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Cardiac Allograft Vasculopathy (CAV) is a leading contributor to late transplant rejection. Although implicated, the mechanisms by which bone marrow-derived cells promote CAV remain unclear. Emerging evidence implicates the cell surface receptor tyrosine kinase AXL to be elevated in rejecting human allografts. AXL protein is found on multiple cell types, including bone marrow-derived myeloid cells. The causal role of AXL from this compartment and during transplant is largely unknown. This is important because AXL is a key regulator of myeloid inflammation. Utilizing experimental chimeras deficient in the bone marrow-derived Axl gene, we report that Axl antagonizes cardiac allograft survival and promotes CAV. Flow cytometric and histologic analyses of Axl-deficient transplant recipients revealed reductions in both allograft immune cell accumulation and vascular intimal thickness. Co-culture experiments designed to identify cell-intrinsic functions of Axl uncovered complementary cell-proliferative pathways by which Axl promotes CAV-associated inflammation. Specifically, Axl-deficient myeloid cells were less efficient at increasing the replication of both antigen-specific T cells and vascular smooth muscle cells (VSMCs), the latter a key hallmark of CAV. For the latter, we discovered that Axl-was required to amass the VSMC mitogen Platelet-Derived Growth Factor. Taken together, our studies reveal a new role for myeloid Axl in the progression of CAV and mitogenic crosstalk. Inhibition of AXL-protein, in combination with current standards of care, is a candidate strategy to prolong cardiac allograft survival., (Copyright © 2021 International Society for Heart and Lung Transplantation. Published by Elsevier Inc. All rights reserved.)

Background: Serological severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibody assays differ in the target antigen specificity, e.g. of antibodies directed against the viral spike or the nucleocapsid protein, and in the spectrum of detected immunoglobulins. The aim of the study was to evaluate the performance of two different routinely used immunoassays in hospitalized and outpatient COVID-19 cases., Methods: The test characteristics of commercially available spike1 protein-based serological assays (Euroimmun, EI-assays), determining IgA or IgG and nucleocapsid-based assays (Virotech, VT-assays) determining IgA, IgM or IgG were compared in 139 controls and 116 hospitalized and outpatient COVID-19 cases., Results: Hospitalized COVID-19 patients (n = 51; 115 samples) showed significantly higher concentrations of antibodies against SARS-CoV-2 and differed from outpatient cases (n = 65) by higher age, higher disease severity scores and earlier follow up blood sampling. Sensitivity of the two IgG assays was comparable in hospitalized patients tested ≥ 14 days (EI-assay: 88%, CI 95% 67.6-99.9; VT-assay: 96%, CI 95% 77.7-99.8). In outpatient COVID-19 cases sensitivity was significantly lower in the VT-assay (86.2%, CI 95% 74.8-93.1) compared with the EI-assay (98.5%, CI 95% 90.6-99.9). Assays for IgA and IgM demonstrated a lack of specificity or sensitivity., Conclusions: Our results indicate that SARS-CoV-2 serological assays may need to be optimized to produce reliable results in outpatient COVID-19 cases who are low or even asymptomatic. Assays for IgA and IgM have limited diagnostic performance and do not prove an additional value for population-based screening approaches., (Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.)

Recipient infusion of donor apoptotic cells is an emerging strategy for inducing robust transplantation tolerance. Daily clearance of billions of self-apoptotic cells relies on homeostatic engagement of phagocytic receptors, in particular, receptors of the tyrosine kinase family TAM (Tyro3, Axl, and MerTK), to maintain self-tolerance. However, an outstanding question is if allogeneic apoptotic cells trigger the same receptor system for inducing allogeneic tolerance. Here, we employed allogeneic apoptotic splenocytes and discovered that the efferocytic receptor MerTK on recipient phagocytes is a critical mediator for transplantation tolerance induced by this strategy. Our findings indicate that the tolerogenic properties of allogeneic apoptotic splenocytes require MerTK transmission of intracellular signaling to suppress the production of inflammatory cytokine interferon α (IFN-α). We further demonstrate that MerTK is crucial for subsequent expansion of myeloid-derived suppressor cells and for promoting their immunomodulatory function, including maintaining graft-infiltrating CD4 + CD25 + Foxp3 + regulatory T cells. Consequently, recipient MerTK deficiency resulted in failure of tolerance by donor apoptotic cells, and this failure could be effectively rescued by IFN-α receptor blockade. These findings underscore the importance of the efferocytic receptor MerTK in mediating transplantation tolerance by donor apoptotic cells and implicate MerTK agonism as a promising target for promoting transplantation tolerance., (© 2018 The American Society of Transplantation and the American Society of Transplant Surgeons.)