Sean Pittock, MD – Mayo Clinic:
Firstly, to clarify, NMO IgG is a term that’s essentially synonymous with aquaporin-4 IgG. Determinable IgG was introduced in the first paper, in the paper that described this antibody for the first time, and that was using a tissue-based immunofluorescence assay. Subsequent to the discovery of its target, then we began to use the term aquaporin-4 IgG.
How does one detect aquaporin-4 IgG? Well, traditionally we used an indirect tissue-based immunofluorescence assay. That was subsequently superseded by immunoprecipitation assays and then in the U.S. we began to use ELISA kit assays to detect this antibody. Those assays are still used in many of the laboratories throughout the U.S.
More recent data from collaborative studies that have been funded by the Guthy-Jackson Charitable Foundation have shown that the best assay for detection of aquaporin-4 antibodies is actually cell-based assays. These are assets where HEK293 cells are transfected with the water channel, and then you look at the binding of the patient’s antibody to that target that’s now expressed on the outside of that transfected cell.
You can either detect the antibodies by visual observation of the binding of the antibody to those cells down a microscope or you can do a flow cytometry analysis. What we’ve shown in much of our work is that the cell-based assays are more sensitive and more specific than the other assays that are currently available. I think it’s important to recognize that the optimal approach to testing for aquaporin-4 antibodies are cell-based assays. There are many laboratories that offer testing for aquaporin-4 antibodies throughout the U.S.