Regulatory macrophages as therapeutic targets and therapeutic agents in solid organ transplantation

M1/M2 macrophages and their overlaps - myth or reality?

Macrophages represent heterogeneous cell population with important roles in defence mechanisms and in homoeostasis. Tissue macrophages from diverse anatomical locations adopt distinct activation states. M1 and M2 macrophages are two polarized forms of mononuclear phagocyte in vitro differentiation with distinct phenotypic patterns and functional properties, but in vivo, there is a wide range of different macrophage phenotypes in between depending on the microenvironment and natural signals they receive. In human infections, pathogens use different strategies to combat macrophages and these strategies include shaping the macrophage polarization towards one or another phenotype. Macrophages infiltrating the tumours can affect the patient's prognosis. M2 macrophages have been shown to promote tumour growth, while M1 macrophages provide both tumour-promoting and anti-tumour properties. In autoimmune diseases, both prolonged M1 activation, as well as altered M2 function can contribute to their onset and activity. In human atherosclerotic lesions, macrophages expressing both M1 and M2 profiles have been detected as one of the potential factors affecting occurrence of cardiovascular diseases. In allergic inflammation, T2 cytokines drive macrophage polarization towards M2 profiles, which promote airway inflammation and remodelling. M1 macrophages in transplantations seem to contribute to acute rejection, while M2 macrophages promote the fibrosis of the graft. The view of pro-inflammatory M1 macrophages and M2 macrophages suppressing inflammation seems to be an oversimplification because these cells exploit very high level of plasticity and represent a large scale of different immunophenotypes with overlapping properties. In this respect, it would be more precise to describe macrophages as M1-like and M2-like.

M2 Macrophages Serve as Critical Executor of Innate Immunity in Chronic Allograft Rejection

Allograft functional failure due to acute or chronic rejection has long been a major concern in the area of solid organ transplantation for decades. As critical component of innate immune system, the macrophages are unlikely to be exclusive for driving acute or chronic sterile inflammation against allografts. Traditionally, macrophages are classified into two types, M1 and M2 like macrophages, based on their functions. M1 macrophages are involved in acute rejection for triggering sterile inflammation thus lead to tissue damage and poor allograft survival, while M2 macrophages represent contradictory features, playing pivotal roles in both anti-inflammation and development of graft fibrosis and resulting in chronic rejection. Macrophages also contribute to allograft vasculopathy, but the phenotypes remain to be identified. Moreover, increasing evidences are challenging traditional identification and classification of macrophage in various diseases. Better understanding the role of macrophage in chronic rejection is fundamental to developing innovative strategies for preventing late graft loss. In this review, we will update the recent progress in our understanding of diversity of macrophage-dominated innate immune response, and reveal the roles of M2 macrophages in chronic allograft rejection as well.

Untangling Local Pro-Inflammatory, Reparative, and Regulatory Damage-Associated Molecular-Patterns (DAMPs) Pathways to Improve Transplant Outcomes

Detrimental inflammatory responses after solid organ transplantation are initiated when immune cells sense pathogen-associated molecular patterns (PAMPs) and certain damage-associated molecular patterns (DAMPs) released or exposed during transplant-associated processes, such as ischemia/reperfusion injury (IRI), surgical trauma, and recipient conditioning. These inflammatory responses initiate and propagate anti-alloantigen (AlloAg) responses and targeting DAMPs and PAMPs, or the signaling cascades they activate, reduce alloimmunity, and contribute to improved outcomes after allogeneic solid organ transplantation in experimental studies. However, DAMPs have also been implicated in initiating essential anti-inflammatory and reparative functions of specific immune cells, particularly Treg and macrophages. Interestingly, DAMP signaling is also involved in local and systemic homeostasis. Herein, we describe the emerging literature defining how poor outcomes after transplantation may result, not from just an over-abundance of DAMP-driven inflammation, but instead an inadequate presence of a subset of DAMPs or related molecules needed to repair tissue successfully or re-establish tissue homeostasis. Adverse outcomes may also arise when these homeostatic or reparative signals become dysregulated or hijacked by alloreactive immune cells in transplant niches. A complete understanding of the critical pathways controlling tissue repair and homeostasis, and how alloimmune responses or transplant-related processes disrupt these will lead to new immunotherapeutics that can prevent or reverse the tissue pathology leading to lost grafts due to chronic rejection.

Regulatory T Cells for the Induction of Transplantation Tolerance

Organ transplantation is the optimal treatment for terminal and irreversible organ failure. Achieving transplantation tolerance has long been the ultimate goal in the field of transplantation. Regulatory T cell (Treg)-based therapy is a promising novel approach for inducing donor organ-specific tolerance. Tregs play critical roles in the maintenance of immune homeostasis and self-tolerance, by promoting transplantation tolerance through a variety of mechanisms on different target cells, including anti-inflammatory cytokine production, induction of apoptosis, disruption of metabolic pathways, and mutual interaction with dendritic cells. The continued success of Treg-based therapy in the clinical setting is critically dependent on preclinical studies that support its translational potential. However, although some initial clinical trials of adoptive Treg therapy have successively demonstrated safety and efficacy for immunosuppressant minimization and transplantation tolerance induction, most Treg-based hematopoietic stem cell and solid organ clinical trials are still in their infancy. These clinical trials have not only focused on safety and efficacy but also included optimization and standardization protocols of good manufacturing practice regarding cell isolation, expansion, dosing, timing, specificity, quality control, concomitant immunosuppressants, and post-administration monitoring. We herein report a brief introduction of Tregs, including their phenotypic and functional characterization, and focus on the clinical translation of Treg-based therapeutic applications in the setting of transplantation.

Macrophages in Organ Transplantation

Current immunosuppressive therapy has led to excellent short-term survival rates in organ transplantation. However, long-term graft survival rates are suboptimal, and a vast number of allografts are gradually lost in the clinic. An increasing number of animal and clinical studies have demonstrated that monocytes and macrophages play a pivotal role in graft rejection, as these mononuclear phagocytic cells recognize alloantigens and trigger an inflammatory cascade that activate the adaptive immune response. Moreover, recent studies suggest that monocytes acquire a feature of memory recall response that is associated with a potent immune response. This form of memory is called “trained immunity,” and it is retained by mechanisms of epigenetic and metabolic changes in innate immune cells after exposure to particular ligands, which have a direct impact in allograft rejection. In this review article, we highlight the role of monocytes and macrophages in organ transplantation and summarize therapeutic approaches to promote tolerance through manipulation of monocytes and macrophages. These strategies may open new therapeutic opportunities to increase long-term transplant survival rates in the clinic.

M2b macrophages protect against myocardial remodeling after ischemia/reperfusion injury by regulating kinase activation of platelet-derived growth factor receptor of cardiac fibroblast

Background

Myocardial injury is a major cause of myocardial remodeling. Macrophages are important in cardiac repair as a result of their interactions with fibroblasts. As regulatory macrophages, M2b macrophages modulate inflammatory immune responses without participating in wound healing and could have enhanced protective effects on myocardial remodeling. Therefore, we tested the hypothesis that M2b macrophages could improve cardiac function and ameliorate myocardial fibrosis after the myocardial ischemia/reperfusion injury (MI/RI).

Methods

In vivo, MI/RI models were established with Sprague-Dawley (SD) rats and either M2b macrophages (MT group) or the same volume of vehicle (CK group) was injected into the ischemic zone. Two weeks after the operation, cardiac function and diameters were determined by echocardiography examination. Level of myocardial fibrosis was measured by Sirius red staining and the expression of fibrosis-related factors. In vitro, cardiac fibroblasts (CFs) were co-cultured with M2b macrophages or cultured with M2b macrophage supernatant. Expression of α-smooth muscle actin (α-SMA) and connective tissue growth factor (CCN2/CTGF) in the CFs were measured by western blotting and immunofluorescence staining. In addition, the expression of platelet-derived growth factors (PDGFs), the expression of platelet-derived growth factor receptors (PDGFRs) and the phosphorylation of PDGFRs was detected by western blotting.

Results

A significantly higher rat survival rate, improved left ventricular (LV) systolic function, decreased diameter of the LV and alleviated myocardial fibrosis were observed in the MT group than in the CK group. In vitro, the activation of CFs was significantly reduced by the M2b macrophages treatments, relative to the blank control. In addition, the kinase activation of PDGFRs was decreased by M2b macrophage treatments both in vivo and in vitro.

Conclusions

Our study demonstrated that the administration of M2b macrophages could attenuate myocardial remodeling after MI/RI. The regulation of the activation of PDGFRs in CFs is an important part of the protective mechanism.

Immunomodulatory Dual-Sized Microparticle System Conditions Human Antigen Presenting Cells Into a Tolerogenic Phenotype In Vitro and Inhibits Type 1 Diabetes-Specific Autoreactive T Cell Responses

Current monotherapeutic agents fail to restore tolerance to self-antigens in autoimmune individuals without systemic immunosuppression. We hypothesized that a combinatorial drug formulation delivered by a poly-lactic-co-glycolic acid (PLGA) dual-sized microparticle (dMP) system would facilitate tunable drug delivery to elicit immune tolerance. Specifically, we utilized 30 µm MPs to provide local sustained release of granulocyte-macrophage colony-stimulating factor (GM-CSF) and transforming growth factor β1 (TGF-β1) along with 1 µm MPs to facilitate phagocytic uptake of encapsulated antigen and 1α,25(OH)2 Vitamin D3 (VD3) followed by tolerogenic antigen presentation. We previously demonstrated the dMP system ameliorated type 1 diabetes (T1D) and experimental autoimmune encephalomyelitis (EAE) in murine models. Here, we investigated the system's capacity to impact human cell activity in vitro to advance clinical translation. dMP treatment directly reduced T cell proliferation and inflammatory cytokine production. dMP delivery to monocytes and monocyte-derived dendritic cells (DCs) increased their expression of surface and intracellular anti-inflammatory mediators. In co-culture, dMP-treated DCs (dMP-DCs) reduced allogeneic T cell receptor (TCR) signaling and proliferation, while increasing PD-1 expression, IL-10 production, and regulatory T cell (Treg) frequency. To model antigen-specific activation and downstream function, we co-cultured TCR-engineered autoreactive T cell "avatars," with dMP-DCs or control DCs followed by β-cell line (ßlox5) target cells. For G6PC2-specific CD8+ avatars (clone 32), dMP-DC exposure reduced Granzyme B and dampened cytotoxicity. GAD65-reactive CD4+ avatars (clone 4.13) exhibited an anergic/exhausted phenotype with dMP-DC presence. Collectively, these data suggest this dMP formulation conditions human antigen presenting cells toward a tolerogenic phenotype, inducing regulatory and suppressive T cell responses.

Manipulation of Regulatory Dendritic Cells for Induction Transplantation Tolerance

Current organ transplantation therapy is life-saving but accompanied by well-recognized side effects due to post-transplantation systematic immunosuppressive treatment. Dendritic cells (DCs) are central instigators and regulators of transplantation immunity and are responsible for balancing allograft rejection and tolerance. They are derived from monocyte-macrophage DC progenitors originating in the bone marrow and are classified into different subsets based on their developmental, phenotypical, and functional criteria. Functionally, DCs instigate allograft immunity by presenting donor antigens to alloreactive T cells via direct, indirect, and semidirect recognition pathways and provide essential signaling for alloreactive T cell activation via costimulatory molecules and pro-inflammatory cytokines. Regulatory DCs (DCregs) are characterized by a relatively low expression of major histocompatibility complex, costimulatory molecules, and altered cytokine production and exert their regulatory function through T cell anergy, T cell deletion, and regulatory T cell induction. In rodent transplantation studies, DCreg-based therapy, by in situ targeting or infusion of ex vivo generated DCregs, exhibits promising potential as a natural, well-tolerated, organ-specific therapeutic strategy for promoting lasting organ-specific transplantation tolerance. Recent early-phase studies of DCregs have begun to examine the safety and efficacy of DCreg-induced allograft tolerance in living-donor renal or liver transplantations. The present review summarizes the basic characteristics, function, and translation of DCregs in transplantation tolerance induction.

Human Mesenchymal Stromal Cell Secretome Promotes the Immunoregulatory Phenotype and Phagocytosis Activity in Human Macrophages

Human mesenchymal stromal/stem cells (hMSCs) show great promise in cell therapy due to their immunomodulatory properties. The overall immunomodulatory response of hMSCs resembles the resolution of inflammation, in which lipid mediators and regulatory macrophages (Mregs) play key roles. We investigated the effect of hMSC cell-cell contact and secretome on macrophages polarized and activated toward Mreg phenotype. Moreover, we studied the effect of supplemented polyunsaturated fatty acids (PUFAs): docosahexaenoic acid (DHA) and arachidonic acid, the precursors of lipid mediators, on hMSC immunomodulation. Our results show that unlike hMSC cell-cell contact, the hMSC secretome markedly increased the CD206 expression in both Mreg-polarized and Mreg-activated macrophages. Moreover, the secretome enhanced the expression of programmed death-ligand 1 on Mreg-polarized macrophages and Mer receptor tyrosine kinase on Mreg-activated macrophages. Remarkably, these changes were translated into improved Candida albicans phagocytosis activity of macrophages. Taken together, these results demonstrate that the hMSC secretome promotes the immunoregulatory and proresolving phenotype of Mregs. Intriguingly, DHA supplementation to hMSCs resulted in a more potentiated immunomodulation with increased CD163 expression and decreased gene expression of matrix metalloproteinase 2 in Mreg-polarized macrophages. These findings highlight the potential of PUFA supplementations as an easy and safe method to improve the hMSC therapeutic potential.

Tolerogenic Dendritic Cells Reduce Cardiac Inflammation and Fibrosis in Chronic Chagas Disease

Chronic Chagas disease cardiomyopathy (CCC) is the most frequent and severe form of this parasitic disease. CCC is caused by a progressive inflammation in the heart, resulting in alterations that can culminate in heart failure and death. The use of dendritic cells (DCs) appears as an option for the development of treatments due to their important role in regulating immune responses. Here, we investigated whether tolerogenic cells (tDCs) could interfere with the progression of CCC in an experimental model of Chagas disease. The tDCs were generated and characterized as CD11b⁺ CD11c⁺ cells, low expression of MHC-II, CD86, CD80, and CD40, and increased expression of PD-L. These cells produced low levels of IL-6 and IL-12p70 and higher levels of IL-10, compared to mature DCs (mDCs). Interestingly, tDCs inhibited lymphoproliferation and markedly increased the population of FoxP3⁺ Treg cells in vitro, compared to mature DCs. In a mouse model of CCC, treatment with tDCs reduced heart inflammation and fibrosis. Furthermore, tDCs treatment reduced the gene expression of pro-inflammatory cytokines (Ifng and Il12) and of genes related to cardiac remodeling (Col1a2 and Lgals3), while increasing the gene expression of IL-10. Finally, administration of tDCs, increased the percentage of Treg cells in the hearts and spleens of chagasic mice. Ours results show that tolerogenic dendritic cells have therapeutic potential on CCC, inhibiting disease progression.

M2b Macrophages Regulate Cardiac Fibroblast Activation and Alleviate Cardiac Fibrosis After Reperfusion Injury

BACKGROUND

Macrophages play an important role in the development of cardiac fibrosis. However, the roles of different macrophage subtypes in cardiac fibroblast (CF) activation and cardiac fibrosis are unknown.

METHODS AND RESULTS

Bone marrow-derived macrophages (BMDMs) were treated with different stimuli to induce differentiation into M1, M2a, M2b, and M2c macrophage subtypes. CFs were co-cultured with different subtypes of macrophages or cultured with macrophage supernatants. Results revealed that M2b macrophages significantly suppressed the proliferation and migration of CFs, the expression of fibrosis-related proteins (collagen I [COL-1] and α-smooth muscle actin [α-SMA]), and differentiation into cardiac myofibroblasts (MFs). The opposite effects were observed with M2a macrophages. A rat model of cardiac ischemia/reperfusion (I/R) injury was used to determine the effect of M2b macrophages transplantation. After cardiac I/R injury, transplantation of M2b macrophages improved cardiac function and reduced cardiac fibrosis. The effect of macrophage subtypes on p-ERK, ERK, p-p38, and p38 phosphorylation was examined by Western blotting. The results showed that M2b macrophages significantly inhibited the mitogen-activated protein kinase (MAPK) signaling pathway.

CONCLUSIONS

These study results demonstrate for the first time that different subtypes of macrophages have different roles in regulating CF activation. M2b macrophages inhibit CF activation, and thus can be considered anti-fibrotic macrophages. M2a macrophages promote CF activation, and thus are pro-fibrotic macrophages.

Organization of the Skin Immune System and Compartmentalized Immune Responses in Infectious Diseases

The skin is an organ harboring several types of immune cells that participate in innate and adaptive immune responses. The immune system of the skin comprises both skin cells and professional immune cells that together constitute what is designated skin-associated lymphoid tissue (SALT). In this review, I extensively discuss the organization of SALT and the mechanisms involved in its responses to infectious diseases of the skin and mucosa. The nature of these SALT responses, and the cellular mediators involved, often determines the clinical course of such infections. I list and describe the components of innate immunity, such as the roles of the keratinocyte barrier and of inflammatory and natural killer cells. I also examine the mechanisms involved in adaptive immune responses, with emphasis on new cytokine profiles, and the role of cell death phenomena in host-pathogen interactions and control of the immune responses to infectious agents. Finally, I highlight the importance of studying SALT in order to better understand host-pathogen relationships involving the skin and detail future directions in the immunological investigation of this organ, especially in light of recent findings regarding the skin immune system.

Innate Immune Determinants of Graft-Versus-Host Disease and Bidirectional Immune Tolerance in Allogeneic Transplantation

The success of tissue transplantation from a healthy donor to a diseased individual (allo-transplantation) is regulated by the immune systems of both donor and recipient. Developing a state of specific non-reactivity between donor and recipient, while maintaining the salutary effects of immune function in the recipient, is called “immune (transplantation) tolerance”. In the classic early post-transplant period, minimizing bidirectional donor ←→ recipient reactivity requires the administration of immunosuppressive drugs, which have deleterious side effects (severe immunodeficiency, opportunistic infections, and neoplasia, in addition to drug-specific reactions and organ toxicities). Inducing immune tolerance directly through donor and recipient immune cells, particularly via subsets of immune regulatory cells, has helped to significantly reduce side effects associated with multiple immunosuppressive drugs after allo-transplantation. The innate and adaptive arms of the immune system are both implicated in inducing immune tolerance. In the present article, we will review innate immune subset manipulations and their potential applications in hematopoietic stem cell transplantation (HSCT) to cure malignant and non-malignant hematological disorders by inducing long-lasting donor ←→ recipient (bidirectional) immune tolerance and reduced graft-versus-host disease (GVHD). These innate immunotherapeutic strategies to promote long-term immune allo-transplant tolerance include myeloid-derived suppressor cells (MDSCs), regulatory macrophages, tolerogenic dendritic cells (tDCs), Natural Killer (NK) cells, invariant Natural Killer T (iNKT) cells, gamma delta T (γδ-T) cells and mesenchymal stromal cells (MSCs).

TIGIT+ iTregs elicited by human regulatory macrophages control T cell immunity

Human regulatory macrophages (Mreg) have shown early clinical promise as a cell-based adjunct immunosuppressive therapy in solid organ transplantation. It is hypothesised that recipient CD4⁺ T cell responses are actively regulated through direct allorecognition of donor-derived Mregs. Here we show that human Mregs convert allogeneic CD4⁺ T cells to IL-10-producing, TIGIT⁺ FoxP3⁺-induced regulatory T cells that non-specifically suppress bystander T cells and inhibit dendritic cell maturation. Differentiation of Mreg-induced Tregs relies on multiple non-redundant mechanisms that are not exclusive to interaction of Mregs and T cells, including signals mediated by indoleamine 2,3-dioxygenase, TGF-β, retinoic acid, Notch and progestagen-associated endometrial protein. Preoperative administration of donor-derived Mregs to living-donor kidney transplant recipients results in an acute increase in circulating TIGIT⁺ Tregs. These results suggest a feed-forward mechanism by which Mreg treatment promotes allograft acceptance through rapid induction of direct-pathway Tregs.

Regulatory macrophages (Mreg) can directly suppress T effector cell responses. Here the authors show that human Mreg also elicit TIGIT⁺ regulatory T cells by integrating multiple differentiation signals, and that donor Mreg-induced recipient Tregs may promote kidney transplant acceptance in patients.

Mesenchymal Stromal Cells and Their Extracellular Vesicles Enhance the Anti-Inflammatory Phenotype of Regulatory Macrophages by Downregulating the Production of Interleukin (IL)-23 and IL-22

Resolution-phase macrophage population orchestrates active dampening of the inflammation by secreting anti-inflammatory and proresolving products including interleukin (IL)-10 and lipid mediators (LMs). We investigated the effects of both human bone marrow-derived mesenchymal stromal cells (MSCs) and MSC-derived extracellular vesicles (MSC-EVs) on mature human regulatory macrophages (Mregs). The cytokines and LMs were determined from cell culture media of Mregs cultivated with MSCs and MSC-EVs. In addition, the alterations in the expression of cell surface markers and the phagocytic ability of Mregs were investigated. Our novel findings indicate that both MSC coculture and MSC-EVs downregulated the production of IL-23 and IL-22 enhancing the anti-inflammatory phenotype of Mregs and amplifying proresolving properties. The levels of prostaglandin E2 (PGE2) were substantially upregulated in MSC coculture media, which may endorse proresolving LM class switching. In addition, our results manifest, for the first time, that MSC-EVs mediate the Mreg phenotype change via PGE2. These data suggest that both human MSC and MSC-EVs may potentiate tolerance-promoting proresolving phenotype of human Mregs.

Transplantation Tolerance Induction: Cell Therapies and Their Mechanisms

Cell-based therapies have been studied extensively in the context of transplantation tolerance induction. The most successful protocols have relied on transfusion of bone marrow prior to the transplantation of a renal allograft. However, it is not clear that stem cells found in bone marrow are required in order to render a transplant candidate immunologically tolerant. Accordingly, mesenchymal stem cells, regulatory myeloid cells, T regulatory cells, and other cell types are being tested as possible routes to tolerance induction, in the absence of donor-derived stem cells. Early data with each of these cell types have been encouraging. However, the induction regimen capable of achieving consistent tolerance, while avoiding unwanted sided effects, and which is scalable to the human patient, has yet to be identified. Here, we present the status of investigations of various tolerogenic cell types and the mechanistic rationale for their use in tolerance induction protocols.

Prospective Clinical Testing of Regulatory Dendritic Cells in Organ Transplantation

Dendritic cells (DC) are rare, professional antigen-presenting cells with ability to induce or regulate alloimmune responses. Regulatory DC (DCreg) with potential to down-modulate acute and chronic inflammatory conditions that occur in organ transplantation can be generated in vitro under a variety of conditions. Here, we provide a rationale for evaluation of DCreg therapy in clinical organ transplantation with the goal of promoting sustained, donor-specific hyporesponsiveness, while lowering the incidence and severity of rejection and reducing patients’ dependence on anti-rejection drugs. Generation of donor- or recipient-derived DCreg that suppress T cell responses and prolong transplant survival in rodents or non-human primates has been well-described. Recently, good manufacturing practice (GMP)-grade DCreg have been produced at our Institution for prospective use in human organ transplantation. We briefly review experience of regulatory immune therapy in organ transplantation and describe our experience generating and characterizing human monocyte-derived DCreg. We propose a phase I/II safety study in which the influence of donor-derived DCreg combined with conventional immunosuppression on subclinical and clinical rejection and host alloimmune responses will be examined in detail.

Clinical Use of Tolerogenic Dendritic Cells-Harmonization Approach in European Collaborative Effort

The number of patients with autoimmune diseases and severe allergies and recipients of transplants increases worldwide. Currently, these patients require lifelong administration of immunomodulatory drugs. Often, these drugs are expensive and show immediate or late-occurring severe side effects. Treatment would be greatly improved by targeting the cause of autoimmunity, that is, loss of tolerance to self-antigens. Accumulating knowledge on immune mechanisms has led to the development of tolerogenic dendritic cells (tolDC), with the specific objective to restrain unwanted immune reactions in the long term. The first clinical trials with tolDC have recently been conducted and more tolDC trials are underway. Although the safety trials have been encouraging, many questions relating to tolDC, for example, cell-manufacturing protocols, administration route, amount and frequency, or mechanism of action, remain to be answered. Aiming to join efforts in translating tolDC and other tolerogenic cellular products (e.g., Tregs and macrophages) to the clinic, a European COST (European Cooperation in Science and Technology) network has been initiated-A FACTT (action to focus and accelerate cell-based tolerance-inducing therapies). A FACTT aims to minimize overlap and maximize comparison of tolDC approaches through establishment of minimum information models and consensus monitoring parameters, ensuring that progress will be in an efficient, safe, and cost-effective way.

The divergent roles of macrophages in solid organ transplantation

PURPOSE OF REVIEW

This review summarizes the phenotype and function of macrophages in the context of solid organ transplantation and will focus on fundamental insights into their paradoxical pro-inflammatory versus suppressive function. We will also discuss the therapeutic potential of regulatory macrophages in tolerance induction.

RECENT FINDINGS

Macrophages are emerging as an essential element of solid organ transplantation. Macrophages are involved in the pathogenesis of ischemia reperfusion injury, as well as both acute and chronic rejection, exacerbating injury through secretion of inflammatory effectors and by amplifying adaptive immune responses. Notably, not all responses associated with macrophages are deleterious to the graft, and graft protection can in fact be conferred by macrophages. This has been attributed to the presence of macrophages with tissue-repair capabilities, as well as the effects of regulatory macrophages.

SUMMARY

The explosion of new information on the role of macrophages in solid organ transplantation has opened up new avenues of research and the possibility of therapeutic intervention. However, the role of myeloid cells in graft rejection, resolution of rejection and tissue repair remains poorly understood. A better understanding of plasticity and regulation of monocyte polarization is vital for the development of new therapies for the treatment of acute and chronic transplant rejection.

Cell therapy for immunosuppression after kidney transplantation

PURPOSE

To give an overview over cell therapeutic approaches to immunosuppression in clinical kidney transplantation. A focus is on myeloid suppressor cell therapy by mitomycin C-induced cells (MICs).

METHODS

Literature review with an emphasis on already existing therapies.

RESULTS

Several cell therapeutic approaches to immunosuppression and donor-specific unresponsiveness are now being tested in early phase I and phase II trials in clinical kidney transplantation. Cell products such as regulatory T cells or regulatory macrophages, or other myeloid suppressor cell therapies, may either consist of donor-specific, third-party, or autologous cell preparations. Major problems are the identification of the suppressive cell populations and their expansion to have sufficient amount of cells to achieve donor unresponsiveness (e.g., with regulatory T cells). We show a simple and safe way to establish donor unresponsiveness in living-donor kidney transplantation by MIC therapy. A phase I clinical trial is now under way to test the safety and efficacy of this cell therapeutic approach.

CONCLUSIONS

Cell therapeutic approaches to immunosuppression after kidney transplantation may revolutionize clinical transplantation in the future.

In question: the scientific value of preclinical safety pharmacology and toxicology studies with cell-based therapies

A new cell-based medicinal product containing human regulatory macrophages, known as Mreg_UKR, has been developed and conforms to expectations of a therapeutic drug. Here, Mreg_UKR was subjected to pharmacokinetic, safety pharmacology, and toxicological testing, which identified no adverse reactions. These results would normally be interpreted as evidence of the probable clinical safety of Mreg_UKR; however, we contend that, owing to their uncertain biological relevance, our data do not fully support this conclusion. This leads us to question whether there is adequate scientific justification for preclinical safety testing of similar novel cell-based medicinal products using animal models. In earlier work, two patients were treated with regulatory macrophages prior to kidney transplantation. In our opinion, the absence of acute or chronic adverse effects in these cases is the most convincing available evidence of the likely safety of Mreg_UKR in future recipients. On this basis, we consider that safety information from previous clinical investigations of related cell products should carry greater weight than preclinical data when evaluating the safety profile of novel cell-based medicinal products. By extension, we argue that omitting extensive preclinical safety studies before conducting small-scale exploratory clinical investigations of novel cell-based medicinal products data may be justifiable in some instances.

Macrophages in solid organ transplantation

Macrophages are highly plastic hematopoietic cells with diversified functions related to their anatomic location and differentiation states. A number of recent studies have examined the role of macrophages in solid organ transplantation. These studies show that macrophages can induce allograft injury but, conversely, can also promote tissue repair in ischemia-reperfusion injury and acute rejection. Therapeutic strategies that target macrophages to improve outcomes in solid organ transplant recipients are being examined in preclinical and clinical models. In this review, we discuss the role of macrophages in different types of injury and rejection, with a focus on macrophage-mediated tissue injury, specifically vascular injury, repair and remodeling. We also discuss emerging macrophage-centered therapeutic opportunities in solid organ transplantation.

Regulatory myeloid cells in transplantation

Regulatory myeloid cells (RMC) are emerging as novel targets for immunosuppressive (IS) agents and hold considerable promise as cellular therapeutic agents. Herein, we discuss the ability of regulatory macrophages, regulatory dendritic cells, and myeloid-derived suppressor cells to regulate alloimmunity, their potential as cellular therapeutic agents, and the IS agents that target their function. We consider protocols for the generation of RMC and the selection of donor- or recipient-derived cells for adoptive cell therapy. Additionally, the issues of cell trafficking and antigen (Ag) specificity after RMC transfer are discussed. Improved understanding of the immunobiology of these cells has increased the possibility of moving RMC into the clinic to reduce the burden of current IS agents and to promote Ag-specific tolerance. In the second half of this review, we discuss the influence of established and experimental IS agents on myeloid cell populations. IS agents believed historically to act primarily on T cell activation and proliferation are emerging as important regulators of RMC function. Better insights into the influence of IS agents on RMC will enhance our ability to develop cell therapy protocols to promote the function of these cells. Moreover, novel IS agents may be designed to target RMC in situ to promote Ag-specific immune regulation in transplantation and to usher in a new era of immune modulation exploiting cells of myeloid origin.

Standardization of whole blood immune phenotype monitoring for clinical trials: panels and methods from the ONE study

Background

Immune monitoring by flow cytometry is a fast and highly informative way of studying the effects of novel therapeutics aimed at reducing transplant rejection or treating autoimmune diseases. The ONE Study consortium has recently initiated a series of clinical trials aimed at using different cell therapies to promote tolerance to renal allografts. To compare the effectiveness of different cell therapies, the consortium developed a robust immune monitoring strategy, including procedures for whole blood (WB) leukocyte subset profiling by flow cytometry.

Methods

Six leukocyte profiling panels computing 7- to 9-surface marker antigens for monitoring the major leukocyte subsets as well as characteristics of T cell, B cell, and dendritic cell (DC) subsets were designed. The precision and variability of these panels were estimated. The assay was standardized within eight international laboratories using Flow-Set Pro beads for mean fluorescence intensity target definition and the flow cytometer setup procedure. Standardization was demonstrated by performing inter-site comparisons.

Results

Optimized methods for sample collection, storage, preparation, and analysis were established, including protocols for gating target subsets. WB specimen age testing demonstrated that staining must be performed within 4 hours of sample collection to keep variability low, meaning less than or equal to 10% for the majority of defined leukocyte subsets. Inter-site comparisons between all participating centers testing shipped normal WB revealed good precision, with a variability of 0.05% to 30% between sites. Intra-assay analyses revealed a variability of 0.05% to 20% for the majority of subpopulations. This was dependent on the frequency of the particular subset, with smaller subsets showing higher variability. The intra-assay variability performance defined limits of quantitation (LoQ) for subsets, which will be the basis for assessing statistically significant differences achieved by the different cell therapies.

Conclusions

Local performance and central analysis of the ONE Study flow cytometry panel yields acceptable variability in a standardized assay at multiple international sites. These panels and procedures with WB allow unmanipulated analysis of changes in absolute cell numbers of leukocyte subsets in single- or multicenter clinical trials. Accordingly, we propose the ONE Study panel may be adopted as a standardized method for monitoring patients in clinical trials enrolling transplant patients, particularly trials of novel tolerance promoting therapies, to facilitate fair and meaningful comparisons between trials.

Redox control of inflammation in macrophages

Macrophages are present throughout the human body, constitute important immune effector cells, and have variable roles in a great number of pathological, but also physiological, settings. It is apparent that macrophages need to adjust their activation profile toward a steadily changing environment that requires altering their phenotype, a process known as macrophage polarization. Formation of reactive oxygen species (ROS), derived from NADPH-oxidases, mitochondria, or NO-producing enzymes, are not necessarily toxic, but rather compose a network signaling system, known as redox regulation. Formation of redox signals in classically versus alternatively activated macrophages, their action and interaction at the level of key targets, and the resulting physiology still are insufficiently understood. We review the identity, source, and biological activities of ROS produced during macrophage activation, and discuss how they shape the key transcriptional responses evoked by hypoxia-inducible transcription factors, nuclear-erythroid 2-p45-related factor 2 (Nrf2), and peroxisome proliferator-activated receptor-γ. We summarize the mechanisms how redox signals add to the process of macrophage polarization and reprogramming, how this is controlled by the interaction of macrophages with their environment, and addresses the outcome of the polarization process in health and disease. Future studies need to tackle the option whether we can use the knowledge of redox biology in macrophages to shape their mediator profile in pathophysiology, to accelerate healing in injured tissue, to fight the invading pathogens, or to eliminate settings of altered self in tumors.

Alternative approaches to myeloid suppressor cell therapy in transplantation: comparing regulatory macrophages to tolerogenic DCs and MDSCs

Several types of myeloid suppressor cell are currently being developed as cell-based immunosuppressive agents. Despite detailed knowledge about the molecular and cellular functions of these cell types, expert opinions differ on how to best implement such therapies in solid organ transplantation. Efforts in our laboratory to develop a cell-based medicinal product for promoting tolerance in renal transplant patients have focused on a type of suppressor macrophage, which we call the regulatory macrophage (M reg). Our favoured clinical strategy is to administer donor-derived M regs to recipients one week prior to transplantation. In contrast, many groups working with tolerogenic dendritic cells (DCs) advocate post-transplant administration of recipient-derived cells. A third alternative, using myeloid-derived suppressor cells, presumably demands that cells are given around the time of transplantation, so that they can infiltrate the graft to create a suppressive environment. On present evidence, it is not possible to say which cell type and treatment strategy might be clinically superior. This review seeks to position our basic scientific and early-stage clinical studies of human regulatory macrophages within the broader context of myeloid suppressor cell therapy in transplantation.

IFN-γ-induced iNOS expression in mouse regulatory macrophages prolongs allograft survival in fully immunocompetent recipients

Mouse monocytes exposed to macrophage colony-stimulating factor (M-CSF) and interferon-γ (IFN-γ) were driven to a novel suppressor phenotype. These regulatory macrophages (M regs) expressed markers distinguishing them from M0-, M1-, and M2-polarized macrophages and monocyte-derived dendritic cells (DCs). M regs completely suppressed polyclonal T cell proliferation through an inducible nitric oxide synthase (iNOS)-dependent mechanism. Additionally, M regs eliminated cocultured T cells in an allospecific fashion. In a heterotopic heart transplant model, a single intravenous administration of 5 × 10(6) donor-strain M regs before transplantation significantly prolonged allograft survival in fully immunocompetent recipients using both the stringent C3H-to-BALB/c (32.6 ± 4.5 versus 8.7 ± 0.2 days) and B6-to-BALB/c (31.1 ± 12 versus 9.7 ± 0.4 days) strain combinations. Nos2-deficient M regs did not prolong allograft survival, proving that M reg function in vivo is iNOS-dependent and mediated by living cells. M regs were detectable for at least 2 weeks postinfusion in allogeneic recipients. In their origin, development, phenotypic relationship with other in vitro-derived macrophages and functions, there are solid grounds to assert a near-equivalence of mouse and human M regs. It is concluded that mouse M regs represent a novel, phenotypically distinct subset of suppressor macrophages. Clinical applications of M reg therapy as an adjunct immunosuppressive therapy are currently being investigated within The ONE Study.