A variety of NS-Ab have been identified in patients with AE or related disorder. Tissue-based assay (TBA) using a rodent brain IHC is used to screen NS-Ab while cell-based assay is used to determine NS antigen. A commercial rat brain IHC is currently available but its clinical relevance remains unclear. Immunostaining patterns of NS antigens have not been extensively studied yet.

We retrospectively reviewed the clinical information of 617 patients who underwent a testing for NS-Ab between January 2007 and September 2022 at the laboratory of Josep Dalmau (Pennsylvania, or Barcelona). Among those, we selected 255 patients whose archived CSF was examined with commercial IHC (Euroimmun AG) at Kitasato University to evaluate an immunostaining pattern. We assessed clinical value of the commercial IHC, and characterized NS antigen-specific immunostaining pattern with both in-house IHC and commercial IHC.

Sensitivity and specificity of “neuropil pattern” for predicting NS-Ab were 63/94 (67.0%), and 158/161 (98.1%), respectively. False-positive rate was 1.9% (3/161) while false-negative rate was 33.0% (31/94). In all 3 false-positive patients, neuropil-like staining was attributed to high titers of GAD65-Ab. In 31 false-negative patients, NMDAR was most frequently identified (n=18 [58.1%], 16/18 [88.9%] had low antibody titers [< 1:32]), followed by GABAaR (n=5). TBA revealed distinctive immunostaining pattern highly characteristic of each NS antigen.

TBA is useful not only for screening NS-Ab but also for estimating NS antigens; however, the negative results should be interpreted cautiously because “neuropil pattern” may be missed on commercial IHC when antibody titers are low.