Working toward immune tolerance in lung transplantation

Insight on immune cells in rejection and infection postlung transplant

OBJECTIVE

The aim of this study is to provide a concise overview of the role of immune cells in rejection and infection after lung transplantation.

METHODS

Based on previous clinical and basic studies, the role of various types of immune cells in the development of rejection and infection after lung transplantation is summarized.

RESULTS

Immune cell functional status is strongly associated with common complications after lung transplantation, such as primary graft dysfunction, infection and occlusive bronchitis syndrome. Targeted balancing of immune cell tolerance and rejection is an important tool for successful lung transplantation.

CONCLUSION

A comprehensive understanding of immune cell function and the mechanisms that balance immune tolerance and immune rejection may be a crucial factor in improving survival after lung transplantation.

Improved Outcomes for Infants and Young Children Undergoing Lung Transplantation at Three Years of Age and Younger

Rationale: Since its inception, older children and adolescents have predominated in pediatric lung transplantation. Most pediatric lung transplant programs around the world have transplanted few infants and young children. Early mortality after lung transplantation and inadequate donor organs have been perceived as limitations for success in lung transplantation at this age. Objectives: Our aim was to describe our experience in a large pediatric lung transplant program with respect to lung transplantation in infants and young children, focusing on diagnosis, waitlist, and mortality. Methods: We performed a retrospective review of infants and young children under 3 years of age at the time of transplant in our program from 2002 through 2020. Results: The patient cohort represented a severely morbid recipient group, with the majority hospitalized in the intensive care unit on mechanical ventilation just before transplantation. There was a marked heterogeneity of diagnoses distinct from diagnoses in an older cohort. Waitlist time was shorter than in older age cohorts. There was a decrease in early mortality, lower incidence of allograft rejection, and satisfactory long-term survival in this age group compared with the older cohort and published experience. Severe viral infection was an important cause of early mortality after transplant. Nonetheless, survival is comparable to older patients, with better enduring survival in those who survive the early transplant period in more recent years. Conclusions: Carefully selected infants and young children with end-stage lung and pulmonary vascular disease are appropriate candidates for lung transplantation and are likely underserved by current clinical practice.

Human leukocyte antigen epitope mismatch loads and the development of de novo donor-specific antibodies in cardiothoracic organ transplantation

De novo donor-specific human leucocyte antigen (HLA) antibodies (dnDSA) are associated with increased risk of rejection and mortality in solid organ transplantation. Such dnDSA is produced in some recipients upon allorecognition of mismatched HLA post-transplant. HLA matching is not currently considered in the allocation of deceased donor hearts and lungs and pre-transplant immunological risk stratification is based entirely on the mean fluorescence intensity (MFI) of circulating donor-directed HLA antibodies. HLA epitope-based matching tools predict B-cell or T-cell HLA epitopes that are present in the donor's HLA but absent in the recipient's HLA. We hypothesized that patients with higher epitope mismatch loads would be at increased risk of dnDSA development. We retrospectively analysed 73 heart and/or lung transplant recipients who were tested for DSA between 2015 and 2020. HLAMatchmaker, PIRCHE-II and HLA epitope mismatch algorithm (HLA-EMMA) were used to calculate eplet mismatch (EpMM) loads, T-cell epitope mismatch (TEpMM) loads and solvent accessible amino acid mismatch (SAMM) loads, respectively. Multivariate analyses showed that HLA-EMMA was the only tool with a significant association between the total score for all HLA loci and dnDSA production [odds ratio (OR) 1.021, 95% confidence interval (CI) 1.003-1.042, p = .0225] though this increased risk was marginal. The majority of dnDSA were directed against HLA-DQ and patients with higher HLA-DQ TEpMM loads (OR = 1.008, CI = 1.002-1.014, p = .007), and HLA-DR+DQ SAMM loads (OR = 1.035, CI = 1.010-1.064, p = .0077) were most at risk of producing dnDSA. We also showed that patients with a risk epitope within the HLA molecule encoded for by HLA-DQA1*05 + HLA-DQB1*02/03:01 were significantly more likely to produce dnDSA. The use of HLA epitope-based matching tools could be used for cardiothoracic transplant risk stratification to enable early intervention and monitoring of patients at increased risk of producing dnDSA.

Trametinib Attenuates Delayed Rejection and Preserves Thymic Function in Rat Lung Transplantation

Delayed immunological rejection after human lung transplantation causes chronic lung allograft dysfunction, which is associated with high mortality. Delayed rejection may be attributable to indirect alloantigen presentation by host antigen-presenting cells; however, its pathophysiology is not fully understood. The mitogen-activated protein kinase pathway is activated in T cells upon stimulation, and we previously showed that the MEK inhibitor, trametinib, suppresses graft-versus-host disease after murine bone marrow transplantation. We investigated whether trametinib suppresses graft rejection after two types of rat lung transplantation and analyzed its immunological mode of action. Major histocompatibility complex-mismatched transplantation from brown Norway rats into Lewis rats and minor histocompatibility antigen-mismatched transplantation from Fischer 344 rats into Lewis rats were performed. Cyclosporine (CsA) and/or trametinib were administered alone or consecutively. Acute and delayed rejection, lymphocyte infiltration, and pulmonary function were evaluated. Administration of trametinib after CsA suppressed delayed rejection, reduced inflammatory cell infiltration and fibrosis within the graft, and preserved pulmonary functions at Day 28. Trametinib suppressed functional differentiation of T and B cells in the periphery but preserved thymic T cell differentiation. Donor B cells within the graft disappeared by Day 14, indicating that delayed graft rejection at Day 28 was mainly due to indirect presentation by host antigen-presenting cells. Finally, trametinib administration without CsA preconditioning suppressed rejection after minor histocompatibility antigen-mismatched transplantation. Trametinib attenuates delayed rejection upon major histocompatibility complex-mismatched transplantation by suppressing indirect presentation and is a promising candidate to treat chronic lung allograft dysfunction in humans.

Messengers of tolerance

The use of immunosuppressant drugs after organ transplantation has brought great success in the field of organ transplantation with respect to short-term outcome. However, major challenges (i.e., limited improvement of long-term survival, immunosuppressant toxicity, infections and carcinoma) demand alternate treatment approaches that minimizes the use of immunosuppressants. Interestingly, few studies have identified groups of transplant patients who developed operational tolerance and thereby keep their allograft without complications in absence of immunosuppressants. These rare groups of patients are of particular interest as study subjects for understanding mechanisms of graft tolerance that could be leveraged in future for inducing tolerance and for understanding mechanisms involved in improving long-term allograft outcomes. Also, biomarkers from these studies could benefit the larger transplant population by their application in immunosuppressant tailoring and identification of tolerant patients among patients with stably functioning allografts. This review compiles several gene expression studies performed in samples from tolerant patients in different solid organ transplantations to identify key genes and associated molecular pathways relevant to tolerance. This review is aimed at putting forth all this important work done thus far and to identify research gaps that need to be filled, in order to achieve the greater purpose of these studies.

Repeated systemic administration of adipose tissue-derived mesenchymal stem cells prevents tracheal obliteration in a murine model of bronchiolitis obliterans

OBJECTIVE

To investigate the effect of adipose tissue-derived mesenchymal stem cell (ASC) administered either systemically or locally in a murine model of bronchiolitis obliterans.

RESULTS

When compared to controls, systemic treatment with 10⁶ ASCs on D0 and a second dose on D7 significantly prevented tracheal obliteration 28 days after heterotopic tracheal transplantation (median of 94 vs. 16%; P < 0.01). A single dose tended towards less stenosis than controls, but did not reach statistical significance (28 vs. 94%; P = 0.054). On the contrary, repeated local injection was incapable of preventing tracheal obliteration when compared to a single injection or controls (37 vs. 71 vs. 87%). Two intravenous doses also tended to be better than two local injections (16 vs. 37%; P = 0.058), and were better than a single local dose (16 vs. 71%; P < 0.01).

CONCLUSION

A second dose of ASC, given systemically after 7 days, reduces luminal obliteration in a heterotopic tracheal transplantation model in mice, suggesting that ASC can be used to prevent obliterative bronchiolitis after lung transplantation.

The emerging role of ECM crosslinking in T cell mobility as a hallmark of immunosenescence in humans

Immunosenescence is thought to result from cellular aging and to reflect exposure to environmental stressors and antigens, including cytomegalovirus (CMV). However, not all of the features of immunosenescence are consistent with this view, and this has led to the emergence of the sister theory of "inflammaging". The recently discovered diffuse tissue distribution of resident memory T cells (T_RM) which don't recirculate, calls these theories into question. These cells account for most T cells residing in barrier epithelia which sit in and travel through the extracellular matrix (ECM). With almost all studies to date carried out on peripheral blood, the age-related changes of the ECM and their consequences for T cell mobility, which is crucial for the function of these cells, have been largely ignored. We propose an update of the theoretical framework of immunosenescence, based on a novel hypothesis: the increasing stiffness and cross-linking of the senescent ECM lead to a progressive immunodeficiency due to an age-related decrease in T cell mobility and eventually the death of these cells. A key element of this mechanism is the mechanical stress to which the cell cytoplasm and nucleus are subjected during passage through the ECM. This hypothesis is based on an "evo-devo" perspective bringing together some major characteristics of aging, to create a single interpretive framework for immunosenescence.