Autoantibodies may serve as glaucoma biomarkers

The loss of autoantibodies may serve as useful biomarkers for glaucoma, according to Nadine von Thun Und Hohenstein-Blaul, Johannes Gutenberg University Mainz, Mainz, Germany, and colleagues.

“We conclude that the absence of some autoantibodies in glaucoma patients reflects a loss of the protective potential of natural autoimmunity and may thus encourage neurodegenerative processes,” they wrote in the journal Eye.

The researchers showed a correlation between changes in autoantibody profiles and the loss of retinal ganglion cells independent of IOP.

While an elevated IOP is the most common known risk factor for glaucoma, most people with an elevated IOP do not develop the disease.

For this reason, researchers have reasoned that other risk factors play a more important role in the development of the condition, especially in those with normal IOP.

Risk factors may include age, sex, ethnicity, oxidative stress, systemic and ocular vascular factors, elevated glutamate concentration or nitric oxide levels, and autoimmune conditions. However, IOP remains the only risk factor susceptible to modification as a treatment for glaucoma.

To date, most potential screening tests for glaucoma have an estimated 85% specificity and most patients suffer from glaucoma for more than 10 years before diagnosis, partly as a result of ineffective screening. During that time, as many as half their retinal ganglion cells may be lost.

Studies suggest up to half of glaucoma cases remain undiagnosed in developed countries, and 9 out of 10 worldwide, they wrote. Earlier diagnosis might prevent many cases of blindness. Researchers are looking for biomarkers that can be used to catch glaucoma cases earlier.

Von Thun Und Hohenstein-Blaul and her colleagues undertook an examination of proteomic changes in the retinas of glaucoma patients using mass spectrometry.

They found distinct changes in 10% of proteins supporting the involvement of three functional classes of in glaucoma: mitochondrial, stress and nucleus proteins, suggesting an impairment of energy metabolism, stress response, and gene expression alterations.

The researchers have also found evidence of autoimmune involvement in the pathogenesis of glaucoma. They have detected disease-specific changes in complex expression levels of immunoglobulin C (IgC) autoantibodies against ocular tissue in the sera and aqueous humour of patients with primary open-angle glaucoma, normal tension glaucoma, and ocular hypertension.

These profiles are stable and can be found in populations from different countries, they added.

Not only are the levels of some antibodies elevated, but others are lowered, the researchers found. For example, they found up-regulation of autoantibodies against alpha-fodrin, HSP70 and myelin basic protein, and down-regulation of antibodies against αB-Cyrstallin and Vimentin.

Exploring role of autoimmune responses

To explore the influence of these autoimmune responses, the researchers immunised animals with antibodies against some of the proteins in question, such as HSP60, HSP27, and MBP, resulting in glaucomatous damage with distinct retinal ganglion cell loss.

In another experiment, they attached a silicone loop around the limbi of animal eyes to temporarily increase the IOP. Pressure returned to normal after this manipulation, demonstrating that this does not create angle-closure glaucoma.

They also found that these intermittent IOP elevations caused degeneration in the optic nerve and the retina and changed IgC autoantibody activity. Attempts to modulate the immune response with the B lymphocyte inhibitor Belimumab did not increase axonal and retinal ganglion cell survival.

However, these authors have done in vitro studies showing that antibodies down-regulated in glaucoma patients, for example, antibodies against glial fibrillary acidic protein, 14-3-3 or gamma (γ)-synuclein, have protective effects on immortalized neuroretinal cells and increase their viability under stress conditions.

By incubating retinal organ culture from adolescent pigs with antibodies for 24 h they found a protective effect on retinal ganglion cells for anti-GFAP and anti- γ-synuclein. They did not find this effect for anti-myoglobin, which they used as a control.

They also found increased levels of glutamine synthetase in the antibody-treated retinal explants, a marker of Müller cells. Müller cells are essential for retinal ganglion cell survival and play a part in recycling neurotransmitters, homeostasis of the extracellular environment, removal of metabolic end products, and the supply of neurotrophic and antioxidant factors.

It is not clear whether such these changes in autoantibodies cause glaucoma or simply result from it, but they do seem promising as biomarkers for the disease, the researchers wrote.

Using a targeted antigen microarray, they differentiated sera of people with primary open angle glaucoma from people without glaucoma based on antibody profiles, with a sensitivity and specificity better than 93%.

“This represents an enormous diagnostic precision when compared to established screening tests,” they wrote. “Furthermore, the correlation between damage and changes in the autoantibody reactivity found in the intermittent ocular hypertension model suggests that autoantibody profiling could also be used as biomarkers for progression and severity of the disease.”