The potential role for uNGAL in monitoring and predicting renal disease activity in LN has been explored in both murine and human LN. In murine models of LN, NGAL has been found to induce apoptosis in mesangial cells and facilitate recruitment of inflammatory cells in the kidney through the upregulation of pro-inflammatory mediators [25
In humans, many cross-sectional studies have examined the role of NGAL in SLE patients for detection of LN activity and relapses [12
]. We too have previously demonstrated in a cross-sectional study that uNGAL levels in patients with active LN were higher than in those with inactive renal disease [16
]. We also found no association between uNGAL with prednisolone and other immunosuppressive medications [16
To date, only four longitudinal studies have been reported on the usefulness of uNGAL as a biomarker for active LN or predictor for impending relapse/ flares. Of these four studies, three were performed in pediatric patients and only one in adults. Suzuki et al. [15
] in a longitudinal study of 85 pediatric patients with SLE showed that uNGAL levels were a novel biomarker for relapse of LN. Hinze et al. [26
] in another longitudinal study of children with SLE (n=111) found that uNGAL levels predicted the impending relapse of LN. An increase in plasma NGAL levels also predicted worsening of global and renal disease activity. In a recent longitudinal study of childhood–onset SLE patients (n=64), Watson et al. [27
] demonstrated uNGAL to be a good predictor of worsening renal disease activity.
In the only reported longitudinal study of 107 adult SLE patients of whom only 25 had biopsy-proven LN, Rubinstein et al. [14
] also found that uNGAL was a significant predictor of renal flare and outperformed anti-ds DNA Ab.
Unlike all the above longitudinal studies, all our adult SLE patients were ‘homogenous’ for biopsy-proven LN albeit with varying grades of activity. These 100 LN patients were followed at shorter 2 monthly intervals over 3 visits in an attempt to diagnose and/or predict ‘early’ flares. uNGAL levels were again higher in patients with active LN compared to those with inactive LN.
With treatment, majority of patients with active LN at baseline (28/47, 60%) achieved remission at end study, a third remained with NR (16/47, 34%) and three patients (6%) relapsed. uNGAL levels fell significantly in all patients in response to treatment in both CR/PR as well as in the persistent NR patients.
In those patients who relapsed (13%), uNGAL levels increased concurrently with LN relapse but decreased progressively with treatment. However, in the single one nonresponder throughout the study, uNGAL increased further on follow up in tandem with nephrotic range proteinuria and rising serum creatinine levels. This patient was subjected to repeat renal biopsy which showed that although the activity index of her class IV LN had decreased from 14/24 to 3/24, she had developed an added membranous component i.e. class IV+V. More intensive remission-induction therapy was instituted. Just like SLE its parent disease, LN can also undergo multiple episodes of relapses-remissions and to perform repeated renal biopsies is such patients is not only highly traumatic, may lead to complications and is possibly unethical. Thus serial uNGAL monitoring in conjunction with the usual clinical parameters can obviate repeated ‘invasive’ renal biopsies as is the current clinical nephrology practice.
Similar to findings reported by Suzuki et al. [15
], uNGAL levels in our study also correlated with proteinuria, SLEDAI-2K global and renal scores. These data suggest that the source of increased uNGAL in LN is most likely due to increased production by the kidneys and not due to extrarenal disease per se. The correlation between uNGAL levels with SLEDAI-2K global score can be explained by the renal components included as criteria in the said score. Like Pitashny et al. [13
] and Suzuki et al. [15
], we too found no correlation between uNGAL levels with SLEDAI-2K extrarenal score or anti-dsDNA Ab titres and serum complements. The role of anti-dsDNA Ab titres and C3 and C4 levels as indicators of LN disease activity or LN outcome remains controversial [28
]. Similar to other studies [29
], we found no association between anti-dsDNA Ab titres and serum complement levels with LN activity.
At all time points, the ROC curves for uNGAL showed it is to be a good noninvasive marker for detection of LN activity and performed better than many of the usual blood and urinary markers such as serum albumin, serum creatinine, eGFR and haematuria. However, it was not as good as proteinuria and SLEDAI-2K renal score for the detection of clinical LN activity. This fact may be due to that the proteinuria and SLEDAI-2K renal score were included as major criteria in the definition of LN activity. uNGAL also outperformed the usual serological markers i.e. anti-dsDNA Ab titres, serum complements for diagnosis of LN activity. This lack of correlation between anti-dsDNA Ab titres and LN activity corroborates with that reported by Rubinstein et al. [14
]. Whereas these same authors [14
] found that using ROC curves, uNGAL showed equivalent performance to those of C3 and C4 for predicting LN flares in patients with a past history of biopsy-proven LN.
Multiple logistic regression analysis showed that only serum albumin and proteinuria were independent predictors of LN activity or relapse but not uNGAL. This is in contradiction to findings by Rubinstein et al. [14
] that uNGAL remained a significant predictor for LN activity even after adjustment for age, sex, and race, class of LN and anti-dsDNA Ab titres.
The major limitation of this study was that the association between uNGAL with the various histological classes of LN were not obtained concurrently. This was due to the delay between urine collection for NGAL and renal biopsies. Perhaps more conclusive results could have been obtained if urine samples for the NGAL were taken concurrently with renal biopsies for those patients who had LN flares or who had persistently active LN. Another limitation was the short follow up of 4 months only due to cost (predominantly) and time constraints. Notwithstanding these shortfalls, this study was purposely designed with the shorter observation intervals of 2 months over the 3 visits as this would have had a better chance of diagnosing/ picking up early renal flares as well as provide closer monitoring of treatment response. A longer follow up for at least 2 - 3 years would be ideal.
In conclusion, uNGAL was significantly increased in active LN especially in LN flares. It had good diagnostic performances with good sensitivities and moderate specificities for detection of LN activity and/ or relapse. Perhaps the sensitivities and specificities of these biomarkers could be improved by incorporating several new biomarkers currently also under study into a panel of biomarkers for assessing LN activity similar to that proposed for acute kidney injury (AKI), interleukin-8 (IL-18), kidney injury molecule-1 (KIM-1) and liver-type fatty acid-binding protein (L-FABP) [31
]. Although uNGAL was not an independent predictor for LN activity, it could serve as an adjunctive marker for the diagnosis of subclinical and early relapses. Larger prospective longitudinal studies for longer periods are indicated.