Outcomes after lung transplantation are markedly inferior to those after other solid organ transplants. A better understanding of cellular and molecular mechanisms contributing to lung graft injury will be critical to improve outcomes. Advances in this field have been hampered by the lack of a mouse model of lung transplantation. Here, we report a mouse model of vascularized aerated single lung transplantation utilizing cuff techniques. We show that syngeneic grafts have normal histological appearance with minimal infiltration of T lymphocytes. Allogeneic grafts show acute cellular rejection with infiltration of T lymphocytes and recipient-type antigen presenting cells. Our data show that we have developed a physiological model of lung transplantation in the mouse, which provides ample opportunity for the study of nonimmune and immune mechanisms that contribute to lung allograft injury.

Background. The novel coronavirus severe acute respiratory syndrome coronavirus 2 [coronavirus disease 2019 (COVID-19)] poses unique challenges for immunosuppressed patients. Solid organ transplant (SOT) recipients comprise a large proportion of this group, yet there is limited knowledge about the presentation, clinical course, and immunosuppression management of this novel infection among heart, lung, liver, pancreas, and kidney transplant recipients. Methods. We present 21 SOT recipients diagnosed with COVID-19 between January 1, 2020 and April 22, 2020 at a US high-volume transplant center. Diagnostic workup, clinical course, immunosuppression/antiviral management, and immediate outcomes are described. Results. Twenty-one (15.9%) of 132 symptomatic patients tested were positive. Mean age at diagnosis was 54.8 ± 10.9 y. Median time from transplant was 5.58 y (interquartile range 2.25, 7.33). Median follow-up was 18 d (interquartile range 13, 30). Fourteen patients required inpatient management, with 7 (50%) placed in the intensive care unit (ICU). All transplant types were represented. Nearly 43% exhibited GI symptoms. Over half (56.2%) presented with elevated serum creatinine suggestive of acute kidney injury. The majority of patients (5/7) with concomitant infections at baseline required the ICU. Eighty percent received hydroxychloroquine ± azithromycin. Ten received toclizumab and/or ribavirin; 1 received remdesivir. Antimetabolites ± calcineurin inhibitors were held or reduced. Over half of hospitalized patients (8/14) were discharged home. Only 1 mortality (4.8%) to date, in a critically ill heart/kidney patient who had been in the ICU before diagnosis. Conclusions. COVID-19 positive SOT at our institution had favorable short-term outcomes. Those with concomitant infections had more severe illness. More data will be available to evaluate long-term outcomes and disease impact on graft function.

The mechanisms by which innate immune signals regulate alloimmune responses remain poorly understood. In the present study, we show by intravital 2-photon microscopy direct interactions between graft-infiltrating neutrophils and donor

Primary graft dysfunction (PGD) is a common early complication after lung transplantation. We conducted a retrospective cohort study of 334 recipients to evaluate the impact of PGD graded at 24, 48 and 72 h on the risk of bronchiolitis obliterans syndrome (BOS) development (stage 1) and progression (stages 2 and 3). We constructed multivariable Cox proportional hazards models to determine the risk of BOS attributable to PGD in the context of other potential risk factors including acute rejection, lymphocytic bronchitis and respiratory viral infections. All grades of PGD at all time points were significant risk factors for BOS development and progression independent of acute rejection, lymphocytic bronchitis and respiratory viral infections. Specifically, PGD grade 1 at T24 was associated with a relative risk of BOS stage 1 of 1.93, grade 2 with a relative risk of 2.29 and grade 3 with a relative risk of 3.31. Furthermore, this direct relationship between the severity of PGD and the risk of BOS persisted at all time points. We conclude that all grades of PGD at all time points are independent risk factors for BOS development and progression. Future strategies that might attenuate the severity of PGD may mitigate the risk of BOS.

Rationale: Although innate immunity is increasingly recognized to contribute to lung allograft rejection, the significance of endogenous innate ligands, such as hyaluronan (HA) fragments, in clinical or experimental lung transplantation is uncertain.Objectives: To determine if HA is associated with clinical bronchiolitis obliterans syndrome (BOS) in lung transplant recipients, and evaluate the effect of low-or high-molecular-weight HA on experimental lung allograft rejection, including dependence on innate signaling pathways or effector cells.Methods: HA concentrations were measured in bronchoalveolar lavage and plasma samples from lung recipients with or without established BOS. BOS and normal lung tissues were assessed for HA localization and expression of HA synthases. Murine orthotopic lung recipients with established tolerance were treated with low-or highmolecular-weight HA under varied experimental conditions, including Toll-like receptor (TLR) 2/4 and myeloid differentiation protein 88 deficiency and neutrophil depletion.Measurements and Main Results: HA localized within areas of intraluminal small airways fibrosis in BOS lung tissue. Moreover, transcripts for HA synthase enzymes were significantly elevated in BOS versus normal lung tissues and both lavage fluid and plasma HA concentrations were increased in recipients with BOS. Treatment with low-molecular-weight HA abrogated tolerance in murine orthotopic lung recipients in a TLR2/4-and myeloid differentiation protein 88-dependent fashion and drove expansion of alloantigen-specific T lymphocytes. Additionally, TLRdependent signals stimulated neutrophilia that promoted rejection. In contrast, high-molecular-weight HA attenuated basal allograft inflammation.Conclusions: These data suggest that accumulation of HA could contribute to BOS by directly activating innate immune signaling pathways that promote allograft rejection and neutrophilia.

Background Previous studies have suggested that azithromycin improves lung function in lung transplant recipients with bronchiolitis obliterans syndrome (BOS). However, these studies did not include a non-treated BOS control cohort or perform survival analysis. This study was undertaken to estimate the effect of azithromycin treatment on survival in lung transplant recipients with BOS. Methods We conducted a retrospective cohort study of consecutive lung transplant recipients who developed BOS between 1999 and 2007. An association between azithromycin treatment and death was assessed using univariate and multivariable time-dependent Cox regression analysis. Results Of the 178 recipients that developed BOS in our study, 78 developed BOS after 2003 and were treated with azithromycin. The azithromycin treated and untreated cohorts had similar baseline characteristics. Univariate analysis demonstrated that azithromycin treatment was associated with a survival advantage and this beneficial treatment effect was more pronounced when treatment was initiated during BOS stage 1. Multivariable analysis demonstrated azithromycin treatment during BOS stage 1 (adjusted hazard ratio=0.23, p=0.01) and absolute FEV1 value at the time of BOS stage 1 (adjusted hazard ratio=0.52, p=0.003) were both associated with a decreased risk for death. Conclusion In lung transplant recipients with BOS stage 1, azithromycin treatment initiated before BOS stage 2 was independently associated with a significant reduction in the risk of death. This finding supports the need for a randomized controlled trial to confirm the impact of azithromycin on survival in lung transplant recipients.

Comparisons between groups were conducted using the chi-square or Fisher exact test as appropriate. COVID-19, coronavirus disease 2019; IgG, immunoglobulin; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Graft rejection remains a formidable problem contributing to poor outcomes after lung transplantation. Blocking chemokine pathways have yielded promising results in some organ transplant systems. Previous clinical studies have demonstrated upregulation of CCR2 ligands following lung transplantation. Moreover, lung injury is attenuated in CCR2-deficient mice in several inflammatory models. In this study, we examined the role of CCR2 in monocyte recruitment and alloimmune responses in a mouse model of vascularized orthotopic lung transplantation. The CCR2 ligand MCP-1 is upregulated in serum and allografts following lung transplantation. CCR2 is critical for the mobilization of monocytes from the bone marrow into the bloodstream and for the accumulation of CD11c+ cells within lung allografts. A portion of graft-infiltrating recipient CD11c+ cells expresses both recipient and donor MHC molecules. Two-photon imaging demonstrates that recipient CD11c+ cells are associated with recipient T cells within the graft. While recipient CCR2 deficiency does not prevent acute lung rejection and is associated with increased graft infiltration by T cells, it significantly reduces CD4+ Th1 indirect and direct allorecognition. Thus, CCR2 may be a potential target to attenuate alloimmune responses after lung transplantation.