion, these studies are the first to demonstrate that AEB prolongs rat heart allograft survival safely as monotherapy and in combination with nonefficacious doses of cyclosporine, everolimus or FTY720. Thus, AEB may have the potential to offer an alternative to calcineurin inhibitor-based therapies.

Towards identification of the mechanisms of action of parasite-derived peptide GK1 on the immunogenicity of an influenza vaccine

Previous studies have shown that the synthetic peptide GK1, derived from Taenia crassiceps cysticerci, enhances the immunogenicity of the commercial inactivated influenza vaccine Fluzone in both young and aged mice. In particular, antibody responses were much improved. Since GK1 is a peptide and is rapidly cleared from the body, it offers the possibility to improve vaccine performance without undesirable effects. This study was therefore designed to understand the mechanisms of action involved in the adjuvant properties of GK1. For this, transgenic mice expressing a T-cell receptor specific for an epitope from the influenza virus hemagglutinin (HA) protein were employed. The GK1 peptide significantly increased the in vivo proliferative response of HA-specific CD4+ T cells when it was coimmunized with the HA epitope. Dendritic cells treated in vitro with GK1 were capable of enhancing T-cell activation. Furthermore, in synergy with lipopolysaccharide, GK1 enhanced the expression of major histocompatibility complex class II and costimulatory molecules of dendritic cells and promoted the secretion of proinflammatory cytokines and chemokines upon antigen-driven T-cell interaction. These data provide important insights into the mechanism that underlies the GK1 adjuvant capacity observed previously and underline the feasibility of using the transgenic mouse model described herein as a tool for investigation of the modes of action of different influenza vaccine adjuvants.

Role of initial protein phosphorylation events and localized release-activated calcium influx in B cell antigen receptor signaling

An increase in intracellular calcium concentration is one of the major initial steps in B cell activation following antigen receptor (BCR) ligation. We show herein that in C57BL/6 murine B lymphocytes and in model cell lines, BCR-mediated calcium ion (Ca(2+)) influx occurs via highly selective Ca(2+) release-activated channels, and stromal interaction molecule 1 (STIM1) plays an important role in this pathway. We also demonstrate the temporal relation between Ca(2+)-dependent signaling events and formation of the immune synapse. Our data indicate that cytoplasmic Ca(2+) levels in areas adjacent to the immune synapse differ from those in the rest of the cytoplasm. Finally, a comparison of phosphorylation patterns of BCR-triggered signaling proteins in the presence or absence of Ca(2+) revealed the unanticipated finding that initial BCR-triggered, Ca(2+)-dependent tyrosine phosphorylation events involve predominantly Ca(2+) released from intracellular stores and that influx-derived Ca(2+) is not essential. This suggests a different role for this phase of Ca(2+) influx.

Human regulatory T cells inhibit polarization of T helper cells toward antigen-presenting cells via a TGF-beta-dependent mechanism

The molecular mechanisms used by regulatory T cells (Treg) to inhibit the effector phase of adaptive immune responses are still elusive. In the present work, we investigated the possibility that Treg may interfere with a basic biological function of T helper cells (T(H)): polarization of secretory machinery for dedicated help delivery. To address this question, we visualized by confocal microscopy different parameters of activation in T(H) and Treg cells interacting simultaneously with individual antigen-presenting cells (APC). Our results show that, although productive TCR engagement in T(H)/APC conjugates was unaffected by the presence of adjacent Treg, the reorientation of T(H) secretory machinery toward APC was strongly inhibited. Blocking TGF-beta completely reverted Treg induced inhibition of T(H) polarization. Our results identify a previously undescribed mechanism by which Treg inhibit effector T cells. TGF-beta produced by adjacent Treg interferes with polarization of T(H) secretory machinery toward APC, thus affecting a crucial step of T(H)-mediated amplification of the immune response.

The molecular makeup and function of regulatory and effector synapses

Physical interactions between T cells and antigen-presenting cells (APCs) form the basis of any specific immune response. Upon cognate contacts, a multimolecular assembly of receptors and adhesion molecules on both cells is created, termed the immunological synapse (IS). Very diverse structures of ISs have been described, yet the functional importance for T-cell differentiation is largely unclear. Here we discuss the principal structure and function of ISs. We then focus on two characteristic T-cell-APC pairs, namely T cells contacting dendritic cells (DCs) or naive B cells, for which extremely different patterns of the IS have been observed as well as fundamentally different effects on the function of the activated T cells. We provide a model on how differences in signaling and the involvement of adhesion molecules might lead to diverse interaction kinetics and, eventually, diverse T-cell differentiation. We hypothesize that the preferred activation of the adhesion molecule leukocyte function-associated antigen-1 (LFA-1) and of the negative regulator for T-cell activation, cytotoxic T-lymphocyte antigen-4 (CTLA-4), through contact with naive B cells, lead to prolonged cell-cell contacts and the generation of T cells with regulatory capacity. In contrast, DCs might have evolved mechanisms to avoid LFA-1 overactivation and CTLA-4 triggering, thereby promoting more dynamic contacts that lead to the preferential generation of effector cells.

CD4+ T cell acquisition of the bystander pMHC I colocalizing in the same immunological synapse comprising pMHC II and costimulatory CD40, CD54, CD80, OX40L, and 41BBL

We previously showed that CD4+ T cells acquired peptide/major histocompatibility complex (pMHC) I and costimulatory molecules by dendritic cell (DC) activation. However, the molecular mechanism for pMHC I acquisition is unclear. In this study, by using a panel of engineered DC2.4 cells or incubation of these cells with Con A-stimulated CD4+ T cells, we conducted capping and synapse formation assay and examined them by confocal fluorescence microscopy. We demonstrated that (i) CD54 and CD80 colocalized with pMHC I/II in the same lipid rafts, whereas CD40, OX40L, and 41BBL localized in the lipid rafts but separately from pMHC I/II, and (ii) MHC I/II colocalized with the costimulatory molecules in the same synapse formed between a DC and a CD4+ T cell, leading to expression of the acquired bystander pMHC I on CD4+ T cells via internalization/recycling pathway. These results provide some useful information in composition and dynamics of immunological synapses.

Naive B cells generate regulatory T cells in the presence of a mature immunologic synapse

Naive B cells are ineffective antigen-presenting cells and are considered unable to activate naive T cells. However, antigen-specific contact of these cells leads to stable cell pairs that remain associated over hours in vivo. The physiologic role of such pairs has not been evaluated. We show here that antigen-specific conjugates between naive B cells and naive T cells display a mature immunologic synapse in the contact zone that is absent in T-cell-dendritic-cell (DC) pairs. B cells induce substantial proliferation but, contrary to DCs, no loss of L-selectin in T cells. Surprisingly, while DC-triggered T cells develop into normal effector cells, B-cell stimulation over 72 hours induces regulatory T cells inhibiting priming of fresh T cells in a contact-dependent manner in vitro. In vivo, the regulatory T cells home to lymph nodes where they potently suppress immune responses such as in cutaneous hypersensitivity and ectopic allogeneic heart transplant rejection. Our finding might help to explain old observations on tolerance induction by B cells, identify the mature immunologic synapse as a central functional module of this process, and suggest the use of naive B-cell-primed regulatory T cells, "bTregs," as a useful approach for therapeutic intervention in adverse adaptive immune responses.

Modulation of immunological synapse by membrane-bound and soluble ligands

An efficient adaptive immune response should prevent pathogen infections and tumor growth without causing significant damage to host constituents. A crucial event determining the balance between tolerance and immunity is antigen recognition by T cells on the surface of antigen presenting cells (APC). Several molecular contacts at the interface between T cells and APCs contribute to define the nature of the adaptive immune response against a particular antigen. Upon TCR engagement by a peptide-MHC complex (pMHC) on the surface of an APC, a specialized supra-molecular structure known as immunological synapse (IS) assembles at the interface between these two cells. This structure involves massive re-distribution of membrane proteins, including TCR and pMHC complexes, as well as co-stimulatory and adhesion molecules. Furthermore, IS assembly leads to several important intracellular events necessary for T cell activation, such as recruitment of signaling molecules and cytoskeleton rearrangements. Because IS assembly leads to major consequences on the function of T cells, several studies have attempted to identify both soluble and membrane-bound molecules that could contribute to modulate the IS function. Here we describe recent literature on the regulation of IS assembly and modulation by TCR/pMHC binding kinetics, chemokines and cytokines focusing on their role at controlling the balance between adaptive immunity and tolerance.

von Willebrand factor and transforming growth factor-beta modulate immune response against coagulation factor VIII in FVIII-deficient mice

In up to 25% haemophilia A patients, the administration of coagulation factor VIII (FVIII) preparations for treatment of haemorrhages results in production of factor VIII specific antibodies. Plasma-derived FVIII preparations contain other plasma proteins, which may modulate the immune response to FVIII. We used FVIII-deficient mice to assess the role of von Willebrand factor (VWF) and cytokine transforming growth factor beta-1 (TGF-beta1) in the immune response against FVIII. Using the FVIII and FVIII in complex with VWF purified from the plasma-derived FVIII preparation, we demonstrated that a lower concentration of FVIII antibody was induced in FVIII-VWF-treated mice compared to FVIII-treated mice (p<0.05). The addition of recombinant latent TGF-beta1 to FVIII decreased the antibody response against FVIII compared to FVIII treatment alone (p<0.01). The obtained results suggest that VWF and latent TGF-beta1 present in plasma-derived FVIII preparations reduce the immune response against FVIII. However, we cannot exlude possible modulatory effects of other plasma proteins.