Rejection of the Renal Allograft in the Absence of Demonstrable Antibody and Complement

Inverted direct allorecognition triggers early donor-specific antibody responses after transplantation

The generation of antibodies against donor-specific major histocompatibility complex (MHC) antigens, a type of donor-specific antibodies (DSAs), after transplantation requires that recipient's allospecific B cells receive help from T cells. The current dogma holds that this help is exclusively provided by the recipient's CD4+ T cells that recognize complexes of recipient's MHC II molecules and peptides derived from donor-specific MHC alloantigens, a process called indirect allorecognition. Here, we demonstrated that, after allogeneic heart transplantation, CD3ε knockout recipient mice lacking T cells generate a rapid, transient wave of switched alloantibodies, predominantly directed against MHC I molecules. This is due to the presence of donor CD4+ T cells within the graft that recognize intact recipient's MHC II molecules expressed by B cell receptor-activated allospecific B cells. Indirect evidence suggests that this inverted direct pathway is also operant in patients after transplantation. Resident memory donor CD4+ T cells were observed in perfusion liquids of human renal and lung grafts and acquired B cell helper functions upon in vitro stimulation. Furthermore, T follicular helper cells, specialized in helping B cells, were abundant in mucosa-associated lymphoid tissue of lung and intestinal grafts. In the latter, more graft-derived passenger T cells correlated with the detection of donor T cells in recipient's circulation; this, in turn, was associated with an early transient anti-MHC I DSA response and worse transplantation outcomes. We conclude that this inverted direct allorecognition is a possible explanation for the early transient anti-MHC DSA responses frequently observed after lung or intestinal transplantations.

Decreased Expression of MicroRNA-107 in B Lymphocytes of Patients with Antibody-Mediated Renal Allograft Rejection

MicroRNAs (miRNAs) are small noncoding RNA molecules that participate in normal B cell lineage development through posttranscriptional gene regulation. Antibody-mediated renal allograft rejection (ABMR) is emerging as one of the most common serious threats to renal transplant patients. In this study, we explored the role of miRNAs in the pathogenesis of ABMR. The differentially expressed miRNAs were identified by Affymetrix miRNA microarray analysis using B lymphocytes from 5 recipients and 5 volunteers. Based on quantitative RT-PCR, the expression levels of miR-107 were lower in the B lymphocytes from recipients than in those from volunteers. Computational analysis predicted that 3'-untranslated region of the autophagy-related protein 12 (ATG12) mRNA was targeted by miR-107, and we identified ATG12 as a target of miR-107 by Luciferase assay. Importantly, the expression levels of ATG12 in B lymphocytes of recipients were higher than those in the volunteer group, and miR-107 mimic significantly decreased ATG12 expression and formation of autolysosomes in B lymphocytes of recipients. Furthermore, we observed that levels of autophagy in B lymphocytes of transplant recipients were higher than those in B cells from volunteers. These findings suggest that miR-107 may contribute to the regulation of autophagy via targeting ATG12. Lastly, treatment with an miR-107 mimic caused the decrease in the secretion of IgG and IgM antibodies from B lymphocytes of transplant recipients, indicating that deregulated miR-107 could be involved in the pathogenesis of ABMR. Taken together, we propose that decreased miR-107 expression is associated with autophagy activation in B lymphocytes from patients with ABMR.

Transplant glomerulopathy

In the renal allograft, transplant glomerulopathy represents a morphologic lesion and not a specific diagnosis. The hallmark pathologic feature is glomerular basement membrane reduplication by light microscopy or electron microscopy in the absence of immune complex deposits. Transplant glomerulopathy results from chronic, recurring endothelial cell injury that can be mediated by HLA alloantibodies (donor-specific antibodies), various autoantibodies, cell-mediated immune injury, thrombotic microangiopathy, or chronic hepatitis C. Clinically, transplant glomerulopathy may be silent, detectable on protocol biopsy, or present with overt manifestations, including up to nephrotic range proteinuria, hypertension, and declining glomerular filtration rate. In either case, transplant glomerulopathy is associated with reduced graft survival. This review details the morphologic features of transplant glomerulopathy found on light microscopy, immunofluorescence microscopy, and electron microscopy. The pathophysiology of the causes and risk factors are discussed. Clinical manifestations are emphasized and potential therapeutic modalities are examined.