Mast cell numbers and protease expression patterns in biopsy specimens following renal transplantation from living-related donors predict long-term graft function

Mast Cells in Kidney Transplant Biopsies With Borderline T Cell-mediated Rejection and Their Relation to Chronicity

Mast cells are potential contributors to chronic changes in kidney transplants (KTx). Here, the role of mast cells (MCs) in KTx is investigated in patients with minimal inflammatory lesions.

Methods

Fourty-seven KTx biopsies (2009-2018) with borderline pathological evidence for T cell-mediated rejection according to the Banff'17 Update were retrospectively included and corresponding clinical data was collected. Immunohistochemistry for tryptase was performed on formalin-fixed paraffin-embedded sections. Cortical MCs were counted and corrected for area (MC/mm²). Interstitial fibrosis was assessed by Sirius Red staining and quantified using digital image analysis (QuPath).

Results

Increased MC number was correlated to donor age (spearman's r = 0.35, P = 0.022), deceased donor kidneys (mean difference = 0.74, t [32.5] = 2.21, P = 0.035), and delayed graft function (MD = 0.78, t [33.9] = 2.43, P = 0.020). Increased MC number was also correlated to the amount of interstitial fibrosis (r = 0.42, P = 0.003) but did not correlate with transplant function over time (r = -0.14, P = 0.36). Additionally, transplant survival 2 y post-biopsy was not correlated to MC number (mean difference = -0.02, t [15.36] = -0.06, P = 0.96).

Conclusions

MC number in suspicious (borderline) for acute T cell-mediated rejection is correlated to interstitial fibrosis and time post-transplantation, suggesting MCs to be a marker for cumulative burden of tissue injury. There was no association between MCs and transplant function over time or transplant survival 2 y post-biopsy. It remains unclear whether MCs are just a bystander or have pro-inflammatory or anti-inflammatory effects in the KTx with minimal lesions.

The mast cell: A Janus in kidney transplants

Mast cells (MCs) are innate immune cells with a versatile set of functionalities, enabling them to orchestrate immune responses in various ways. Aside from their known role in allergy, they also partake in both allograft tolerance and rejection through interaction with regulatory T cells, effector T cells, B cells and degranulation of cytokines and other mediators. MC mediators have both pro- and anti-inflammatory actions, but overall lean towards pro-fibrotic pathways. Paradoxically, they are also seen as having potential protective effects in tissue remodeling post-injury. This manuscript elaborates on current knowledge of the functional diversity of mast cells in kidney transplants, combining theory and practice into a MC model stipulating both protective and harmful capabilities in the kidney transplant setting.

Intrarenal Renin-Angiotensin-System Dysregulation after Kidney Transplantation

Angiotensin-converting enzyme inhibitors (ACEis) are beneficial in patients with chronic kidney disease (CKD). Yet, their clinical effects after kidney transplantation (KTx) remain ambiguous and local renin-angiotensin system (RAS) regulation including the ‘classical’ and ‘alternative’ RAS has not been studied so far. Here, we investigated both systemic and kidney allograft-specific intrarenal RAS using tandem mass-spectrometry in KTx recipients with or without established ACEi therapy (n = 48). Transplant patients were grouped into early (<2 years), intermediate (2–12 years) or late periods after KTx (>12 years). Patients on ACEi displayed lower angiotensin (Ang) II plasma levels (P < 0.01) and higher levels of Ang I (P < 0.05) and Ang-(1–7) (P < 0.05) compared to those without ACEi independent of graft vintage. Substantial intrarenal Ang II synthesis was observed regardless of ACEi therapy. Further, we detected maximal allograft Ang II synthesis in the late transplant vintage group (P < 0.005) likely as a consequence of increased allograft chymase activity (P < 0.005). Finally, we could identify neprilysin (NEP) as the central enzyme of ‘alternative RAS’ metabolism in kidney allografts. In summary, a progressive increase of chymase-dependent Ang II synthesis reveals a transplant-specific distortion of RAS regulation after KTx with considerable pathogenic and therapeutic implications.

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 controversial role of mast cells in fibrosis

Fibrosis is a medical condition characterized by an excessive deposition of extracellular matrix compounds such as collagen in tissues. Fibrotic lesions are present in many diseases and can affect all organs. The excessive extracellular matrix accumulation in these conditions can often have serious consequences and in many cases be life-threatening. A typical event seen in many fibrotic conditions is a profound accumulation of mast cells (MCs), suggesting that these cells can contribute to the pathology. Indeed, there is now substantialv evidence pointing to an important role of MCs in fibrotic disease. However, investigations from various clinical settings and different animal models have arrived at partly contradictory conclusions as to how MCs affect fibrosis, with many studies suggesting a detrimental role of MCs whereas others suggest that MCs can be protective. Here, we review the current knowledge of how MCs can affect fibrosis.

Allograft rejection is associated with development of functional IgE specific for donor MHC antigens

BACKGROUND

Donor-specific antibodies of the IgG isotype are measured routinely for diagnostic purposes in renal transplant recipients and are associated with antibody-mediated rejection and long-term graft loss.

OBJECTIVE

This study aimed to investigate whether MHC-specific antibodies of the IgE isotype are induced during allograft rejection.

METHODS

Anti-MHC/HLA IgE levels were measured in sera of mice grafted with skin or heart transplants from various donor strains and in sera of kidney transplant patients with high levels of HLA IgG. Mediator release was triggered in vitro by stimulating basophils that were coated with murine or human IgE-positive serum, respectively, with specific recombinant MHC/HLA antigens. Kidney tissue samples obtained from organ donors were analyzed by using flow cytometry for cells expressing the high-affinity receptor for IgE (FcεRI).

RESULTS

Donor MHC class I- and MHC class II-specific IgE was found on acute rejection of skin and heart grafts in several murine strain combinations, as well as during chronic antibody-mediated heart graft rejection. Anti-HLA IgE, including donor HLA class I and II specificities, was identified in a group of sensitized transplant recipients. Murine and human anti-MHC/HLA IgE triggered mediator release in coated basophils on stimulation with specific MHC/HLA antigens. HLA-specific IgE was not linked to atopy, and allergen-specific IgE present in allergic patients did not cross-react with HLA antigens. FcεRI+ cells were found in the human renal cortex and medulla and provide targets for HLA-specific IgE.

CONCLUSION

These results demonstrate that MHC/HLA-specific IgE develops during an alloresponse and is functional in mediating effector mechanisms.

Mast cell phenotypes in the allograft after lung transplantation

BACKGROUND

The burden of mast cell (MC) infiltration and their phenotypes, MC-tryptase (MCT ) and MC-tryptase/chymase (MCTC ), after lung transplantation (LT) has not been evaluated in human studies.

METHODS

We reviewed 20 transbronchial lung biopsy (TBLB) specimen from patients with early normal allograft (<6 months post-LT, n=5), late normal allograft (>6 months, n=5), A2 or worse acute cellular rejection (ACR, n=5), and chronic lung allograft dysfunction (CLAD, n=5). Slides were immunostained for tryptase and chymase. Total MC, MCT , MCTC and MCTC to-MCT ratio were compared between the four groups using a generalized linear mixed model.

RESULTS

Irrespective of clinicopathologic diagnosis, MC burden tends to increase with time (r(2) =.56, P=.009). MCTC phenotype was significantly increased in the CLAD group (8.2±4.9 cells per HPF) in comparison with the other three groups (early normal: 1.6±1.7, P=.0026; late normal: 2.5±2.3, P=.048; ACR: 2.7±3.5, P=.021). Further, the ratio of MCTC to MCT was significantly increased in CLAD group as compared to the other three groups (P<.001 for all comparisons).

CONCLUSIONS

The burden of MC may increase in the allograft as function of time. Patients with CLAD have an increased relative and absolute burden of MCTC phenotype MC. Future studies are needed to confirm these findings and evaluate the potential pathologic role of MCTC in allograft dysfunction.

The putative role of mast cells in lung transplantation

Mast cells (MCs) were primarily recognized as effector cells of allergy. These cells are acting predominantly at the interface between the host and the external environment, such as skin, gastrointestinal and the respiratory tract. Only recently, MCs have gained increased recognition as cells of functional plasticity with immune-regulatory properties that influence both the innate and the adaptive immune response in inflammatory disorders, cancer and transplantation. Through the secretion of both proinflammatory and antiinflammatory mediators, MCs can either ameliorate or deteriorate the course and outcome in lung transplantation. Recent research from other models recognized the immune-protective activity of MCs including its role as an important source of IL-10 and TGF-β for the modulation of alloreactive T cell responses or assistance in Treg activity. This paper summarizes the current understanding of MCs in lung transplantation and discusses MC-mediated immune-mechanisms by which the outcome of the engrafted organ is modulated.

Mast cells in human health and disease

Mast cells are primarily known for their role in defense against pathogens, particularly bacteria; neutralization of venom toxins; and for triggering allergic responses and anaphylaxis. In addition to these direct effector functions, activated mast cells rapidly recruit other innate and adaptive immune cells and can participate in "tuning" the immune response. In this review we touch briefly on these important functions and then focus on some of the less-appreciated roles of mast cells in human disease including cancer, autoimmune inflammation, organ transplant, and fibrosis. Although it is difficult to formally assign causal roles to mast cells in human disease, we offer a general review of data that correlate the presence and activation of mast cells with exacerbated inflammation and disease progression. Conversely, in some restricted contexts, mast cells may offer protective roles. For example, the presence of mast cells in some malignant or cardiovascular diseases is associated with favorable prognosis. In these cases, specific localization of mast cells within the tissue and whether they express chymase or tryptase (or both) are diagnostically important considerations. Finally, we review experimental animal models that imply a causal role for mast cells in disease and discuss important caveats and controversies of these findings.

The role of mast cells after solid organ transplantation

Mast cells are best known as primary responders in allergic reactions, including anaphylaxis and asthma. However, recent studies have shown that mast cells are functionally diverse cells with immunoregulatory properties that influence both the innate and adaptive immunities. Mast cells are capable of producing an array of both proinflammatory and anti-inflammatory mediators, acting as antigen-presenting cells, and expressing a spectrum of costimulatory molecules. Moreover, mast cells seem to confer a certain degree of immune privilege to tissues in concert with T-regulatory cells and are essential players in fibrotic conditions. The following review of the literature serves to further define the role of mast cells in the immunologic reactions affecting transplanted solid organ grafts.

Late kidney allograft loss: what we know about it, and what we can do about it

Despite dramatic improvements in immunosuppression, late graft loss after kidney transplantation remains a common and difficult problem. Histologic evaluation may reveal changes related to BK polyomavirus infection, hypertension, or calcineurin inhibitor toxicity, which can help to guide therapy. The designation chronic allograft nephropathy should thus be reserved for biopsies with tubular atrophy and interstitial fibrosis without an apparent cause. Although the cause clearly includes both antigen-dependent and antigen-independent events, the approach remains largely to exclude immune mechanisms. Although this review discusses the potential contribution of antibody to chronic injury, it focuses on the basic elements of kidney injury, the role of parenchymal cells in promoting injury, and the proliferative and inflammatory responses that accompanying injury. Strategies to manage these recipients include close attention to accompanying hypertension, diabetes, and hyperlipidemia, as well as consideration for altering immunosuppression; however, therapies that limit epithelial-to-mesenchymal transition or directly block fibrosis pathways may reduce chronic allograft fibrosis and may prove to be useful. Understanding the basic pathogenesis sufficiently to allow early intervention may finally benefit patients who are at high risk for tubular atrophy and interstitial fibrosis and promote their long-term graft function.