IL-6 induced by Staphylococcus aureus infection prevents the induction of skin allograft acceptance in mice

Modulatory immune responses in fungal infection associated with organ transplant - advancements, management, and challenges

Organ transplantation stands as a pivotal achievement in modern medicine, offering hope to individuals with end-stage organ diseases. Advancements in immunology led to improved organ transplant survival through the development of immunosuppressants, but this heightened susceptibility to fungal infections with nonspecific symptoms in recipients. This review aims to establish an intricate balance between immune responses and fungal infections in organ transplant recipients. It explores the fundamental immune mechanisms, recent advances in immune response dynamics, and strategies for immune modulation, encompassing responses to fungal infections, immunomodulatory approaches, diagnostics, treatment challenges, and management. Early diagnosis of fungal infections in transplant patients is emphasized with the understanding that innate immune responses could potentially reduce immunosuppression and promise efficient and safe immuno-modulating treatments. Advances in fungal research and genetic influences on immune-fungal interactions are underscored, as well as the potential of single-cell technologies integrated with machine learning for biomarker discovery. This review provides a snapshot of the complex interplay between immune responses and fungal infections in organ transplantation and underscores key research directions.

Heterogeneity in allospecific T cell function in transplant-tolerant hosts determines susceptibility to rejection following infection

Even when successfully induced, immunological tolerance to solid organs remains vulnerable to inflammatory insults, which can trigger rejection. In a mouse model of cardiac allograft tolerance in which infection with Listeria monocytogenes (Lm) precipitates rejection of previously accepted grafts, we showed that recipient CD4+ TCR75 cells reactive to a donor MHC class I-derived peptide become hypofunctional if the allograft is accepted for more than 3 weeks. Paradoxically, infection-induced transplant rejection was not associated with transcriptional or functional reinvigoration of TCR75 cells. We hypothesized that there is heterogeneity in the level of dysfunction of different allospecific T cells, depending on duration of their cognate antigen expression. Unlike CD4+ TCR75 cells, CD4+ TEa cells specific for a peptide derived from donor MHC class II, an alloantigen whose expression declines after transplantation but remains inducible in settings of inflammation, retained function in tolerant mice and expanded during Lm-induced rejection. Repeated injections of alloantigens drove hypofunction in TEa cells and rendered grafts resistant to Lm-dependent rejection. Our results uncover a functional heterogeneity in allospecific T cells of distinct specificities after tolerance induction and reveal a strategy to defunctionalize a greater repertoire of allospecific T cells, thereby mitigating a critical vulnerability of tolerance.

Gut Microbiome in Patients after Heart Transplantation-Current State of Knowledge

The human gut microbiota include over 10 trillion microorganisms, such as bacteria, fungi, viruses, archaea, and protozoa. Many reports indicate the strong correlation between dysbiosis and the severity of cardiovascular diseases. Microbiota seem to interact with the host's alloimmunity and may have an immunomodulatory role in graft rejection processes. In our study, we present the current state of the knowledge of microbiota in heart transplant recipients. We present up-to-date microbiota diagnostic methods, interactions between microbiota and immunosuppressive drugs, the immunomodulatory effects of dysbiosis, and the available strategies (experimental and clinical strategies) to modulate host microbiota.

Autoimmunity: A New Focus on Nasal Polyps

Chronic rhinosinusitis with nasal polyps (CRSwNP) has long been considered a benign, chronic inflammatory, and hyperplastic disease. Recent studies have shown that autoimmune-related mechanisms are involved in the pathology of nasal polyps. Activated plasma cells, eosinophils, basophils, innate type 2 lymphocytes, mast cells, and proinflammatory cytokine in polyp tissue indicate the mobilization of innate and adaptive immune pathways during polyp formation. The discovery of a series of autoantibodies further supports the autoimmune nature of nasal polyps. Local homeostasis dysregulation, infection, and chronic inflammation may trigger autoimmunity through several mechanisms, including autoantigens overproduction, microbial translocation, molecular mimicry, superantigens, activation or inhibition of receptors, bystander activation, dysregulation of Toll-Like Receptors (TLRs), epitope spreading, autoantigens complementarity. In this paper, we elaborated on the microbiome-mediated mechanism, abnormal host immunity, and genetic changes to update the role of autoimmunity in the pathogenesis of chronic rhinosinusitis with nasal polyps.

Bee venom as an alternative for antibiotics against Staphylococcus aureus infections

The misuse of antibiotics has led to antibiotic-resistant bacterial strains, making it even harder to combat and eliminate their infections. Staphylococcus aureus causes various adverse infections and diseases, including skin abscesses, bloodstream infections, pneumonia, and joint infections. In this study, we aimed to test the cytotoxic and antibacterial effects of bee venom-loaded chitosan nanoparticles (BV-loaded CS-NPs) in comparison to gamma-irradiated BV and native BV from Apis mellifera. The physiochemical characterizations of our treatments were determined by Fourier Transform Infrared Spectroscopy (FTIR), Transmission Electron Microscope (TEM), zeta-potential, release rate, and Encapsulation Efficiency (EE). Our study was conducted on both levels, in-vitro and in-vivo. For the in-vitro study, a bacterial model of Staphylococcus aureus with an ATCC number of 6538 was grown in tryptic soy agar (TSA) medium, and the inhibition zones of our drug candidates were measured with the appropriate statistical analysis performed. For the in-vivo study, levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), Creatinine, Urea, and interleukin 6 (IL-6) were analyzed. BV-loaded CS-NPs showed relatively better results than the other alternatives, which are native BV and gamma-irradiated BV. The results showed that the antibacterial effect of BV-loaded CS-NPs was greater than the alternatives. Furthermore, its cytotoxic effect was far less than the native and irradiated bee venom. These outcomes ensure that loading BV on CS-NPs makes it a promising drug candidate for an antibiotic alternative with minimal cytotoxicity and enhanced antibacterial activity.

Trained immunity - basic concepts and contributions to immunopathology

Trained immunity is a functional state of the innate immune response and is characterized by long-term epigenetic reprogramming of innate immune cells. This concept originated in the field of infectious diseases - training of innate immune cells, such as monocytes, macrophages and/or natural killer cells, by infection or vaccination enhances immune responses against microbial pathogens after restimulation. Although initially reported in circulating monocytes and tissue macrophages (termed peripheral trained immunity), subsequent findings indicate that immune progenitor cells in the bone marrow can also be trained (that is, central trained immunity), which explains the long-term innate immunity-mediated protective effects of vaccination against heterologous infections. Although trained immunity is beneficial against infections, its inappropriate induction by endogenous stimuli can also lead to aberrant inflammation. For example, in systemic lupus erythematosus and systemic sclerosis, trained immunity might contribute to inflammatory activity, which promotes disease progression. In organ transplantation, trained immunity has been associated with acute rejection and suppression of trained immunity prolonged allograft survival. This novel concept provides a better understanding of the involvement of the innate immune response in different pathological conditions, and provides a new framework for the development of therapies and treatment strategies that target epigenetic and metabolic pathways of the innate immune system.

Development of an in vitro homeostasis model between airway epithelial cells, bacteria and bacteriophages: a time-lapsed observation of cell viability and inflammatory response

Bacteriophages represent the most extensive group of viruses within the human virome and have a significant impact on general health and well-being by regulating bacterial population dynamics. Staphylococcus aureus, found in the anterior nostrils, throat and skin, is an opportunistic pathobiont that can cause a wide range of diseases, from chronic inflammation to severe and acute infections. In this study, we developed a human cell-based homeostasis model between a clinically isolated strain of S. aureus 141 and active phages for this strain (PYO^Sa141) isolated from the commercial Pyophage cocktail (PYO). The cocktail is produced by Eliava BioPreparations Ltd. (Tbilisi, Georgia) and is used as an add-on therapy for bacterial infections, mainly in Georgia. The triptych interaction model was evaluated by time-dependent analysis of cell death and inflammatory response of the nasal and bronchial epithelial cells. Inflammatory mediators (IL-8, CCL5/RANTES, IL-6 and IL-1β) in the culture supernatants were measured by enzyme-linked immunosorbent assay and cell viability was determined by crystal violet staining. By measuring trans-epithelial electrical resistance, we assessed the epithelial integrity of nasal cells that had differentiated under air-liquid interface conditions. PYO^Sa141 was found to have a prophylactic effect on airway epithelial cells exposed to S. aureus 141 by effectively down-regulating bacterial-induced inflammation, cell death and epithelial barrier disruption in a time-dependent manner. Overall, the proposed model represents an advance in the way multi-component biological systems can be simulated in vitro.

New insights into ischemia-reperfusion injury signaling pathways in organ transplantation

PURPOSE OF REVIEW

Ischemia-reperfusion injury (IRI) leading to allograft rejection in solid organ transplant recipients is a devastating event that compromises graft and patient survival. As our clinical knowledge regarding its definition and presentation has significantly improved over the last years, adequate biomarkers translating to important therapeutic intervention remains a challenge. This review will summarize recent findings in this area.

RECENT FINDINGS

In the past 18 months, our understanding of organ transplantation IRI has improved. IRI involves a positive amplification feedback loop encompassing damaged cells at the graft site, the activity of redox-sensitive damage-associated molecular patterns, and local sequestration of recipient-derived monocytes, lymphocytes and polymorphonuclear leukocytes, like neutrophils, to sustain the immunological cascade and to enhance the destruction of the foreign tissue. Recent studies have identified critical components leading to IRI, including the oxidation state of high mobility group box 1, a classic danger signal, its role in the Toll-like receptor 4-interleukin (IL)-23-IL-17A signaling axis, and the role of neutrophils and CD321, a marker for transmigration of circulating leukocytes into the inflamed tissue. In addition, recent findings imply that the protective functions mediated by autophagy activation counterbalance the detrimental nucleotide-binding domain-like receptor family, pyrin domain containing 3 inflammasome pathway. Finally, clinical studies reveal the posttransplant variables associated with early allograft dysfunction and IRI.

SUMMARY

The future challenge will be understanding how crosstalk at the molecular and cellular levels integrate prospectively to predict which peri-transplant signals are essential for long-term clinical outcomes.

Targeting inflammation and immune activation to improve CTLA4-Ig-based modulation of transplant rejection

For the last few decades, Calcineurin inhibitors (CNI)-based therapy has been the pillar of immunosuppression for prevention of organ transplant rejection. However, despite exerting effective control of acute rejection in the first year post-transplant, prolonged CNI use is associated with significant side effects and is not well suited for long term allograft survival. The implementation of Costimulation Blockade (CoB) therapies, based on the interruption of T cell costimulatory signals as strategy to control allo-responses, has proven potential for better management of transplant recipients compared to CNI-based therapies. The use of the biologic cytotoxic T-lymphocyte associated protein 4 (CTLA4)-Ig is the most successful approach to date in this arena. Following evaluation of the BENEFIT trials, Belatacept, a high-affinity version of CTLA4-Ig, has been FDA approved for use in kidney transplant recipients. Despite its benefits, the use of CTLA4-Ig as a monotherapy has proved to be insufficient to induce long-term allograft acceptance in several settings. Multiple studies have demonstrated that events that induce an acute inflammatory response with the consequent release of proinflammatory cytokines, and an abundance of allograft-reactive memory cells in the recipient, can prevent the induction of or break established immunomodulation induced with CoB regimens. This review highlights advances in our understanding of the factors and mechanisms that limit CoB regimens efficacy. We also discuss recent successes in experimentally designing complementary therapies that favor CTLA4-Ig effect, affording a better control of transplant rejection and supporting their clinical applicability.

Gut microbiome dysbiosis in the setting of solid organ transplantation: What we have gleaned from human and animal studies

The human gut microbiome refers to all of the microorganisms present throughout the length of the gastrointestinal tract. Gut flora influence host metabolic and immune processes in myriad ways. They also play an important role in maturation and modulation of the immune system. Dysbiosis or a pathologic alteration in gut flora has been implicated in a number of diseases ranging from metabolic, autoimmune and degenerative. Whether dysbiosis has similar implications in organ transplant has been the focus of a number of pre-clinical and clinical studies. Researchers have observed significant microbiome changes after solid organ transplantation in humans that have been associated with clinical outcomes such as post-transplant urinary tract infections and diarrhea. In this article, we will discuss the available data regarding pathologic alterations in gut microbiome (dysbiosis) in solid organ transplant recipients as well as some of challenges in this field. We will also discuss animal studies focusing on mouse models of transplantation that shed light on the underlying mechanisms that explain these findings.

Tolerance of a Vascularized Composite Allograft Achieved in MHC Class-I-mismatch Swine via Mixed Chimerism

Background

Vascularized composite allografts (VCAs) allow reconstruction of devastating injuries and amputations, yet require lifelong immunosuppression that is associated with significant morbidity. Induction of immune tolerance of VCAs would permit widespread use of these procedures. VCAs are acquired from deceased donors most likely to be fully-MHC-mismatched (in contrast to living-related renal transplant donor-recipient pairs matched at one MHC haplotype). After achieving VCA tolerance in a swine model equivalent to clinical living-related renal transplants (single-haplotype MHC mismatches: e.g., “mother-daughter”/haploidentical), we tested our protocol in MHC class I, class II, and fully-MHC-mismatched pairs. Although class II mismatched swine demonstrated similar results as the haploidentical scenario (stable mixed chimerism and tolerance), our protocol failed to prevent rejection of class I and full mismatch VCAs. Here, we describe a new adapted conditioning protocol that successfully achieved tolerance across MHC class-I-mismatch barriers in swine.

Methods

Swine were treated with non-myeloablative total body and thymic irradiation two days prior to infusion of bone marrow cells from an MHC class I-mismatched donor. They also received a short-term treatment with CTLA4-Ig (Belatacept^®) and anti-IL6R mAb (Tociluzimab^®) and were transplanted with an osteomyocutaneous VCA from the same donor.

Results

Stable mixed chimerism and tolerance of MHC class-I-mismatched VCAs was achieved in 3 recipients. Allograft tolerance was associated with a sustained lack of anti-donor T cell response and a concomitant expansion of double negative CD4^-CD8^- T cells producing IL-10.

Conclusions

This study demonstrates the first successful mixed chimerism-induced VCA tolerance in a large animal model across a MHC class-I-mismatch. Future studies aimed at fully-mismatched donor-recipient pairs are under investigation with this protocol.

Pregnancy-induced humoral sensitization overrides T cell tolerance to fetus-matched allografts in mice

Immunological tolerance to semiallogeneic fetuses is necessary to achieving successful first pregnancy and permitting subsequent pregnancies with the same father. Paradoxically, pregnancy is an important cause of sensitization, resulting in the accelerated rejection of offspring-matched allografts. The underlying basis for divergent outcomes following reencounter of the same alloantigens on transplanted organs versus fetuses in postpartum females is incompletely understood. Using a mouse model that allows concurrent tracking of endogenous fetus-specific T and B cell responses in a single recipient, we show that semiallogeneic pregnancies simultaneously induce fetus-specific T cell tolerance and humoral sensitization. Pregnancy-induced antibodies, but not B cells, impeded transplantation tolerance elicited by costimulation blockade to offspring-matched cardiac grafts. Remarkably, in B cell-deficient mice, allogeneic pregnancy enabled the spontaneous acceptance of fetus-matched allografts. The presence of pregnancy-sensitized B cells that cannot secrete antibodies at the time of heart transplantation was sufficient to precipitate rejection and override pregnancy-established T cell tolerance. Thus, while induction of memory B cells and alloantibodies by pregnancies establishes formidable barriers to transplant success for multigravid women, our observations raise the possibility that humoral desensitization will not only improve transplantation outcomes, but also reveal an unexpected propensity of multiparous recipients to achieve tolerance to offspring-matched allografts.

A short course of tofacitinib sustains the immunoregulatory effect of CTLA4-Ig in the presence of inflammatory cytokines and promotes long-term survival of murine cardiac allografts

Costimulation blockade-based regimens are a promising strategy for management of transplant recipients. However, maintenance immunosuppression via CTLA4-Ig monotherapy is characterized by high frequency of rejection episodes. Recent evidence suggests that inflammatory cytokines contribute to alloreactive T cell activation in a CD28-independent manner, a reasonable contributor to the limited efficacy of CTLA4-Ig. In this study, we investigated the possible synergism of a combined short-term inhibition of cytokine signaling and CD28 engagement on the modulation of rejection. Our results demonstrate that the JAK/STAT inhibitor tofacitinib restored the immunomodulatory effect of CTLA4-Ig on mouse alloreactive T cells in the presence of inflammatory cytokines. Tofacitinib exposure conferred dendritic cells with a tolerogenic phenotype reducing their cytokine secretion and costimulatory molecules expression. JAK inhibition also directly affected T cell activation. In vivo, the combination of CTLA4-Ig and tofacitinib induced long-term survival of heart allografts and, importantly, it was equally effective when using grafts subjected to prolonged ischemia. Transplant survival correlated with a reduction in effector T cells and intragraft accumulation of regulatory T cells. Collectively, our studies demonstrate a powerful synergism between CTLA4-Ig and tofacitinib and suggest their combined use is a promising strategy for improved management of transplanted patients.

Microbiota and immunoregulation: A focus on regulatory B lymphocytes and transplantation

The microbiota plays a major role in the regulation of the host immune functions thus establishing a symbiotic relationship that maintains immune homeostasis. Among immune cells, regulatory B cells (Bregs), which can inhibit effector T cell responses, may be involved in the intestinal homeostasis. Recent works suggest that the interaction between the microbiota and Bregs appears to be important to limit autoimmune diseases and help to maintain tolerance in transplantation. Short-chain fatty acids (SCFAs), recognized as major metabolites of the microbiota, seem to be involved in the generation of a pro-tolerogenic environment in the gut, particularly through the regulation of B cell differentiation, limiting mature B cells and promoting the function of Bregs. In this review, we show that this B cells-microbiota interaction may open a path toward new potential therapeutic applications not only for patients with autoimmune diseases but also in transplantation.

Monotherapy With Anti-CD70 Antibody Causes Long-Term Mouse Cardiac Allograft Acceptance With Induction of Tolerogenic Dendritic Cells

Allograft rejection has been an obstacle for the long-term survival of patients. CD70, a tumor necrosis factor (TNF) family member critically expressed on antigen-presenting cells and strongly but transiently up-regulated during lymphocyte activation, represents an important co-stimulatory molecule that induces effective T cell responses. We used a mouse heterotopic cardiac transplantation model to evaluate the effects of monotherapy with the antibody targeting mouse CD70 (FR70) on transplantation tolerance and its immunoregulatory activity. FR70-treated C3H recipient mice permanently accepted B6 fully mismatched cardiac allografts. Consistent with the graft survival, the infiltration of CD8⁺ T cells in the graft was reduced, dendritic cells were differentiated into a tolerogenic status, and the number of regulatory T cells was elevated both in the graft and the recipient’s spleen. In addition, naïve C3H given an adoptive transfer of spleen cells from the primary recipients with FR70 treatment accepted a heart graft from a matching B6 donor but not third-party BALB/c mice.　Our findings show that treatment with FR70 induced regulatory cells and inhibited cytotoxic T cell proliferation, which led to long-term acceptance of mouse cardiac allografts. These findings highlight the potential role of anti-CD70 antibodies as a clinically effective treatment for allograft rejection.

Acute murine cytomegalovirus disrupts established transplantation tolerance and causes recipient allo-sensitization

We have previously shown that acute cytomegalovirus (CMV) infection disrupts the induction of transplantation tolerance. However, what impact acute CMV infection would have on the maintenance of established tolerance and on subsequent recipient allo-sensitization is a clinically important unanswered question. Here we used an allogeneic murine islet transplantation tolerance model to examine the impact of acute CMV infection on: (a) disruption of established transplantation tolerance during tolerance maintenance; and (b) the possibility of recipient allo-sensitization by CMV-mediated disruption of stable tolerance. We demonstrated that acute CMV infection abrogated transplantation tolerance during the maintenance stage in 50%-60% recipients. We further demonstrated that acute CMV infection-mediated tolerance disruption led to recipient allo-sensitization by reverting the tolerant state of allo-specific T cells and promoting their differentiation to allo-specific memory cells. Consequently, a second same-donor islet allograft was rejected in an accelerated fashion by these recipients. Our study therefore supports close monitoring for allo-sensitization in previously tolerant transplant recipients in whom tolerance maintenance is disrupted by an episode of acute CMV infection.

Bacterial products in donor airways prevent the induction of lung transplant tolerance

Although postoperative bacterial infections can trigger rejection of pulmonary allografts, the impact of bacterial colonization of donor grafts on alloimmune responses to transplanted lungs remains unknown. Here, we tested the hypothesis that bacterial products present within donor grafts at the time of implantation promote lung allograft rejection. Administration of the toll-like receptor 2 (TLR2) agonist Pam₃ Cys₄ to Balb/c wild-type grafts triggered acute cellular rejection after transplantation into B6 wild-type recipients that received perioperative costimulatory blockade. Pam₃ Cys₄ -triggered rejection was associated with an expansion of CD8⁺ T lymphocytes and CD11c⁺ CD11b^hi MHC (major histocompatibility complex) class II⁺ antigen-presenting cells within the transplanted lungs. Rejection was prevented when lungs were transplanted into TLR2-deficient recipients but not when MyD88-deficient donors were used. Adoptive transfer of B6 wild-type monocytes, but not T cells, following transplantation into B6 TLR2-deficient recipients restored the ability of Pam₃ Cys₄ to trigger acute cellular rejection. Thus, we have demonstrated that activation of TLR2 by a bacterial lipopeptide within the donor airways prevents the induction of lung allograft tolerance through a process mediated by recipient-derived monocytes. Our work suggests that donor lungs harboring bacteria may precipitate an inflammatory response that can facilitate allograft rejection.

Inducing Transient Mixed Chimerism for Allograft Survival Without Maintenance Immunosuppression With Combined Kidney and Bone Marrow Transplantation: Protocol Optimization

BACKGROUND

Tolerance induction is an important goal in the field of organ transplantation. We have sequentially modified our conditioning regimen for induction of donor-specific tolerance in recipients of major histocompatibility complex-mismatched combined kidney and bone marrow transplantation (CKBMT).

METHODS

From December 2011 to May 2017, 8 major histocompatibility complex-mismatched patients received CKBMT. The initial conditioning regimen (protocol 1) consisted of cyclophosphamide (CP), rituximab, rabbit antithymocyte globulin, and thymic irradiation. Tacrolimus and steroids were used for the maintenance of immunosuppression (IS).

RESULTS

This regimen was complicated by transient acute kidney injury, which has been the major clinical feature of engraftment syndrome and side effects of CP, although one of 2 subjects successfully discontinued his IS for 14 months. The conditioning regimen was modified by reducing the CP dose and adding fludarabine (protocol 2). The final modification was reducing the fludarabine and rabbit antithymocyte globulin doses (protocol 3). Mixed chimerism, detected by the short tandem repeat method, was achieved transiently in all subjects for 3-20 weeks. Among the 3 subjects treated with protocol 2, IS was successfully discontinued for >35 months in one subject, but the other 2 subjects suffered from severe BK virus-associated nephritis. All 3 subjects treated with protocol 3 tolerated the protocol well and have successfully discontinued IS for >4-41 months. Interestingly, de novo donor-specific antibody was not detected in any subject during all the follow-up periods.

CONCLUSIONS

Our clinical trial has shown that long-term renal allograft survival without maintenance IS can be achieved by induction of mixed chimerism following CKBMT.

Localized mandibular infection affects remote in vivo bioreactor bone generation

Mandibular reconstruction requires functional and aesthetic repair and is further complicated by contamination from oral and skin flora. Antibiotic-releasing porous space maintainers have been developed for the local release of vancomycin and to promote soft tissue attachment. In this study, mandibular defects in six sheep were inoculated with 106 colony forming units of Staphylococcus aureus; three sheep were implanted with unloaded porous space maintainers and three sheep were implanted with vancomycin-loaded space maintainers within the defect site. During the same surgery, 3D-printed in vivo bioreactors containing autograft or xenograft were implanted adjacent to rib periosteum. After 9 weeks, animals were euthanized, and tissues were analyzed. Antibiotic-loaded space maintainers were able to prevent dehiscence of soft tissue overlying the space maintainer, reduce local inflammatory cells, eliminate the persistence of pathogens, and prevent the increase in mandibular size compared to unloaded space maintainers in this sheep model. Animals with an untreated mandibular infection formed bony tissues with greater density and maturity within the distal bioreactors. Additionally, tissues grown in autograft-filled bioreactors had higher compressive moduli and higher maximum screw pull-out forces than xenograft-filled bioreactors. In summary, we demonstrated that antibiotic-releasing space maintainers are an innovative approach to preserve a robust soft tissue pocket while clearing infection, and that local infections can increase local and remote bone growth.

MerTK negatively regulates Staphylococcus aureus induced inflammatory response via SOCS1/SOCS3 and Mal

OBJECTIVE

Staphylococcus aureus (S. aureus), one of Gram-positive pathogen, is frequently associated with acute lung inflammation. The central feature of S. aureus acute lung inflammation are pulmonary dysfunctioning and impeded host defence response, which cause failure in inflammatory cytokines homeostasis and leads to serious tissue damage. However, the role of the Mer receptor tyrosine kinase (MerTK) in the lung following S. aureus infection remains elusive. Here, we investigate whether MerTK alleviates S. aureus induced uncontrolled inflammation through negatively regulating toll-like receptor 2 and 6 (TLR2/ TLR6) via suppressor of cytokine signalling 1, 3 (SOCS1/SOCS3).

METHODS AND RESULTS

We found in mice lung tissues and RAW 264.7 macrophages upon S. aureus infection activates TLR2 and TLR6 driven mitogen-activated protein kinases (MAPKs) and nuclear factor kappa B (NF-κB) signalling pathways, resulting in production of inflammatory cytokines including tumour necrosis factor-α (TNF-α), interleukin 1β (IL-1β), interleukin 6 (IL-6). Furthermore, S. aureus-infection groups showed a significant up-regulation of MerTK which serves as mediator of SOCS1 and SOCS3. Subsequently, through feedback mechanism SOCS1/3 degrade Mal, resulting in inhibition of downstream TLR mediated inflammatory pathways. Moreover, MerTK^-/- mice lung tissues and silencing MerTK in RAW 264.7 inhibited the S. aureus-induced activation of MerTK, which significantly upregulated the phosphorylation of crucial protein in MAPKs (ERK, JNK, p38) and NF-κB (IĸBα, p65) signalling pathways, as well as the production of pro-inflammatory cytokines.

CONCLUSION

Collectively, these findings indicate the important role of MerTK in self-regulatory resolution of S. aureus-induced inflammatory pathways and cytokines through intrinsic SOCS1 and SOCS3 repressed feedback on TLR2, TLR6 both in vivo and in vitro.

Microbiota in organ transplantation: An immunological and therapeutic conundrum?

The gastrointestinal (GI) tract microbiota is an environmental factor that regulates host immunity in allo-transplantation (allo-Tx). It is required for the development of resistance against pathogens and the stabilization of mucosa-associated lymphoid tissue. The gut-microbiota axis may also precipitate allograft rejection by producing metabolites that activate host cell-mediated and humoral immunity. Here, we discuss new insights into microbial immunomodulation, highlighting ongoing attempts to affect commensal colonization in an attempt to ameliorate allograft rejection cascade. Recent progress on the use of antibiotics to modulate GI microbiota diversity and innate-adaptive immune interface are discussed. Our focus on the microbiota's influence of endoplasmic reticulum (ER) stress and autophagy signaling through hepatic EP4/CHOP/LC3B platforms reveals a novel molecular pathway and potential biomarkers determining the progression of allo-Tx damage. Understanding and harnessing the potential of microbiome/bacteriophage therapies may offer safe and effective means for personalized treatment to reduce risks of infections and immunosuppression in allo-Tx.

Trained immunity in organ transplantation

Consistent induction of donor-specific unresponsiveness in the absence of continuous immunosuppressive therapy and toxic effects remains a difficult task in clinical organ transplantation. Transplant immunologists have developed numerous experimental treatments that target antigen-presentation (signal 1), costimulation (signal 2), and cytokine production (signal 3) to establish transplantation tolerance. While promising results have been obtained using therapeutic approaches that predominantly target the adaptive immune response, the long-term graft survival rates remain suboptimal. This suggests the existence of unrecognized allograft rejection mechanisms that contribute to organ failure. We postulate that trained immunity stimulatory pathways are critical to the immune response that mediates graft loss. Trained immunity is a recently discovered functional program of the innate immune system, which is characterized by nonpermanent epigenetic and metabolic reprogramming of macrophages. Since trained macrophages upregulate costimulatory molecules (signal 2) and produce pro-inflammatory cytokines (signal 3), they contribute to potent graft reactive immune responses and organ transplant rejection. In this review, we summarize the detrimental effects of trained immunity in the context of organ transplantation and describe pathways that induce macrophage training associated with graft rejection.

Impact of infection on transplantation tolerance

Allograft tolerance is the ultimate goal of organ transplantation. Current strategies for tolerance induction mainly focus on inhibiting alloreactive T cells while promoting regulatory immune cells. Pathogenic infections may have direct impact on both effector and regulatory cell populations, therefore can alter host susceptibility to transplantation tolerance induction as well as impair the quality and stability of tolerance once induced. In this review, we will discuss existing data demonstrating the effect of infections on transplantation tolerance, with particular emphasis on the role of the stage of infection (acute, chronic, or latent) and the stage of tolerance (induction or maintenance) in this infection-tolerance interaction. While the deleterious effect of acute infection on tolerance is mainly driven by proinflammatory cytokines induced shortly after the infection, chronic infection may generate exhausted T cells that could in fact facilitate transplantation tolerance. In addition to pathogenic infections, commensal intestinal microbiota also has numerous significant immunomodulatory effects that can shape the host alloimmunity following transplantation. A comprehensive understanding of these mechanisms is crucial for the development of therapeutic strategies for robustly inducing and stably maintaining transplantation tolerance while preserving host anti-pathogen immunity in clinically relevant scenarios.

Exercise increases skin graft resistance to rejection

Regular exercise reduces risk of various chronic diseases and can prevent the development and recurrence of cancer, making it a promising nonpharmacological modulator of disease. Yet the effect of regular exercise on solid organ transplant outcome remains uncertain. Using a model of voluntary wheel-running exercise and skin transplantation in mice, we hypothesized that exercise strengthens the alloimmune response, leading to an increased rate of rejection. Instead, we found that regular exercise in mice resulted in prolonged graft survival, with mean allograft survival time increasing by almost 50%. We observed this graft survival extension in exercised mice despite evidence of a slightly enhanced alloimmune response, comprised of increased proliferation of alloreactive CD4⁺ T cells, as well as increased interferon-γ production by these cells. Exercise was not associated with significant changes in numbers of conventional CD4⁺ or CD8⁺ T cells, NK cells, or Foxp3⁺ regulatory T cells. In conclusion, our study suggests that exercise increases skin graft resistance to a similar or slightly higher level of alloimmunity and supports regular exercise as an important beneficial pursuit for transplant recipients.

Urinary microbiome associated with chronic allograft dysfunction in kidney transplant recipients

BACKGROUND

We performed a study to identify differences in the urinary microbiome associated with chronic allograft dysfunction (CAD) and compared the urinary microbiome of male and female transplant recipients with CAD.

METHODS

This case-control study enrolled 67 patients within the Deterioration of Kidney Allograft Function (DeKAF) Genomics cohort at two transplant centers. CAD was defined as a greater than 25% rise in serum creatinine relative to a 3 month post-transplant baseline. Urine samples from patients with and without CAD were analyzed using 16S V4 bacterial ribosomal DNA sequences.

RESULTS

Corynebacterium was more prevalent in female and male patients with CAD compared to non-CAD female patients (P = 0.0005). A total 21 distinct Operational Taxonomic Unit (OTUs) were identified as significantly different when comparing CAD and non-CAD patients using Kruskal-Wallis (P < 0.01). A subset analysis of female patients with CAD compared to non-CAD females identified similar differentially abundant OTUs, including the genera Corynebacterium and Staphylococcus (Kruskal-Wallis; P = 0.01; P = 0.004, respectively). Male CAD vs female CAD analysis showed greater abundance of phylum Proteobacteria in males.

CONCLUSION

There were differences in the urinary microbiome when comparing female and male CAD patients with their female non-CAD counterparts and these differences persisted in the subset analysis limited to female patients only.

Development of transplant immunosuppressive agents - considerations in the use of animal models

INTRODUCTION

The development of all immunosuppressant agents to date has involved the experimental use of large and small animal models. Over the last half-century, immunosuppressive drugs have extended the lives of transplant patients worldwide. However, the use of animal models in the development of these drugs is not perfect, and this has brought to light a number of issues including idiosyncratic reactions that are found in animal models but not in humans. The 2006 highly publicized case of the 'elephant man' TGN 1412 drug trial highlights the importance of being cogent of the limitations of animal models. Areas covered: This review covers the utility and limitations of the use of animal models for the development of immunosuppressant agents. This includes both large and small animal models, particularly rodent models in the transplant setting. Expert opinion: The use of animal models represents a critical stage in the development of immunosuppressive drugs. Limitations include physiological differences to humans; this is especially true of immunologically naïve lab rodents with small memory cell populations. Toxic drug levels may differ widely between species. Animal models are also costly and raise ethical concerns. However, there is currently no way to recreate the complex environment of the human immune system purely in vitro.

Maintaining T cell tolerance of alloantigens: Lessons from animal studies

Achieving host immune tolerance of allogeneic transplants represents the ultimate challenge in clinical transplantation. It has become clear that different cells and mechanisms participate in acquisition versus maintenance of allograft tolerance. Indeed, manipulations which prevent tolerance induction often fail to abrogate tolerance once it has been established. Hence, elucidation of the immunological mechanisms underlying maintenance of T cell tolerance to alloantigens is essential for the development of novel interventions that preserve a robust and long lasting state of allograft tolerance that relies on T cell deletion in addition to intra-graft suppression of inflammatory immune responses. In this review, we discuss some essential elements of the mechanisms involved in the maintenance of naturally occurring or experimentally induced allograft tolerance, including the newly described role of antigen cross-dressing mediated by extracellular vesicles.

Gut microbes contribute to variation in solid organ transplant outcomes in mice

BACKGROUND

Solid organ transplant recipients show heterogeneity in the occurrence and timing of acute rejection episodes. Understanding the factors responsible for such variability in patient outcomes may lead to improved diagnostic and therapeutic approaches. Rejection kinetics of transplanted organs mainly depends on the extent of genetic disparities between donor and recipient, but a role for environmental factors is emerging. We have recently shown that major alterations of the microbiota following broad-spectrum antibiotics, or use of germ-free animals, promoted longer skin graft survival in mice. Here, we tested whether spontaneous differences in microbial colonization between genetically similar individuals can contribute to variability in graft rejection kinetics.

RESULTS

We compared rejection kinetics of minor mismatched skin grafts in C57BL/6 mice from Jackson Laboratory (Jax) and Taconic Farms (Tac), genetically similar animals colonized by different commensal microbes. Female Tac mice rejected skin grafts from vendor-matched males more quickly than Jax mice. We observed prolonged graft survival in Tac mice when they were exposed to Jax mice microbiome through co-housing or fecal microbiota transplantation (FMT) by gastric gavage. In contrast, exposure to Tac mice did not change graft rejection kinetics in Jax mice, suggesting a dominant suppressive effect of Jax microbiota. High-throughput sequencing of 16S rRNA gene amplicons from Jax and Tac mice fecal samples confirmed a convergence of microbiota composition after cohousing or fecal transfer. Our analysis of amplicon data associated members of a single bacterial genus, Alistipes, with prolonged graft survival. Consistent with this finding, members of the genus Alistipes were absent in a separate Tac cohort, in which fecal transfer from Jax mice failed to prolong graft survival.

CONCLUSIONS

These results demonstrate that differences in resident microbiome in healthy individuals may translate into distinct kinetics of graft rejection, and contribute to interpersonal variability in graft outcomes. The association between Alistipes and prolonged skin graft survival in mice suggests that members of this genus might affect host physiology, including at sites distal to the gastrointestinal tract. Overall, these findings allude to a potential therapeutic role for specific gut microbes to promote graft survival through the administration of probiotics, or FMT.

New Insights into Graft-Versus-Host Disease and Graft Rejection

Allogeneic transplantation of foreign organs or tissues has lifesaving potential, but can lead to serious complications. After solid organ transplantation, immune-mediated rejection mandates the use of prolonged global immunosuppression and limits the life span of transplanted allografts. After bone marrow transplantation, donor-derived immune cells can trigger life-threatening graft-versus-host disease. T cells are central mediators of alloimmune complications and the target of most existing therapeutic interventions. We review recent progress in identifying multiple cell types in addition to T cells and new molecular pathways that regulate pathogenic alloreactivity. Key discoveries include the cellular subsets that function as potential sources of alloantigens, the cross talk of innate lymphoid cells with damaged epithelia and with the recipient microbiome, the impact of the alarmin interleukin-33 on alloreactivity, and the role of Notch ligands expressed by fibroblastic stromal cells in alloimmunity. While refining our understanding of transplantation immunobiology, these findings identify new therapeutic targets and new areas of investigation.

Mechanisms of Mixed Chimerism-Based Transplant Tolerance

Immune responses to allografts represent a major barrier in organ transplantation. Immune tolerance to avoid chronic immunosuppression is a critical goal in the field, recently achieved in the clinic by combining bone marrow transplantation (BMT) with kidney transplantation following non-myeloablative conditioning. At high levels of chimerism such protocols can permit central deletional tolerance, but with a significant risk of graft-versus-host (GVH) disease (GVHD). By contrast, transient chimerism-based tolerance is devoid of GVHD risk and appears to initially depend on regulatory T cells (Tregs) followed by gradual, presumably peripheral, clonal deletion of donor-reactive T cells. Here we review recent mechanistic insights into tolerance and the development of more robust and safer protocols for tolerance induction that will be guided by innovative immune monitoring tools.

Fifty Shades of Tolerance: Beyond a Binary Tolerant/Non-Tolerant Paradigm

Purpose of review

It has long been considered that tolerance in a transplant recipient is a binary all-or-none state: either the graft is accepted without immunosuppression identifying the recipient as tolerant, or the recipient rejects the graft and is not tolerant. This tolerance paradigm, however, does not accurately reflect data emerging from animal models and patients and requires revision.

Recent Findings

It is becoming appreciated that there may be different gradations in the quality of tolerance based on underlying cellular mechanisms of immunological tolerance, and that individuals may enhance their tolerance by strengthening or combining different cellular mechanisms. Furthermore, evidence suggests that even if tolerance is lost, the loss may be only temporary, and in some circumstances tolerance can be restored.

Summary

Shifting our focus from an all-or-nothing tolerance paradigm to one with many shades may help us better understand how tolerance operates, and how this state may be tracked and enhanced for better patient outcomes.

Erosion of Transplantation Tolerance After Infection

Recent clinical studies suggest that operational allograft tolerance can be persistent, but long-term surviving allografts can be rejected in a subset of patients, sometimes after episodes of infection. In this study, we examined the impact of Listeria monocytogenes (Lm) infection on the quality of tolerance in a mouse model of heart allograft transplantation. Lm infection induced full rejection in 40% of tolerant recipients, with the remaining experiencing a rejection crisis or no palpable change in their allografts. In the surviving allografts on day 8 postinfection, graft-infiltrating cell numbers increased and exhibited a loss in the tolerance gene signature. By day 30 postinfection, the tolerance signature was broadly restored, but with a discernible reduction in the expression of a subset of 234 genes that marked tolerance and was down-regulated at day 8 post-Lm infection. We further demonstrated that the tolerant state after Lm infection was functionally eroded, as rejection of the long-term surviving graft was induced with anti-PD-L1 whereas the same treatment had no effect in noninfected tolerant mice. Collectively, these observations demonstrate that tolerance, even if initially robust, exists as a continuum that can be eroded following bystander immune responses that accompany certain infections.

Tracking of TCR-Transgenic T Cells Reveals That Multiple Mechanisms Maintain Cardiac Transplant Tolerance in Mice

Solid organ transplantation tolerance can be achieved following select transient immunosuppressive regimens that result in long-lasting restraint of alloimmunity without affecting responses to other antigens. Transplantation tolerance has been observed in animal models following costimulation or coreceptor blockade therapies, and in a subset of patients through induction protocols that include donor bone marrow transplantation, or following withdrawal of immunosuppression. Previous data from our lab and others have shown that proinflammatory interventions that successfully prevent the induction of transplantation tolerance in mice often fail to break tolerance once it has been stably established. This suggests that established tolerance acquires resilience to proinflammatory insults, and prompted us to investigate the mechanisms that maintain a stable state of robust tolerance. Our results demonstrate that only a triple intervention of depleting CD25⁺ regulatory T cells (Tregs), blocking programmed death ligand-1 (PD-L1) signals, and transferring low numbers of alloreactive T cells was sufficient to break established tolerance. We infer from these observations that Tregs and PD-1/PD-L1 signals cooperate to preserve a low alloreactive T cell frequency to maintain tolerance. Thus, therapeutic protocols designed to induce multiple parallel mechanisms of peripheral tolerance may be necessary to achieve robust transplantation tolerance capable of maintaining one allograft for life in the clinic.

The Microbiome, Systemic Immune Function, and Allotransplantation

Diverse effects of the microbiome on solid organ transplantation are beginning to be recognized. In allograft recipients, microbial networks are disrupted by immunosuppression, nosocomial and community-based infectious exposures, antimicrobial therapies, surgery, and immune processes. Shifting microbial patterns, including acute infectious exposures, have dynamic and reciprocal interactions with local and systemic immune systems. Both individual microbial species and microbial networks have central roles in the induction and control of innate and adaptive immune responses, in graft rejection, and in ischemia-reperfusion injury. Understanding the diverse interactions between the microbiome and the immune system of allograft recipients may facilitate clinical management in the future.

Selenium Deficiency Facilitates Inflammation Following S. aureus Infection by Regulating TLR2-Related Pathways in the Mouse Mammary Gland

Selenium (Se) is an essential micronutrient affecting various aspects of health. Se deficiency has been associated with inflammation and immune responses. Mastitis poses a serious problem for humans and animals in the postpartum period. Staphylococcus aureus (S. aureus) is the most common infectious bacterial pathogen associated with mastitis. The present study sought to determine the effects and underlying mechanism of dietary Se on S. aureus-induced inflammation using a model of mouse mastitis. ELISA and Western blotting were performed to detect protein levels. Quantitative PCR (qPCR) was performed to detect messenger RNA (mRNA) levels. The histopathological changes indicated that Se deficiency resulted in increased inflammatory lesions in S. aureus mastitis, whereas Se deficiency did not induce inflammatory lesions in the mammary gland. Myeloperoxidase (MPO) activity was increased in Se-deficient mice with S. aureus mastitis. Analysis of cytokine mRNA and protein showed that Se deficiency leads to increased TNF-α, IL-1β, and IL-6 production in S. aureus mastitis. In addition, Se deficiency enhanced the mRNA and protein expressions of toll-like receptor 2 (TLR2), which were originally upregulated by S. aureus in the mammary gland tissues and human embryonic kidney 293 (HEK293)-mTLR2 cells. When Se-deficient mice were infected with S. aureus, the phosphorylation of IκB, nuclear factor-κB (NF-κB), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 was greatly increased. The results indicate that Se deficiency could intensify the inflammatory reaction in S. aureus mastitis. This work contributes to the exploration of new methods of preventing or treating of S. aureus mastitis and other infectious diseases.

Micromanaging alloimmunity

Increasing evidence indicates that microbes have a large influence on immune function. Previous studies have linked pathogenic microorganisms with decreased allograft tolerance and subsequent rejection. In this issue of the JCI, Lei and colleagues demonstrate that commensal organisms also influence the host response to allograft transplantation. Using murine skin and cardiac transplant models, the authors demonstrate that allograft rejection is accelerated in mice with a normal microbiome compared with germ-free animals and antibiotic-treated mice. The increased graft rejection observed in conventional animals was due to enhanced T cell priming and was mediated through type I IFN. Together, these results suggest that altering a patient's microbial community prior to transplant could improve allograft acceptance.

The composition of the microbiota modulates allograft rejection

Transplantation is the only cure for end-stage organ failure, but without immunosuppression, T cells rapidly reject allografts. While genetic disparities between donor and recipient are major determinants of the kinetics of transplant rejection, little is known about the contribution of environmental factors. Because colonized organs have worse transplant outcome than sterile organs, we tested the influence of host and donor microbiota on skin transplant rejection. Compared with untreated conventional mice, pretreatment of donors and recipients with broad-spectrum antibiotics (Abx) or use of germ-free (GF) donors and recipients resulted in prolonged survival of minor antigen-mismatched skin grafts. Increased graft survival correlated with reduced type I IFN signaling in antigen-presenting cells (APCs) and decreased priming of alloreactive T cells. Colonization of GF mice with fecal material from untreated conventional mice, but not from Abx-pretreated mice, enhanced the ability of APCs to prime alloreactive T cells and accelerated graft rejection, suggesting that alloimmunity is modulated by the composition of microbiota rather than the quantity of bacteria. Abx pretreatment of conventional mice also delayed rejection of major antigen-mismatched skin and MHC class II-mismatched cardiac allografts. This study demonstrates that Abx pretreatment prolongs graft survival, suggesting that targeting microbial constituents is a potential therapeutic strategy for enhancing graft acceptance.

Repeated Injections of IL-2 Break Renal Allograft Tolerance Induced via Mixed Hematopoietic Chimerism in Monkeys

Tolerance of allografts achieved in mice via stable mixed hematopoietic chimerism relies essentially on continuous elimination of developing alloreactive T cells in the thymus (central deletion). Conversely, while only transient mixed chimerism is observed in nonhuman primates and patients, it is sufficient to ensure tolerance of kidney allografts. In this setting, it is likely that tolerance depends on peripheral regulatory mechanisms rather than thymic deletion. This implies that, in primates, upsetting the balance between inflammatory and regulatory alloimmunity could abolish tolerance and trigger the rejection of previously accepted renal allografts. In this study, six monkeys that were treated with a mixed chimerism protocol and had accepted a kidney allograft for periods of 1-10 years after withdrawal of immunosuppression received subcutaneous injections of IL-2 cytokine (0.6-3 × 10(6) IU/m(2) ). This resulted in rapid rejection of previously tolerated renal transplants and was associated with an expansion and reactivation of alloreactive pro-inflammatory memory T cells in the host's lymphoid organs and in the graft. This phenomenon was prevented by anti-CD8 antibody treatment. Finally, this process was reversible in that cessation of IL-2 administration aborted the rejection process and restored normal kidney graft function.

Resistance of spontaneously diabetic Ins2(akita) mice to allograft tolerance induced by anti-CD154 therapy

BACKGROUND

Despite ongoing advances in the clinical islet transplant field, progressive decline in graft function continues to reduce the long-term success of islet transplantation for restoring euglycemia in type 1 diabetic recipients. To preserve graft function and avoid the use of chronic immunosuppressive drug therapy, a key goal is to induce donor-specific immune tolerance to islet transplants. Preclinical rodent studies of islet transplantation largely utilize models of diabetes either induced experimentally with beta cell toxins or spontaneously occurring in strains genetically prone to autoimmune diabetes. In this study, we sought to determine if chronic, severe hyperglycemia itself, independent of both beta cell toxins and host autoimmunity, influenced acute allograft rejection and/or the capacity to induce allograft tolerance.

METHOD

To this end, we studied the response to islet allografts in severely diabetic, non-autoimmune C57Bl/6 Ins2(akita) recipients.

RESULTS

Results indicate that diabetic Ins2(akita) mice display higher levels of blood glucose, show more rapid acute islet allograft rejection, and are resistant to allograft prolongation induced with anti-CD154 therapy relative to wild-type littermates rendered diabetic with streptozotocin. As such, results suggest that severe hyperglycemia may be an independent risk factor impacting the capacity to induce tolerance to islet allografts. Thus, Ins2(akita) mice represent a stringent model for evaluating anti-rejection strategies in the setting of severe metabolic demand on islet transplants.

Clinical Implications of Basic Science Discoveries: Immune Homeostasis and the Microbiome-Dietary and Therapeutic Modulation and Implications for Transplantation

Links between the human microbiome and the innate and adaptive immune systems and their impact on autoimmune and inflammatory diseases are only beginning to be recognized. Characterization of the complex human microbial community is facilitated by culture-independent nucleic acid sequencing tools and bioinformatics systems. Specific organisms and microbial antigens are linked with initiation of innate immune responses that, depending on the context, may be associated with tolerogenic or effector immune responses. Further complexity is introduced by preclinical data that demonstrate the impacts of dietary manipulation on the prevention of genetically determined, systemic autoimmune disorders and on gastrointestinal microbiota. Investigation of interactions of complex microbial populations with the human immune system may provide new targets for clinical management in allotransplantation.

Inflammatory triggers of acute rejection of organ allografts

Solid organ transplantation is a vital therapy for end stage diseases. Decades of research have established that components of the adaptive immune system are critical for transplant rejection, but the role of the innate immune system in organ transplantation is just emerging. Accumulating evidence indicates that the innate immune system is activated at the time of organ implantation by the release of endogenous inflammatory triggers. This review discusses the nature of these triggers in organ transplantation and also potential mediators that may enhance inflammation resolution after organ implantation.

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.

Taming inflammation by targeting cytokine signaling: new perspectives in the induction of transplantation tolerance

Transplantation tolerance remains an elusive goal, partly due to limitations in our understanding of the interplay between inflammatory mediators and their role in the activation and regulation of T lymphocytes. Although multiple mechanisms acting both centrally and peripherally are responsible for tolerance induction, the signaling pathways leading to activation or regulation of adaptive immunity are often complex, branched, redundant and modulated by the microenvironment's inflammatory milieu. Accumulating evidence clearly indicates that inflammatory cytokines limit the tolerogenic potential of immunomodulatory protocols by supporting priming of the immune system and counteracting regulatory mechanisms, ultimately promoting rejection. In this review, we summarize recent progress in the development of novel therapeutics to manipulate this inflammatory environment and achievements in targeted inhibition of inflammatory cytokine signaling. Ultimately, robust transplant tolerance induction will probably require a multifaceted, holistic approach that integrates the various mechanisms of tolerance induction, incorporates the dynamic alterations in costimulatory requirements of alloreactive T cells, while maintaining endogenous mechanisms of immune regulation.

Microbes and allogeneic transplantation

Microbial products can be recognized by pattern recognition receptors expressed by immune and parenchymal cells and drive innate immunity that can in turn shape adaptive immune responses to microbial and transplant antigens. In transplanted patients, the signals and their downstream inflammatory cytokines elicited in response to infections can modulate ongoing alloimmune responses and modify the fate of transplanted organs. In recent years, it has become apparent that microbial signals can be generated not only by active pathogenic infections but also by commensal microbiota, thus opening a new field of research into the interplay between the microbiota and the immune system in homeostasis and disease. The wide use of antibiotics and immunosuppressive drugs in transplanted patients can have dramatic consequences on the microbiota that can in turn shape immune responses and perhaps alloresponses, whereas the ongoing immune responses can in turn affect the commensal or pathogenic microorganisms in a feed-forward circle. Here, we discuss known and hypothesized mechanisms for how infections or microbiota-derived signals may affect local or systemic alloimmunity and briefly review data on downstream effects of antibiotics and vaccinations.

The cholinergic anti-inflammatory pathway delays TLR-induced skin allograft rejection in mice: cholinergic pathway modulates alloreactivity

Activation of innate immunity through Toll-like receptors (TLR) can abrogate transplantation tolerance by revealing hidden T cell alloreactivity. Separately, the cholinergic anti-inflammatory pathway has the capacity to dampen macrophage activation and cytokine release during endotoxemia and ischemia reperfusion injury. However, the relevance of the α7 nicotinic acetylcholine receptor (α7nAChR)-dependent anti-inflammatory pathway in the process of allograft rejection or maintenance of tolerance remains unknown. The aim of our study is to investigate whether the cholinergic pathway could impact T cell alloreactivity and transplant outcome in mice. For this purpose, we performed minor-mismatched skin allografts using donor/recipient combinations genetically deficient for the α7nAChR. Minor-mismatched skin grafts were not rejected unless the mice were housed in an environment with endogenous pathogen exposure or the graft was treated with direct application of imiquimod (a TLR7 ligand). The α7nAChR-deficient recipient mice showed accelerated rejection compared to wild type recipient mice under these conditions of TLR activation. The accelerated rejection was associated with enhanced IL-17 and IFN-γ production by alloreactive T cells. An α7nAChR-deficiency in the donor tissue facilitated allograft rejection but not in recipient mice. In addition, adoptive T cell transfer experiments in skin-grafted lymphopenic animals revealed a direct regulatory role for the α7nAChR on T cells. Taken together, our data demonstrate that the cholinergic pathway regulates alloreactivity and transplantation tolerance at multiple levels. One implication suggested by our work is that, in an organ transplant setting, deliberate α7nAChR stimulation of brain dead donors might be a valuable approach for preventing donor tissue inflammation prior to transplant.

Mycobacterium tuberculosis PstS1 amplifies IFN-γ and induces IL-17/IL-22 responses by unrelated memory CD4+ T cells via dendritic cell activation

The immunological mechanisms that modulate protection during Mycobacterium tuberculosis (Mtb) infection or vaccination are not fully understood. Secretion of IFN-γ and, to a lesser extent, of IL-17 by CD4(+) T cells plays a major role both in protection and immunopathology. Few Mtb Ags interacting with DCs affect priming, activation, and regulation of Ag-unrelated CD4(+) T-cell responses. Here we demonstrate that PstS1, a 38 kDa-lipoprotein of Mtb, promotes Ag-independent activation of memory T lymphocytes specific for Ag85B or Ag85A, two immunodominant protective Ags of Mtb. PstS1 expands CD4(+) and CD8(+) memory T cells, amplifies secretion of IFN-γ and IL-22 and induces IL-17 production by effector memory cells in an Ag-unrelated manner in vitro and in vivo. These effects were mediated through the stimulation of DCs, particularly of the CD8α(-) subtype, which respond to PstS1 by undergoing phenotypic maturation and by secreting IL-6, IL-1β and, to a lower extent, IL-23. IL-6 secretion by PstS1-stimulated DCs was required for IFN-γ, and to a lesser extent for IL-22 responses by Ag85B-specific memory T cells. These results may open new perspectives for immunotherapeutic strategies to control Th1/Th17 immune responses in Mtb infections and in vaccinations against tuberculosis.

Vaccine review: "Staphyloccocus aureus vaccines: problems and prospects"

Staphylococcus aureus is a leading cause of both healthcare- and community-associated infections globally. S. aureus exhibits diverse clinical presentations, ranging from benign carriage and superficial skin and soft tissue infections to deep wound and organ/space infections, biofilm-related prosthesis infections, life-threatening bacteremia and sepsis. This broad clinical spectrum, together with the high incidence of these disease manifestations and magnitude of the diverse populations at risk, presents a high unmet medical need and a substantial burden to the healthcare system. With the increasing propensity of S. aureus to develop resistance to essentially all classes of antibiotics, alternative strategies, such as prophylactic vaccination to prevent S. aureus infections, are actively being pursued in healthcare settings. Within the last decade, the S. aureus vaccine field has witnessed two major vaccine failures in phase 3 clinical trials designed to prevent S. aureus infections in either patients undergoing cardiothoracic surgery or patients with end-stage renal disease undergoing hemodialysis. This review summarizes the potential underlying reasons why these two approaches may have failed, and proposes avenues that may provide successful vaccine approaches to prevent S. aureus disease in the future.