Macrophages as Effectors of Acute and Chronic Allograft Injury

Innate and Adaptive Immune Responses in Intestinal Transplant Rejection: Through the Lens of Inflammatory Bowel and Intestinal Graft-Versus-Host Diseases

Intestinal transplantation is a life-saving procedure utilized for patients failing total parenteral nutrition. However, intestinal transplantattion remains plagued with low survival rates and high risk of allograft rejection. The authors explore roles of innate (macrophages, natural killer cells, innate lymphoid cells) and adaptive immune cells (Th1, Th2, Th17, Tregs) in inflammatory responses, particularly inflammatory bowel disease and graft versus host disease, and correlate these findings to intestinal allograft rejection, highlighting which effectors exacerbate or suppress intestinal rejection. Better understanding of this immunology can open further investigation into potential biomolecular targets to develop improved therapeutic treatment options and immunomonitoring techniques to combat allograft rejection and enhance patient lives.

CCR7 and CD48 as Predicted Targets in Acute Rejection Related to M1 Macrophage after Pediatric Kidney Transplantation

Background

Kidney transplantation (KT) is the best treatment for end-stage renal disease. Although long and short-term survival rates for the graft have improved significantly with the development of immunosuppressants, acute rejection (AR) remains a major risk factor attacking the graft and patients. The innate immune response plays an important role in rejection. Therefore, our objective is to determine the biomarkers of congenital immunity associated with AR after KT and provide support for future research.

Materials and Methods

A differential expression genes (DEGs) analysis was performed based on the dataset GSE174020 from the NCBI gene Expression Synthesis Database (GEO) and then combined with the GSE5099 M1 macrophage-related gene identified in the Molecular Signatures Database. We then identified genes in DEGs associated with M1 macrophages defined as DEM1Gs and performed gene ontology (GO) and Kyoto Encyclopedia of Genomes (KEGG) enrichment analysis. Cibersort was used to analyze the immune cell infiltration during AR. At the same time, we used the protein-protein interaction (PPI) network and Cytoscape software to determine the key genes. Dataset, GSE14328 derived from pediatric patients, GSE138043 and GSE9493 derived from adult patients, were used to verify Hub genes. Additional verification was the rat KT model, which was used to perform HE staining, immunohistochemical staining, and Western Blot. Hub genes were searched in the HPA database to confirm their expression. Finally, we construct the interaction network of transcription factor (TF)-Hub genes and miRNA-Hub genes.

Results

Compared to the normal group, 366 genes were upregulated, and 423 genes were downregulated in the AR group. Then, 106 genes related to M1 macrophages were found among these genes. GO and KEGG enrichment analysis showed that these genes are mainly involved in cytokine binding, antigen binding, NK cell-mediated cytotoxicity, activation of immune receptors and immune response, and activation of the inflammatory NF-κB signaling pathway. Two Hub genes, namely CCR7 and CD48, were identified by PPI and Cytoscape analysis. They have been verified in external validation sets, originated from both pediatric patients and adult patients, and animal experiments. In the HPA database, CCR7 and CD48 are mainly expressed in T cells, B cells, macrophages, and tissues where these immune cells are distributed. In addition to immunoinfiltration, CD4+T, CD8+T, NK cells, NKT cells, and monocytes increased significantly in the AR group, which was highly consistent with the results of Hub gene screening. Finally, we predicted that 19 TFs and 32 miRNAs might interact with the Hub gene.

Conclusions

Through a comprehensive bioinformatic analysis, our findings may provide predictive and therapeutic targets for AR after KT.

Repositioning baloxavir marboxil as VISTA agonist that ameliorates experimental asthma

V-type immunoglobulin domain-containing suppressor of T-cell activation (VISTA), a novel negative checkpoint regulator, plays an essential role in allergic pulmonary inflammation in mice. Treatment with a VISTA agonistic antibody could significantly improve asthma symptoms. Thus, for allergic asthma treatment, VISTA targeting may be a compelling approach. In this study, we examined the functional mechanism of VISTA in allergic pulmonary inflammation and screened the FDA-approved drugs for VISTA agonists. By using mass cytometry (CyTOF), we found that VISTA deficiency primarily increased lung macrophage infiltration in the OVA-induced asthma model, accompanied by an increased proportion of M1 macrophages (CD11b+F4/80+CD86+) and a decreased proportion of M2 macrophages (CD11b+F4/80+CD206+). Further in vitro studies showed that VISTA deficiency promoted M1 polarization and inhibited M2 polarization of bone marrow-derived macrophages (BMDMs). Importantly, we discovered baloxavir marboxil (BXM) as a VISTA agonist by virtual screening of FDA-approved drugs. The surface plasmon resonance (SPR) assays revealed that BXM (KD = 1.07 µM) as well as its active form, baloxavir acid (BXA) (KD = 0.21 µM), could directly bind to VISTA with high affinity. Notably, treatment with BXM significantly ameliorated asthma symptoms, including less lung inflammation, mucus secretion, and the generation of Th2 cytokines (IL-5, IL-13, and IL-4), which were dramatically attenuated by anti-VISTA monoclonal antibody treatment. BXM administration also reduced the pulmonary infiltration of M1 macrophages and raised M2 macrophages. Collectively, our study indicates that VISTA regulates pulmonary inflammation in allergic asthma by regulating macrophage polarization and baloxavir marboxil, and an old drug might be a new treatment for allergic asthma through targeting VISTA.

An agent-based model of cardiac allograft vasculopathy: toward a better understanding of chronic rejection dynamics

Cardiac allograft vasculopathy (CAV) is a coronary artery disease affecting 50% of heart transplant (HTx) recipients, and it is the major cause of graft loss. CAV is driven by the interplay of immunological and non-immunological factors, setting off a cascade of events promoting endothelial damage and vascular dysfunction. The etiology and evolution of tissue pathology are largely unknown, making disease management challenging. So far, in vivo models, mostly mouse-based, have been widely used to study CAV, but they are resource-consuming, pose many ethical issues, and allow limited investigation of time points and important biomechanical measurements. Recently, agent-based models (ABMs) proved to be valid computational tools for deciphering mechanobiological mechanisms driving vascular adaptation processes at the cell/tissue level, augmenting cost-effective in vivo lab-based experiments, at the same time guaranteeing richness in observation time points and low consumption of resources. We hypothesize that integrating ABMs with lab-based experiments can aid in vivo research by overcoming those limitations. Accordingly, this work proposes a bidimensional ABM of CAV in a mouse coronary artery cross-section, simulating the arterial wall response to two distinct stimuli: inflammation and hemodynamic disturbances, the latter considered in terms of low wall shear stress (WSS). These stimuli trigger i) inflammatory cell activation and ii) exacerbated vascular cell activities. Moreover, an extensive analysis was performed to investigate the ABM sensitivity to the driving parameters and inputs and gain insights into the ABM working mechanisms. The ABM was able to effectively replicate a 4-week CAV initiation and progression, characterized by lumen area decrease due to progressive intimal thickening in regions exposed to high inflammation and low WSS. Moreover, the parameter and input sensitivity analysis highlighted that the inflammatory-related events rather than the WSS predominantly drive CAV, corroborating the inflammatory nature of the vasculopathy. The proof-of-concept model proposed herein demonstrated its potential in deepening the pathology knowledge and supporting the in vivo analysis of CAV.

Eschar dissolution and the immunoregulator effect of keratinase on burn wounds

At present, enzyme debridement preparation has shown a good curative effect on eschar removal of burn wounds. Keratinase has shown great potential in enzymatic debridement because of its good fibrin-degrading ability. In this study, the debridement of keratinase was examined by using a third degree burn wound model in rats. We observed the wound, and keratinase shortened the time of eschar dissolution after debridement. Histopathology and immunofluorescence staining showed that the eschar in the keratinase group became thinner, inflammatory cell infiltration in the wound increased, the fluorescence intensity of the macrophage surface marker CD68 increased, and the CD163/CD86 ratio increased. In bone marrow-derived macrophages (BMDMs), there was no significant difference in the activity of CCK-8 in cells in the keratinase group compared with the control group. The fluorescence intensity of the keratinase group was higher than that of the control group. At 12 h, the cell scratches were obviously closed. The number of migrated Transwell cells increased. Flow cytometry and immunofluorescence analysis showed increased expression of CD206 and Arg-1 and decreased expression of CD86 and iNOS. The gene expression of the Arg-1, iNOS and IL-10 was increased, as shown by qPCR. The secretion of IL-10 was increased and TNF-α was decreased, as shown by ELISA. We concluded that keratinase dissolution of eschar not only has a hydrolytic effect on eschar but may also affect immune regulation to enhance the migration and phagocytosis of macrophages, promote the polarization of macrophages, and further enhance the effect of eschar dissolution. Therefore, keratinase may have good prospects for the debridement of burn wounds.

Machine learning identifies signatures of macrophage reactivity and tolerance that predict disease outcomes

BACKGROUND

Single-cell transcriptomic studies have greatly improved organ-specific insights into macrophage polarization states are essential for the initiation and resolution of inflammation in all tissues; however, such insights are yet to translate into therapies that can predictably alter macrophage fate.

METHOD

Using machine learning algorithms on human macrophages, here we reveal the continuum of polarization states that is shared across diverse contexts. A path, comprised of 338 genes accurately identified both physiologic and pathologic spectra of "reactivity" and "tolerance", and remained relevant across tissues, organs, species, and immune cells (>12,500 diverse datasets).

FINDINGS

This 338-gene signature identified macrophage polarization states at single-cell resolution, in physiology and across diverse human diseases, and in murine pre-clinical disease models. The signature consistently outperformed conventional signatures in the degree of transcriptome-proteome overlap, and in detecting disease states; it also prognosticated outcomes across diverse acute and chronic diseases, e.g., sepsis, liver fibrosis, aging, and cancers. Crowd-sourced genetic and pharmacologic studies confirmed that model-rationalized interventions trigger predictable macrophage fates.

INTERPRETATION

These findings provide a formal and universally relevant definition of macrophage states and a predictive framework (http://hegemon.ucsd.edu/SMaRT) for the scientific community to develop macrophage-targeted precision diagnostics and therapeutics.

FUNDING

This work was supported by the National Institutes for Health (NIH) grant R01-AI155696 (to P.G, D.S and S.D). Other sources of support include: R01-GM138385 (to D.S), R01-AI141630 (to P.G), R01-DK107585 (to S.D), and UG3TR003355 (to D.S, S.D, and P.G). D.S was also supported by two Padres Pedal the Cause awards (Padres Pedal the Cause/RADY #PTC2017 and San Diego NCI Cancer Centers Council (C3) #PTC2017). S.S, G.D.K, and D.D were supported through The American Association of Immunologists (AAI) Intersect Fellowship Program for Computational Scientists and Immunologists. We also acknowledge support from the Padres Pedal the Cause #PTC2021 and the Torey Coast Foundation, La Jolla (P.G and D.S). D.S, P.G, and S.D were also supported by the Leona M. and Harry B. Helmsley Charitable Trust.

Intragraft immune cells: accomplices or antagonists of recipient-derived macrophages in allograft fibrosis?

Organ fibrosis caused by chronic allograft rejection is a major concern in the field of transplantation. Macrophage-to-myofibroblast transition plays a critical role in chronic allograft fibrosis. Adaptive immune cells (such as B and CD4⁺ T cells) and innate immune cells (such as neutrophils and innate lymphoid cells) participate in the occurrence of recipient-derived macrophages transformed to myofibroblasts by secreting cytokines, which eventually leads to fibrosis of the transplanted organ. This review provides an update on the latest progress in understanding the plasticity of recipient-derived macrophages in chronic allograft rejection. We discuss here the immune mechanisms of allograft fibrosis and review the reaction of immune cells in allograft. The interactions between immune cells and the process of myofibroblast formulation are being considered for the potential therapeutic targets of chronic allograft fibrosis. Therefore, research on this topic seems to provide novel clues for developing strategies for preventing and treating allograft fibrosis.

HLA class II antibody activation of endothelial cells induces M2 macrophage differentiation in peripheral blood

BACKGROUND

Donor-specific human leukocyte antigen (HLA) class II antibodies (HLA-II Abs) combined with allogeneic endothelial cells (ECs) mediate high-risk rejection in kidney transplant patients. Macrophage accumulation is a significant histological feature of antibody-mediated rejection (AMR) in kidney transplant patients. Here, we further investigated the effect of HLA-II Abs on macrophage phenotypes to provide theoretical basis for clinical treatment of AMR.

METHODS

We prepared an experimental model containing HLA-II Ab-stimulated microvascular ECs and peripheral blood mononuclear cells (PBMCs) co-culture and explored the potential relationship of HLA-II Ab, ECs activation, and macrophage differentiation. Immune phenotype of macrophage subsets was analyzed and quantified by flow cytometry. HLA-II Ab activation of ECs induces M2 macrophage differentiation signal pathways which were investigated by qPCR and western blotting.

RESULTS

The stimulation of ECs by F(ab')₂ fragment of HLA-II Abs led to phosphorylation of PI3K, Akt, and mTOR, which mediated IL-10, ICAM-1, VCAM-1 secretion. The enhanced ICAM-1 and IL-10 promoted the migration of PBMCs and their differentiation into CD68⁺ and CD163⁺ (M2-type) macrophages, respectively, but not CD86⁺ macrophages.

CONCLUSION

These findings revealed the PI3K/Akt/mTOR signal pathways activated by HLA-II Abs in ECs and the immune regulation ability of HLA-II Abs to induce PBMC differentiation.

Donor-But Not Recipient-Derived Cells Produce Collagen-1 in Chronically Rejected Cardiac Allografts

Fibrosis is a prominent feature of chronic allograft rejection, caused by an excessive production of matrix proteins, including collagen-1. Several cell types produce collagen-1, including mesenchymal fibroblasts and cells of hematopoietic origin. Here, we sought to determine whether tissue-resident donor-derived cells or allograft-infiltrating recipient-derived cells are responsible for allograft fibrosis, and whether hematopoietic cells contribute to collagen production. A fully MHC-mismatched mouse heterotopic heart transplantation model was used, with transient depletion of CD4⁺ T cells to prevent acute rejection. Collagen-1 was selectively knocked out in recipients or donors. In addition, collagen-1 was specifically deleted in hematopoietic cells. Tissue-resident macrophages were depleted using anti-CSF1R antibody. Allograft fibrosis and inflammation were quantified 20 days post-transplantation. Selective collagen-1 knock-out in recipients or donors showed that tissue-resident cells from donor hearts, but not infiltrating recipient-derived cells, are responsible for production of collagen-1 in allografts. Cell-type-specific knock-out experiments showed that hematopoietic tissue-resident cells in donor hearts substantially contributed to graft fibrosis. Tissue resident macrophages, however, were not responsible for collagen-production, as their deletion worsened allograft fibrosis. Donor-derived cells including those of hematopoietic origin determine allograft fibrosis, making them attractive targets for organ preconditioning to improve long-term transplantation outcomes.

SIRT6 inhibition delays peripheral nerve recovery by suppressing migration, phagocytosis and M2-polarization of macrophages

Background

Silent information regulator 6 (SIRT6) is a mammalian homolog of the nicotinamide adenine dinucleotide (NAD)-dependent deacetylase sirtuin family. Prior evidences suggested that the anti-inflammatory function of SIRT6 after spinal cord and brain injury, and it plays a crucial role in macrophages polarization of adipose tissue and skin. However, the role of SIRT6 in macrophages involved peripheral nerve injury is still unknown. Given the prominent role of macrophages in peripheral nerve recovery, we aim to investigate the role of SIRT6 in the regulation of phenotypes shift and functions in macrophages after peripheral nerve injury.

Results

In the present study, we first identified a significant increase of SIRT6 expression during nerve degeneration and macrophages phagocytosis. Next, we found nerve recovery was delayed after SIRT6 silencing by injected shRNA lentivirus into the crushed sciatic nerve, which exhibited a reduced expression of myelin-related proteins (e.g., MAG and MBP), severer myoatrophy of target muscles, and inferior nerve conduction compared to the shRNA control injected mice. In vitro, we found that SIRT6 inhibition by being treated with a selective inhibitor OSS_128167 or lentivirus transfection impairs migration and phagocytosis capacity of bone marrow-derived macrophages (BMDM). In addition, SIRT6 expression was discovered to be reduced after M1 polarization, but SIRT6 was enhanced after M2 polarization in the monocyte-macrophage cell line RAW264.7 and BMDM. Moreover, SIRT6 inhibition increased M1 macrophage polarization with a concomitant decrease in M2 polarization both in RAW264.7 and BMDM via activating NF-κB and TNF-α expression, and SIRT6 activation by UBCS039 treatment could shift the macrophages from M1 to M2 phenotype.

Conclusion

Our findings indicate that SIRT6 inhibition impairs peripheral nerve repair through suppressing the migration, phagocytosis, and M2 polarization of macrophages. Therefore, SIRT6 may become a favorable therapeutic target for peripheral nerve injury.

The Potential Role of Human NME1 in Neuronal Differentiation of Porcine Mesenchymal Stem Cells: Application of NB-hNME1 as a Human NME1 Suppressor

This study aimed to investigate the effects of the human macrophage (MP) secretome in cellular xenograft rejection. The role of human nucleoside diphosphate kinase A (hNME1), from the secretome of MPs involved in the neuronal differentiation of miniature pig adipose tissue-derived mesenchymal stem cells (mp AD-MSCs), was evaluated by proteomic analysis. Herein, we first demonstrate that hNME1 strongly binds to porcine ST8 alpha-N-acetyl-neuraminide alpha-2,8-sialyltransferase 1 (pST8SIA1), which is a ganglioside GD3 synthase. When hNME1 binds with pST8SIA1, it induces degradation of pST8SIA1 in mp AD-MSCs, thereby inhibiting the expression of ganglioside GD3 followed by decreased neuronal differentiation of mp AD-MSCs. Therefore, we produced nanobodies (NBs) named NB-hNME1 that bind to hNME1 specifically, and the inhibitory effect of NB-hNME1 was evaluated for blocking the binding between hNME1 and pST8SIA1. Consequently, NB-hNME1 effectively blocked the binding of hNME1 to pST8SIA1, thereby recovering the expression of ganglioside GD3 and neuronal differentiation of mp AD-MSCs. Our findings suggest that mp AD-MSCs could be a potential candidate for use as an additive, such as an immunosuppressant, in stem cell transplantation.

Macrophage-specific RhoA knockout delays Wallerian degeneration after peripheral nerve injury in mice

Background

Plenty of macrophages are recruited to the injured nerve to play key roles in the immunoreaction and engulf the debris of degenerated axons and myelin during Wallerian degeneration, thus creating a conducive microenvironment for nerve regeneration. Recently, drugs targeting the RhoA pathway have been widely used to promote peripheral axonal regeneration. However, the role of RhoA in macrophage during Wallerian degeneration and nerve regeneration after peripheral nerve injury is still unknown. Herein, we come up with the hypothesis that RhoA might influence Wallerian degeneration and nerve regeneration by affecting the migration and phagocytosis of macrophages after peripheral nerve injury.

Methods

Immunohistochemistry, Western blotting, H&E staining, and electrophysiology were performed to access the Wallerian degeneration and axonal regeneration after sciatic nerve transection and crush injury in the Lyz^Cre+/−; RhoA^flox/flox (cKO) mice or Lyz2^Cre+/− (Cre) mice, regardless of sex. Macrophages’ migration and phagocytosis were detected in the injured nerves and the cultured macrophages. Moreover, the expression and potential roles of ROCK and MLCK were also evaluated in the cultured macrophages.

Results

1. RhoA was specifically knocked out in macrophages of the cKO mice; 2. The segmentation of axons and myelin, the axonal regeneration, and nerve conduction in the injured nerve were significantly impeded while the myoatrophy was more severe in the cKO mice compared with those in Cre mice; 3. RhoA knockout attenuated the migration and phagocytosis of macrophages in vivo and in vitro; 4. ROCK and MLCK were downregulated in the cKO macrophages while inhibition of ROCK and MLCK could weaken the migration and phagocytosis of macrophages.

Conclusions

Our findings suggest that RhoA depletion in macrophages exerts a detrimental effect on Wallerian degeneration and nerve regeneration, which is most likely due to the impaired migration and phagocytosis of macrophages resulted from disrupted RhoA/ROCK/MLCK pathway. Since previous research has proved RhoA inhibition in neurons was favoring for axonal regeneration, the present study reminds us of that the cellular specificity of RhoA-targeted drugs is needed to be considered in the future application for treating peripheral nerve injury.

High-resolution phenotyping of early acute rejection reveals a conserved alloimmune signature

Alloimmune responses in acute rejection are complex, involving multiple interacting cell types and pathways. Deep profiling of these cell types has been limited by technology that lacks the capacity to resolve this high dimensionality. Single-cell mass cytometry is used to characterize the alloimmune response in early acute rejection, measuring 37 parameters simultaneously, across multiple time points in two models: a murine cardiac and vascularized composite allotransplant (VCA). Semi-supervised hierarchical clustering is used to group related cell types defined by combinatorial expression of surface and intracellular proteins, along with markers of effector function and activation. This expression profile is mapped to visualize changes in antigen composition across cell types, revealing phenotypic signatures in alloimmune T cells, natural killer (NK) cells, and myeloid subsets that are conserved and that firmly distinguish rejecting from non-rejecting grafts. These data provide a comprehensive, high-dimensional profile of cellular rejection after allograft transplantation.

High-resolution visualization and quantification of nucleic acid-based therapeutics in cells and tissues using Nanoscale secondary ion mass spectrometry (NanoSIMS)

Nucleic acid therapeutics (NATs) have proven useful in promoting the degradation of specific transcripts, modifying gene expression, and regulating mRNA splicing. In each situation, efficient delivery of nucleic acids to cells, tissues and intracellular compartments is crucial—both for optimizing efficacy and reducing side effects. Despite successes in NATs, our understanding of their cellular uptake and distribution in tissues is limited. Current methods have yielded insights into distribution of NATs within cells and tissues, but the sensitivity and resolution of these approaches are limited. Here, we show that nanoscale secondary ion mass spectrometry (NanoSIMS) imaging can be used to define the distribution of 5-bromo-2′-deoxythymidine (5-BrdT) modified antisense oligonucleotides (ASO) in cells and tissues with high sensitivity and spatial resolution. This approach makes it possible to define ASO uptake and distribution in different subcellular compartments and to quantify the impact of targeting ligands designed to promote ASO uptake by cells. Our studies showed that phosphorothioate ASOs are associated with filopodia and the inner nuclear membrane in cultured cells, and also revealed substantial cellular and subcellular heterogeneity of ASO uptake in mouse tissues. NanoSIMS imaging represents a significant advance in visualizing uptake and distribution of NATs; this approach will be useful in optimizing efficacy and delivery of NATs for treating human disease.

RhoA- and Actin-Dependent Functions of Macrophages from the Rodent Cardiac Transplantation Model Perspective -Timing Is the Essence

Simple Summary

The functions of animal and human cells depend on the actin cytoskeleton and its regulating protein called the RhoA. The actin cytoskeleton and RhoA also regulate the response of the immune cells such as macrophages to the microbial invasion and/or the presence of a non-self, such as a transplanted organ. The immune response against transplant occurs in several steps. The early step occurring within days post-transplantation is called the acute rejection and the late step, occurring months to years post-transplantation, is called the chronic rejection. In clinical transplantation, acute rejection is easily manageable by the anti-rejection drugs. However, there is no cure for chronic rejection, which is caused by the macrophages entering the transplant and promoting blockage of its blood vessels and destruction of tissue. We discuss here how the inhibition of the RhoA and actin cytoskeleton polymerization in the macrophages, either by genetic interference or pharmacologically, prevents macrophage entry into the transplanted organ and prevents chronic rejection, and also how it affects the anti-microbial function of the macrophages. We also focus on the importance of timing of the macrophage functions in chronic rejection and how the circadian rhythm may affect the anti-chronic rejection and anti-microbial therapies.

Abstract

The small GTPase RhoA, and its down-stream effector ROCK kinase, and the interacting Rac1 and mTORC2 pathways, are the principal regulators of the actin cytoskeleton and actin-related functions in all eukaryotic cells, including the immune cells. As such, they also regulate the phenotypes and functions of macrophages in the immune response and beyond. Here, we review the results of our and other’s studies on the role of the actin and RhoA pathway in shaping the macrophage functions in general and macrophage immune response during the development of chronic (long term) rejection of allografts in the rodent cardiac transplantation model. We focus on the importance of timing of the macrophage functions in chronic rejection and how the circadian rhythm may affect the anti-chronic rejection therapies.

Macrophage Proinflammatory Responses to Microorganisms and Transplanted Organs

Tissue-resident macrophages and those conscripted from the blood/bone marrow are professional phagocytes. They play a role in tissue homeostasis, replacement, and healing, and are the first-line responders to microbial (viral, bacterial, and fungi) infections. Intrinsic ameboid-type motility allows non-resident macrophages to move to the site of inflammation or injury, where, in response to the inflammatory milieu they perform the anti-microbial and/or tissue repair functions. Depending on the need and the signaling from the surrounding tissue and other immune cells, macrophages acquire morphologically and functionally different phenotypes, which allow them to play either pro-inflammatory or anti-inflammatory functions. As such, the macrophages are also the major players in the rejection of the transplanted organs making an excellent target for the novel anti-rejection therapies in clinical transplantation. In this review, we describe some of the less covered aspects of macrophage response to microbial infection and organ transplantation.

Siponimod (Mayzent) Downregulates RhoA and Cell Surface Expression of the S1P1 and CX3CR1 Receptors in Mouse RAW 264.7 Macrophages

The Siponimod (Mayzent) is a newly developed drug, similar to Fingolimod (FTY720) but with fewer side effects, approved by the Food and Drug Administration for the treatment of multiple sclerosis (MS). The therapeutic effect of siponimod and FTY720 in MS relies on their inhibitory effect on the sphingosine 1-phosphate (S1P) signaling. These drugs bind to the S1P receptors and block the CCL2 chemokine pathway that is responsible for the exit of the immune cells from the lymphoid organs, and circulation, thus preventing immune cell-dependent injury to the nervous system. We recently found that FTY720 beside its effect on the S1P pathway also blocks the RhoA pathway, which is involved in the actin cytoskeleton-related function of macrophages, such as expression/recycling of fractalkine (CX3CL1) receptors (CX3CR1), which direct macrophages to the transplanted organs during the development of the long-term (chronic) rejection. Here we tested the effects of siponimod on the RhoA pathway and the expression of the S1P1 and CX3CR1 receptors in mouse RAW 264.7 macrophages. We found that siponimod downregulates the expression of RhoA protein and decreases the cell surface expression of S1P1 and CX3CR1 receptors. This newly discovered crosstalk between S1P and RhoA/CX3CR1 pathways may help in the development of novel anti-chronic rejection therapies in clinical transplantation.

Role of donor macrophages after heart and lung transplantation

Since the 1960s, heart and lung transplantation has remained the optimal therapy for patients with end-stage disease, extending and improving quality of life for thousands of individuals annually. Expanding donor organ availability and immunologic compatibility is a priority to help meet the clinical demand for organ transplant. While effective, current immunosuppression is imperfect as it lacks specificity and imposes unintended adverse effects such as opportunistic infections and malignancy that limit the health and longevity of transplant recipients. In this review, we focus on donor macrophages as a new target to achieve allograft tolerance. Donor organ-directed therapies have the potential to improve allograft survival while minimizing patient harm related to global suppression of recipient immune responses. Topics highlighted include the role of ontogenically distinct donor macrophage populations in ischemia-reperfusion injury and rejection, including their interaction with allograft-infiltrating recipient immune cells and potential therapeutic approaches. Ultimately, a better understanding of how donor intrinsic immunity influences allograft acceptance and survival will provide new opportunities to improve the outcomes of transplant recipients.

A Review on the Function and Regulation of ARHGDIB/RhoGDI2 Expression Including the Hypothetical Role of ARHGDIB/RhoGDI2 Autoantibodies in Kidney Transplantation

Challenging and still unsolved problems in kidney transplantation are risk stratification and the treatment of humoral rejection. Antibody-mediated rejection is an important cause of early and chronic rejection. The impact of donor-specific HLA antibodies on antibody-mediated rejection–causing graft damage is well known, but the clinical relevance of non-HLA antibodies remains unclear. Recently, in 2 independent studies, a new correlation was found between the presence of non-HLA anti-Rho guanosine diphosphate dissociation inhibitor 2 (ARHGDIB) antibodies and increased graft failure. RhoGDI2, another name for ARHGDIB, is a negative regulator of the Rho guanosine triphosphate (RhoGTP)ases RhoA, Rac1m, and Cdc42, whose main function is regulating the actin network in a variety of cells. RhoGDI2 is mainly expressed intracellularly, and some expression is observed on the cell surface. Currently, there is no mechanism known to explain this correlation. Additionally, the reason why the antibodies are produced is unknown. In this review, we will address these questions, provide an overview of other diseases in which these antibodies are prevalent, and describe the physiological role of RhoGDI2 itself. If the mechanism and impact of RhoGDI2 antibodies in kidney graft failure are known, improved risk stratification can be provided to decrease the rate of donor kidney graft failure.

Macrophage phenotype bioengineered by magnetic, genetic, or pharmacologic interference

In all eukaryotes, the cell shape depends on the actin filament cytoskeleton, which is regulated by the small GTPase RhoA. It is well known that the cell shape determines cell function and behavior. Inversely, any change in the cell behavior and/or function reverberates at the cell shape. In this review, we describe how mechanical/magnetic, genetic, or pharmacologic interference with the actin cytoskeleton enforces changes in cell shape and function and how such techniques can be used to control the phenotype and functions of immune cells such as macrophages and to develop novel anti-cancer and anti-rejection clinical therapies.

Long Non-coding RNA Expression Profiling in Biopsy to Identify Renal Allograft at Risk of Chronic Damage and Future Graft Loss

The loss of allograft from chronic damage is still the major risk that renal transplant recipients face today. Biomarkers for early detection of chronic damage are needed to improve the long-term graft survival. This study aimed to identify long non-coding RNA (lncRNA) biomarkers associated with chronic damage and graft loss after renal transplantation. Gene Expression Omnibus (GEO) datasets including GSE57387 (n = 101), GSE21374 (n = 282), and GSE25902 (n = 24) from three high-quality studies were analyzed. By repurposing the publicly available array-based data coupled with Affymetrix Human Exon 1.0 ST and Human U133 Plus 2.0 arrays, we obtained expression profiles of 11323 and 3383 lncRNAs in biopsies after renal transplantation, respectively. The logistic regression model and Cox regression model were applied to identify lncRNAs associated with chronic damage and graft survival. High AC093673.5 expression was identified as significantly associated with the three endpoints including chronic damage, progressive chronic histological damage, and graft failure across these three datasets. A six-lncRNA signature was created to predict renal allograft at risk of chronic damage with a high predictive ability (AUC = 0.94). Gene set enrichment analysis (GSEA) indicated that our lncRNA signature was related with allograft rejection and immunity. Our study highlights the importance of lncRNAs in chronic graft damage and allograft loss, supporting their potential role as prognosis biomarkers.

Mast cell-mediated mechanistic pathways in organ transplantation

Adaptive immunity has gained importance in transplant immunology for years, based on models in which T-cells orchestrate the immune responses during rejection. Most recently, researches revealed that innate immune cells, including mast cells (MCs) also play a pivotal role in allograft rejection. MC mediated immunoregulatory responses influence the innate and adaptive immune responses. Their capability to produce an array of both pro-inflammatory and anti-inflammatory mediators, expressing a wide range of costimulatory molecules in addition to acting as antigen-presenting cells (APCs), make them effective immune cells far beyond their classical role as primary orchestrator cells of allergy. Activated regulatory Tcells (Treg) cells contribute to MC recruitment into grafts by releasing interleukin (IL)-9. Tregs are capable of stabilizing MCs and suppressing IgE mediated degranulation through interaction of Treg expressing OX40 with MCs expressing OX40L. MCs in turn release transforming growth factor (TGF)-β and IL-10 which possess suppressive properties. Thus, these cells can suppress the proliferation of T-cells and support the generation of Tregs. MCs in addition to orchestrating immune responses in grafts by cell-to-cell interactions with variety of immune cells, cause histologic changes, mainly fibrosis by releasing mediators such as histamine, fibroblast growth factor-2 (FGF-2), TGF-β, chymase, and cathepsin G. The role of MCs in transplant rejection remains controversial. The accumulation of MCs in rejected grafts suggests that they play a role in preventing graft tolerance, and contribute to the progression of chronic rejection of allografts. However, high expression of MC-related gene products in tolerant grafts and their known interaction with Tregs on the other hand, support the notion that they are an integral component in achieving peripheral tolerance.

The Evolving Roles of Macrophages in Organ Transplantation

Organ transplantation is a life-saving strategy for patients with end-stage organ failure. Over the past few decades, organ transplantation has achieved an excellent success in short-term survival but only a marginal improvement in long-term graft outcomes. The pathophysiology of graft loss is multifactorial and remains incompletely defined. However, emerging evidence suggests macrophages as crucial mediators of acute and chronic allograft immunopathology. In this process, macrophage-mediated mobilization of first-line defenses, particularly phagocytosis and the release of acute inflammatory mediators, is important, but macrophages also launch adaptive alloimmune reactions against grafts through antigen processing and presentation, as well as providing costimulation. Additionally, crosstalk with other immune cells and graft endothelial cells causes tissue damage or fibrosis in transplanted organs, contributing to graft loss or tolerance resistance. However, some macrophages function as regulatory cells that are capable of suppressing allogeneic T cells, inhibiting DC maturation, inducing the differentiation of Tregs, and subsequently promoting transplant tolerance. This functional diversity of macrophages in organ transplantation is consistent with their heterogeneity. Although our knowledge of the detrimental or beneficial effects of macrophages on transplants has exponentially increased, the exact mechanisms controlling macrophage functions are not yet completely understood. Here, we review recent advances in our understanding of the multifaceted nature of macrophages, focusing on their evolving roles in organ transplantation and the mechanisms involved in their activation and function in allograft transplantation. We also discuss potential therapeutic options and opportunities to target macrophage to improve the outcomes of transplant recipients.

Macrophage functions in wound healing

Macrophages play a crucial role in regeneration and consecutive phases of wound healing. In this review, we summarise current knowledge on the ontogeny, origin, phenotypical heterogeneity, and functional exchangeability of macrophages participating in these processes. We also describe the genetic, pharmacologic, and bioengineering methods for manipulation of macrophage phenotype and functions and their potential for development of the novel, clinically applicable therapies.

Inhibition of RhoA and mTORC2/Rictor by Fingolimod (FTY720) induces p21-activated kinase 1, PAK-1 and amplifies podosomes in mouse peritoneal macrophages

Macrophage functions in the immune response depend on their ability to infiltrate tissues and organs. The penetration between and within the tissues requires degradation of extracellular matrix (ECM), a function performed by the specialized, endopeptidase- and actin filament- rich organelles located at the ventral surface of macrophage, called the podosomes. Podosome formation requires local inhibition of small GTPase RhoA activity, and depends on Rac 1/Rho guanine nucleotide exchange factor 7, β-PIX and its binding partner the p21-activated kinase (PAK-1). The activity of RhoA and Rac 1 is in turn regulated by mTOR/mTORC2 pathway. Here we showed that a fungus metabolite Fingolimod (FTY720, Gilenya), which is clinically approved for the treatment of multiple sclerosis, down-regulates Rictor, which is a signature molecule of mTORC2 and dictates its substrate (actin cytoskeleton) specificity, down-regulates RhoA, up-regulates PAK-1, and causes amplification of podosomes in mouse peritoneal macrophages.

Macrophages and RhoA Pathway in Transplanted Organs

RhoA is a small GTPase that, via its downstream effectors, regulates a variety of cell functions such as cytokinesis, cell migration, vesicular trafficking, and phagocytosis. As such the RhoA pathway is also pivotal for proper functioning of immune cells including macrophages. By controlling actin cytoskeleton organization, RhoA pathway modulates macrophage's polarity and basic functions: phagocytosis, migration, and extracellular matrix degradation. Numerous studies indicate that macrophages are very important effectors contributing to acute and chronic rejection of transplanted organs. In this review we discuss the role of RhoA pathway in governance of macrophage's functions in terms of transplanted organs.