The role of annexin II in the pathogenesis of lupus nephritis

Abstract

Lupus nephritis is a severe organ manifestation of systemic lupus

erythematosus (SLE), and is characterized by the production of anti-dsDNA

antibodies. It is an important cause of renal failure. The mechanism through which

anti-dsDNA antibodies bind to tissues and mediate kidney injury remains to be fully

elucidated. Emerging evidence suggests that anti-dsDNA antibodies can bind to cells

and extracellular antigens directly through cross-reactivity, independent of bridging

chromatin material.

Mesangial cells play an important role in normal kidney structure and

functions, and its pathophysiology. Mesangial abnormalities in lupus nephritis

precede more severe injuries such as lesions in the glomerular capillary loop. We

previously demonstrated that the binding of human anti-dsDNA antibodies to

mesangial cells (HMC) correlated with disease activity and induced inflammatory as

well as fibrotic pathways. The aim of this project is to identify the cross-reactive

antigen(s) on the mesangial cell surface that mediates anti-dsDNA antibody binding

and the alterations in cell functions that result from this interaction.

HMC plasma membrane proteins were purified. Using proteomic and

biochemical approaches, we identified annexin II as the predominant cross-reactive

antigen on the HMC surface that mediated human polyclonal anti-dsDNA antibody

binding. Following this interaction, anti-dsDNA antibodies were internalized in a

time- and temperature-dependent manner, and translocated to both the cytoplasm and

nucleus within 30 min. This resulted in induction of annexin II synthesis, IL-6

secretion and cell proliferation, which was mediated through the activation of p38

MAPK, JNK and AKT. The binding activity to annexin II in the serum

immunoglobulin fraction correlated with the titre of anti-dsDNA antibody. Binding

activity of anti-dsDNA antibodies to annexin II correlated with clinical disease

activity and circulating anti-dsDNA antibody levels. These correlations were more

prominent in male patients with lupus nephritis. Glomerular annexin II expression

was increased in patients with active lupus nephritis and co-localized with IgG and

C3 deposition. Gene silencing of annexin II in HMC reduced anti-dsDNA antibody

binding, which was accompanied by reduced IL-6 secretion and cell proliferation.

Using female NZB/W F1 mice, an established murine model of lupus

nephritis, we demonstrated that intra-glomerular annexin II expression increased

with disease progression and was accompanied by an increase in the expression of

p11, its cellular protein ligand. Our data suggest that annexin II may exist in the

kidney as a heterotetramer and is involved in disease pathogenesis. At the

ultrastructural level, annexin II was detected in the mesangial matrix, amongst

electron dense deposits in the glomerular basement membrane, on the foot processes

in podocytes and within the Bowman’s capsule.

In conclusion, our data demonstrated that annexin II is the major cell

surface antigen on HMC that mediates the cross-reactive binding of human anti-DNA

antibodies. Through this interaction, cellular processes are triggered that contribute to

the pathogenesis of lupus nephritis.