Proteomic Portrayal of Transplant Pathologies
Authors: Peter Nickerson, Peter S. Heeger
Journal: Journal of the American Society of Nephrology (2009)

Despite recent improvements in 1-yr outcomes for kidney transplantation, long-term allograft survival has not changed significantly in the past several decades.1 The survival half-life for kidneys from deceased donors is approximately 11 yr, and the pathogenesis of these late graft losses is multifactorial.2 Analyses of graft histology reflected in the revised Banff criteria indicate chronic allograft injury can be subcategorized, in part, on the basis of evidence of local inflammation and the presence or absence of interstitial fibrosis and tubular atrophy (IF/TA).3 Although specific inciting factors are difficult to define in each situation, distinct histopathologic entities often correlate with likely causes. For example, calcineurin inhibitor toxicity frequently manifests as IF/TA without inflammation; ongoing cellular alloimmunity presents histologically with tubulitis with or without IF/TA; C4d staining suggests transplant glomerulopathy with or without IF/TA; and detectable, donor-specific serum antibodies underlie antibody-mediated allograft injury.3 Because only a subset of patients develop chronic injury and because at present physicians do not have the ability to reverse chronic fibrotic kidney damage, it is essential that the transplant community develop reliable and noninvasive approaches to predict which patients are most likely to develop graft failure so that appropriate interventions can be instituted before graft failure becomes clinically apparent. At present, protocol biopsies of well-functioning grafts provide the best assessment tool to predict risk for subsequent graft failure (e.g., the presence of subclinical acute rejection, IF/TA, transplant glomerulopathy).4–6 Defining likely causes of an ongoing pathologic process is similarly important because this information guides subsequent treatment regimens. For example, calcineurin inhibitors are withdrawn from patients at risk for calcineurin inhibitor–induced toxicity, whereas individuals at risk for cellular and/or antibody-mediated immune injury are candidates for altered immunosuppression aimed at inhibiting the relevant component of the immune system. A significant worldwide research effort is now focused on delineating assays and approaches to meet the laudatory goal of predicting outcomes after transplantation. These endeavors include detailed characterizations of serum alloantibody levels,7 measurements of individual soluble or cell-surface markers (proteins or genes) in peripheral blood and/or urine,8 and evaluations of peripheral cellular alloimmunity,9 among others. Additional unbiased approaches being used by numerous research groups include profiling gene expression10 and/or proteome expression patterns11,12 in patient cohorts to assess the utility of these profiles as surrogate markers of injury. In this issue of JASN, Quintana et al.13 provide new evidence that assessment of the urine proteome can noninvasively distinguish two types of chronic allograft pathology. The authors performed urine proteomic analysis using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) in 32 patients with biopsy-proven chronic allograft injury (GFR <35 ml/min, proteinuria, >3 yr after transplantation). These patients had either chronic active antibody–associated rejection (CAAR) or significant IF/TA without evidence of antibody-mediated pathology. Intriguing, the urine proteomes obtained from patients with the two pathologies were distinctly different from one another, and both differed from those found in urine of 10 healthy control subjects and from eight patients with excellent allograft function 1 mo after transplantation. Having defined a distinctive pattern in their “test sets” of patients, the authors then studied separate, small validation sets and confirmed that the urine proteome patterns were indeed distinct. Statistical analyses indicate that in this latter cohort, IF/TA and CAAR are readily differentiated from each other with high precision. This “first step” description of an accurate method to differentiate noninvasively subtypes of chronic allograft histopathology is intriguing and could ultimately be clinically useful. Still, a number of caveats need to be noted when interpreting this work. Despite the inclusion of a validation set, the study is small, encompassing only 32 patients, and thus needs further validation in a larger cohort. Although the authors show that sera from all patients with CAAR contained donor-specific antibody, they did not report serum alloantibody specificities in the patients without CAAR and with IF/TA. It is possible that the presence or absence of a donor-specific antibody in the serum could be just as useful in differentiating IF/TA from CAAR as the proteomic results identified by this research team. This point is particularly relevant because alloantibody testing is available clinically, and the complexities involved in MALDI-TOF-MS proteomic analysis make it unlikely to become a routine testing strategy used in many hospital laboratories. The authors also have not yet characterized the individual proteins that comprise the specific proteomic patterns associated with each histopathology. Defining these proteins could shed light on the pathogenesis of each disease process and might identify a specific target protein that could be a reliable surrogate marker, a result that would negate the requirement for routine proteomic analyses. Although the authors show the proteomes of those with chronic injury differ from those from stable control subjects, the latter were also not matched for time after transplantation. It is possible that the duration of the transplant itself affects the urine proteome, thereby confounding data interpretation. Finally, it is notable that the authors have yet to demonstrate that the proteomic patterns observed in established CAAR or IF/TA are useful in predicting impending or subclinical CAAR or IF/TA. A prospective, controlled trial will be required to determine whether serial assessments of the urine proteome suggest patterns of injury before the development of hypertension, proteinuria, or elevation in serum creatinine or whether urine proteome patterns are superior to other clinical and experimental monitoring strategies for this purpose. The results delineated by Quintana et al.13 represent one more step toward development of tactics to individualize the care of transplant recipients on the basis of objective surrogate markers. Using this approach among other immune monitoring techniques, the transplant community will, in due course, be able to assess accurately the risk for impending graft injury and thereby rationally guide decisions to alter therapy, with the ultimate goals of prolonging transplant survival and improving patient health. DISCLOSURES None.