Autoimmune B Cell Repertoire in a Mouse Model of Sjögren’s Syndrome (1)
One of the hallmarks of Sjögren’s syndrome (SjS) is inflammation of the exocrine tissue, termed focal lymphocytic sialadenitis, and the presence of anti-nuclear antibodies such as anti-dsDNA and anti-Ro52 that are useful diagnostic markers. In susceptible individuals, chronic B-cell activation may lead to the expansion of autoreactive lymphocytes that can lead to organ damage developing autoimmune disorders. The frequency of IgM and IgG autoreactive antibodies was significantly increased in autoantibodies from SjS mice as compared to B6 wild-type mice, especially in mice with an age over 60 weeks.
The percentage of anti-Ro52 IgG antibodies in the SjS mice increased dramatically with age to the point that more than 50% of the B cell clones were reactive with this antigen in 66-week old animals.
A higher frequency of polyreactivity in SjS mice was shown as compared with B6 mice. These differences were observed for both IgM and IgG antibodies at all tested ages.
Although most antibodies presented distinct patterns of polyreactivity, an increase in the antigen binding reactivity of the IgG antibodies from aged SjS mice was detected.
The vast majority of the polyreactive IgG antibodies were of the IgG2b subclass, whereas most of the polyreactive antibodies from the B6 mice corresponded to the IgG3. An increase trend in the number of non-synonymous mutations affecting amino acids in the heavy chain variable region of antibodies with age in SjS mice.
Antigen reactivity strength of the five polyreactive antibodies that correspond to the two expanded clones was tested at several concentrations with the set of antigens. The antibodies with the same sequence presented identical reactivity patterns. Although the staining pattern was the same between clonally related hybridomas, H2h4.7.94 presented a stronger cytoplasmic staining.
All polyreactive IgG antibodies in the SjS mice corresponded to IgG2b and IgG2c isotypes as compared to the B6 mice where IgG3 isotype was the most abundant. IgG2b and IgG2c are known to be isotypes more pathogenic than the IgG3 antibodies in SLE mouse models. The activation of autoreactive B cells in these mice may occur in extrafollicular localizations. Immature B T1 cells can produce isotype class-switch to IgG2b and IgG2c in a T-independent TLR7 dependent way. All anti-Ro52 autoantibodies were polyreactive, the same as anti-Ro52 antibodies from SjS patients presenting high levels of polyreactivity. Monospecific anti-DNA antibodies did not generate lupus nephritis, polyreactive anti-DNA antibodies did. Polyreactivity is a pathologic characteristic for lupus nephritis. Anti-DNA/anti-NMDAR (N-methyl-D-aspartate receptor) cross-reactive antibodies generate damage in the hippocampus and also cause a flexible memory deficit, when penetrating the blood brain barrier.