Transplant glomerulopathy (TG) usually has been described as part of a constellation of late chronic histologic abnormalities associated with proteinuria and declining function. The current study used both protocol and clinically-indicated biopsies to investigate clinical and subclinical TG, their prognosis and possible association with alloantibody. We retrospectively studied 582 renal transplants with a negative pre-transplant T-cell complement dependent cytotoxicity crossmatch. TG was diagnosed in 55 patients, 27 (49%) based on protocol biopsy in well-functioning grafts. The cumulative incidence of TG increased over time to 20% at 5 years. The prognosis of subclinical TG was equally as poor as TG diagnosed with graft dysfunction, with progressive worsening of histopathologic changes and function. Although TG was associated with both acute and chronic histologic abnormalities, 14.5% of TG biopsies showed no interstitial fibrosis or tubular atrophy, while 58% (7/12) of biopsies with severe TG showed only minimal abnormalities. TG was associated with acute rejection, pretransplant hepatitis C antibody positivity and anti-HLA antibodies (especially anti-Class II), with the risk increasing if the antibodies were donor specific. We suggest that subclinical TG is an under-recognized cause of antibody-mediated, chronic renal allograft injury which may be mechanistically distinct from other causes of nephropathy.
Transplant glomerulopathy (TG) is a condition associated with poor outcome, characterized by duplication of glomerular basement membranes, mesangial matrix expansion, and mesangial cell interposition (1,2). Originally classified as a variant of chronic allograft nephropathy of unknown etiology, TG is now recognized with increased frequency in patients with a prior history of humoral rejection, and is also associated with deposition of the complement degradation product C4 d, suggesting that TG may be one manifestation of antibody-mediated injury (3–5).
Previous reports of TG have commonly described TG as a late manifestation of allograft injury, occurring years after transplantation, typically associated with decreased function and proteinuria, as well as severe interstitial fibrosis and tubular atrophy (IF/TA) (2,6). These reports were based on biopsies performed to investigate renal allograft dysfunction. Recently we have identified the characteristic lesion of TG early following transplantation on protocol biopsies performed in patients with well-functioning allografts, often with minimal or no other histologic abnormalities (7). Despite the fact that this early presentation may be associated with relatively mild degrees of allograft dysfunction at diagnosis, these individuals have poor outcome (8). The purpose of this investigation is: (1) to determine the incidence of TG in a large cohort of kidney transplant recipients studied with surveillance biopsies to compare clinical and subclinical TG, (2) to evaluate allograft histology and function at the time of diagnosis of TG, (3) to assess the progression of histologic and functional abnormalities and (4) to investigate factors associated with the development of TG, including its association with anti-HLA antibodies.
Patient selection and evaluation
This study was conducted with informed consent using a protocol approved by the Institutional Review Board of the Mayo Foundation and Clinic. Included in this analysis are 582 kidney allograft recipients transplanted between January 2000 and December 2004. Clinical and laboratory data from donors and recipients were extracted from electronic databases and from the patient's medical record. Recipients transplanted using protocols addressing ABO blood group or HLA antibody (‘positive crossmatch’) incompatibilities were excluded.
Histologic data were obtained from biopsies performed by protocol, as well as for the evaluation of allograft dysfunction. Protocol biopsies form an integral part of the management of kidney transplant recipients at our institution. Typically, biopsies are done at the time of transplantation (time 0) and at 4, 12, 24 and 60 months posttransplantation. All patients in this study had surveillance biopsies done at least at the time of transplantation and 1 year after transplantation.
All biopsies were evaluated by routine light microscopy by one of five dedicated nephropathologists and scored using the Banff 97 classification (3). The diagnosis of acute cellular or humoral rejection was based on light and immunofluorescence microscopic findings using published criteria in conjunction with allograft dysfunction (3,9). The diagnosis of TG for this study was based on the presence of duplication of the glomerular basement membrane detected using silver-methenamine staining. Electron microscopic evaluation was not systematically performed. The Banff 97 classification system scores TG severity based on the percentage of glomerular peripheral capillary loops involved in affected, nonsclerotic glomeruli, ranging from cg0 (no involvement), cg1 (mild TG, with double contours affecting up to 25% of capillary loops of the most affected glomeruli), cg2 (moderate, 26–50% of capillary loops involved) to cg3 (severe, more than 50% of peripheral capillary loops affected). All biopsies given a ‘cg’ score greater than 0 were reviewed and re-scored by one of the authors (SS). Those patients with duplication of glomerular basement membrane (GBM) due to recurrent disease were excluded.
During the period of time covered by this study, C4 d staining was not done routinely in all biopsies but rather based on clinical indication. At the time of diagnosis of TG, some biopsies were evaluated for C4 d deposition using an immunofluorescence technique. Biopsies diagnostic of TG that had not previously been evaluated for C4 d deposition were retrospectively studied using an immunoperoxidase technique (polyclonal rabbit C4 d antibody (Alpco Diagnostics, Salem, NH) diluted 1/50, processed following steam EDTA pretreatment and slide preparation using a DAKO Autostainer plus (DAKO North America, Inc. Carpinteria, CA).
Renal allograft function
Graft function was measured by serum creatinine on a frequent basis and by glomerular filtration rate (GFR) measured by non radiolabeled iothalamate clearance 3 weeks posttransplantation and yearly thereafter (10). GFR was estimated (eGFR) from the concentration of serum creatinine using the modification of diet in renal disease (MDRD) formula (11). Twenty-four-hour urine protein excretion was obtained at the same time points as the GFR measurements and when clinically indicated.
Immunosuppression consisted of induction with ATG (Thymoglobulin®, Sangstadt, Menlo Park, CA, 1.5 mg/kg/d for 5–7 doses) in 78% of cases; 13% received anti-CD25 receptor antibodies (Zenapax®, Roche, Nutley, NJ, or Simulect®, Novartis Pharmaceuticals, East Hanover, NJ); 8% did not receive induction and the remaining received OKT3 (Orthoclone OKT3®, Ortho-Biotech, Bridgewater, NJ) (0.4%) or alemtuzumab (Campath®, Genzyme, Cambridge, MA) (0.6%). These differences reflect an evolution of the immunosuppressive protocols in our institution. Maintenance immunosuppression consisted of prednisone, tacrolimus (Prograf®, Fujisawa, Deerfield, IL) and mycophenolate mofetil (Cellcept®, 750–1000 mg twice daily, Roche, Nutley, NJ) in 81% of patients. Instead of tacrolimus, cyclosporine (Neoral®, Novartis Pharmaceuticals or Gengraf®, Abbott Laboratories, Abbott Park, IL) was used in 9% of patients and sirolimus (Rapamune®, Wyeth Pharmaceuticals, Cambridge, MA) in 9%.
Characterization of anti-HLA antibodies
Patients previously identified as having donor-specific anti-HLA antibodies prior to transplant (positive crossmatch) or ABO blood group incompatibility were excluded from this study. All patients had a negative T-cell crossmatch at transplantation using an anti-human globulin enhanced complement-dependent cytotoxicity assay (12).
Five hundred twenty-four of 582 (90%) patients in the study had final crossmatch sera analyzed using solid phase antibody assays. LABScreen I and II® multi-antigen synthetic flow bead (One Lambda, Inc., Canoga Park, CA) testing was performed according to the manufacturer instructions to detect anti-HLA antibodies. Sera found to have anti-HLA antibodies were then tested using single HLA antigen-coated flow beads (Labscreen® PRA I and II, One Lambda) to determine anti-HLA antibody single antigen specificity.
Data are expressed as means and standard deviation throughout the manuscript. Means of normally distributed data were compared by Student's t-test, paired t. Data that were not normally distributed were compared by nonparametric tests. Chi-square was used to compare proportions. Survival was analyzed by Cox and Kaplan-Meier plots. Three end points were analyzed: diagnosis of TG, patient survival and death-censored graft survival. To assess the incidence of TG, patients were analyzed at the time of the diagnostic biopsy. Individuals without TG were analyzed at the time of their last biopsy. Additional end points of the survival analysis included death and death-censored graft loss. For these survival analyses TG was considered as a time dependent variable because this event was diagnosed at different time points following transplantation.
Compared to patients without TG, patients with TG were younger, more commonly allosensitized, and had a higher incidence of hepatitis C antibodies, acute rejection episodes and prior transplantation (Table 1). Other variables not statistically related to TG included recipient sex and race; donor age, sex, race or type (LD vs. DD); pretransplant dialysis; HLA mismatches; pretransplant diabetes; polyoma virus nephropathy; maintenance or induction immunosuppression.
Table 1. Characteristics of patients with or without the diagnosis of TG
1Student's t-test; 2Chi square; 3Non-parametric t test (Wilcoxon).
Number of patients
Recipient age (years)
51 ± 14
46 ± 17
Dialysis pretransplant (%)
PRA peak (%)
2.4 ± 12.7
6.5 ± 19.6
Donor age (years)
42 ± 13
40 ± 12
Donor type (%)
First transplant (%)
Hepatitis C antibodies (%)
Acute rejection (AR)
AR type (%)
41 ± 16.5
43 ± 15.7
Clinical and subclinical TG
Overall, 9.5% of patients (55/582) were diagnosed with TG at a mean of 21 + 14 months (range 4–61 months) after transplantation (mean follow up 41 + 17 months). The cumulative incidence of TG increased with time, from 4.0% at 1 year; 7.7% at 2 years, 14.4% at 3 years, and 20.2% at 5 years posttransplant (Figure 1). Nevertheless, slightly over one half of the patients diagnosed with TG were detected with biopsies performed for clinical cause, and the mean follow-up period was less than 5 years. Twenty-seven of 55 patients (49%) were diagnosed with TG based on protocol biopsies, 20 within 15 months of transplantation. When biopsies performed by protocol were analyzed separately from those performed for clinical dysfunction, the cumulative incidence of TG was 2.8% at 1 year, increasing to 6.1% at 2 years, 8.5% at 3 years and 11.5% at 5 years. The overall incidence of TG at 1-year posttransplant was 4%. The demographics of patients with clinical versus subclinical TG did not differ (data not shown).
TG and other histologic lesions
One of the major aims of this investigation using protocol biopsies was to study the relationship between TG and other histologic lesions. At diagnosis, 25 of 55 patients (45.5%) had mild, 18 (32.7%) moderate and 12 (21.8%) severe TG (cg1, cg2, cg3, respectively). Although the Banff 97 cg score is based on the extent of peripheral capillary loop duplication seen in the most affected glomeruli of the biopsy, we found that as the cg score increased from cg1–cg3, the percentage of glomeruli involved increased, from 26% in cg1, to 57% in cg2 and 73% in cg3 (Figure 2A: statistical information in Figure 2 legend). The severity of acute glomerulitis (Banff 97 ‘g’ score, defined as glomerular mononuclear cell infiltration and endothelial cell enlargement) also increased with increasing cg scores (Figure 2B).
Interstitial inflammation and tubulitis (Banff 97 ‘i’ and ‘t’, respectively) were common, and in six patients the biopsy diagnostic of TG was also diagnostic of acute cellular rejection (Figure 2C and 2D). Nevertheless, the severity of interstitial inflammation and tubulitis was not significantly associated with severity of TG. Similarly, although interstitial fibrosis and tubular atrophy (IF/TA; Banff ‘ci’ and ‘ct’) were commonly seen in biopsies with TG, their severity did not relate significantly to increasing cg scores (Figure 2E and 2F). Notably, 8/55 biopsies with TG (14.5%) had no significant fibrosis or atrophy (ci0 ct0), and in seven of 12 patients with severe TG (cg3) IF/TA was minimal (ci1 ct0–1). Overall, the incidence and severity of other histologic lesions (Banff i, t, ci and ct) did not increase significantly as the cg score increased, with mild abnormalities being present in the majority of biopsies.
C4 d immunostaining was done in 51 of the 55 (93%) patients with TG (51 by immunofluorescence and 28 by immunoperoxidase). Peritubular capillary C4 d deposition was identified in 13 (25%) patients with TG at the time of diagnosis. Glomerular C4 d deposition was evaluated in the 28 biopsies stained by immunoperoxidase and was positive in nine patients (32%). Five of these nine patients with C4 d deposition in glomerular capillaries were also positive for peritubular capillary deposition.
Although there was no increase in the severity of nonglomerular histopathology associated with increasing cg scores in biopsies with TG, an important question is whether these abnormalities are increased when compared to non-TG biopsies. To assess this, in a second analysis we compared the incidence and severity of Banff scores in 546 1 year surveillance biopsies from 527 patients without and 19 patients with TG (cg1 = 12, cg2 = 4, and cg3 = 3; Figure 3A). All patients had stable renal allograft function, and none of the biopsies were performed to investigate allograft dysfunction. As shown in Figure 3B–F, Banff scores for interstitial inflammation, tubulitis, chronic interstitial fibrosis and arteriolar hyalinosis were significantly higher in biopsies with TG compared to biopsies without TG. No difference was observed between groups for tubular atrophy or chronic vasculopathy (‘ct’, ‘cv’).
Allograft function and TG
Higher cg scores were associated with poorer graft function and increased proteinuria (Table 2). However, there was a wide spread of observed values such that the differences among groups did not reach statistical significance. Importantly, 26/55 patients (47%) with TG had serum creatinine concentrations < 1.5 mg/dL at diagnosis, and 22/55 (40%) had proteinuria less than 500 mg/day. In the 546 patients (527 without and 19 with TG) evaluated at the time of a 1 year biopsy, no significant difference was seen in serum creatinine concentration or estimated GFR, while a marginally significant difference was seen in proteinuria (Table 2).
Table 2. Functional parameters at the time of diagnosis of TG, and comparison of function at one year posttransplant in TG vs. non-TG patients. (*= values observed at the time of 1-year posttransplant protocol biopsies)
Serum Creatinine (mg/dL)
Proteinuria (mg/24 h)
1Student's t-test; 2Kruskal-Wallis; 3ANOVA.
TG at diagnosis (n = 55)
1.7 ± 0.7
42 ± 16
1483 ± 2090
cg1 (n = 25)
1.7 ± 0.7
45 ± 16
904 ± 1323
cg2 (n = 18)
1.8 ± 0.7
44 ± 17
1744 ± 2658
cg3 (n = 12)
2.0 ± 0.6
33 ± 9
1860 ± 2313
TG 1 year (n = 19)*1
1.6 ± 0.4
47 ± 17
497 ± 749
Non-TG 1 year (n = 527)*
1.5 ± 0.5
52 ± 15
335 ± 796
p = 0.27 1
p = 0.15 1
p = 0.052
Histologic and functional progression of TG
Thirty-three of the 55 patients with TG had follow-up biopsies a mean of 15 + 14.7 months (range 1– 59) following diagnosis. Six of 17 (35%) with cg1 at diagnosis progressed to cg2 or 3, while five of nine (56%) with cg2 progressed to cg3. In 8/33 patients, the follow-up biopsy failed to show TG (cg = 0). Interstitial fibrosis, tubular atrophy and chronic vasculopathy significantly worsened between diagnosis and follow-up biopsies (Figure 4).
After a follow-up of 41.4 ± 16 months, 37/582 patients had expired (6.4%) and 33 had lost their allograft from causes other than patient death (5.7%), including 15/55 (27%) patients with and 18/527 (3.4%) patients without TG. When evaluated by both univariate as well as multivariate models including recipient age, acute rejection, proteinuria and function at 1 year, TG was strongly and independently associated with allograft loss (Table 3).
Table 3. Correlates to death-censored graft survival
1TG was used as a time dependent variable.
2The correlation between graft survival by univariate analysis was similar whether using measured GFR, serum creatinine or eGFR. However, by multivariate analysis serum creatinine was a stronger predictor of graft survival in these patients.
3HR for proteinuria is calculated per gram of protein in the urine.
Creatinine2 (1 year)
Proteinuria3 (1 year)
Factors associated with the development of TG
By univariate Cox analysis the risk of developing TG was increased in younger recipients, patients with a history of acute rejection, hepatitis C antibodies prior to transplantation, positive panel reactive antibodies (PRA), prior transplantation and HLA class I and II antibodies (Table 4). Additional variables tested and found to be not significantly associated with TG included donor age, sex and race, donor type (living vs. deceased), recipient sex or race, diabetes, polyoma associated nephropathy, transplant year and type of induction or maintenance immunosuppression. When evaluated by multivariate analysis, the risk of TG remained significantly associated with acute rejection, hepatitis C antibody positivity, prior transplantation and HLA antibodies present at transplant (Table 4). Prior to the diagnosis of TG, 18/55 (33%) of patients had been diagnosed with acute rejection (nine borderline acute cellular rejection, six acute cellular rejection, and three humoral rejection). Four of thirteen (31%) patients with hepatitis C antibodies prior to transplantation developed TG, compared to 51/569 (9%) hepatitis C antibody-negative patients. In these four patients we differentiated TG from hepatitis C associated glomerulopathy by excluding immune complex deposition by immunofluorescence staining in four, and electron microscopy in three of three patients evaluated.
Table 4. Variables related to TG
1HLA class I antibodies (detected using LABScreen I ® multi-antigen synthetic flow beads).
2HLA class II antibodies (detected using LABScreen II® multi-antigen synthetic flow beads).
3PRA, HLA class I and class II antibodies were added to separate multivariate models.
HLA class I Ab3
HLA class II Ab3
The variables that relate to development of TG were reanalyzed considering only those patients detected on protocol biopsies (N = 27). This sub analysis was done to assess the possibility that TG diagnosed by protocol biopsy may differ from TG diagnosed in the setting of allograft dysfunction. Risk factor analysis of TG detected by protocol biopsy revealed that acute rejection, particularly humoral rejection (HR 3.08 (CI 1.3–7.1), p = 0.008), pretransplant HLA class I antibodies (2.3 (1.09–5.0), p = 0.028), and pretransplant HLA class II antibodies (8.2 (3.6–19), p < 0.0001) related significantly to the development of TG, while the relationship with hepatitis C approached statistical significance (p = 0.068). By multivariate analysis acute rejection and antibodies to HLA class II related independently to TG. Thus, the variables associated with TG are similar regardless of whether all patients are analyzed or when protocol biopsy detected TG is analyzed separately.
Anti-HLA antibodies and TG
To investigate the relationship between anti-HLA antibodies and TG, final crossmatch sera available from 524/582 (90%) patients were analyzed using HLA antigen coated flow beads (LABScreen I and II® multi-antigen synthetic flow beads). One hundred ninety four of 524 sera (37%) had detectable anti-HLA antibodies, with 81 positive for both class I and II, 58 positive for class I, and 55 positive for class II antibodies (Table 5). There was a strong association between anti-HLA antibodies and TG. Forty-one of 54 (76%) final crossmatch sera from patients who developed TG had detectable anti-HLA antibodies. Figure 5A shows the cumulative incidence of TG in patients with anti-HLA class II antibodies alone, anti-HLA class I antibodies alone, and both class I and II antibodies at the time of transplantation compared to patients without detectable antibodies. The risk of TG was significantly higher in patients with anti-HLA class II antibodies alone (HR = 6.15, CI: 2.8–13.4, p < 0.0001) and this risk increased further when class I and II antibodies were both present (HR = 9.7, CI: 4.9–19, p < 0.0001). Interestingly, the presence of anti-HLA class I antibodies in the absence of class II antibodies was not significantly associated with subsequent development of TG (HR = 1.65, CI: 0.54–5.0, NS).
Table 5. Anti-HLA antibodies at the time of transplantation (DSA = donor specific antibody)
1p < 0.0001, chi square.
N sera tested
Class I +/II +
Class I –/II +
Class I +/II −
Sera found to contain HLA class I or II antibodies were tested using single HLA antigen coated flow beads (LABScreen PRA I and II®) to determine specificity. Donor-specific antibodies (DSA) were identified in 93/194 anti-HLA antibody containing-sera (48%). Among 139 patients with anti-HLA class I antibodies pretransplant, class I DSA were identified in 41 (30%). One hundred thirty-six patients had anti-HLA class II antibodies at transplant, and donor specificity could be identified in 69 (51%). In order to study the impact of HLA class II DSA on subsequent risk for TG, we compared three groups of patients: (1) individuals with anti-HLA class II DSA (DSA+) detected using single antigen coated flow beads (n = 70); (2) patients with anti-HLA class II antibodies but without donor specificity (DSA-; n = 67); and (3) individuals without identifiable anti-HLA class II antibodies (HLA class II negative; n = 387). For this analysis, we combined individuals with HLA class II antibodies into one group, including those with and without class I antibodies. As shown in Figure 5B, the risk of TG was increased in patients with anti-HLA class II antibodies at transplant, even when DSA were not identified (DSA- vs. HLA class II antibody negative: HR = 4.7, CI: 2.2–9.9, p < 0.0001). Importantly, there was a further significant risk for TG in patients with identifiable DSA, compared to DSA- patients (HR = 9.8, CI: 5.3–18.2, p < 0.0001).
Seventeen of 54 tested TG patients had no detectable anti-HLA class II antibodies at transplant. Thirteen of these 17 patients were tested using multi-antigen coated flow beads at least 1 year after transplantation, and 8/13 (62%) were found to have developed class II antibodies. Thus, at the end of the follow-up period, 45/55 (82%) patients with TG had detectable anti-HLA class II antibodies. Conversely, 235 non-TG patients without class II antibodies at transplant had posttransplant sera analyzed, and only 22/235 (9%) were found to have developed them.
In this study we have continued our initial studies using both protocol and clinically-indicated biopsies to focus on the clinical and histologic presentation and pathogenesis of TG (7,8). The main results of these studies can be summarized as follows: 1. the incidence of TG in conventional kidney transplants increases with time beginning within the first year following transplantation; 2. early TG is often subclinical, but is nevertheless associated with reduced graft survival; 3. the histologic features of TG appear to be distinct from other forms of chronic allograft nephropathy; and 4. the pathogenesis of TG is closely linked to anti-HLA class II antibodies.
TG is generally thought to occur as a late complication of kidney transplantation in association with decreased function and proteinuria, although it can occur within months of transplantation, and in the absence of proteinuria (1,2,6,13). Previous studies have consistently described an incidence of 5–10% in conventional kidney transplant recipients based on biopsies performed to investigate allograft dysfunction. In the current study we observed that the cumulative incidence of TG increases over time, from 4% at 1 year to 20.2% in patients followed over 5 years posttransplantation. When protocol biopsies were analyzed separately, the cumulative incidence was 11.5% at 5 years. Although not all patients underwent protocol biopsies at all time points, and the mean follow-up period of the study cohort is less than 5 years, our results support the contention that TG is an under recognized clinical entity. The cumulative incidence of TG is higher than previously reported likely because in this study we used both clinical as well as protocol biopsies to diagnose the disease. Indeed, nearly half of the cases reported here were diagnosed with protocol biopsies. Nevertheless, the point prevalence of TG identified in biopsies 1 year posttransplant (4%) is consistent with the prevalence identified in previous studies (1,6,14,15). A major question addressed by this study is the relationship between TG and other histopathology. TG was associated with greater acute and chronic histologic abnormalities compared to non-TG biopsies, including interstitial inflammation, tubulitis, and IF/TA. Nevertheless, the severity of these abnormalities varied, and some individuals with severe TG had relatively minor changes otherwise. The fact that TG can be found in the absence of other histologic abnormalities suggests that it may be mechanistically distinct from other causes of allograft nephropathy.
Despite the fact that TG was often associated with otherwise mild histologic abnormalities and minimal signs of allograft dysfunction at the time of diagnosis, it was a progressive condition associated with shortened allograft survival. This finding is consistent with our previous study of 292 transplant recipients undergoing protocol biopsies 1 year after transplantation, in which Cosio et. al. reported that over 50% of patients diagnosed with TG achieved an end point of allograft loss or loss of >50% GFR over the next 36 months (8). Thus, even in the absence of graft dysfunction or proteinuria at the time of diagnosis, TG is associated with poor outcome.
We have previously reported that the incidence of TG is significantly increased in recipients of positive crossmatch kidney transplants, especially in those complicated by humoral rejection (4). In the current study we have extended these observations by noting that those patients with pretransplant anti-HLA antibodies, and particularly HLA class II antigens, have a markedly increased risk of developing TG. Individuals in whom HLA class II DSA are identified are at even greater risk for TG. A potential explanation for this strong association relates to the expression of HLA class II antigens in the kidney. Endothelial cells in peritubular as well as glomerular capillaries, but not larger renal blood vessels, express HLA class II antigens under normal conditions (16–18).
Previous studies identified an association between hepatitis C and TG in the absence of cryoglobulins (6,19). The current study confirms this association and suggests that the higher incidence of TG in these patients is due to the presence of anti-HLA antibodies in patients with this viral infection.
Only one third of the patients with TG in this study had C4 d deposition in peritubular capillaries and/or glomerular capillaries. Two different techniques were used to identify C4 d deposition in this study, raising the possibility that variations in sensitivity between the assays may have had an impact on the observed frequency. Nevertheless, the findings in this investigation are consistent with previous studies (5,20,21). Although C4 d deposition is associated with alloantibody-mediated endothelial cell injury, its absence does not rule out the participation of antibodies in the pathogenesis of TG. The interaction of DSA with allograft endothelium may result in injury unrelated to complement activation. Alloantibodies have been shown to produce endothelial cell apoptosis independent of the action of complement (22). Alternatively, chronic exposure to low levels of DSA may produce injury due to a degree of complement activation insufficient to result in C4 d detection.
The findings of this and other studies indicate that TG is a unique clinical entity strongly associated with alloantibody-mediated injury. This is supported by an animal model of kidney transplantation, in which chronic antibody-mediated rejection of renal allografts was associated with glomerular basement membrane duplication (23). The early lesion of TG consists of endothelial and mesangial cell swelling, with later mesangial matrix expansion, and eventual distortion and duplication of glomerular capillary basement membranes. Interestingly, these pathologic changes are not isolated to the glomerulus but also affect peritubular capillaries (PTC) (24). A similar progression of histologic abnormalities has been reported in PTC in the setting of humoral rejection (25). In that study, 2– 8 months following the rejection episode, obliteration of PTCs was demonstrable, resulting in interstitial ischemia and development of fibrosis and tubular atrophy. Thus, while in its early stages TG may be limited to a predominantly glomerular lesion, chronic ischemia resulting in worsening IF/TA may develop as the vasculopathic process progresses. In addition, TG is associated with acute rejection and interstitial inflammation, capable of causing progressive deterioration of kidney structure and function (8). Despite the fact that in the current study TG was diagnosed at a relatively early stage, 32.7% of the group suffered graft loss over the follow-up period of 41 months. This suggests that even at this early stage, TG reflects a pathologic process with severe negative implications for allograft survival.
In conclusion, TG can present early after transplantation, often with minimal allograft dysfunction or proteinuria. It is a common condition, demonstrable in an increasing number of patients within 5 years after transplantation and likely beyond. Early TG can present as a relatively isolated histologic abnormality, preceding the IF/TA and chronic vasculopathy that may evolve over time as a result of progression of capillary damage and subsequent ischemic injury. TG is highly associated with allosensitization prior to transplantation, especially against HLA class II antigens. We conclude that TG represents a clinical entity that is distinct from other causes of IF/TA, and which often corresponds to chronic alloantibody-mediated allograft injury. Further studies are needed to evaluate the association of TG with low levels of alloantibody. Nevertheless, the current investigation suggests that even very low levels of anti-HLA antibodies detectable using highly sensitive solid phase assays, such as Luminex represent an important risk factor. TG appears to be an under- recognized cause of renal allograft loss which needs to be addressed in order to improve long- term renal allograft survival.