Central Role of Macrophages and Nucleic Acid Release in Myasthenia Gravis Thymus

Myasthenia gravis (MG) is a neuromuscular disease mediated by antibodies against the acetylcholine receptor (AChR). The thymus plays a primary role in AChR-MG and is characterized by a type I interferon (IFN) signature linked to IFN-β. We investigated if AChR-MG was characterized by an IFN-I signature in the blood, and further investigated the chronic thymic IFN-I signature.

Serum levels of IFN-β and IFN-α subtypes, and mRNA expression for IFN-I subtypes and IFN-stimulated genes in peripheral mononuclear blood cells (PBMCs) were analyzed. The contribution of endogenous nucleic acids in thymic expression of IFN-I subtypes was investigated in human thymic epithelial cell cultures and the mouse thymus. By immunohistochemistry, thymic CD68⁺ and CD163⁺ macrophages were analyzed in AChR-MG. To investigate the impact of a decrease in thymic macrophages, mice were treated with an anti-CSF1R antibody.

No IFN-I signature was observed in the periphery emphasizing that the IFN-I signature is restricted to the MG thymus. Molecules mimicking endogenous dsDNA signalization (Poly(dA:dT) and 2'3'-cGAMP), or dexamethasone-induced necrotic thymocytes increased IFN-β and α-AChR expression by thymic epithelial cells, and in the mouse thymus. A significant decrease in thymic macrophages was demonstrated in AChR-MG. In mice, a decrease in thymic macrophages led to an increase of necrotic thymocytes associated with IFN-β and α-AChR expression.

These results suggest that the decrease of thymic macrophages in AChR-MG impairs the elimination of apoptotic thymocytes favoring the release of endogenous nucleic acids from necrotic thymocytes. In this inflammatory context, thymic epithelial cells may overexpress IFN-β, which specifically induces α-AChR, resulting in self-sensitization and thymic changes leading to AChR-MG. ANN NEUROL 2023;93:643-654.