Absence of IFN-γ or IL-12 Has Different Effects on Experimental Myasthenia Gravis in C57BL/6 Mice (1)
Th1 cells have been implicated in the pathogenesis of Ab-mediated autoimmune diseases, because they help synthesis of Ab able to fix complement, that should be especially effective in causing tissue damage. IL-12 facilitates Th1 responses by stimulating the differentiation of naive Th cells into Th1 cells, and by serving as a costimulus for maximum IFN-γ secretion by Ag-activated Th1 cells. Activated Th1 cells secrete IFN-γ, which promotes further Th1 cell proliferation and activates macrophages. Myasthenia gravis (MG)4 and its animal model, experimental MG (EMG), are caused by Ab to the muscle acetylcholine receptor (AChR).
Th2 cells modulate immune responses by secreting anti-inflammatory cytokines, like IL-4 and IL-10, that down-regulate the function of APC and Th1 cells.
Two WT mice developed persistent myasthenic weakness from week 10 onward. The other three mice had holding times indicative of EMG between 11 and 12 wk and recovered by week 14 (i.e., 6 wk after the third TAChR immunization). None of the IL-12−/− mice developed stable EMG weakness. All IFN-γ−/− mice developed EMG. Three had abnormally reduced holding time after the second TAChR injection when the WT mice still had normal holding times.
IFN-γ−/− and WT mice developed EMG with similar frequency and severity.
Substantial variations in the concentration of anti-TAChR Ab from mouse to mouse are reflected in the large SDs of the values. The average concentration of serum anti-TAChR Ab was similar for all three groups. After three immunizations with TAChR, WT and IFN-γ−/− mice had average serum concentrations of anti-TAChR Ab of 2.8 ± 1.1 and 2.1 ± 0.7 μM: the difference was not statistically significant.
WT mice synthesized primarily anti-TAChR IgG2b and IgG2c, to a lesser extent IgG1, while IgG3 was undetectable; the anti-IgG2a mAb yielded a very small signal that likely resulted from cross-reactivity with the highly homologous IgG2c. IFN-γ−/− and IL-12−/− mice had anti-TAChR IgG1 in amounts similar to those of the WT mice, whereas their anti-TAChR IgG2c (and IgG2a) was strongly reduced in both strains.
Comparable amounts of Ab-complexed AChR in all three groups were obtained. The EMG resistance of the IL-12−/− mice was not due to inability of their anti-TAChR Ab to bind mouse muscle AChR. All mice had IgG bound to the neuromuscular junctions. All WT and IFN-γ−/− mice also had complement, whereas we could not detect the presence of complement in any muscle section from the IL-12−/− mice. only WT and IFN-γ mice had complement. When stained for the different IgG2 subclasses, the neuromuscular junctions of the WT mouse stained for both IgG2b and IgG2c, those of the IFN-γ−/− mouse stained only for IgG2b, whereas those of the IL-12−/− mouse did not stain for either IgG2b or IgG2c.
All strains recognized the TAChR vigorously and to comparable extents. They all recognized the peptides spanning the sequence region 146–169 and peptide 360–378. Other peptides were recognized inconsistently. The cells from IFN-γ−/− mice secreted IL-4 and IL-10 in amounts comparable to those observed for the cells from the WT mice; they did not secrete IFN-γ and secreted moderate, yet significantly reduced amounts of IL-2 as compared with WT mice. The cells from IL-12−/− mice secreted much more IL-4 and IL-10 than those from either WT or IFN-γ−/− mice; they secreted IL-2 in amounts that were slightly, but not significantly, reduced as compared with WT mice and significantly less IFN-γ than the WT mice.
The 129/SvEv and the C57BL/6 strains both have the H-2b allele, and the I-A allele influences EMG susceptibility; however, genetic factors unrelated to the MHC haplotype also influence susceptibility to EMG. The serum concentration of anti-AChR Ab does not correlate with symptom severity in EMG or MG, indicating that not every subpopulation of anti-AChR Ab causes the disease. Development of several T cell-mediated experimental autoimmune diseases requires IL-12. IL-12 has been implicated also in the pathogenesis of multiple sclerosis. IFN-γ has complex and contrasting effects in the development of T cell-mediated autoimmune diseases. Most EAE-inducing T cell clones isolated from diseased mice secreted IFN-γ and Ab that neutralized IFN-γ-inducing factor prevented EAE.