Immunomodulatory functions of hyaluronate in the LEW-to-F344 model of chronic cardiac allograft rejection

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.

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.

Hyaluronan as a therapeutic target in human diseases

Accumulation and turnover of extracellular matrix is a hallmark of tissue injury, repair and remodeling in human diseases. Hyaluronan is a major component of the extracellular matrix and plays an important role in regulating tissue injury and repair, and controlling disease outcomes. The function of hyaluronan depends on its size, location, and interactions with binding partners. While fragmented hyaluronan stimulates the expression of an array of genes by a variety of cell types regulating inflammatory responses and tissue repair, cell surface hyaluronan provides protection against tissue damage from the environment and promotes regeneration and repair. The interactions of hyaluronan and its binding proteins participate in the pathogenesis of many human diseases. Thus, targeting hyaluronan and its interactions with cells and proteins may provide new approaches to developing therapeutics for inflammatory and fibrosing diseases. This review focuses on the role of hyaluronan in biological and pathological processes, and as a potential therapeutic target in human diseases.

Inflammatory triggers of acute rejection of organ allografts

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

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.

Genetic modulation of CD44 expression by intragraft fibroblasts

This study investigated the genetic composition and the functional implication of CD44 species expressed by intragraft fibroblasts. An LEW-to-F344 heart transplant model of chronic rejection was used. Intragraft fibroblasts recovered from the chronically rejecting allografts displayed a 4.5-fold increase in expression of CD44 mRNA when compared with that of the fibroblasts isolated from non-rejecting heart allografts (P < 0.01). The intragraft fibroblasts preferentially expressed CD44 variant isoforms containing v1 exon transcript. Automated nucleotide sequence analysis revealed that the majority (90.12%) of the CD44 v1 isoforms expressed by the rejecting graft fibroblasts were encoded by a mutated CD44 mRNA, which contained two point mutations and a codon deletion in the v1 coding region. Histochemistry demonstrated a massive deposition of extracellular HA in the rejecting heart allografts. Hyaluronic acid (HA) was able to promote in vitro fibroblast adhesion, migration in a CD44-dependent manner, and survival in a serum-free culture condition. The study concludes that up-regulation of CD44 v1 isoforms expressed by the intragraft fibroblasts is associated with an increase in the deposition of extracellular HA, the principal ligand for CD44, in the allografts, suggesting that CD44-HA interaction plays an important role in regulating fibroblast recruitment and growth in allografts developing chronic rejection.

Vascular Remodeling in Transplant Vasculopathy

As therapeutic strategies to prevent acute rejection progressively improve, transplant vasculopathy (TV) constitutes the single most important limitation for long-term functioning of solid organ allografts. In TV, allograft arteries characteristically develop severe, diffuse intimal hyperplastic lesions that eventually compromise luminal flow and cause ischemic graft failure. Traditional immunosuppressive strategies that check acute allograft rejection do not prevent TV; indeed 50% of transplant recipients will have significant disease within five years of organ transplantation, and 90% will have significant TV a decade after their surgery. TV can involve the entire length of the transplanted arterial bed, including penetrating intraorgan arterioles. Indeed, the luminal narrowing of such penetrating vessels may be the most functionally significant because arterioles represent the major contributors to tissue vascular resistance. Because of the diffuseness of TV involvement in the allograft vascular bed, the only currently definitive therapy requires re-transplantation. Nevertheless, as we better understand the pathogenesis and critical mediators of these lesions, pharmacological advances can be anticipated. Other articles in this thematic review series focus on the specifics of the inciting injury, the cytokines and chemokines that drive TV development, and the nature of the recruited cells in TV lesions, as well as the pathogenic similarities between TV and other vascular lesions such as atherosclerosis. This review focuses on the mechanisms of vascular wall remodeling in TV, including the intimal accumulation of smooth muscle-like cells and associated extracellular matrix, medial smooth muscle cell degeneration, and adventitial fibrosis. A brief overview highlights the aneurysmal changes that can accrue when vessel wall inflammation has a cytokine profile distinct from the typical proinflammatory interferon-gamma-dominated milieu.

The role of hyaluronan degradation products as innate alloimmune agonists

Dendritic cells (DCs) play a key role in initiating alloimmunity yet the substances that activate them during the host response to transplantation remain elusive. In this study we examined the potential roles of endogenous innate immune agonists in activating dendritic cell-dependent alloimmunity. Using a murine in vitro culture system, we show that 135 KDa fragments of the extracellular matrix glycosaminoglycan hyaluronan induce dendritic cell maturation and initiate alloimmunity. Priming of alloimmunity by hyaluronan-activated DCs was dependent on signaling via TIR-associated protein, a Toll-like receptor (TLR) adaptor downstream of TLRs 2 and 4. However, this effect was independent of alternate TLR adaptors, MyD88 or Trif. Using an in vivo murine transplant model, we show that hyaluronan accumulated during skin transplant rejection. Examination of human lung transplant recipients demonstrated that increased levels of intragraft hyaluronan were associated with bronchiolitis obliterans syndrome. In conclusion, our study suggests that fragments of hyaluronan can act as innate immune agonists that activate alloimmunity.

Viral IL-10-mediated immune regulation in pancreatic islet transplantation

Protection of transplanted pancreatic islet grafts in recipients with autoimmune diabetes depends on the suppression of autoimmune recurrence and allogeneic rejection. The aim of this study was to investigate the efficiency of viral IL-10 gene delivery in the prevention of autoimmune recurrence following islet transplantation. We evaluated the effectiveness of a systemically delivered adeno-associated viral vector (AAV vIL-10) carrying viral IL-10 in protecting islet engraftment. We observed significant prolongation of graft survival after treatment with AAV vIL-10 when using islets from donors lacking autoimmunity. We found that the mechanism of vIL-10-mediated protection was associated with suppression of T cell activation and that donor immune cells that were simultaneously transferred with the islet grafts could induce autoimmune recurrence. AAV vIL-10 gene transfer suppressed previously activated T cells and protected grafted islets from autoimmune-mediated destruction. We conclude that vIL-10 can regulate autoimmune activity and that transfer of its gene may have potential for therapeutic islet transplantation.

The role of the hyaluronan receptor CD44 in mesenchymal stem cell migration in the extracellular matrix

In a previous investigation, we demonstrated that mesenchymal stem cells (MSCs) actively migrated to cardiac allografts and contributed to graft fibrosis and, to a lesser extent, to myocardial regeneration. The cellular/molecular mechanism responsible for MSC migration, however, is poorly understood. This paper examines the role of CD44-hyaluronan interaction in MSC migration, using a rat MSC line Ap8c3 and mouse CD44-/- or CD44+/+ bone marrow stromal cells (BMSCs). Platelet-derived growth factor (PDGF) stimulation of MSC Ap8c3 cells significantly increased the levels of cell surface CD44 detected by flow cytometry. The CD44 standard isoform was predominantly expressed by Ap8c3 cells, accounting for 90% of the CD44 mRNA determined by quantitative real-time polymerase chain reaction. Mouse CD44-/- BMSCs bonded inefficiently to hyaluronic acid (HA), whereas CD44+/+ BMSC and MSC Ap8c3 adhered strongly to HA. Adhesions of MSC Ap8c3 cells to HA were suppressed by anti-CD44 antibody and by CD44 small interfering RNA (siRNA). HA coating of the migration chamber significantly promoted passage of CD44+/+ BMSC or Ap8c3 cells, but not CD44-/- BMSCs, through the insert membranes (p < .01). Migration of MSC Ap8c3 was significantly inhibited by anti-CD44 antibodies (p < .01) and to a lesser extent by CD44 siRNA (p = .05). The data indicate that MSC Ap8c3 cells, in response to PDGF stimulation, express high levels of CD44 standard (CD44s) isoform, which facilitates cell migration through interaction with extracellular HA. Such a migratory mechanism could be critical for recruitment of MSCs into wound sites for the proposition of tissue regeneration, as well as for migration of fibroblast progenitors to allografts in the development of graft fibrosis.

Tubular CD44 expression in renal allograft biopsies

CD44 is a type I transmembrane glycoprotein serving as a cell adhesion receptor, whose main ligand is hyaluronic acid, but also may interact with collagen, laminin, fibronectin, and osteopontin. This marker is involved in cell migration, homing, activation, metastasis, and inflammation. Tubular CD44 expression has been shown to correlate with scarring in renal diseases, but there is little data on allograft biopsies. This deficiency is important since experimental studies have shown that blockade of the CD44-hyaluronic acid interaction may prolong allograft survival. In an attempt to clarify the role of tubular CD44 expression in renal allografts, CD44 expression was determined immunohistochemically in 37 allograft and 10 implantation biopsies, as the percentage of tubules expressing this marker. For implantation biopsies the mean tubular CD44 expression was 6% +/- 14%; for allograft biopsies, 13% +/- 20% (P =.17, Mann-Whitney U). By the Spearman correlation test, CD44 expression did not correlate with Banff scores, but was moderately correlated with serum creatinine values at the time of biopsy (P =.017, r =.4). These findings suggest an important role of tubular CD44 expression in renal allografts. It appears to be induced by more than one pathway, resulting in a pattern of expression that correlated with renal function. However larger series are required before recommending the routine use of this marker.

Hyaluronan-independent adhesion of CD44H+ and CD44v10+ lymphocytes to dermal microvascular endothelial cells and keratinocytes

We have recently shown the CD44 variant isoform 10 (CD44v10) to be expressed on reactive as well as malignant cutaneous lymphocytes; however, the functional consequences of CD44v10 expression on lymphocytes are not elucidated. By using appropriately transfected lymphatic cells we analyzed the role of CD44v10 on lymphocytes in cell-matrix adhesion and homotypic and heterotypic cell-cell adhesion assays. Despite a low binding affinity to hyaluronan, CD44v10-expressing lymphocytes exhibited heterotypic cell-cell adhesion to inflamed dermal microvascular endothelium and keratinocytes, as indicated by Stamper-Woodruff assays on tissue sections of delayed type hypersensitivity reactions and adhesion assays with cultured keratinocytes and cytokine-stimulated human dermal microvascular endothelial cells. Antibody-blocking assays excluded interaction of CD44v10 with the principal CD44 ligand hyaluronan as well as involvement of selectins or integrins in these heterotypic cell-cell adhesion assays. In contrast, cellular aggregation assays with fluorescence-labeled CD44v10- and CD44H-expressing lymphocytes revealed homotypic CD44v10/CD44v10 binding as well as binding of CD44v10 with CD44H. Heterotypic cell-cell adhesion assays with ultraviolet-A-irradiated CD44v-negative cytokine-stimulated endothelial cells demonstrated binding kinetics of CD44v10-expressing lymphocytes paralleling those of endothelial CD44H expression. These results imply that a hyaluronan-independent CD44v10/CD44H-mediated pathway is involved in lymphocyte infiltration into the dermis and epidermis of inflamed skin and suggest modulation of CD44H expression on inflamed dermal microvascular endothelium as a mechanism of ultraviolet-A-induced therapeutic effects on the skin.

Monitoring of intragraft pressure of rejecting organs: increased tissue pressure can be reduced by hyaluronidase therapy

BACKGROUND

The present study was undertaken in order to: (a) develop a new technique for measurement of interstitial pressure, (b) study the intragraft pressure of rejecting and non-rejecting organs, and (c) study the effect of treatment with the hyaluronan-degrading enzyme hyaluronidase on intragraft pressure. Treatment with hyaluronidase has previously been demonstrated to result not only in reduction of tissue hyaluronan but also in ameliorated interstitial edema, and we suggested that the diminished edema would lead to a reduced interstitial pressure as well.

METHODS

At day 5 after syngeneic or allogeneic rat heterotopic heart transplantation, the interstitial pressure of the cardiac grafts was measured using a microtip pressure sensor. Subsequently, the allogeneically grafted animals received a continuous intravenous infusion of either hyaluronidase (total dose: 60,000 U/kg) or vehicle during 2 hr; meanwhile, the interstitial pressure was monitored.

RESULTS

The intragraft pressure measurement technique was found to give reproducible results. The interstitial pressure of the rejecting (allogeneic) grafts was considerably higher than that of the non-rejecting (syngeneic), i.e., 12.3+/-1.6 mmHg vs. 1.1+/-0.6 mmHg (P<0.001). Hyaluronidase infusion effectively reduced the interstitial pressure as compared with vehicle treatment. By 20 min, the pressure had been reduced by 28% (P<0.01 compared with vehicle treatment); after 1 hr, by 49% (P<0.001); and after 2 hr, by 68% (P<0.01).

CONCLUSIONS

By using modern technology for tissue pressure measurements, we found that the strongly increased interstitial pressure of rejecting organs can be instantly reduced by intravenous administration of the hyaluronan-degrading enzyme hyaluronidase.

Induction of hyaluronan metabolism after mechanical injury of human peritoneal mesothelial cells in vitro

BACKGROUND

Hyaluronan (HA) is an important extracellular matrix component that is involved in cell movement and tissue repair. In vertebrates, HA synthase genes (HAS 1, HAS 2, and HAS 3) that control the synthesis of HA have been identified. In this article, we investigated HA synthesis in the response of human peritoneal mesothelial cells (HPMCs) to injury.

METHODS

The expression of HAS 1, HAS 2, and HAS 3 mRNA and the synthesis of [(3)H]-labeled HA were examined in an in vitro model of peritoneal mesothelial cell damage. The staining for uridine diphosphoglucose dehydrogenase, a key enzyme in the synthesis of HA, and biotinylated HA-binding protein was used to determine the cellular location of HA synthesis and its site of deposition.

RESULTS

Growth-arrested human HPMCs expressed low levels of mRNA for HAS 2 and HAS 3 but not HAS 1. Following injury to the monolayer, HAS 2 was up-regulated by 6 hours, reaching maximal expression between 12 and 24 hours. In contrast, the expression of HAS 3 was down-regulated. During the same time period, synthesis of HA was increased in the injured monolayer. This synthetic activity appeared to be restricted to cells at the edge of the wound and to cells entering the wound. In a separate series of experiments, the addition of HA to the injured monolayer at a concentration range found in peritoneal fluid (50 to 3300 ng/mL) increased the migration of cells into the wound in a dose-dependent manner.

CONCLUSIONS

These studies provide evidence that HA is an important component of peritoneal mesothelial cell migration. The results also suggest that in this process, there is differential regulation of HAS gene expression and that the synthesis of HA is limited to cells located at the leading edge of the wound.

Hyaluronan anchoring and regulation on the surface of vascular endothelial cells is mediated through the functionally active form of CD44

CD44 on lymphocytes binding to its carbohydrate ligand hyaluronan can mediate primary adhesion (rolling interactions) of lymphocytes on vascular endothelial cells. This adhesion pathway is utilized in the extravasation of activated T cells from the blood into sites of inflammation and therefore influences patterns of lymphocyte homing and inflammation. Hyaluronan is a glycosaminoglycan found in the extracellular matrix and is involved in a number of biological processes. We have shown that the expression of hyaluronan on the surface of endothelial cells is inducible by proinflammatory cytokines. However, the manner through which hyaluronan is anchored to the endothelial cell surface so that it can resist shear forces and the mechanism of the regulation of the level of hyaluronan on the cell surface has not been investigated. In order to characterize potential hyaluronan receptors on endothelial cells, we performed analyses of cell surface staining by flow cytometry on intact endothelial cells and ligand blotting assays using membrane fractions. Hyaluronan binding activity was detected as a major species corresponding to the size of CD44, and this was confirmed to be the same by Western blotting and immunoprecipitation. Moreover, alterations in the surface level of hyaluronan after tumor necrosis factor-alpha stimulation is regulated primarily by changes in the cell surface levels of the hyaluronan-binding form of CD44. In laminar flow assays, lymphoid cells specifically roll on hyaluronan anchored by purified CD44 coated on glass tubes, indicating that the avidity of the endothelial CD44/hyaluronan interaction is sufficient to support rolling adhesions under conditions mimicking physiologic shear forces. Together these studies show that CD44 serves to anchor hyaluronan on endothelial cell surfaces, that activation of CD44 is a major regulator of endothelial surface hyaluronan expression, and that the non-covalent interaction between CD44 and hyaluronan is sufficient to provide resistance to shear under physiologic conditions and thereby support the initial steps of lymphocyte extravasation.

The CD44-initiated pathway of T-cell extravasation uses VLA-4 but not LFA-1 for firm adhesion

Leukocytes extravasate from the blood in response to physiologic or pathologic demands by means of complementary ligand interactions between leukocytes and endothelial cells. The multistep model of leukocyte extravasation involves an initial transient interaction ("rolling" adhesion), followed by secondary (firm) adhesion. We recently showed that binding of CD44 on activated T lymphocytes to endothelial hyaluronan (HA) mediates a primary adhesive interaction under shear stress, permitting extravasation at sites of inflammation. The mechanism for subsequent firm adhesion has not been elucidated. Here we demonstrate that the integrin VLA-4 is used in secondary adhesion after CD44-mediated primary adhesion of human and mouse T cells in vitro, and by mouse T cells in an in vivo model. We show that clonal cell lines and polyclonally activated normal T cells roll under physiologic shear forces on hyaluronate and require VCAM-1, but not ICAM-1, as ligand for subsequent firm adhesion. This firm adhesion is also VLA-4 dependent, as shown by antibody inhibition. Moreover, in vivo short-term homing experiments in a model dependent on CD44 and HA demonstrate that superantigen-activated T cells require VLA-4, but not LFA-1, for entry into an inflamed peritoneal site. Thus, extravasation of activated T cells initiated by CD44 binding to HA depends upon VLA-4-mediated firm adhesion, which may explain the frequent association of these adhesion receptors with diverse chronic inflammatory processes.

Blocking of CD44-hyaluronic acid interaction prolongs rat allograft survival

BACKGROUND

Lymphocyte activation and infiltration into a transplanted organ is an integral component of the rejection process. Graft infiltration of lymphocytes requires adhesion of leukocytes to the endothelium, diapedesis, and transmigration. One of several proteins involved in this process is CD44, which is known to interact with endothelial hyaluronan (HA). Blockade of cell-matrix and cell-cell interactions have been used extensively for modulation of immune responses and graft rejection. Based on these observations, we evaluated the effects of blocking CD44-HA interactions in a transplantation model.

METHODS

We used a low molecular weight hyaluronic acid formulation (LMWHA) for the treatment of rat renal and cardiac allograft recipients. LMWHA was administered intraperitoneally at 0.5-5 mg/kg for 5-10 days after transplantation with or without a subtherapeutic dose of cyclosporine.

RESULTS

LMWHA monotherapy prolonged allograft survival significantly, but only for a few days. In combination with low-dose cyclosporine, long-term survival of allografts was observed in some of recipients.

CONCLUSION

Further definition of the underlying mechanism of LMWHA therapy may provide a rationale for the development of novel, nontoxic, nonimmunogenic immunotherapies.