Regulatory B (Breg) cells are immunosuppressive cells that support immunological tolerance. Through the production of interleukin-10 (IL-10), IL-35, and transforming growth factor β (TGF-β), Breg cells suppress immunopathology by prohibiting the expansion of pathogenic T cells and other pro-inflammatory lymphocytes. In clinical transplantation, Bregs have been linked to operational tolerance, a state in which recipients maintain stable graft function in absence of immunosuppression. In this study, the authors simultaneously quantify cytokine production and immunophenotype in human peripheral B cells across a range of stimulatory conditions and time points using mass cytometry. The analysis shows that multiple functional B cell subsets produce IL-10 and that no phenotype uniquely identifies IL-10+ B cells. Critically, it was observed that a significant portion of IL-10+ B cells co-express the pro-inflammatory cytokines IL-6 and tumor necrosis factor alpha (TNFα). However, operationally tolerant liver transplant recipients have a unique enrichment of IL-10+, but not TNFα+ or IL-6+, B cells compared with transplant recipients receiving immunosuppression. Thus, human IL-10-producing B cells constitute an induced, transient state arising from a diversity of B cell subsets that may contribute to maintenance of immune homeostasis. It is critical to note, that in the clinical cohort, transplant controls were on immunosuppression, while graft-tolerant subjects were not, which is an unavoidable limitation in these studies due to standard of care. Likely, the absence of immunosuppression could contribute to some differences in B cell phenotypes and frequencies. Critically, while the results from this analysis suggests CD9 expression and IL-10:TNFα ratios of B cells could serve as biomarkers for allograft tolerance, it remains to be determined if these features are causative or merely correlated.

Kidney fibrosis is the hallmark of end stage kidney disease. The critical improvements observed in short term outcomes in kidney transplantation have not been mirrored by long-tern outcomes. Understanding the mechanisms and cell-specific injury pathways that associate with fibrosis development is critical to prolong the longevity of the graft. Comprehensive genome-wide human kidney tissue transcriptomics identified changes in the expression of many genes in fibrosis. Differentially expressed genes (DEGs) were enriched for metabolism and proximal tubule cell changes, indicating tubule cell dysfunction and loss in fibrosis. As might be expected, inflammation and immune cell response signatures represented the second-largest gene cluster associated with kidney fibrosis. However, a critical knowledge gap remains as the immune cells associated with kidney fibrosis have not been systematically characterized and the trigger for immune cell influx is poorly understood. Consequently, the immune cell interactions that contribute to the initiation and development of fibrotic processes in fibrosis development are not well understood. This is a critical issue in kidney transplantation, where the kidney graft is under continuous alloimmune injury. In this study, using single-cell sequencing to a mouse model of kidney fibrosis, the authors identified a subset of kidney tubule cells with a profibrotic-inflammatory phenotype characterized by the expression of cytokines and chemokines associated with immune cell recruitment. Receptor–ligand interaction analysis and experimental validation indicate that CXCL1 secreted by profibrotic tubules recruits CXCR2+ basophils. In mice, these basophils are an important source of interleukin-6 and recruitment of the TH17 subset of helper T cells. Genetic deletion or antibody-based depletion of basophils results in reduced renal fibrosis. Human kidney single-cell, bulk gene expression and immunostaining validate a function for basophils in patients with kidney fibrosis. The unexpected and important contribution of basophils to renal fibrosis opens new potential therapeutic avenues for the management of kidney fibrosis. Although these studies were done using chronic kidney disease models, they are highly relevant to the evaluation of fibrosis in the kidney graft.

Acute kidney injury is a critical clinical condition associated with a high degree of morbidity and mortality despite the best supportive care in the absence of effective and/or causal treatment. Ischemia-reperfusion injury is one of the main causes of acute kidney injury in renal transplantation. In this manuscript, using a mouse model, the authors found that IL24 is upregulated in the kidney after renal ischemia-reperfusion injury and that tubular epithelial cells and infiltrating inflammatory cells are the source of IL24. Mice lacking IL24 are protected from renal injury and inflammation. Cell culture studies were also used and showed that IL24 induces apoptosis in renal tubular epithelial cells, which is accompanied by an increased endoplasmatic reticulum stress response. Moreover, IL24 induces robust expression of endogenous IL24 in tubular cells, fostering ER-stress and apoptosis. In kidney transplant recipients with delayed graft function and patients at high risk to develop acute kidney injury after cardiac surgery IL24 is upregulated in the kidney and serum. In summary, the study shows for the first time that IL24 is not only a biomarker for acute kidney injury but plays an important mechanistic role involving both extracellular and intracellular targets and pathways. The authors propose that IL24 is a promising new therapeutic target in patients at risk of or with ischemia-induced acute kidney injury

Antibody-mediated lymphoablation is a commonly used therapy in solid organ and stem cell transplantation and autoimmunity. Its efficacy is determined by the balance between the removal of pathogenic lymphocytes and preserving the individual's ability to respond to opportunistic infections. In transplant recipients, T cell reconstitution following lymphoablation is the sum of potentially detrimental homeostatic proliferation of depletion-resistant memory T cells and beneficial thymopoiesis that replenishes non-alloreactive T cells necessary for host protection. Diminishing the extent of homeostatic T cell proliferation is, therefore, essential for improving the efficacy of clinical lymphoablation. The goal of the current study was to identify mediators of B cell activation following lymphoablation in allograft recipients. Transcriptome analysis revealed that macrophage-inducible C-type lectin (Mincle, Clec4e) expression is up-regulated in B cells from heart allograft recipients treated with murine anti-thymocyte globulin (mATG). Recipient Mincle deficiency diminishes B cell production of pro-inflammatory cytokines and impairs T lymphocyte reconstitution. Mixed bone marrow chimeras lacking Mincle only in B lymphocytes have similar defects in T cell recovery. Conversely, treatment with a synthetic Mincle ligand enhances T cell reconstitution after lymphoablation in non-transplanted mice. Treatment with agonistic CD40 mAb facilitates T cell reconstitution in CD4 T cell-depleted, but not in Mincle-deficient, recipients indicating that CD40 signaling induces T cell proliferation via a Mincle-dependent pathway. In summary, this study indicates that following lymphoablation in the settings of transplantation, Mincle is required for recipient B cells to initiate production of proinflammatory cytokines and thus facilitate homeostatic T cell reconstitution. These findings identify Mincle and its endogenous ligands as potential therapeutic targets for minimizing the effects of posttransplant inflammation and improving the efficacy of lymphoablation in transplant recipients.