In this serial histological analysis, embedded in a randomized multicenter clinical trial of steroid avoidance in pediatric renal allograft recipients treated with tacrolimus, MMF and prolonged induction with daclizumab, we found significant progression of chronic graft injury in the first 2 years post-transplantation in both study arms. Complete steroid avoidance with prolonged IL-2 receptor blockade was associated with neither a higher risk for clinical rejection nor with a higher risk for subclinical inflammation of renal allografts. There was no difference between the SB and the SF treatment arms with respect to chronic histological damage, graft function or graft survival. In addition, this study confirmed that in pediatric kidney transplant recipients, small recipient size and higher donor age are the primary risk factors for progressive chronic tubulo-interstitial damage of renal allografts. The histological appearance of the grafts in was significantly correlated to absolute graft function, although it should be noted that this correlation was rather weak.
The current study showed no increased risk of clinical acute T-cell mediated rejection with a steroid avoidance protocol in a pediatric population. Moreover, we demonstrated for the first time that a SF immunosuppressive regimen does not lead to higher prevalence of subclinical inflammation in this population, as detected in protocol biopsies (all with stable graft function). As both subclinical inflammation in non-atrophic areas and inflammation in atrophic areas have been associated with worse long-term renal allograft outcome (24
), this is a reassuring finding that suggests relatively long-term safety of the steroid-free immunosuppressive protocol (Sarwal et al, submitted). No statistically significant differences were observed between the SF and SB patients in terms of antibody-mediated rejection, although the numerically higher prevalence of acute antibody-mediated rejection in the SF group is of potential clinical importance in recipients with an immunologically higher risk profile. Studies with longer follow-up are needed to elucidate whether this numeric difference translates in statistically significant differences on the long term. Studies in adult recipients have shown increased risk of graft rejection in steroid avoidance studies (26
), which differs by the findings in the current pediatric study, and another recent multicenter, randomized trial in pediatric recipients (15
). Whether this reflects the duration of daclizumab use, the pediatric population or other as yet undefined factors cannot be answered from the current study. Finally, as daclizumab has been withdrawn from the market, it will be necessary to separately evaluate the safety and efficacy of the IL-2 receptor blocker basiliximab for induction in steroid-free immunosuppressive regimens in pediatric kidney transplantation.
Importantly, the current study showed that donor age is a correlate of graft histology after transplantation and that progressive histological damage occurs even in kidneys from relatively young donors transplanted in young recipients, independent of the occurrence of acute rejection, these events being noted fairly soon after transplantation. The impact of older donor age and acceleration of chronic histological injury in the post-transplant allograft has been previously shown in published studies in adult recipients (2
) and with single-center data in pediatric recipients (4
The etiology of this histological damage is probably multifactorial (5
), and the current study offers new insights into possible clinical determinants of this progressive injury. Importantly, we observed that there were no differences between the treatment arms in terms of immune injury. We also observed that steroids do not appear to have a protective role in the avoidance of chronic histological IF/TA damage after transplantation. Earlier animal model studies have suggested that steroids may have a protective role against calcineurin inhibitor induced nephrotoxicity (29
), and one recent study suggested that acute calcineurin inhibitor nephrotoxicity is more prevalent in a steroid withdrawal group than in patients maintained on steroids, although the definition of acute CNIT was not detailed (16
). In our current study, there was no significant difference in the incidence of chronic histological damage or histological lesions suggestive of calcineurin inhibitor nephrotoxicity between the SF and SB treatment groups, although we acknowledge that the power to detect minor histological effects of steroid avoidance is insufficient in the current study. This is related to attrition over time of the number of protocol biopsies in each treatment arm, a common phenomenon in protocol biopsy studies, even in adult recipients (2
). In addition, lack of treatment blinding and lack of data on medication adherence and drug levels could have further decreased the study power, especially since medication adherence can be problematic in adolescents (31
). The fact that graft loss was very low in the current study (in contrast to the protocol biopsy studies in adults), and the equal percentage of biopsies in each study arm, avoids any systematic bias that could arise from the declining proportion of biopsies over time. Larger protocol biopsy studies with lower attrition rates would be very welcome, but accrual and expense could compromise the feasibility of larger clinical trials of this nature in pediatric recipients.
Furthermore, we evaluated other clinical determinants of progressive chronic histological damage of renal allografts. We demonstrated that higher donor age is independently associated with increased chronic histological damage, despite the pristine nature of the kidneys at time of implantation. This suggests that the effects of higher donor age reach beyond the quality of the graft at implantation and continue to be important for the histological and functional evolution in the post-transplantation period (32
). Some of these cellular and molecular mechanisms of aging include subcellular structural changes, DNA mutation accumulation, telomere shortening, oxidative stress, accumulation of advanced glycosylation end products etc resulting in distinct gene expression profiles. The functional limitations such as tubular dysfunction, increased susceptibility to ischemia and drug toxicity, likely persist even in the absence of overt structural changes in implantation biopsies.
Finally, in the current multicenter study, we independently validated the previously described association between smaller pediatric renal allograft recipient size and increased risk for chronic tubulo-interstitial injury (4
). This finding could explain the absence of improvement of absolute GFR (mL/min) in association with growth of the smallest pediatric renal allograft recipients, in contrast to the findings in older children where the absolute GFR increases with growing (34
). As we confirmed an association between small recipient size and chronic (irreversible) histological damage, the irreversibility of the functional adaptation of renal allograft function to the size of the pediatric recipients is likely caused by chronic renal graft ischemia, insufficient renal auto regulation and secondary chronic irreversible tubulo-interstitial damage associated with the donor-recipient size discrepancy. One could hypothesize that the aortic blood flow in smaller pediatric recipients is insufficient to provide adequate perfusion of the adult-sized kidney with adult-sized vasculature, as previously demonstrated in vivo (37
). The independent effect of donor-recipient size discrepancy on both irreversible histological damage of the transplanted kidney and on its functional evolution, suggest that the current strategies to assure optimal intravascular volume (37
) are not sufficient, and exploration of additional therapeutic approaches to increase allograft perfusion could further extend the graft survival benefit (39
) of adult-sized kidneys transplanted into small children. It remains unclear how this accelerated progression of chronic histological damage in infants relates to the superb graft survival in this age group (39
). It could be hypothesized that this apparent contradiction relates to the lower prevalence of late severe rejection phenomena in this age group compared to adolescent recipients, which is often associated with problems with medical adherence (31
). Longer-time follow-up studies should be performed to unravel the impact of the progressive chronic histological damage on long-term graft outcome.