The role of CD44 in the pathogenesis of lupus nephritis

Abstract

Lupus nephritis (LN) is a serious manifestation of systemic lupus erythematosus (SLE), a debilitating autoimmune disease resulting in significant morbidity and mortality. CD44 is a transmembrane glycoprotein expressed in connective and lymphoid tissue and is implicated in inflammation and fibrosis. It is also a component of the endothelial glycocalyx and is shed into the circulation during endothelial cell activation. Emerging evidence suggests that CD44 contributes to autoimmune and renal diseases, but the mechanism remains obscure. In this study, we investigated the clinico-pathological association of serum CD44 level in patients with biopsy-proven Class III/IV } V LN and its involvement in kidney inflammation and fibrosis in NZB/W F1 mice, an established murine model of LN. Serum CD44 level, determined by ELISA, was significantly increased in LN patients with active disease compared to healthy subjects or patients with quiescent LN, non-renal SLE, or non-lupus CKD. Serum CD44 level showed higher sensitivity and specificity than anti-dsDNA antibody and C3 levels in distinguishing between patients with active LN and LN remission. Serum CD44 level correlated with clinical and serological parameters of disease, including anti-dsDNA antibody level, proteinuria, serum creatinine and serum urea levels, and both SLEDAI-2K and renal SLEDAI- 2K scores, and inversely correlated with C3 level, eGFR, and serum albumin level. All episodes of LN flare were accompanied by increased serum CD44 level, which decreased after treatment with immunosuppressive agents. When compared to conventional serological markers of LN activity (anti-dsDNA antibody and C3 levels) and kidney damage (proteinuria, serum creatinine and renal SLEDAI-2K score), CD44 level demonstrated better correlation with eGFR, and comparable correlation with conventional serological and clinical parameters of disease. Our longitudinal analysis included 410 sera from 39 patients, collected serially at intervals of 3-4 months over 3 years of follow-up, with at least 1 sample collected during active disease. Increased serum CD44 level preceded clinical flare by 4.5 months and its level was reduced after 9 months following treatment. Renal biopsies from patients with active proliferative LN showed induction of CD44 expression in tubular epithelial cells and infiltrating immune cells. Serum CD44 level correlated with leukocyte infiltration and interstitial inflammation scores in active LN kidney biopsies, whereas anti-dsDNA antibody and C3 levels showed no association with renal activity or chronicity indices. Female NZB/W F1 mice were randomised to receive either anti-CD44 antibody or non-immune rat anti-mouse IgG by weekly intravenous injections, for four weeks. Administration of anti-CD44 antibody to mice significantly decreased the rate of progression of proteinuria, preserved kidney structure, and reduced mediators of inflammation and fibrosis including HA and collagen expression. Our data demonstrated the involvement of CD44 in LN pathogenesis by mediating tubulo-interstitial inflammation and fibrosis. Our findings also suggested the potential of serum CD44 level serving as a serological biomarker for early diagnosis of LN flare and monitoring of disease activity and treatment response.