The ability to non-invasively follow changes in kidney pathology during the treatment of LN would be an important step forward in improving disease management and outcome. Here we have demonstrated that a composite biomarker, uMCP1 +SCr, accurately reflects renal interstitial inflammation in a moderately-sized cohort of SLE patients. Although individual candidate urine biomarkers were, on average, differentially expressed relative to the level of interstitial inflammation in a population, there was significant overlap among cases with and without interstitial inflammation, and this attenuated the performance of single urine proteins as biomarkers.
misclassified 14% of the biopsies. With additional training data this rate may become lower, but will likely not go to zero. Although the kidney biopsy is the gold-standard comparator for Equation (1)
, there is a finite rate of misclassification with tissue readings. The accuracy of a kidney biopsy depends on the size of the tissue sample obtained. For example, the correct diagnosis of glomerular disease or kidney allograft rejection requires an adequate biopsy defined by a minimum number of glomeruli and blood vessels 18, 19
. There is no information on correct classification of tubulointerstitial lesions by biopsy in SLE, however in a study of paired kidney transplant biopsies, interstitial fibrosis identified on the first biopsy was not seen in 12% of second biopsies 20
. Because it was not felt that regression of fibrosis had occurred, this was thought to be an estimate of misclassification of tubulointerstitial disease by biopsy, and is close to that of our composite biomarker. It is conceivable that urine biomarkers could be less likely to misclassify kidney pathology because they reflect the total renal environment and are not subject to biopsy sampling errors and size variations.
Although this biomarker of interstitial inflammation was developed for the evaluation of LN, it is likely it can be used to describe the renal interstitium in other types of kidney disease. While this will need to be tested in a larger group of non-lupus patients than included here, it is relevant because tubulointerstitial injury, including interstitial inflammation and fibrosis is a risk factor for renal functional decline and poor response to therapy in a variety of disorders. These include membranous nephropathy, focal segmental glomerulosclerosis, IgA nephropathy, diabetic nephropathy, and renal transplant failure 21–27
. Similar to LN, interstitial inflammation appears to be a precursor to interstitial fibrosis in these diseases 27
Given the relationship between interstitial inflammation and interstitial fibrosis, we used uMCP-1, uLFABP, uHepcidin, uPCR, and serum creatinine to derive a composite biomarker of renal interstitial fibrosis. Here uPCR and uHepcidin were informative, but the composite biomarker did not perform as well as Equation (1)
, suggesting the addition of other component markers is needed for a final composite fibrosis biomarker.
Our markers of interstitial inflammation and fibrosis were derived from biopsies read for clinical use. Thus, the markers were based on a semi-quantitative evaluation of interstitial inflammation and fibrosis, rather than precise morphometric measurements and enumeration of individual sub-types of interstitial leukocytes. Reassessment of interstitial inflammation and fibrosis in a more quantitative fashion may further improve biomarker models.
The misclassification of biopsies did not appear to be due to increased use of corticosteroids or immunosuppressive medications, or a delay in the collection of urine after biopsy in the misclassified patients.
In summary, this pilot work demonstrates that combinations of urine proteins and clinical variables can be used to derive potentially useful composite biomarkers that reflect specific pathologic lesions in the kidneys of patients with LN. A limitation of our study is the relatively modest sample size of LN biopsies used for linear discriminant analysis. With larger test sets and more precise (quantitative) measurements of pathology, we expect these biomarker equations can be more finely tuned to increase accuracy. Another important limitation is that we do not have an independent set of LN biopsies to use as a validation cohort. This will be necessary before the biomarkers can be used clinically. Despite these limitations, it is envisioned that eventually these biomarkers will be used to follow patients with LN and possibly other forms of kidney disease over time, and individualize treatment decisions.