Interleukin (IL) 16: a candidate urinary biomarker for proliferative lupus nephritis

Abstract

Objective: Lupus nephritis (LN) is a severe manifestation of systemic lupus erythematosus (SLE). The pathogenesis is incompletely understood and diagnostic biomarkers are scarce. We investigated interleukin (IL) 16 as a potential biomarker for LN in a well-characterised cohort of patients with SLE.

Methods: We measured urinary (u-) and plasma (p-) levels of IL-16 in predefined patient groups using ELISA: LN (n=84), active non-renal SLE (n=63), inactive non-renal SLE (n=73) and matched population controls (n=48). The LN group included patients with recent biopsy-confirmed proliferative (PLN, n=47), mesangioproliferative (MES, n=11) and membranous (MLN, n=26) LN. Renal expression of IL-16 was investigated by immunohistochemistry. Associations between IL-16 measurements and clinical parameters and the diagnostic value for LN were explored.

Results: p-IL-16 was detected in all investigated cases and high p-IL-16 levels were observed in patients with active SLE. u-IL-16 was detected (dt-u-IL-16) in 47.6% of patients with LN, while only up to 17.8% had dt-u-IL-16 in other groups. In the LN group, 68% of patients with PLN had dt-u-IL-16, while the proportions in the MLN and MES groups were lower (11.5% and 45.5%, respectively). The highest u-IL-16 levels were detected in the PLN group. In the regression model, u-IL-16 levels differentiated PLN from other LN patient subgroups (area under the curve 0.775-0.896, p<0.0001). dt-u-IL-16 had superior specificity but slightly lower sensitivity than elevated anti-double-stranded DNA and low complement C3 or C4 in diagnosing PLN. A high proportion of LN kidney infiltrating cells expressed IL-16.

Conclusions: We demonstrate that detectable u-IL-16 can differentiate patients with PLN from those with less severe LN subtypes and active non-renal SLE. Our findings suggest that u-IL-16 could be used as a screening tool at suspicion of severe LN. Furthermore, the high IL-16 levels in plasma, urine and kidney tissue imply that IL-16 could be explored as a therapeutic target in SLE.

Keywords: cytokines; inflammation; lupus nephritis; systemic lupus erythematosus.

Figure 1

Distribution of plasma (p-) and urine (u-) IL-16 levels among the investigated groups. (A) Patients with LN had higher plasma levels of IL-16 in comparison with patients with INR-SLE and pCs. (B) Patients with LN had the highest urine levels of IL-16 in comparison with other investigated groups: ANR-SLE, INR-SLE and pCs. (C) p-IL-16 levels did not differ among the investigated LN classes: PLN, MLN and MES. (D) Patients with PLN had higher u-IL-16 levels than patients with MLN or MES. ANR, active non-renal; IL-16, interleukin 16; INR, inactive non-renal; LN, lupus nephritis; MES, mesangioproliferative lupus nephritis; MLN, membranous lupus nephritis; ns, not significant; pCs, population controls; PLN, proliferative lupus nephritis; SLE, systemic lupus erythematosus.

Figure 2

Logistic regression analysis of the diagnostic value of plasma (p-) and urinary (u-) IL-16 in subgroups of patients with LN. (A) ROC curves for u-IL-16, p-IL-16 and u-ACR demonstrate how these variables differentiate LN from ANR-SLE. (B) AUC demonstrates the diagnostic value of u-IL-16 and p-IL-16 for PLN compared with u-ACR among patients with other LN classes. (C) AUC demonstrates the diagnostic value of u-IL-16 and p-IL-16 for active PLN compared with u-ACR among patients with LN. (D) ROC analysis demonstrates the discriminative value of u-IL-16 and p-IL-16 in differentiating active PLN from MLN. (E) Discriminative value of u-IL-16 and p-IL-16 in differentiating PLN from other LN classes in patients with significant albuminuria of u-ACR >30 (corresponding to proteinuria of 500 mg/24 hours). (F) Discriminative value of u-IL-16 and p-IL-16 in differentiating PLN from other LN classes in patients with high albuminuria of u-ACR >50. ACR, albumin to creatinine ratio; ANR-SLE, active non-renal SLE; AUC, area under the curve; IL, interleukin 16; LN, lupus nephritis; MES, mesangioproliferative lupus nephritis; MLN, membranous lupus nephritis; PLN, proliferative lupus nephritis; ROC, receiver operator characteristic curve; SLE, systemic lupus erythematosus.

Figure 3

Expression of IL-16 and CD3 in LN kidney biopsies. (A) A high proportion of kidney infiltrating mononuclear cells express IL-16 in untreated PLN. (B) Numerically, a lower proportion of IL-16 positive cells were observed in patients with PLN with ongoing treatment (example of a patient on 10 mg of steroids). (C) A proportion of IL-16 positive kidney infiltrating cells were observed in MLN. (D) A proportion of the infiltrating cells express T cell marker CD3. IL-16, interleukin 16; LN, lupus nephritis; MLN, membranous lupus nephritis; PLN, proliferative lupus nephritis.