**5. Intrathecal Ig synthesis in animal models—technical limitations**

IgG synthesis level in CSF is highly correlated with CD19+CD138+ plasmablast levels [133], but although ITS is commonly thought to be associated with CSF floating cells, this minute cell number may account for less than 0.1% of the whole ITS, meaning that the bulk of IgG synthesis is provided by resident IgG‐secreting cells, residing either in the meninges or in the perivascular areas [134]. In experimental allergic encephalomyelitis (EAE) models, the parenchymal level of B‐cell infiltration increases from null to 134 B‐cell/cm<sup>2</sup> [135] and the whole number of infiltrating B‐cells is in the range of 10<sup>3</sup> –105 cells per mouse or rat brain [136, 137]. In MS, B‐cells remain a minor proportion of the infiltrating lymphocytes (<5%), are virtually absent from the parenchyma, and are mostly observed around the small veins and meninges (4.6–6 cells/mm<sup>2</sup> )[1, 138]. Plasma cells are occasionally found in demyelinated areas. No data are available regarding a possible role of meningeal B‐cell aggregates in ITS.

IT Ig synthesis in animal models has received little attention since the very small volume of CSF does not facilitate sampling. Although older experiments based on low‐sensitivity techniques failed to demonstrate any ITS during EAE, optimal techniques (IEF followed by anti‐IgG staining) have subsequently confirmed the local synthesis of OCB and/or an elevated IgG index [139–142]. In viral models of demyelination (i.e., measles or JHM strain of corona‐ virus infection), a mirror pattern of OCB is common but local OCBs are always revealed by immunoblot against viral antigens [21, 143]. We are not aware of any EAE experiments aimed at demonstrating the nonspecific pattern of ITS observed in MS, especially the animal coun‐ terpart of the human MRZ pattern using common animal viruses or vaccines. From a theoret‐ ical point of view, animals receiving intrathecal vaccination with foreign antigens such as albumin are able to mount a strong intrathecal response [144–146]. In a unique case challenged intrathecally with ovalbumin (OVA), an increase in AI‐herpes was also obtained apart from the expected increase in AI‐ovalbumin, suggesting that animal models are promising for future studies of nonspecific ITS [145].
