Live Cell-Based Assays May Find Antibodies in Some Seronegative MG Cases

Although a number of therapies are available to treat people with the neuromuscular junction disorder myasthenia gravis (MG), some patients do not receive the diagnosis because existing assays cannot detect disease-associated antibodies in their blood.

In a new study published in Neuromuscular Disorders, investigators sought to see whether a different type of assay might better identify patients with MG. Their results highlight the significant need for better diagnostic strategies.

The study authors explained that the most common autoantibodies in people with MG are directed against the acetylcholine receptor (AChR-ab), the muscle-specific tyrosine kinase (MuSK-ab), and the lipoprotein-related protein 4 (LRP4-ab). Of those, AChR-ab is the most prevalent, present in about 75% to 80% of cases.

“However, in approximately 15% of MG patients no known serum antibodies can be detected with the currently available assays in clinical practice and are therefore termed to be ‘seronegative’ (SNMG),” they wrote.

The investigators added that in the United States and Europe, access to monoclonal antibody therapy is only available to patients who test positive for AChR-ab.

They said one way to solve the problem would be to test different diagnostic assays. They noted that fixed and live cell-based assays are believed to have higher sensitivity than more routinely used assays like the enzyme-linked immunosorbent assay (ELISA), the indirect immunofluorescence test (IIFT), and the radioimmunoprecipitation assay (RIA). Live cell-based assays (L-CBAs) have recently been developed that were designed to identify the antibodies associated with MG. In one, “acetylcholine receptors are expressed in high density on the cell membrane of human embryonic kidney cells by means of the ‘cluster protein’ rapsyn,” they wrote.

Further, one report suggests that among people with SNMG who were tested by RIA, the rate of antibody binding to rapsyn-clustered AChR was 66%.

The investigators wanted to know how frequently live cell-based assays might be able to detect clustered AChR, MuSK, and LRP4 autoantibodies in people with SNMG. They recruited 67 German patients who were seronegative via ELISA and IIFT and tested them by RIA and L-CBAs.

Only 2 patients tested positive for AChR via RIA, and just 3 patients were positive for clustered AChR. The patients who showed binding to the adult AChR were also tested for binding to the fetal AchR, and 2 of those 3 tests were positive. No patients were positive for MuSK or LRP4.

The authors noted that their rate of clustered AChR-ab (4.5%) was significantly lower than the rate found in previous studies, although they said that may be partly due to the fact that their cohort included patients with ocular MG and not generalized MG.

“It is well known that the rate of AChR-ab detection is lower in patients with purely ocular symptoms compared to patients with generalized MG,” they wrote. They added, however, that even after adjusting for ocular symptoms, their rate of clustered AChR-ab was still lower.

Still, the authors said the strategy used in this study is important because it can help differentiate patients who are truly “triple negative” for the 3 main MG antibodies from those who are positive for the antibodies but who cannot be identified through the most routinely used assays.

“It appears likely that targets for autoantibodies in addition to AChR, MuSK, and LRP4 exist in rare cases of MG,” they wrote. “Affected patients would benefit from early diagnosis and the possibility of a personalized, antibody-specific treatment strategy.”

Reference

Hoffmann S, Waters P, Jacobson L, et al. Autoantibody detection by a live cell-based assay in conventionally antibody-tested triple seronegative Myasthenia gravis. Neuromuscul Disord. 2023;33(2):139-144. doi:10.1016/j.nmd.2023.01.002