To characterize immunostaining pattern of individual neuronal surface (NS) antigens using in-house and/or commercial rat brain immunohistochemistry.

Tissue-based assay (TBA) is used for screening of NS antibodies (NS-Abs), while cell-based assay (CBA) is used to determine the NS antigens. Commercial CBA is currently available for many, but not all, NS antigens. Accordingly, when only CBA is used to determine the etiology of autoimmune encephalitis (AE), NS antibodies not examined with CBA may be missed.

We retrospectively reviewed the clinical information of 531 patients with suspected AE or its related disorder who underwent testing for NS-Abs between January 2007 and August 2021 at Dalmau’s Lab (Pennsylvania or Barcelona); NS-Abs were positive in 123 patients. We selected 47 patients with antibodies against diverse NS antigens (NMDAR, GABAaR, GABAbR, LGI1, Caspr2, GluK2, and AMPAR) and performed TBA with either in-house or commercial rat brain immunohistochemistry (EUROIMMUN AG). We also performed TBA in 78 antibody-negative patients. In-house immunohistochemistry was developed at Kitasato University.

We performed TBA in 47 antibody-positive patients with either in-house (n=6), commercial (n=23), or both immunohistochemistry (n=18), and in 78 antibody-negative patients with either in-house (n=14), commercial (n=55), or both (n=9). TBA showed antigen-specific distinctive immunostaining pattern on both in-house and commercial immunohistochemistry with similar distribution; however, when antibody titers were low, in-house immunohistochemistry showed more clear reactivity with NS antigens than commercial immunohistochemistry. In antibody-negative patients, TBA did not show apparent reactivity. TBA disclosed a glial (GFAP) or synaptic (GAD65) pattern in a small group independent of NS-Ab positivity.     

Both In-house and commercial immunohistochemistry are useful not only for screening of NS-Abs but also for estimation of the NS antigen; however, the results of commercial immunohistochemistry should be interpreted carefully because NS-Abs may be missed when antibody titers are low. TBA provides additional clues, such as GFAP positivity.