Lupus nephritis (LN) is among the main determinants of poor prognosis in systemic lupus erythematosus (SLE). The objective of this study was to 1) isolate and identify proteins contained in the LN urinary protein signature (PS) of children with SLE; 2) assess the usefulness of the PS-proteins for detecting activity of LN over time. Using surface-enhanced or matrix assisted laser desorption/ ionization time of flight mass spectrometry, the proteins contained in the LN urinary PS were identified. They were transferrin (Tf), ceruloplasmin (Cp), α1-acid-glycoprotein (AGP), lipocalintype prostaglandin-D synthetase (L-PGDS), albumin and albumin-related fragments. Serial plasma and urine samples were analyzed using immunonephelometry or ELISA in 98 children with SLE (78% African-American) and 30 controls with juvenile idiopathic arthritis. All urinary PS-proteins were significantly higher with active versus inactive LN or in patients without LN (all p<0.005), and their combined area under the receiver operating characteristic curve was 0.85. As early as 3 months before a clinical diagnosis of worsening LN, significant increases of urinary Tf, AGP (both p < 0.0001) and L-PGDS (p < 0.01) occurred, indicating that these PS-proteins are biomarkers of LN activity and may help anticipate the future course of LN.
Systemic Lupus Erythematosus (SLE) is an inflammatory autoimmune disease and lupus nephritis (LN) is one of the main determinants of poor prognosis (1). Currently, LN is gauged by measuring circulating and excreted indicators of renal dysfunction, with supporting information from kidney biopsies. The latter constitute the current standard for diagnosing LN, providing a direct assessment of the presence, severity and activity of LN, and the degree of renal damage (2). Due to the invasive nature of kidney biopsies, clinicians base LN activity and its therapy on the results of urinary protein excretion, urinary sediment, creatinine clearance and serum albumin. These traditional markers are not accurate in assessing whether active LN is present or not, and none of them is predictive, i.e. can anticipate the course of LN.
Using Surface-Enhanced Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (SELDI-TOF MS) technology, we previously identified a LN urinary protein signature (PS), consisting of eight candidate biomarkers at the mass-to-charge ratios (m/z) of 2.763, 22, 23, 44, 56, 79, 100, and 133 kDa (3).
In this study, we present the identification of the specific proteins contained in this PS of children with LN. We further assayed plasma and urine samples of SLE patients and controls with juvenile idiopathic arthritis (JIA) to investigate the concurrent and predictive validity of the PS-proteins to serve as biomarkers of LN activity.