Urinary ALCAM as a biomarker for lupus nephritis
by Dr Cherie Ng and Dr Chaim Putterman
Lupus nephritis is an immune-mediated glomerulonephritis that causes significant morbidity and mortality in patients with systemic lupus erythematous, a multisystem autoimmune disease. Non-invasive biomarkers are necessary to track disease and treatment response. Urinary ALCAM is a potential biomarker that samples the kidney and is relevant to the mechanisms driving disease.
Introduction
Systemic lupus erythematosus (SLE) is a chronic autoimmune inflammatory disease that predominantly affects women of reproductive age. Approximately half of SLE patients develop lupus nephritis (LN), a serious complication in which autoimmunity-driven inflammation involves the kidney. Presentation of LN can vary from subclinical abnormalities to overt nephritis and nephrotic syndrome, and can result in significant morbidity and mortality despite the use of anti-inflammatory and immunosuppressive treatments. Of SLE patients with LN, ∼40% progress to chronic kidney disease and ∼10% progress to end-stage kidney disease [1] requiring renal replacement therapy (dialysis or a kidney transplant). Arresting renal disease is important to improving long-term prognosis; biomarkers of LN for diagnosis and moni-toring of renal inflammation and disease progression are essential tools in working towards this objective.
The established biomarkers for monitoring LN disease arelimited by either the invasiveness of the procedure or the inability to accurately track disease activity, predict disease flares, or monitor response to treatment. Renal biopsy has long been the gold standard for determining diagnosis and disease severity of LN. Although biopsy is highly informative due to the ability to directly assess the diseased tissue, the highly invasive nature of the procedure is not appropriate for repeated use in monitoring disease over time. Other less-invasive sampling methods are collection of serum and urine. Urine is the most attractive spe-
cimen type because it samples the kidney directly and collection is non-invasive and is, thus, the most amenable to the patient for frequent sampling over time. Currently, the main urine biomarker utilized is proteinuria as assessed by the spot urine protein-to-creatinine ratio (UPCR) and 24-hour proteinuria. Although proteinuria does predict the long-term prognosis of LN, it lacks sufficient sensitivity and specificity for differentiating renal inflammation versus chronic irreversible damage. More-over, significant kidney damage can occur before renal function is measurably impaired, therefore impeding early detection and treatment of nephritis. Finally, proteinuria levels are not always indicative of ongoing inflammation or the level of inflammation in the kidneys, which makes it less reliable for detection of disease flares. Consequently, it is necessary to identify new biomarkers that are able to assess disease severity, predict renal flares, and monitor treatment response and disease progression.
ALCAM as a diagnostic biomarker in LN
Much of the search for new urinary biomarkers has been focused on soluble cytokines, chemokines, and urinary enzymes. One such potential biomarker is activated leukocyte adhesion molecule (ALCAM). ALCAM is an approximately 100-kD glycoprotein that is typically membrane bound but can be shed from the cell surface to create a soluble version of the protein [2]. Functionally, ALCAM has roles in cell–cell adhesion through homotypic interactions and in T-cell activation and migration through heterotypic interactions with the CD6 receptor. ALCAM was first identified as a possible renal biomarker in a proteomic screen of urine from patients with LN [3]. A soluble form of the protein was found to be elevated in the urine of SLE patients with active LN compared to patients with quiescent nephritis or no prior nephritis. The ability of ALCAM to differentiate renal disease severity has now been tested in multiple cohorts across multiple ethnicities, including Black, Asian and Hispanic and Caucasian patients [3–5].
The consensus of these analyses is that urine levels of ALCAM
as a biomarker may outperform traditional/standard measures of SLE/LN in regards to sensitivity, specificity, and predictive power to detect active renal disease. Urinary ALCAM levels positively correlate with both general and renal disease activity, as measured by general disease severity scoring (systemic lupus erythematosus disease activity index; SLEDAI) and renal disease activity scoring [renal SLEDAI, Systemic Lupus International Collaborating Clinics registry for Atherosclerosis (SLICC RAS), British Isles Lupus Assessment Group (BILAG)] [4,5] suggesting that this protein is associated with renal disease progression. When compared to gold standard renal histopathology, urinary ALCAM levels have been found to correlate with activity index; in addition, urinary ALCAM levels were significantly increased in class III and IV (proliferative) LN as compared to those in class V (membranous) LN [5]. Given that proliferative LN has a less favourable prognosis for renal survival compared to membranous LN [6], the ability of ALCAM to distinguish the two states suggests that it is aligned with disease severity. When compared to the standard lupus biomarkers including serum anti-dsDNA antibody, complement 3 (C3), complement 4 (C4), and proteinuria, urinary ALCAM not only correlates with these measures but is better able to differentiate between the proliferative and membranous LN [5]. The ability of ALCAM to indicate disease severity is limited to the urine; serum ALCAM has not been able to discriminate active LN from other SLE patients [5].
Mechanistically, urinary ALCAM is a highly relevant renal biomarker because it is expressed by the tissues and cells involved in LN and may be a factor in the disease pathology. Within renal tissue, ALCAM is notably expressed by cells of the loop of Henle, the proximal tubule, and collecting ducts as well as by endothelial cells and antigen-presenting cells [7–9]. In other diseases with renal inflammation, ALCAM has also been shown to be elevated
in the kidney, including in the glomeruli with high expression in tubules and podocytes [10,11]. Thus, when renal inflammation is initiated, cells in the inflamed tissue increase their expression of ALCAM – some of which is subsequently shed into the urine, resulting in increased soluble ALCAM levels.
The increase in urinary ALCAM may not only represent the increase in tissue expression, but may also be indicative of the pro-inflammatory interaction between ALCAM and CD6. CD6 is highly expressed on T cells which are key cells in driving the pathogenic inflammation in lupus. The interaction of ALCAM and CD6 can occur in several ways: (i) Interaction of CD6+ T cells with ALCAM+ antigen-presenting cells resulting in re-stimulation and maintenance of pathogenic T-cell responses in the kidney; (ii) Interaction of CD6+ T cells with ALCAM+ kidney structural cells, which are known to express major histocompatibility class I
(MHCI) and/or MHCII proteins, also resulting in re-stimulation of T cells; or (iii) CD6+ T cells interacting with ALCAM on renal cells to aide in infiltration into the inflamed tissue. The synergy of these interactions may aid in sustaining high levels of pathogenic T-cell activity and trafficking, including release of pro-inflammatory cytokines, recruitment of inflammatory cells, and increased destruction of renal tissue, all of which further perpetuate the inflammatory cycle. Though it is unlikely that ALCAM or any other urinary biomarker will provide sufficient information alone to track and treat LN, ALCAM may provide a mechanistically relevant method of assessing the inflammatory state of LN.
Summary
Although ALCAM as a biomarker still needs to be assessed prospectively in longer term studies, studies of existing samples from multiple SLE and LN cohorts validate ALCAM as a relevant marker to disease that may provide more information than proteinuria alone on the inflammatory events in the kidney. The ability to detect early renal inflammation prior to impaired renal function is key to arresting or delaying LN disease.
The authors
Cheri Ng1 PhD, MPH and Chaim Putterman*2,3,4 MD
1 Equillium, La Jolla, CA, USA
2 Division of Rheumatology and Department of Microbiology & Immunology, Albert Einstein College of Medicine, Bronx, NY, USA
3 Azrieli Faculty of Medicine, Bar-Ilan Universty, Safed, Israel
4 Research Institute, Galilee Medical Center, Nahariya, Israel
* Corresponding author
E-mail: chaim.putterman@einsteinmed.edu
Figure 1. Schematic of ALCAM levels as lupus nephritis (LN)-associated renal inflammation increases
(a) Schematic of events leading to increased levels of soluble activated leukocyte adhesion molecule (ALCAM) in urine during renal inflammation. (b) Relative urinary ALCAM levels in healthy subjects and subjects with inactive systemic lupus erythematosus (SLE), active SLE without renal involvement, and LN. (C. Ng)
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