Transplantation Tolerance in NF-κB-Impaired Mice Is Not Due to Regulation but Is Prevented by Transgenic Expression of Bcl-xL

Transplantation tolerance and its outcome during infections and inflammation

Much progress has been made toward understanding the mechanistic basis of transplantation tolerance in experimental models, which implicates clonal deletion of alloreactive T and B cells, induction of cell-intrinsic hyporesponsiveness, and dominant regulatory cells mediating infectious tolerance and linked suppression. Despite encouraging success in the laboratory, achieving tolerance in the clinic remains challenging, although the basis for these challenges is beginning to be understood. Heterologous memory alloreactive T cells generated by infections prior to transplantation have been shown to be a critical barrier to tolerance induction. Furthermore, infections at the time of transplantation and tolerance induction provide a pro-inflammatory milieu that alters the stability and function of regulatory T cells as well as the activation requirements and differentiation of effector T cells. Thus, infections can result in enhanced alloreactivity, resistance to tolerance induction, and destabilization of the established tolerance state. We speculate that these experimental findings have relevance to the clinic, where infections have been associated with allograft rejection and may be a causal event precipitating the loss of grafts after long periods of stable operational tolerance. Understanding the mechanisms by which infections prevent and destabilize tolerance can lead to therapies that promote stable life-long tolerance in transplant recipients.

B cells assist allograft rejection in the deficiency of protein kinase c-theta

We have previously shown that mice deficient in protein kinase C theta (PKCθ) have the ability to reject cardiac allografts, but are susceptible to tolerance induction. Here we tested role of B cells in assisting alloimmune responses in the absence of PKCθ. Mouse cardiac allograft transplantations were performed from Balb/c (H-2d) to PKCθ knockout (PKCθ(-/-)), PKCθ and B cell double-knockout (PBDK, H-2b) mice and wild-type (WT) C57BL/6 (H-2b) mice. PBDK mice spontaneously accepted the allografts with the inhibition of NF-κB activation in the donor cardiac allograft. Anti-B cell antibody (rituximab) significantly delayed allograft rejection in PKCθ(-/-), but not in WT mice. Co-transfer of PKCθ(-/-) T plus PKCθ(-/-) B cells or primed sera triggered allograft rejection in Rag1(-/-) mice, and only major histocompatibility complex class II-enriched B cells, but not class I-enriched B cells, were able to promote rejection. This, together with the inability of PKCθ(-/-) and CD28(-/-) double-deficient (PCDK) mice to acutely reject allografts, suggested that an effective cognate interaction between PKCθ(-/-) T and B cells for acute rejection is CD28 molecule dependent. We conclude that T-B cell interactions synergize with PKCθ(-/-) T cells to mediate acute allograft rejection.

OX40 signaling favors the induction of T(H)9 cells and airway inflammation

The mechanisms that regulate the T(H)9 subset of helper T cells and diseases mediated by T(H)9 cells remain poorly defined. Here we found that the costimulatory receptor OX40 was a powerful inducer of T(H)9 cells in vitro and T(H)9 cell-dependent airway inflammation in vivo. In polarizing conditions based on transforming growth factor-β (TGF-β), ligation of OX40 inhibited the production of induced regulatory T cells and the T(H)17 subset of helper T cells and diverted CD4(+)Foxp3(-) T cells to a T(H)9 phenotype. Mechanistically, OX40 activated the ubiquitin ligase TRAF6, which triggered induction of the kinase NIK in CD4(+) T cells and the noncanonical transcription factor NF-κB pathway; this subsequently led to the generation of T(H)9 cells. Thus, our study identifies a previously unknown mechanism for the induction of T(H)9 cells and may have important clinical implications in allergic inflammation.

Role of T-cell-specific nuclear factor κB in islet allograft rejection

BACKGROUND

Pancreatic islet transplantation has the potential to cure type 1 diabetes, a chronic lifelong disease, but its clinical applicability is limited by allograft rejection. Nuclear factor κB (NF-κB) is a transcription factor important for survival and differentiation of T cells. In this study, we tested whether NF-κB in T cells is required for the rejection of islet allografts.

METHODS

Mice expressing a superrepressor form of NF-κB selectively in T cells (IκBαΔN-Tg mice) with or without the antiapoptotic factor Bcl-xL, or mice with impaired T-cell receptor (TCR)- and B cell receptor-driven NF-κB activity (CARMA1-KO mice) were rendered diabetic and transplanted with islet allografts. Secondary skin transplantation in long-term acceptors of islet allografts was used to test for the development of donor-specific tolerance. Immune infiltration of the transplanted islets was examined by immunofluorescence. TCR-transgenic CD4 T cells were used to follow T-cell priming and differentiation.

RESULTS

Islet allograft survival was prolonged in IκBαΔN-Tg mice, although the animals did not develop donor-specific tolerance. Reduced NF-κB activity did not prevent T-cell priming or differentiation but reduced survival of activated T cells, as transgenic expression of Bcl-xL restored islet allograft rejection in IκBαΔN-Tg mice. Abolishing TCR- and B cell receptor-driven activation of NF-κB selectively by CARMA1 deficiency prevented T-cell priming and islet allograft rejection.

CONCLUSIONS

Our data suggest that T cell-NF-κB plays an important role in the rejection of islet allografts. Targeting NF-κB selectively in lymphocytes seems a promising approach to facilitate acceptance of transplanted islets.

Role of T cell-nuclear factor κB in transplantation

Nuclear factor (NF) κB is a pleiotropic transcription factor that is ubiquitously expressed. After transplantation of solid organs, NF-κB in the graft is activated within a few hours as a consequence of ischemia/reperfusion and then again after a few days in intragraft infiltrating cells during the process of acute allograft rejection. In the present article, we review the components of the NF-κB pathway, their mechanisms of activation, and their role in T cell and antigen-presenting cell activation and differentiation and in solid organ allograft rejection. Targeted inhibition of NF-κB in selected cell types may promote graft survival with fewer adverse effects compared with global immunosuppressive therapies.

Cell-intrinsic NF-κB activation is critical for the development of natural regulatory T cells in mice

Background

Naturally occurring CD4⁺CD25⁺Foxp3⁺ regulatory T (Treg) cells develop in the thymus and represent a mature T cell subpopulation critically involved in maintaining peripheral tolerance. The differentiation of Treg cells in the thymus requires T cell receptor (TCR)/CD28 stimulation along with cytokine-promoted Foxp3 induction. TCR-mediated nuclear factor kappa B (NF-κB) activation seems to be involved in differentiation of Treg cells because deletion of components of the NF-κB signaling pathway, as well as of NF-κB transcription factors, leads to markedly decreased Treg cell numbers in thymus and periphery.

Methodology/Principal Findings

To investigate if Treg cell-intrinsic NF-κB activation is required for thymic development and peripheral homeostasis of Treg cells we used transgenic (Tg) mice with thymocyte-specific expression of a stable IκBα mutant to inhibit NF-κB activation solely within the T cell lineage. Here we show that Treg cell-intrinsic NF-κB activation is important for the generation of cytokine-responsive Foxp3⁻ thymic Treg precursors and their further differentiation into mature Treg cells. Treg cell development could neither be completely rescued by the addition of exogenous Interleukin 2 (IL-2) nor by the presence of wild-type derived cells in adoptive transfer experiments. However, peripheral NF-κB activation appears to be required for IL-2 production by conventional T cells, thereby participating in Treg cell homeostasis. Moreover, pharmacological NF-κB inhibition via the IκB kinase β (IKKβ) inhibitor AS602868 led to markedly diminished thymic and peripheral Treg cell frequencies.

Conclusion/Significance

Our results indicate that Treg cell-intrinsic NF-κB activation is essential for thymic Treg cell differentiation, and further suggest pharmacological NF-κB inhibition as a potential therapeutic approach for manipulating this process.

c-Rel phenocopies PKCtheta but not Bcl-10 in regulating CD8+ T-cell activation versus tolerance

Elucidating the signaling events that promote T-cell tolerance versus activation provides important insights for manipulating immunity in vivo. Previous studies have suggested that the absence of PKCtheta results in the induction of anergy and that the balance between the induction of the transcription factors NFAT, AP1 and NF-kappaB plays a key role in determining whether T-cell anergy or activation is induced. Here, we examine whether Bcl-10 and specific family members of NF-kappaB act downstream of PKCtheta to alter CD8(+) T-cell activation and/or anergy. We showed that T cells from mice deficient in c-Rel but not NF-kappaB1 (p50) have increased susceptibility to the induction of anergy, similar to T cells from PKCtheta-deficient mice. Surprisingly T cells from Bcl-10-deficient mice showed a strikingly different phenotype to the PKCtheta-deficient T cells, with a severe block in TCR-mediated activation. Furthermore, we have also shown that survival signals downstream of NF-kappaB, are uncoupled from signals that mediate T-cell anergy. These results suggest that c-Rel plays a critical role downstream of PKCtheta in controlling CD8(+) T-cell anergy induction.

CARMA1 regulation of regulatory T cell development involves modulation of interleukin-2 receptor signaling

T cell receptor-stimulated NF-kappaB activation requires CARMA1 and is negatively regulated by the deubiquitinase CYLD. Recent studies suggest that CARMA1 regulates regulatory T cell (Treg) development, although the role of NF-kappaB in this event is incompletely understood. We show that CYLD deficiency causes constitutive NF-kappaB activation in thymocytes, which is associated with enhanced frequency of Treg cells. The NF-kappaB activation in CYLD-deficient thymocytes is independent of CARMA1, because the NF-kappaB activation was also detected in CYLD/CARMA1 double knock-out thymocytes. Interestingly, although loss of CYLD causes NF-kappaB activation in the CARMA1-deficient thymocytes, the CYLD deficiency fails to rescue the defect of CARMA1 knock-out mice in Treg development. Furthermore, inhibition of canonical NF-kappaB by an IkappaBalpha transgene only partially inhibits Treg development. We demonstrate that CARMA1 regulates IL-2 receptor signaling and controls the IL-2-stimulated maturation of Treg precursors to mature Tregs. These results suggest that the role of CARMA1 in Treg regulation involves both NF-kappaB activation and IL-2 receptor signaling.

Protosappanin A induces immunosuppression of rats heart transplantation targeting T cells in grafts via NF-kappaB pathway

Protosappanin A as one major and effective ingredient from Caesalpinia sappan L. exhibited antirejection activity obviously in heart-transplanted rat. The present study was designed to screen out the potential target genes of protosappanin A with microarray technology and reveal some molecular mechanism of immunosuppressive effect. Rats performed with ectopic peritoneal heart transplantation were randomized into three groups receiving different treatments for 7 days: protosappanin A group (25 mg kg(-1)), cyclosporine A group (10 mg kg(-1)), and control group. The differentially expressed genes responding to protosappanin A were analyzed with microarrays. Among common differentially expressed genes, the ones of interest were selected for further evaluation by real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), Western blot, immunochemistry, immunofluorescence, and ELISA. Among the 146 common differentially expressed genes, NF-kappaB and related genes like IkappaBa, IFN-r, and IP10 were selected for verification. The results of qRT-PCR, Western blot, immunochemistry, and ELISA showed that protosappanin A significantly reduced the expression of NF-kappaB, IFN-r, and IP10 (p < 0.05) and increased IkappaBa expression (p < 0.05) in graft. Moreover, the immunochemistry staining of NF-kappaB and IkappaBa was mainly observed in infiltrating mononuclear cells. Strikingly, immunofluorescent staining localized NF-kappaB to the TCR-positive T cells in graft. Furthermore, protosappanin A exhibited inhibitory effect on T cell proliferation in recipients after 7-day treatment. In conclusion, protosappanin A might act on T cells through inhibiting NF-kappaB activation and downstream gene expressions of IFN-r and IP10, meanwhile reducing T cell proliferation responding to alloantigen, so as to induce immunosuppressive effect. The results encourage a potential therapeutic evaluation of protosappanin A for clinical organ transplantation or other T cell-mediated immune disorders. Additionally, our study also verified the feasibility of microarray utilization in Chinese herb research to explore molecular mechanism and promote development of scientific theories.

Fas mediates cardiac allograft acceptance in mice with impaired T-cell-intrinsic NF-kappaB signaling

The transcription factor NF-kappaB is critical for T-cell activation and survival. We have shown that mice expressing a T-cell-restricted NF-kappaB superrepressor (IkappaBalphaDeltaN-Tg) permanently accept heart but not skin allografts. Overexpression of the prosurvival factor Bcl-x(L) in T cells restored heart rejection, suggesting that graft acceptance in IkappaBalphaDeltaN-Tg mice was attributable to deletion of alloreactive T cells.In vitro, the increased death of IkappaBalphaDeltaN-Tg T cells upon TCR stimulation when compared with wildtype T cells was mostly because of Fas/FasL interaction. Similarly, Fas played a key role in cardiac allograft acceptance by IkappaBalphaDeltaN-Tg mice as both genetic and antibody-mediated inhibition of Fas-signaling restored cardiac allograft rejection. Rejection correlated with graft infiltration by T cells and splenic production of IFN-gamma upon allostimulation. These results indicate that T-cell inhibition of NF-kappaB results in cardiac allograft acceptance because of increased susceptibility to Fas-mediated cell death.

Bacteriophage interactions with phagocytes and their potential significance in experimental therapy

Bacteriophages are among the most numerous creatures on earth and they are omnipresent. They are thus in constant natural contact with humans and animals. However, the clinical and technological use of bacteriophages has also become more frequent, which is why all aspects of phage–mammal interactions need to be explored. Bacteriophages are able to interact with mammalian phagocytes. They may inhibit the phagocytosis of bacteria, but they may also undergo phagocytosis themselves. The ability of bacteriophages to reduce reactive oxygen species production by polymorphonuclear leukocytes in the presence of bacteria or their endotoxins was also confirmed. Studies show that the high immunogenicity of bacteriophages may also be employed in anti-tumor treatment. The present knowledge of phage interactions with cellular components of the mammalian immune system is sparse and insufficient, especially considering the increasing interest in the application of these viruses in human life. We believe that continuation of such research is indispensable.

A critical role for protein kinase C-theta-mediated T cell survival in cardiac allograft rejection

Protein kinase C (PKC)-theta mediates the critical TCR signals required for T cell activation. Previously, we have shown that in response to TCR stimulation, PKC-theta-/- T cells undergo apoptosis due to greatly reduced levels of the anti-apoptotic molecule, Bcl-xL. In this study, we demonstrate that PKC-theta-regulated expression of Bcl-xL is essential for T cell-mediated cardiac allograft rejection. Rag1-/- mice reconstituted with wild-type T cells readily rejected fully mismatched cardiac allografts, whereas Rag1-/- mice reconstituted with PKC-theta-/- T cells failed to promote rejection. Transgenic expression of Bcl-xL in PKC-theta-/- T cells was sufficient to restore cardiac allograft rejection, suggesting that PKC-theta-regulated survival is required for T cell-mediated cardiac allograft rejection in this adoptive transfer model. In contrast to adoptive transfer experiments, intact PKC-theta-/- mice displayed delayed, but successful cardiac allograft rejection, suggesting the potential compensation for PKC-theta function. Finally, a subtherapeutic dose of anti-CD154 Ab or CTLA4-Ig, which was not sufficient to prevent cardiac allograft rejection in the wild-type mice, prevented heart rejection in the PKC-theta-/- mice. Thus, in combination with other treatments, inhibition of PKC-theta may facilitate achieving long-term survival of allografts.

Epidermal Langerhans cells promote skin allograft rejection in mice with NF-kappa B-impaired T cells

T cells play a major role in the acute rejection of transplanted organs. Using mice transgenic for a T-cell-restricted NF-kappaB super-repressor (IkappaBalphaDeltaN-Tg mice), we have previously shown that T-cell-NF-kappaB is essential for the acute rejection of cardiac but not skin allografts. In this study, we investigated the mechanism by which skin grafts activate IkappaBalphaDeltaN-Tg T cells. Rejection was not due to residual T-cell-NF-kappaB activity as mice with p50/p52(-/-) T cells successfully rejected skin grafts. Rather, skin but not cardiac allografts effectively induced proliferation of graft-specific IkappaBalphaDeltaN-Tg T cells. Rejection of skin grafts by IkappaBalphaDeltaN-Tg mice was in part dependent on the presence of donor Langerhans cells (LC), a type of epidermal dendritic cells (DC), as lack of LC in donor skin grafts resulted in prolongation of skin allograft survival and injection of LC at the time of cardiac transplantation was sufficient to promote cardiac allograft rejection by IkappaBalphaDeltaN-Tg mice. Our results suggest that LC allow NF-kappaB-impaired T cells to reach an activation threshold sufficient for transplant rejection. The combined blockade of T-cell-NF-kappaB with that of alternative pathways allowing activation of NF-kappaB-impaired T cells may be an effective strategy for tolerance induction to highly immunogenic organs.

TLR engagement prevents transplantation tolerance

In many experimental models, heart, pancreas and kidney allografts are accepted long-term following costimulation-targeting therapies, whereas skin, lung and intestine resist the induction of tolerance under the same regimens. We noted that a common feature of the resistant organs is their constant exposure to commensal microbes and hypothesized that these microorganisms may stimulate Toll-like receptors (TLRs), promote alloresponses and prevent tolerance induction. This hypothesis prompts the predictions that TLR engagement at the time of transplantation should avert tolerance to heart allografts in animals treated with costimulation-targeting therapies, whereas inhibition of TLR signaling should promote tolerance to skin allografts under the same conditions. Indeed, engagement of a single TLR was sufficient to prevent anti-CD154-mediated long-term cardiac allograft acceptance and correlated with abolished intragraft recruitment of CD4+/FoxP3+ regulatory T cells and the development of linked-suppression. Conversely, a lack of donor and recipient MyD88-dependent signaling led to successful skin allograft acceptance in anti-CD154-treated animals. Thus, the status of TLR signaling contributes to the resistance versus susceptibility of organs to transplantation tolerance.

NF-kappaB and the regulation of hematopoiesis

This review will focus on the role of nuclear factor kappaB (NF-kappaB) signaling in hematopoietic differentiation. We will also discuss several hematopoietic pathologies associated with deregulation of NF-kappaB and their potential therapies.

Bacteriophages and transplantation tolerance

Our recent findings suggest that bacteriophages (phages) may not only eliminate bacteria, but also modulate immune functions. In this communication, we demonstrate that phages may strongly inhibit human T-cell activation and proliferation as well as activation of the nuclear transcription factor NF-kappaB in response to a viral pathogen. Phage administration in vivo can diminish cellular infiltration of allogeneic skin allografts. Thus, phage treatment should be considered in antibiotic-resistant posttransplantation infections. Furthermore, phages could find a broader application in clinical transplantation.

Role of natural killer cell subsets in cardiac allograft rejection

To achieve donor-specific immune tolerance to allogeneic organ transplants, it is imperative to understand the cell types involved in acute allograft rejection. In wild-type mice, CD4(+) T cells are necessary and sufficient for acute rejection of cardiac allografts. However, when T-cell responses are suboptimal, such as in mice treated with costimulation-targeting agents or in CD28-deficient mice, and perhaps in transplanted patients taking immunosuppressive drugs, the participation of other lymphocytes such as CD8(+) T cells and NK1.1(+) cells becomes apparent. We found that host NK but not NKT cells were required for cardiac rejection. Ly49G2(+) NK cells suppressed rejection, whereas a subset of NK cells lacking inhibitory Ly49 receptors for donor MHC class I molecules was sufficient to promote rejection. Notably, rejection was independent of the activating receptors Ly49D and NKG2D. Finally, our experiments supported a mechanism by which NK cells promote expansion and effector function of alloreactive T cells. Thus, therapies aimed at specific subsets of NK cells may facilitate transplantation tolerance in settings of impaired T-cell function.