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C4d, a split product of C4 activation in classical and lectin pathways of the complement system activation, has been regarded as a footprint of tissue damage in antibody-mediated rejection in transplantology. The introduction of C4d staining into daily clinical practice aroused an ever-increasing interest in the role of antibody-mediated mechanisms in kidney allograft rejection. However, this marker of complement activation is also important in other various kidney glomerular pathologies such as immunoglobulin A nephropathy, membranoproliferative glomerulonephritis, lupus nephritis, and others. In routine histopathological practice, C4d staining can be done by two his tological methods, specifically by immunofluorescence on frozen tissue using monoclonal antibody to C4d (with the downside of unsteady availability of frozen tissue) or by immunohistochemistry using C4d antibodies on formalin-fixed and paraffin-embedded renal tissue. The aim of this narrative review is to summarize recent knowled ge about the complement fragment C4d and its significance in differe nt kidney pathologies, focusing on its immunohistochemical detection in renal tissue biopsies. We have supplemented this review with our experience with our proprietary methodology of preparation and practical use of antibodies such as anti-C4d, on a small national level. Immunohistochemical staining for C4d has revolutionized the field of renal histopathology. Despite being a simple diagnostic test, its utility can be of utmost importance, especially in a resource-poor setting where immunofluorescence and frozen tissue may not be available [ABSTRACT FROM AUTHOR]

Background: Glomerular deposition of C4d is a widely used biomarker for activation of the lectin pathway in the complement system and is reported to be associated with kidney progression in immunoglobulin A nephropathy (IgAN). The aim of this study was to evaluate whether glomerular C4d deposition, as a new biomarker, improves the prediction of kidney prognosis in IgAN. Study Design: Systematic review and meta-analysis. Setting & Population: Patients with biopsy-proven primary IgAN without age limitations.Selection Criteria for Studies: Cross-sectional or cohort studies reporting the prevalence of glomerular C4d deposition or evaluating its association with IgAN progression. Predictor: Glomerular C4d deposition. Outcome: Composite progression event of a >30% decline in estimated glomerular filtration rate or end-stage kidney disease. Results: 12 studies with 1,251 patients were included. The prevalence of glomerular C4d deposition was 34% (95% CI, 27%-41%), with large heterogeneity (I2 = 86%; P

Abstract Background To evaluate renal expression of C4d, a complement component in the classical/mannose binding lectin (MBL) pathway, in patients with primary Sjögren’s syndrome (pSS)-associated renal impairments. Methods We retrospectively reviewed the clinical and pathological data from 39 patients with pSS presenting with renal impairments. C4d was examined in paraffin-embedded biopsy tissues using immunohistochemistry. Glomerular C4d positive was defined when > 75% glomeruli were globally stained. Tubulointerstitial C4d (TI-C4d) were scored semi-quantitatively as 0 (absent), 1 (spotty or weak), 2 (patchy) and 3 (diffuse). A TI-C4d score ≥ 2 was considered TI-C4d positive and included in the TI-C4d+ group and vice versa. Peritubular capillary (PTC) C4d was scored as 0 (absent), 1 (0~10%, minimal), 2 (10%~ 50%, focal), and 3 (> 50%, diffuse). Results Glomerular C4d deposition was observed in all 8 patients with pSS-related membranous nephropathy (MN) without obvious C1q deposition. Two of 5 patients with mesangial proliferative glomerulonephritis and 1 of 2 patients with IgA nephropathy had mild mesangial C4d deposition. Sixteen patients (6 glomerular dominant and 10 tubulointerstitial dominant) presented TI-C4d score ≥ 2. Patients in the TI-C4d+ group exhibited a higher serum creatinine level at the time of renal biopsy (TI-C4d+ 132.5 [89.7, 165.5] vs. TI-C4d− 83.0 [70.7, 102.0] μmol/L, P = 0.008). PTC C4d was observed in 12 patients, with each of minimal, focal and diffuse staining being noted in 4 patients. Conclusions The MBL pathway of complement activation was potentially involved in pSS-related MN. Tubulointerstitial C4d might be a pathological marker of severe renal injury in patients with pSS-related renal impairments.

HSCT-associated thrombotic microangiopathy ( TA- TMA) is a severe complication with a poor prognosis. Recently, it has been reported that complement system dysregulation, such as CFH autoantibodies and deletions CFH-related genes 3 and 1, induced TA- TMA. In addition, C4d-positive renal arterioles are both a good marker of complement system activation and a useful diagnostic tool for TA- TMA. Because dysregulation of the complement system is associated with TA- TMA, the complement system might be a therapeutic target, such as eculizumab, a terminal complement inhibitor. Herein, we describe an eight-yr-old boy who developed TA- TMA accompanied by severe renal dysfunction. His renal specimen showed diffuse C4d deposition in the renal arterioles, which is consistent with TA- TMA. Although the patient gradually improved without eculizumab, renal arteriolar C4d staining in sample with TA- TMA shows the complement system activation and may guide the target therapy using the eculizumab. [ABSTRACT FROM AUTHOR]

Background: Antibody-mediated rejection (AMR) after lung transplantation remains enigmatic, and there is no consensus on the characteristic clinical, immunologic and histologic features. Methods: We performed a retrospective, single-center cohort study and identified cases of acute AMR based on the presence of circulating donor-specific human leukocyte antigen (HLA) antibodies (DSA), histologic evidence of acute lung injury, C4d deposition and clinical allograft dysfunction. Results: We identified 21 recipients with acute AMR based on the aforementioned criteria. AMR occurred a median 258 days after transplantation; 7 recipients developed AMR within 45 days of transplantation. All patients had clinical allograft dysfunction, DSA, histology of acute lung injury and capillary endothelial C4d deposition. Fifteen recipients improved clinically and survived to hospital discharge, but 6 died of refractory AMR. One survivor had bronchiolitis obliterans syndrome at the time of AMR diagnosis; 13 of the 14 remaining survivors developed chronic lung allograft dysfunction (CLAD) during follow-up. Overall, 15 recipients died during the study period, and the median survival after the diagnosis of AMR was 593 days. Conclusions: Acute AMR can be a fulminant form of lung rejection, and survivors are at increased risk of developing CLAD. The constellation of acute lung injury, DSA and capillary endothelial C4d deposition is compelling for acute AMR in recipients with allograft dysfunction. This clinicopathologic definition requires validation in a multicenter cohort, but may serve as a foundation for future studies to further characterize AMR. [ABSTRACT FROM AUTHOR]

Aim: Association between C4d deposition and renal allograft survival is still uncertain. We retrospectively evaluated the clinical outcome of C4d deposition in allograft renal biopsies. Methods: One hundred and fifty biopsies from 150 patients with a histological diagnosis of acute rejection from December 1997 to March 2007 were included. Paraffin-embedded sections were stained with a polyclonal antibody using an immunoperoxidase technique. Detailed clinical data were obtained by retrospective review. Results: C4d was stained positively in 74 (49.3%) of 150 cases: 47 (61.5%) biopsies showed diffuse C4d deposition and 27 (38.5%) showed focal C4d deposition. During follow up, significantly more C4d-positive patients (24/74 patients, 32.4%) lost their grafts, compared with the C4d-negative group (10/76 patients, 13.2%) ( P = 0.005). After a Kaplan–Meier analysis, grafts from the C4d-negative group had a markedly higher survival as compared with the C4d-positive group ( P = 0.003, log–rank test). Graft survival among C4d-negative, C4d diffuse-positive, and C4d focal-positive groups was significantly different ( P = 0.007, log–rank test). The graft survival rate among C4d-negative patients in early (<6 months) and later biopsies (>6 months), and C4d-positive patients in early and in later biopsies was different ( P = 0.028, log–rank test). The adjusted risk ratio of graft failure after Cox proportional hazards multivariate analyses for C4d-positive patients was 3.309 (95% confidence interval, 1.413–6.537; P = 0.004). Conclusion: Patients with C4d deposition had an inferior graft survival, especially with diffused C4d deposition, and later experienced acute rejection. C4d deposition was an independent risk factor for graft survival. [ABSTRACT FROM AUTHOR]

Suppression of antibody-mediated rejection (AMR) is mandatory for the acceptance of renal allograft in ABO blood type incompatible and pre-sensitized combinations. The aim of this study was to evaluate the difference in histopathology of AMR between ABO incompatible (ABOI) and pre-sensitized cases. Among 69 kidney recipients who underwent transplant surgery at our institute since 2002, four patients who manifested AMR were included in this study. They initially received quadrant immunosuppressants, tacrolimus, mycophenolate mofetil, methylprednisolone and basiliximab. Two patients received grafts from ABOI donors and the other two received grafts from flow T-cell crossmatch-positive donors. Although satisfying antibody removal was achieved by pre-transplant plasmapheresis, all four cases manifested acute AMR, within two wk post-transplant. Antibody titer and panel reactive antibody increased at the time of AMR. ABOI cases showed slight cellular infiltration. These cases showed diffuse, strong and linear deposition of C4d at peritubular capillaries (PTC). On the other hand, pre-sensitized cases showed more intense cellular infiltration, especially in glomerulus but only faint and focal deposition of C4d at PTC. All four cases were treated with corticosteroid pulse therapy in conjunction with several sessions of plasmapheresis. [ABSTRACT FROM AUTHOR]

Aims: To determine whether C4d immunopositivity helps recognition of humoral rejection in dysfunctional liver allografts. Methods and results: C4d immunopositivity was retrospectively evaluated in liver allografts. There were three staining patterns: portal venular plexus, sinusoidal and hepatocellular. The latter was related to ischaemic necrosis and not scored as positive. C4d immunopositivity was not encountered in 10 preperfusion or 15 consecutive early protocol biopsies. However, three of 12 early protocol biopsy specimens from crossmatch-positive patients were C4d+, two showing repeated positivity on at least one further biopsy specimen, while others remained negative. C4d was also positive in 2/16 early moderate acute cellular rejections, 3/14 cases of centrilobular necroinflammation, 3/11 biliary obstructions, 3/13 chronic rejections and 1/10 primary non-functional allografts. Conclusion: C4d immunopositivity is uncommon in liver allografts. There is a weak positive correlation with a positive lymphocytotoxic crossmatch and some patterns of allograft dysfunction. The morphological associations resemble those reported in lymphocytotoxic crossmatch-positive patients, plus occasional sinusoidal and hepatocellular injury. Although the practical utility of C4d immunohistochemistry seems limited, it may identify a small subgroup of individuals in whom chronic humoral microvascular injury contributes to allograft dysfunction. [ABSTRACT FROM AUTHOR]

Abstract: The role of humoral immunity in causing antibody-mediated rejection (AMR) of organ allografts has been extensively documented. For this reason, negative complement-dependent cytotoxicity (CDC) cross-matches between recipient sera and donor T and B lymphocytes have become a mandatory requirement for cadaveric kidney transplantation. However, the significance of donor-specific antibodies (DSAs) detectable only by flow cytometry (FC) or solid phase assays (SPA) but not CDC is still controversial. We have performed a retrospective analysis of FC cross-matching results in 80 consecutive cadaver kidney allograft recipients. Antibodies against HLA class I and class II antigens were measured by CDC and SPA in sequential samples of sera obtained prior to transplantation. The preoperative cross-match was performed by CDC using magnetically sorted T and B cells from donor spleen. Sera obtained from each patient before and at the time of transplantation were included in the final cross-match. The sample of serum obtained at the time of transplantation was cross-matched retrospectively by FC and analyzed for anti-HLA antibody specificity on high resolution SPA. The actuarial kidney allograft survival at one year was 98%. Two of these eighty patients lost the graft, one due to AMR, the other for reasons unrelated to DSAs. Donor-specific antibodies were detected by FC in 17 of 80 patients, yet only 6 of 17 had an early episode of AMR. This episode was successfully reversed by desensitization therapy using intravenous immunoglobin (IVIG) and plasmapheresis. Flow cytomery cross-matching showed 95% specificity but only 35% sensitivity for prediction of AMR (p = 0.002). There was a significant correlation between high panel reactive antibodies (PRA) and positive FC cross-matching (p = 0 .0001), as well as high PRA and AMR (p = 0.0004 by CDC and 0.0011 by Luminex). Reversible AMR occurred 12–30 days post-transplantation in 8 patients. Of these 8 patients, 3 had no detectable DSAs in spite of C4d positivity, 4 had C4d deposition in conjunction with anti-HLA antibodies, and 1 patient had DSAs (anti-MICA) yet no C4d deposition. We conclude that early initiation of desensitization protocols can prevent transplant failure and that retrospective FC cross-matches may facilitate the diagnosis of AMR. Extensive analysis of patients’ sera using a comprehensive set of tests may contribute to early treatment and better understanding of the mechanism underlying humoral rejection. [Copyright &y& Elsevier]

Abstract: The pathogenesis of antibody-mediated rejection has been investigated, but the precise mechanism of chronic glomerular rejection remains unclear. We have followed the clinicopathological course of a patient with pre-existing anti-donor antibody only detected by flow-cytometry crossmatch for over 3 years. Glomerular endothelial injuries and peculiar glomerular lesions were noted in biopsy specimen of postoperative year 3; however, both typical chronic vascular rejection lesions and peritubular capillary multilayered lesions were not revealed. We consider that the presence of weak anti-donor antibody leading early onset of acute humoral rejection played a role in the pathogenesis of early onset of chronic transplant glomerulopathy. [ABSTRACT FROM AUTHOR]

Abstract: Background: Alloantibodies and C4d deposition in peritubular capillaries (PTCs) are thought to be related to antibody-mediated acute rejection. The purpose of this study was to evaluate the relationship between C4d deposition in PTCs and alloantibodies at various days after allograft dysfunction due to severe acute rejection. Method: There were 620 renal transplantations (Tx) performed. Forty patients diagnosed with acute humoral and/or vascular rejection showed graft dysfunction with anuria or dysuria. The patients were divided into four groups by ABO compatibility and clinical course after graft dysfunction: compatible recipients with graft loss (c-GL; n = 6); compatible recipients with recovery from graft dysfunction (c-RE; n = 10); incompatible recipients with graft loss (i-GL; n = 9); and incompatible recipients with recovery from graft dysfunction (i-RE; n = 15). Results: C4d depositions in 4/6 c-GL recipients increased, and those in 8/10 c-RE recipients decreased after graft dysfunction. These changes in C4d deposition between the c-GL and the c-RE groups were significantly different (P < 0.01). These titres of anti-A/B IgG antibody in 7/9 i-GL recipients increased and those in 8/15 i-RE recipients decreased after graft dysfunction. These changes in titre between the i-GL and the i-RE groups were significantly different (P < 0.01). All c-GL recipients and 4/10 c-RE recipients had anti-HLA antibody at the last biopsy. There was a significant difference in the number of recipients who had anti-HLA antibody between the c-GL and the c-RE groups (P < 0.05). Conclusions: These results indicate that changes in C4d deposition in PTCs in the c-ABO group and titre of anti-A/B IgG antibody in the ABO-incompatible groups exert a strong impact on graft survival after dysfunction in the early period after Tx. [ABSTRACT FROM AUTHOR]

Background: Chronic histiocytic intervillositis (chronic intervillositis) is defined by a diffuse infiltration of monocytes into the intervillous space, which often leads to poor obstetrical outcomes, including recurrent intrauterine growth restriction, miscarriage, and fetal death. The pathogenesis of chronic intervillositis is still poorly defined, and there is an unmet medical need for improved management.Objective: This study aimed to demonstrate the role of anti-human leukocyte antigen alloantibodies in the pathogenesis of chronic intervillositis through the application of criteria used in solid-organ transplantation for the diagnosis of antibody-mediated rejection.Study Design: A multidisciplinary research study based on thorough immunologic and pathologic investigations was carried out for 2 separate couples who experienced recurrent secondary fetal losses following a first normal pregnancy associated with histologic evidence of chronic intervillositis.Results: Very high levels of complement-fixing, fetus-specific antibodies targeting mismatched human leukocyte antigen alleles, harbored by the 2 paternal haplotypes, were identified in both cases. Polymorphic human leukocyte antigens were expressed on the surface of trophoblastic villi of the inflamed placenta but not in healthy placental tissue. The binding of alloantibodies to paternal human leukocyte antigens induced dramatic activation of the complement classical pathway in trophoblastic villi, leading to C4d deposition and formation of the terminal complex C5b-9. All requirements for the diagnosis of antibody-mediated placental rejection were fulfilled according to the criteria used in the Banff classification of allograft pathology. In silico analysis was performed using a human leukocyte antigen epitope viewer to reconstitute the human leukocyte antigen sensitization history. Reactivity against a single mismatched epitope present in the first-born healthy child accounted for a broad sensitization to human leukocyte antigens, including those harbored by the 2 paternal haplotypes. This finding explained the high rates of chronic intervillositis recurrence during subsequent pregnancies.Conclusion: This study provides novel mechanistic insights into the pathogenesis of chronic intervillositis and provides new avenues for individualized counseling and therapeutic options. [ABSTRACT FROM AUTHOR]

Background: Glomerular deposition of C4d is a widely used biomarker for activation of the lectin pathway in the complement system and is reported to be associated with kidney progression in immunoglobulin A nephropathy (IgAN). The aim of this study was to evaluate whether glomerular C4d deposition, as a new biomarker, improves the prediction of kidney prognosis in IgAN., Study Design: Systematic review and meta-analysis., Setting & Population: Patients with biopsy-proven primary IgAN without age limitations.Selection Criteria for Studies: Cross-sectional or cohort studies reporting the prevalence of glomerular C4d deposition or evaluating its association with IgAN progression., Predictor: Glomerular C4d deposition., Outcome: Composite progression event of a >30% decline in estimated glomerular filtration rate or end-stage kidney disease., Results: 12 studies with 1,251 patients were included. The prevalence of glomerular C4d deposition was 34% (95% CI, 27%-41%), with large heterogeneity ( I 2  < 0.001). Patients with C4d deposition had lower estimated glomerular filtration rates (mean difference [MD], -11.48; 95% CI, -18.27 to -4.70; P  < 0.001). Patients with C4d deposition had lower estimated glomerular filtration rates (mean difference [MD], -11.48; 95% CI, -18.27 to -4.70; P  < 0.001) as well as higher urinary protein-creatinine ratios (MD, 0.87; 95% CI, 0.53-1.21; P  < 0.001) or 24-hour urinary protein excretion (MD, 0.99; 95% CI, 0.50-1.47; P  = 0.02) than patients without C4d deposition. Glomerular C4d deposition was associated with a high Oxford classification score, including M1, E1, S1, and T1/2 lesions (all P  ≤ 0.006). Patients with C4d deposition had higher rates of use of renin-angiotensin system blockers and immunosuppressants. Glomerular C4d was found to be a risk factor for the composite kidney event (RR, 3.17; 95% CI, 2.29-4.40; P  ≤ 0.006). Patients with C4d deposition had higher rates of use of renin-angiotensin system blockers and immunosuppressants. Glomerular C4d was found to be a risk factor for the composite kidney event (RR, 3.17; 95% CI, 2.29-4.40; P  < 0.001; adjusted HR, 2.05; 95% CI, 1.53-2.76; P  < 0.001) and end-stage kidney disease (RR, 4.37; 95% CI, 3.15-6.07; P  < 0.001) without evidence of heterogeneity., Limitations: The definition of positive C4d was not uniform and not all studies provided data about kidney outcomes., Conclusions: Glomerular C4d deposition is associated with an adverse prognosis and may be a useful biomarker of disease prediction in IgAN., (© 2021 Published by Elsevier Inc. on behalf of the National Kidney Foundation, Inc.)

Aim: To investigate the incidence, risk factors and clinical outcomes of acute antibody-mediated rejection (ABMR) after intestinal transplantation (ITx)., Methods: A retrospective single-center analysis was performed to identify cases of acute ABMR after ITx, based on the presence of donor-specific antibody (DSA), acute tissue damage, C4d deposition, and allograft dysfunction., Results: Acute ABMR was identified in 18 (10.3%) out of 175 intestinal allografts with an average occurrence of 10 d (range, 4-162) after ITx. All acute ABMR cases were presensitized to donor human leukocyte antigens class I and/or II antigens with a detectable DSA. A positive cross-match was seen in 14 (77.8%) cases and twelve of 18 patients (66.7%) produced newly-formed DSA following ITx. Histological characteristics of acute ABMR include endothelial C4d deposits, interstitial hemorrhage, and severe congestion with focal fibrin thrombin in the lamina propria capillaries. Multivariate analysis identified a liver-free graft and high level of panel reactive antibody as a significant independent risk factor. Despite initial improvement after therapy, eleven recipients (61.1%) lost transplant secondary to rejection. Of those, 9 (50%) underwent graft removal and 4 (22.2%) received second transplantation following acute ABMR. At an average follow-up of 32.3 mo (range, 13.3-76.4), 8 (44.4%) recipients died., Conclusion: Our results indicate that acute ABMR is an important cause of intestine graft dysfunction, particularly in a liver-exclusive graft and survivors are at an increased risk of developing refractory acute rejection and chronic rejection. More effective strategies to prevent and manage acute ABMR are needed to improve outcomes., Competing Interests: Conflict-of-interest statement: The author declares no conflict of interests for this article.

Antibody-mediated rejection (ABMR) has increasingly emerged as an important cause of allograft loss after intestinal transplantation (ITx). Compelling evidence indicates that donor-specific antibodies can mediate and promote acute and chronic rejection after ITx. However, diagnostic criteria for ABMR after ITx have not been established yet and the mechanisms of antibody-mediated graft injury are not well-known. Effective approaches to prevent and treat ABMR are required to improve long-term outcomes of intestine recipients. Clearly, ABMR after ITx has become an important area for research and clinical investigation., Competing Interests: Conflict-of-interest statement: The author declares no conflict of interests for this article.

Complement-mediated glomerulonephritis, which includes C3 glomerulopathy, is characterized by dominant staining of C3 with minimal or no immunoglobulin deposits on immunofluorescence studies. We describe a new entity of complement-mediated glomerulonephritis that is characterized by bright C4d staining but with no or minimal C3 or immunoglobulin deposits on immunofluorescence studies. We label this entity as C4 glomerulopathy. C4 glomerulopathy includes C4 dense deposit disease and C4 glomerulonephritis. C4 dense deposit disease is characterized by bright C4d staining and dense deposits along glomerular basement membranes. C4 glomerulonephritis is characterized by bright C4d staining and many mesangial electron-dense deposits, with or without rare intramembranous electron-dense deposits. We describe clinical features and kidney biopsy results in a short series of 3 patients to highlight these findings. All 3 patients presented with proteinuria, and 2 patients also had hematuria. Kidney function was preserved in 2 patients, whereas 1 patient presented with declining kidney function. Evaluation for autoimmune disease, infection, and paraprotein yielded negative results in all patients. Complement levels were normal, although 1 patient had borderline low C4 levels. Kidney biopsy showed mesangial proliferative or membranoproliferative glomerulonephritis with bright C4d staining and absent or minimal C1q, C3, and immunoglobulin. Laser microdissection and mass spectrometry of glomeruli in all 3 patients showed large to moderate numbers of spectra matching C4. Furthermore, analysis of amino acid sequences showed that they were localized to the C4d portion of C4, consistent with immunofluorescence findings. Further studies are required to determine the underlying cause. In summary, we describe a novel complement-mediated glomerulonephritis that is characterized by bright glomerular C4d staining with minimal or absent staining for C1q, C3, and immunoglobulin., (Copyright © 2016 National Kidney Foundation, Inc. Published by Elsevier Inc. All rights reserved.)