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.

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.