Protective role of anti-synthetic hinge peptide antibody for glomerular deposition of hypoglycosylated IgA1

History of IgA Nephropathy Mouse Models

IgA nephropathy (IgAN) is the most common primary glomerulonephritis in the world. It was first described in 1968 by Jean Berger and Nicole Hinglais as the presence of intercapillary deposits of IgA. Despite this simple description, patients with IgAN may present very broad clinical features ranging from the isolated presence of IgA in the mesangium without clinical or biological manifestations to rapidly progressive kidney failure. These features are associated with a variety of histological lesions, from the discrete thickening of the mesangial matrix to diffuse cell proliferation. Immunofluorescence on IgAN kidney specimens shows the isolated presence of IgA or its inconsistent association with IgG and complement components. This clinical heterogeneity of IgAN clearly echoes its complex and multifactorial pathophysiology in humans, inviting further analyses of its various aspects through the use of experimental models. Small-animal models of IgAN provide the most pertinent strategies for studying the multifactorial aspects of IgAN pathogenesis and progression. Although only primates have the IgA1 subclass, several murine models have been developed in which various aspects of immune responses are deregulated and which are useful in the understanding of IgAN physiopathology as well as in the assessment of IgAN therapeutic approaches. In this manuscript, we review all murine IgAN models developed since 1968 and discuss their remarkable contribution to understanding the disease.

Mesangial Deposition Can Strongly Involve Innate-Like IgA Molecules Lacking Affinity Maturation

BACKGROUND

IgA nephropathy (IgAN) often follows infections and features IgA mesangial deposition. Polymeric IgA deposits in the mesangium seem to have varied pathogenic potential, but understanding their pathogenicity remains a challenge. Most mesangial IgA1 in human IgAN has a hypogalactosylated hinge region, but it is unclear whether this is required for IgA deposition. Another important question is the role of adaptive IgA responses and high-affinity mature IgA antibodies and whether low-affinity IgA produced by innate-like B cells might also yield mesangial deposits.

METHODS

To explore the effects of specific qualitative variations in IgA and whether altered affinity maturation can influence IgA mesangial deposition and activate complement, we used several transgenic human IgA1-producing models with IgA deposition, including one lacking the DNA-editing enzyme activation-induced cytidine deaminase (AID), which is required in affinity maturation. Also, to explore the potential role of the IgA receptor CD89 in glomerular inflammation, we used a model that expresses CD89 in a pattern observed in humans.

RESULTS

We found that human IgA induced glomerular damage independent of CD89. When comparing mice able to produce high-affinity IgA antibodies with mice lacking AID-enabled Ig affinity maturation, we found that IgA deposition and complement activation significantly increased and led to IgAN pathogenesis, although without significant proteinuria or hematuria. We also observed that hinge hypoglycosylation was not mandatory for IgA deposition.

CONCLUSIONS

In a mouse model of IgAN, compared with high-affinity IgA, low-affinity innate-like IgA, formed in the absence of normal antigen-driven maturation, was more readily involved in IgA glomerular deposition with pathogenic effects.

O-linked oligosaccharides of the IgA1 hinge region: roles of its aberrant structure in the occurrence and/or progression of IgA nephropathy

Primary IgA nephropathy (IgAN) has been regarded as an immune complex-mediated glomerulonephritis characterized immunohistologically by the predominant deposition of IgA in the glomerular mesangial area with a variety of histopathologic injuries (Clarkson et al. in Ann Rev Med 38:157-168, 1987). In 1992, the characteristic structure of O-linked oligosaccharides (O-glycans) in the IgA1 hinge and its possible aberrancy were simultaneously and independently proposed by Mesteckey et al. (Cont Nephrol 104:172-182, 1993), and our group (Cont Nephrol 104:217, 1993) at the International Congress of Nephrology (IgA Nephropathy 25th year) held in Nancy, France. Since then, the aberrancy has been confirmed by several research groups and is suspected to play a role in the occurrence and/or the progression of IgAN. At the end of the 1980s, I took an interest in the existence of O-glycans in the hinge region of IgA1 and have pursued the structure of the carbohydrate chains. Since an excellent review on the structure and the role of the carbohydrate in IgA molecules was recently published by Narita et al. (Clin Ex Nephrol 12:332-338, 2008), this review focuses on the process by which I developed the idea of aberrant O-glycosylation in IgA1 molecules in IgAN patients and summarizes our recent observations concerning IgA1 molecules.