Effect of anti-muscarinic autoantibodies on leukocyte function in Sjögren's syndrome

Patients with primary Sjögren's syndrome, a systemic autoimmune disease, have been shown to have serum autoantibodies that react with the muscarinic acetylcholine type 3 receptor (M3R).Primary Sjögren's syndrome is a systemic autoimmune disease. Patients with primary Sjögren's syndrome have been shown to have serum autoantibodies that react with the muscarinic acetylcholine type 3 receptor (M3R). Leukopenia has been reported to be significantly more common in primary Sjögren's syndrome patients who have anti-M3R-autoantibodies in their sera. In this study, we investigated whether these anti-M3R autoantibodies have effects on M3R and MHCI expression in Jurkat T cells. Purified IgG antibodies were isolated from the serum of healthy individuals and primary Sjögren's syndrome patients. Jurkat cell line was used to represent T lymphocytes. In situ immunofluorescence confocal microscopy was used to confirm the binding reactivity of primary Sjögren's syndrome IgG antibodies to M3R. Co-immunoprecipitation and immunofluorescence results suggested a direct interaction between M3R and MHC I. Co-internalization of M3R and MHC I was observed when Jurkat cells were exposed to the primary Sjögren's syndrome IgG, but this primary Sjögren's syndrome IgG-induced co-internalization of M3R and MHC I was prevented by the presence of exogenous IFN-γ. Primary Sjögren's syndrome IgG itself did not affect the viability of Jurkat cells, but Jurkat cells exposed to primary Sjögren's syndrome IgG were observed to undergo significant cell death when co-cultured with primary Natural Killer cells. Our results suggest that anti-M3R autoantibodies in primary Sjögren's syndrome induce downregulation of plasma membrane-resident M3R and MHC class I molecules in leukocytes followed by NK cell-mediated cell death. This mechanism may explain the frequency of leukopenia occurrence in patients with primary Sjögren's syndrome.