Modular activation of macrophage-like cells by beta-2-microglobulin via mitochondria and the cGAS-STING pathway

Beta-2-microglobulin (β2m) is a component of the major histocompatibility complex class I. β2m is released into cellular fluids in response to various stimuli, including pro-inflammatory cytokines. Elevated β2m levels have been found associated with autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and Crohn's disease, as well as in various hematological cancers and viral infections. Despite an established correlation between immune activation of especially monocytes and macrophages, and circulating β2m levels, the causative relationship remains unclear. Here, we investigate the effects of exogenous β2m and a complement C1s cleaved form, dK58β2m, on two murine macrophage-like cell lines J774 and RAW. We demonstrate that β2m, and to a greater extent dK58β2m, can affect mitochondrial activity. Furthermore, the presence of IFN-γ amplifies the effect, causing altered bioenergetics, and increased production of mitochondrial reactive oxygen species and nitric oxide. In addition, we found activation of the cGAS-STING pathway by β2m and dK58β2m in the presence of IFN-γ. Only dK58β2m in combination with IFN-γ caused apoptosis and cell death. Our findings highlight the modular nature of a β2m-induced macrophage response, potentiated by dK58β2m and IFN-γ, and provide information on the underlying mechanisms responsible for the immune activation properties of β2m.