Characterization of fibroblasts from rejecting tissue: the hyaluronan production is increased

Interferons and interferon-related pathways in heart disease

Interferons (IFNs) and IFN-related pathways play key roles in the defence against microbial infection. However, these processes may also be activated during the pathogenesis of non-infectious diseases, where they may contribute to organ injury, or function in a compensatory manner. In this review, we explore the roles of IFNs and IFN-related pathways in heart disease. We consider the cardiac effects of type I IFNs and IFN-stimulated genes (ISGs); the emerging role of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway; the seemingly paradoxical effects of the type II IFN, IFN-γ; and the varied actions of the interferon regulatory factor (IRF) family of transcription factors. Recombinant IFNs and small molecule inhibitors of mediators of IFN receptor signaling are already employed in the clinic for the treatment of some autoimmune diseases, infections, and cancers. There has also been renewed interest in IFNs and IFN-related pathways because of their involvement in SARS-CoV-2 infection, and because of the relatively recent emergence of cGAS-STING as a pattern recognition receptor-activated pathway. Whether these advances will ultimately result in improvements in the care of those experiencing heart disease remains to be determined.

Regulation of cardiac fibroblasts by lymphocytes after a myocardial infarction: playing in the major league

Cardiac fibrosis is a pathological condition characterized by excessive accumulation of extracellular matrix components within the myocardium, which can lead to impaired cardiac function and heart failure. Studies have shown that lymphocytes including B and T cells play important roles in the development and progression of cardiac fibrosis after a myocardial infarction. In this review, we focus on the regulation of cardiac fibrosis by lymphocyte subsets, with a particular emphasis on CD4+ and CD8+ T cells and their effects on fibroblasts and cardiac remodeling. We also highlight areas for further exploration of the interactions between T cells and fibroblasts necessary for understanding and treating cardiac fibrosis and heart failure.

Sterile inflammation in thoracic transplantation

The life-saving benefits of organ transplantation can be thwarted by allograft dysfunction due to both infectious and sterile inflammation post-surgery. Sterile inflammation can occur after necrotic cell death due to the release of endogenous ligands [such as damage-associated molecular patterns (DAMPs) and alarmins], which perpetuate inflammation and ongoing cellular injury via various signaling cascades. Ischemia-reperfusion injury (IRI) is a significant contributor to sterile inflammation after organ transplantation and is associated with detrimental short- and long-term outcomes. While the vicious cycle of sterile inflammation and cellular injury is remarkably consistent amongst different organs and even species, we have begun understanding its mechanistic basis only over the last few decades. This understanding has resulted in the developments of novel, yet non-specific therapies for mitigating IRI-induced graft damage, albeit with moderate results. Thus, further understanding of the mechanisms underlying sterile inflammation after transplantation is critical for identifying personalized therapies to prevent or interrupt this vicious cycle and mitigating allograft dysfunction. In this review, we identify common and distinct pathways of post-transplant sterile inflammation across both heart and lung transplantation that can potentially be targeted.

Absent in Melanoma 2 (AIM2) limits pro-inflammatory cytokine transcription in cardiomyocytes by inhibiting STAT1 phosphorylation

Interferon (IFN)-γ is highly upregulated during heart inflammation and enhances the production of pro-inflammatory cytokines. Absent in Melanoma 2 (AIM2) is an IFN-inducible protein implicated as a component of the inflammasome. Here we seek to determine the role of AIM2 during inflammation in cardiac cells. We found that the presence of AIM2, but not of the other inflammasome components Nod-like receptor (NLR) NLRP3 or NLRC4, specifically limited the transcription of the pro-inflammatory cytokines interleukin (IL)-6, IP-10, and tumor necrosis factor (TNF)-α in HL-1 mouse cardiomyocytes stimulated with IFN-γ and lipopolysaccharides (LPS). Similarly, AIM2 reduced pro-inflammatory cytokine transcription in primary mouse neonatal cardiomyocytes (MNC), but not in primary mouse neonatal cardiac fibroblasts (MNF). Interestingly, AIM2-dependent reduction of pro-inflammatory cytokines in cardiomyocytes was independent of Caspase-1. Mechanistically, AIM2 reduced pro-inflammatory cytokine transcription in cardiomyocytes by interacting with and inhibiting the phosphorylation of STAT1. In AIM2-depleted cardiomyocytes, increased STAT1 phosphorylation enhanced the NF-κB pathway by promoting NF-κB p65 phosphorylation and acetylation. These results show for the first time that AIM2 plays an important anti-inflammatory, yet inflammasome-independent function in cardiomyocytes. Our findings will help to further understand how the various heart cell types differently react to inflammatory stimuli.

Could interferon-gamma be a therapeutic target for treating heart failure?

The cytokine interferon-gamma (IFN-γ) is the only known member of the type II family of interferons, and as such, binds to its own distinct receptor. It is important in host defense against infection, as well as adaptive immune responses. While a wide array of cytokines are known to be involved in adverse remodeling of the heart and the progression to heart failure, the role of IFN-γ is unclear. Recent evidence from clinical studies, animal models of myocarditis and hypertension, as well as isolated cell studies, provide conflicting data as to whether IFN-γ is pathological or protective in the heart. Thus, it is important to highlight these discrepant findings so that areas of future investigation can be identified to more clearly determine the precise role of IFN-γ in the heart. Accordingly, this review will (1) discuss the source of IFN-γ in the diseased heart; (2) summarize the data from animal studies; (3) discuss the effects of IFN-γ on isolated cardiac fibroblasts and cardiomyocytes; (4) identify signaling mechanisms that may be invoked by IFN-γ in the heart; and (5) present the clinical evidence supporting a role for IFN-γ in heart failure.

Graft rejection - endogenous or allogeneic?

The presence and persistence of alloantigen is necessary for graft-specific T-cell-mediated immunity. However, specificity comprises only a single facet of an extremely complex process. Evidence is accruing to suggest that immunogenicity could be manipulated by endogenous ligands released during tissue injury. Stress molecules are significantly up-regulated following transplantation and stimulate conserved receptors on a range of leucocytes, including dendritic cells (DCs). The DCs are essential for co-stimulation and the induction of adaptive immunity. Stress signals can act as an adjuvant leading to DC maturation and activation. DCs stimulated by endogens exhibit enhanced alloantigen presentation, co-stimulation and production of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18. Inflammasomes have a major role in IL-1β/IL-18 production and secretion, and can be stimulated by endogens. Importantly, the polarization toward inflammatory T helper type 17 cells as opposed to regulatory T cells is dependent upon, among other factors, IL-1β. This highlights an important differentiation pathway that may be influenced by endogenous signals. Minimizing graft damage and stress expression should hypothetically be advantageous, and we feel that this area warrants further research, and may provide novel treatment modalities with potential clinical benefit.

Hyaluronan in intra-operative edema of NF1-associated neurofibromas

The tumor suppressor disorder neurofibromatosis type 1 (NF1) is associated with development of multiple neurofibromas which may grow intraneurally as plexiform neurofibromas (PNF) or intracutaneously (CNF). Upon surgery neurofibromas may show prominent swelling hindering skin-edge approximation. To assess whether the water binding glycosaminoglycan hyaluronan is involved in intra-operative swelling, 51 neurofibromas from 33 NF1-patients were investigated. Hyaluronan was histologically demonstrated and was quantified by ELISA. Molecular weight of hyaluronan was determined by gel filtration. Further, hyaluronan content was measured in cultivated Schwann cells and fibroblasts. Clinically, 67% of PNF were associated with moderate or severe intra-operative swelling, whereas only 36% of CNF showed this feature. Significantly higher levels of hyaluronan content were found in PNF compared to CNF (P < 0.05). Mast cell density did not correlate with any of the parameters. Molecular weight of hyaluronan in PNF and CNF ranged from higher than 10⁶ Da to approximately 10⁵ Da. Fibroblasts produced less hyaluronan than Schwann cells. The findings support the view that hyaluronan plays an important role in intra-operative swelling in neurofibroma surgery.

Graft morphology correlates with fibroblast activity in cardiac allograft rejection

Several extracellular matrix substances, such as hyaluronan and fibronectin, may affect graft viability by their involvement in cell adhesion and in migration. These substances are produced locally in the tissue by fibroblasts. The aim of this study was to investigate the activation state of intragraft fibroblasts under various immunosuppressive treatments and to correlate these with morphological parameters. Syngeneic (n = 5) and allogeneic rat (n = 5-6/group) heterotopic heart transplantations were performed. Allogeneically transplanted animals were immunosuppressed with cyclosporine, mycophenolate mofetil or prednisolone. After 10 days, the transplanted hearts were removed for subsequent isolation of intragraft fibroblasts and for evaluation of graft morphology. The hyaluronan synthesis of graft fibroblasts correlated with the cellular infiltration (p < 0.05) and the interstitial oedema (p < 0.05) of the cardiac grafts. In general, proliferation rate and hyaluronan production were of the same magnitude in fibroblasts from allogeneic hearts under immunosuppression with cyclosporine, mycophenolate mofetil or prednisolone as in fibroblasts from syngeneic grafts. A pool of fibroblasts isolated from cardiac grafts of non-immunosuppressed, allogeneically transplanted rats (n = 4) showed considerably higher levels. We concluded that fibroblast activity correlates to the viability of the tissue rather than to the specific drug used for immunosuppression.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

Liver sinusoidal endothelial cell function in rejected and spontaneously accepted rat liver allografts

Studies have suggested that liver sinusoidal endothelial cells (LSEC) may play an important role in tolerance induction. In this study, we evaluated the functional difference of LSEC in rejection and spontaneous acceptance of liver allografts by using rat liver transplant model. LSEC function was determined by circulating hyaluronic acid (HA) levels and fluorescein isothiocyanate-labeled formaldehyde-treated serum albumin (FITC-FSA) uptake. Additional parameters include the number of circulating lymphocytes and LSEC apoptosis. In spontaneously accepted group, we found (i) significantly lower serum HA levels (P = 0.002), (ii) a more rapid uptake of FITC-FSA, and (iii) a reduced number of circulating CD8a+ cells when compared with the rejection group. Strikingly, HA levels in spontaneously accepted group are even lower than syngeneic control group. Further investigation revealed that interleukin-1beta, a cytokine that promotes LSEC function, was higher in DA than in Lewis rats. In summary, our study demonstrates that LSEC function is better preserved in spontaneously accepted rat liver allografts than in those which are rejected. These findings warrant further studies to verify if LSEC actively contributes to liver transplant outcome or just a target of different immunologic responses.

The role of hyaluronic acid in wound healing: assessment of clinical evidence

Hyaluronic acid (hyaluronan), a naturally occurring polymer within the skin, has been extensively studied since its discovery in 1934. It has been used in a wide range of medical fields as diverse as orthopedics and cosmetic surgery, but it is in tissue engineering that it has been primarily advanced for treatment. The breakdown products of this large macromolecule have a range of properties that lend it specifically to this setting and also to the field of wound healing. It is non-antigenic and may be manufactured in a number of forms, ranging from gels to sheets of solid material through to lightly woven meshes. Epidermal engraftment is superior to most of the available biotechnologies and, as such, the material shows great promise in both animal and clinical studies of tissue engineering. Ongoing work centers around the ability of the molecule to enhance angiogenesis and the conversion of chronic wounds into acute wounds.

Regulation of fibroblasts by activated and non-activated immune cells

BACKGROUND

Rejection of transplanted tissue is characterized by cell infiltration and interstitial edema. Graft fibroblasts and fibroblast products are partly involved in the regulation of both these phenomena. Knowledge about the mechanisms behind fibroblast activation may lead to new strategies to prevent rejection. This study investigated whether cells of the immune system have the capacity to regulate fibroblast activation.

METHODS

Fibroblasts isolated from rejecting heart transplants or from normal heart tissue were cultured in the presence of supernatants of stimulated or non-stimulated immune cells. The immune cells were challenged either in vitro (incubation with phytohemagglutinin) or in vivo (organ transplantation). Fibroblast proliferation and hyaluronan production were measured.

RESULTS

Normal, sub-confluent heart fibroblasts showed an increased proliferation rate in the presence of supernatants of activated immunocompetent cells, irrespective of if these cells had been stimulated in vitro or in vivo. As expected, proliferation rate and hyaluronan production were upregulated in fibroblasts isolated from rejecting tissue. However, supernatants of biopsy specimens obtained from non-rejecting organs (syngeneic transplants or normal hearts) had an inhibitory effect on the growth rate of confluent fibroblasts isolated from rejecting tissue.

CONCLUSIONS

We conclude that graft-infiltrating cells and immune cells activated in vitro have the capacity to stimulate fibroblasts, most probably as a result of the production and secretion of fibroblast-stimulating factors.

Effects of commonly used immunosuppressants on graft-derived fibroblasts

In acute rejection of transplanted organs intragraft fibroblasts increase their production of hyaluronan. Hyaluronan has strong water binding capacity and an increased tissue content of hyaluronan thus contributes to the development of interstitial oedema. The present study examined the effects of commonly used immunosuppressants (prednisolone, cyclosporin, tacrolimus, mycophenolic acid and sirolimus) on fibroblast proliferation, hyaluronan production and cell surface receptor expression. Fibroblasts isolated from rejecting tissue and from normal, non-transplanted tissue were studied in parallel. All substances investigated, except tacrolimus, were found to affect fibroblasts in one way or another. The most striking effect was the almost total inhibition of fibroblast proliferation in the presence of mycophenolic acid. Cyclosporin reduced the proliferation by about 50% and prednisolone had an inhibiting effect on hyaluronan production (50% reduction). These effects were observed on fibroblasts isolated from rat cardiac allografts undergoing rejection as well as on fibroblasts obtained from normal heart tissue. In contrast, sirolimus was found to stimulate the proliferation of fibroblasts from rejecting tissue (100% increase), but not that of normal fibroblasts. The majority of the fibroblasts expressed the hyaluronan receptor CD44, with a more intense expression in cultures of fibroblasts derived at rejection. None of the immunosuppressants affected the staining pattern (number of positive cells or intensity). The inhibitory effects of prednisolone, cyclosporin and mycophenolic acid on fibroblasts may contribute to the overall beneficial effects of these drugs when used for prevention or treatment of rejection.

The graft content of hyaluronan is increased during xenograft rejection

BACKGROUND

Hyaluronan, a macromolecule with strong water binding capacity, is associated with interstitial oedema during rejection of allogeneic transplants. However, the involvement of hyaluronan during xenograft rejection has previously not been investigated. The aims of this study were to characterize hyaluronan content and distribution during rejection of concordant mouse-to-rat cardiac xenografts, and to explore the effects of hyaluronidase (HAse) on xenograft survival.

METHODS

Graft recipients were treated with 15-deoxyspergualin (DSG) or both HAse and DSG. Grafts were removed on day 5 from some of the animals to analyse hyaluronan and water content, while other animals were used to investigate graft survival. The hyaluronan content was measured by a radiometric assay and the distribution was analysed by histochemical staining.

RESULTS

In xenografts undergoing rejection (the DSG group) there was a strong increase of the hyaluronan [555 +/- 93 microg/g dry weight (dw)] and water (82.7 +/- 0.4%) contents compared with normal mouse heart tissue (166 +/- 10 microg/g dw; P < 0.01 and 78.6 +/- 0.5%; P < 0.001, respectively). The combined use of HAse and DSG reduced the accumulation of hyaluronan (284 +/- 43 microg/g dw; P < 0.05 vs. DSG) but did not affect the average water content. The average graft survival time did not differ between the groups; however, three grafts in the HAse + DSG-treatment group survived much longer than the longest-surviving grafts in the DSG group.

CONCLUSIONS

These data suggest that the graft content of hyaluronan considerably increases during xenograft rejection. HAse effectively reduces this accumulation, but does not affect the average water content.