Innate allorecognition in transplantation

Targeting Macrophages in Organ Transplantation: A Step Toward Personalized Medicine

Organ transplantation remains the most optimal strategy for patients with end-stage organ failure. However, prevailing methods of immunosuppression are marred by adverse side effects, and allograft rejection remains common. It is imperative to identify and comprehensively characterize the cell types involved in allograft rejection, and develop therapies with greater specificity. There is increasing recognition that processes mediating allograft rejection are the result of interactions between innate and adaptive immune cells. Macrophages are heterogeneous innate immune cells with diverse functions that contribute to ischemia-reperfusion injury, acute rejection, and chronic rejection. Macrophages are inflammatory cells capable of innate allorecognition that strengthen their responses to secondary exposures over time via "trained immunity." However, macrophages also adopt immunoregulatory phenotypes and may promote allograft tolerance. In this review, we discuss the roles of macrophages in rejection and tolerance, and detail how macrophage plasticity and polarization influence transplantation outcomes. A comprehensive understanding of macrophages in transplant will guide future personalized approaches to therapies aimed at facilitating tolerance or mitigating the rejection process.

Posttransplant complications: molecular mechanisms and therapeutic interventions

Posttransplantation complications pose a major challenge to the long-term survival and quality of life of organ transplant recipients. These complications encompass immune-mediated complications, infectious complications, metabolic complications, and malignancies, with each type influenced by various risk factors and pathological mechanisms. The molecular mechanisms underlying posttransplantation complications involve a complex interplay of immunological, metabolic, and oncogenic processes, including innate and adaptive immune activation, immunosuppressant side effects, and viral reactivation. Here, we provide a comprehensive overview of the clinical features, risk factors, and molecular mechanisms of major posttransplantation complications. We systematically summarize the current understanding of the immunological basis of allograft rejection and graft-versus-host disease, the metabolic dysregulation associated with immunosuppressive agents, and the role of oncogenic viruses in posttransplantation malignancies. Furthermore, we discuss potential prevention and intervention strategies based on these mechanistic insights, highlighting the importance of optimizing immunosuppressive regimens, enhancing infection prophylaxis, and implementing targeted therapies. We also emphasize the need for future research to develop individualized complication control strategies under the guidance of precision medicine, ultimately improving the prognosis and quality of life of transplant recipients.

Cyclic guanosine monophosphate-adenosine monophosphate synthetase/stimulator of interferon genes signaling aggravated corneal allograft rejection through neutrophil extracellular traps

The activation of innate immunity following transplantation has been identified as a crucial factor in allograft inflammation and rejection. However, the role of cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)/stimulator of interferon genes (STING) signaling-mediated innate immunity in the pathogenesis of allograft rejection remains unclear. Utilizing a well-established murine model of corneal transplantation, we demonstrated increased expression of cGAS and STING in rejected-corneal allografts compared with syngeneic (Syn) and normal (Nor) corneas, along with significant activation of the cGAS/STING pathway, as evidenced by the enhanced phosphorylation of TANK-binding kinase 1and interferon regulatory factor 3. Pharmacological and genetic inhibition of cGAS/STING signaling markedly delayed corneal transplantation rejection, resulting in prolonged survival time and reduced inflammatory infiltration. Furthermore, we observed an increase in the formation of neutrophil extracellular traps (NETs) in rejected allografts, and the inhibition of NET formation through targeting peptidylarginine deiminase 4 and DNase I treatment significantly alleviated immune rejection and reduced cGAS/STING signaling activity. Conversely, subconjunctival injection of NETs accelerated corneal transplantation rejection and enhanced the activation of the cGAS/STING pathway. Collectively, these findings demonstrate that NETs contribute to the exacerbation of allograft rejection via cGAS/STING signaling, highlighting the targeting of the NETs/cGAS/STING signaling pathway as a potential strategy for prolonging allograft survival.

Nanotechnology-based Strategies for Molecular Imaging, Diagnosis, and Therapy of Organ Transplantation

Organ transplantation is the preferred paradigm for patients with end-stage organ failures. Despite unprecedented successes, complications such as immune rejection, ischemia-reperfusion injury, and graft dysfunction remain significant barriers to long-term recipient survival after transplantation. Conventional immunosuppressive drugs have limited efficacy because of significant drug toxicities, high systemic immune burden, and emergence of transplant infectious disease, leading to poor quality of life for patients. Nanoparticle-based drug delivery has emerged as a promising medical technology and offers several advantages by enhancing the delivery of drug payloads to their target sites, reducing systemic toxicity, and facilitating patient compliance over free drug administration. In addition, nanotechnology-based imaging approaches provide exciting diagnostic methods for monitoring molecular and cellular changes in transplanted organs, visualizing immune responses, and assessing the severity of rejection. These noninvasive technologies are expected to help enhance the posttransplantation patient survival through real time and early diagnosis of disease progression. Here, we present a comprehensive review of nanotechnology-assisted strategies in various aspects of organ transplantation, including organ protection before transplantation, mitigation of ischemia-reperfusion injury, counteraction of immune rejection, early detection of organ dysfunction posttransplantation, and molecular imaging and diagnosis of immune rejection.

Innate Allorecognition in Transplantation: Ancient Mechanisms With Modern Impact

Through the effective targeting of the adaptive immune system, solid organ transplantation became a life-saving therapy for organ failure. However, beyond 1 y of transplantation, there is little improvement in transplant outcomes. The adaptive immune response requires the activation of the innate immune system. There are no modalities for the specific targeting of the innate immune system involvement in transplant rejection. However, the recent discovery of innate allorecognition and innate immune memory presents novel targets in transplantation that will increase our understanding of organ rejection and might aid in improving transplant outcomes. In this review, we look at the latest developments in the study of innate allorecognition and innate immune memory in transplantation.

Lung Transplant Immunomodulation with Genetically Engineered Mesenchymal Stromal Cells-Therapeutic Window for Interleukin-10

Lung transplantation results are compromised by ischemia-reperfusion injury and alloimmune responses. Ex vivo lung perfusion (EVLP) is used to assess marginal donor lungs before transplantation but is also an excellent platform to apply novel therapeutics. We investigated donor lung immunomodulation using genetically engineered mesenchymal stromal cells with augmented production of human anti-inflammatory hIL-10 (MSCsIL-10). Pig lungs were placed on EVLP for 6 h and randomized to control (n = 7), intravascular delivery of 20 × 106 (n = 5, low dose) or 40 × 106 human MSCs IL-10 (n = 6, high dose). Subsequently, single-lung transplantation was performed, and recipient pigs were monitored for 3 days. hIL-10 secretion was measured during EVLP and after transplantation, and immunological effects were assessed by cytokine profile, T and myeloid cell characterization and mixed lymphocyte reaction. MSCIL-10 therapy rapidly increased hIL-10 during EVLP and resulted in transient hIL-10 elevation after lung transplantation. MSCIL-10 delivery did not affect lung function but was associated with dose-related immunomodulatory effects, with the low dose resulting in a beneficial decrease in apoptosis and lower macrophage activation, but the high MSCIL-10 dose resulting in inflammation and cytotoxic CD8+ T cell activation. MSCIL-10 therapy during EVLP results in a rapid and transient perioperative hIL-10 increase and has a therapeutic window for its immunomodulatory effects.

The fate of adipose tissue and adipose-derived stem cells in allograft

Utilizing adipose tissue and adipose-derived stem cells (ADSCs) turned into a promising field of allograft in recent years. The therapeutic potential of adipose tissue and ADSCs is governed by their molecular secretions, ability to sustain multi-differentiation and self-renewal which are pivotal in reconstructive, genetic diseases, and cosmetic goals. However, revisiting the existing functional capacity of adipose tissue and ADSCs and their intricate relationship with allograft is crucial to figure out the remarkable question of safety to use in allograft due to the growing evidence of interactions between tumor microenvironment and ADSCs. For instance, the molecular secretions of adipose tissue and ADSCs induce angiogenesis, create growth factors, and control the inflammatory response; it has now been well determined. Though the existing preclinical allograft studies gave positive feedback, ADSCs and adipose tissue are attracted by some factors of tumor stroma. Moreover, allorecognition is pivotal to allograft rejection which is carried out by costimulation in a complement-dependent way and leads to the destruction of the donor cells. However, extensive preclinical trials of adipose tissue and ADSCs in allograft at molecular level are still limited. Hence, comprehensive immunomodulatory analysis could ensure the successful allograft of adipose tissue and ADSCs avoiding the oncological risk.

Single-cell analysis of graft-infiltrating host cells identifies caspase-1 as a potential therapeutic target for heart transplant rejection

Aims

Understanding the cellular mechanisms underlying early allograft rejection is crucial for the development of effective immunosuppressant strategies. This study aims to investigate the cellular composition of graft-infiltrating cells during the early rejection stage at a single-cell level and identify potential therapeutic targets.

Methods

A heterotopic heart transplant model was established using enhanced green fluorescent protein (eGFP)-expressing mice as recipients of allogeneic or syngeneic grafts. At 3 days post-transplant, eGFP-positive cells infiltrating the grafts were sorted and subjected to single-cell RNA-seq analysis. Potential molecular targets were evaluated by assessing graft survival and functions following administration of various pharmacological inhibitors.

Results

A total of 27,053 cells recovered from syngrafts and allografts were classified into 20 clusters based on expression profiles and annotated with a reference dataset. Innate immune cells, including monocytes, macrophages, neutrophils, and dendritic cells, constituted the major infiltrating cell types (>90%) in the grafts. Lymphocytes, fibroblasts, and endothelial cells represented a smaller population. Allografts exhibited significantly increased proportions of monocyte-derived cells involved in antigen processing and presentation, as well as activated lymphocytes, as compared to syngrafts. Differential expression analysis revealed upregulation of interferon activation-related genes in the innate immune cells infiltrating allografts. Pro-inflammatory polarization gene signatures were also enriched in these infiltrating cells of allografts. Gene profiling and intercellular communication analysis identified natural killer cells as the primary source of interferon-γ signaling, activating inflammatory monocytes that displayed strong signals of major histocompatibility complexes and co-stimulatory molecules. The inflammatory response was also associated with promoted T cell proliferation and activation in allografts during the early transplant stages. Notably, caspase-1 exhibited specific upregulation in inflammatory monocytes in response to interferon signaling. The regulon analysis also revealed a significant enrichment of interferon-related motifs within the transcriptional regulatory network of downstream inflammatory genes including caspase-1. Remarkably, pharmacological inhibition of caspase-1 was shown to reduce immune infiltration, prevent acute graft rejection, and improve cardiac contractile function.

Conclusion

The single-cell transcriptional profile highlighted the crucial role of caspase-1 in interferon-mediated inflammatory monocytes infiltrating heart transplants, suggesting its potential as a therapeutic target for attenuating rejection.

Innate Immune Memory in Macrophages

Macrophages have been recognized as the primary mediators of innate immunity starting from embryonic/fetal development. Macrophage-mediated defenses may not be as antigen-specific as adaptive immunity, but increasing information suggests that these responses do strengthen with repeated immunological triggers. The concept of innate memory in macrophages has been described as "trained immunity" or "innate immune memory (IIM)." As currently understood, this cellular memory is rooted in epigenetic and metabolic reprogramming. The recognition of IIM may be particularly important in the fetus and the young neonate who are yet to develop protective levels of adaptive immunity, and could even be of preventive/therapeutic importance in many disorders. There may also be a possibility of therapeutic enhancement with targeted vaccination. This article presents a review of the properties, mechanisms, and possible clinical significance of macrophage-mediated IIM.

The multifunctional roles of autophagy in the innate immune response: Implications for regulation of transplantation rejection

Organ transplantation is the main treatment for end-stage organ failure, which has rescued tens of thousands of lives. Immune rejection is the main factor affecting the survival of transplanted organs. How to suppress immune rejection is an important goal of transplantation research. A graft first triggers innate immune responses, leading to graft inflammation, tissue injury and cell death, followed by adaptive immune activation. At present, the importance of innate immunity in graft rejection is poorly understood. Autophagy, an evolutionarily conserved intracellular degradation system, is proven to be involved in regulating innate immune response following graft transplants. Moreover, there is evidence indicating that autophagy can regulate graft dysfunction. Although the specific mechanism by which autophagy affects graft rejection remains unclear, autophagy is involved in innate immune signal transduction, inflammatory response, and various forms of cell death after organ transplantation. This review summarizes how autophagy regulates these processes and proposes potential targets for alleviating immune rejection.

[Current insights on monitoring of renal transplants-Banff and beyond]

Renal transplantation represents the best treatment for end-stage renal disease. Much effort has been invested in improvement of longevity of the transplanted organ including a comprehensive and regularly updated histological scoring system (Banff classification) for surveillance; however, survival of transplanted kidneys is still limited to median 15 years. Molecular analyses have increased the understanding of damaging mechanisms within the transplant, especially antibody-mediated rejection, which can be difficult to identify using histological methods. Changes in the Banff classification necessitate to reclassify biopsies included in studies according to current consensus. Digital and molecular innovations as well as new immunologic mechanisms are anticipated.

Immunoglobulin-like receptors and the generation of innate immune memory

Host immunity is classically divided into "innate" and "adaptive." While the former has always been regarded as the first, rapid, and antigen-nonspecific reaction to invading pathogens, the latter represents the more sophisticated and antigen-specific response that has the potential to persist and generate memory. Recent work however has challenged this dogma, where murine studies have successfully demonstrated the ability of innate immune cells (monocytes and macrophages) to acquire antigen-specific memory to allogeneic major histocompatibility complex (MHC) molecules. The immunoreceptors so far identified that mediate innate immune memory are the paired immunoglobulin-like receptors (PIRs) in mice, which are orthologous to human leukocyte immunoglobulin-like receptors (LILRs). These receptor families are mainly expressed by the myelomonocytic cell lineage, suggesting an important role in the innate immune response. In this review, we will discuss the role of immunoglobulin-like receptors in the development of innate immune memory across species.

Overview of Memory NK Cells in Viral Infections: Possible Role in SARS-CoV-2 Infection

NK cells have usually been defined as cells of the innate immune system, although they are also involved in adaptative responses. These cells belong to the innate lymphocyte cells (ILC) family. They remove unwanted cells, tumoral cells and pathogens. NK cells are essential for viral infection clearance and are involved in tolerogenic responses depending on the dynamic balance of the repertoire of activating and inhibitory receptors. NK plasticity is crucial for tissue function and vigilant immune responses. They directly eliminate virus-infected cells by recognising viral protein antigens using a non-MHC dependent mechanism, recognising viral glycan structures and antigens by NCR family receptors, inducing apoptosis by Fas-Fas ligand interaction, and killing cells by antibody-dependent cell cytotoxicity via the FcγIII receptor. Activating receptors are responsible for the clearance of virally infected cells, while inhibitory KIR receptor activation impairs NK responses and facilitates virus escape. Effective NK memory cells have been described and characterised by a low NKG2A and high NKG2C or NKG2D expression. NK cells have also been used in cell therapy. In SARS-CoV-2 infection, several contradicting reports about the role of NK cells have been published. A careful analysis of the current data and possible implications will be discussed.

Failing Heart Transplants and Rejection-A Cellular Perspective

The median survival of patients with heart transplants is relatively limited, implying one of the most relevant questions in the field—how to expand the lifespan of a heart allograft? Despite optimal transplantation conditions, we do not anticipate a rise in long-term patient survival in near future. In order to develop novel strategies for patient monitoring and specific therapies, it is critical to understand the underlying pathological mechanisms at cellular and molecular levels. These events are driven by innate immune response and allorecognition driven inflammation, which controls both tissue damage and repair in a spatiotemporal context. In addition to immune cells, also structural cells of the heart participate in this process. Novel single cell methods have opened new avenues for understanding the dynamics driving the events leading to allograft failure. Here, we review current knowledge on the cellular composition of a normal heart, and cellular mechanisms of ischemia-reperfusion injury (IRI), acute rejection and cardiac allograft vasculopathy (CAV) in the transplanted hearts. We highlight gaps in current knowledge and suggest future directions, in order to improve cellular and molecular understanding of failing heart allografts.