Age-dependent effects of the recombinant spike protein/SARS-CoV-2 on the M–CSF– and IL-34-differentiated macrophages in vitro

The SARS-CoV-2 virus causes elevated production of senescence-associated secretory phenotype (SASP) markers by macrophages. SARS-CoV-2 enters macrophages through its Spike-protein aided by cathepsin (Cat) B and L, which also mediate SASP production. Since M-CSF and IL-34 control macrophage differentiation, we investigated the age-dependent effects of the Spike-protein on SASP-related pro-inflammatory-cytokines and nuclear-senescence-regulatory-factors, and CatB, L and K, in mouse M–CSF– and IL-34-differentiated macrophages. The Spike-protein upregulated SASP expression in young and aged male M–CSF–macrophages. In contrast, only young and aged male IL-34-macrophages demonstrated significantly reduced pro-inflammatory cytokine expression in response to the Spike-protein in vitro. Furthermore, the S-protein elevated CatB expression in young male M–CSF–macrophages and young female IL-34-macrophages, whereas CatL was overexpressed in young male IL-34- and old male M–CSF–macrophages. Surprisingly, the S-protein increased CatK activity in young and aged male M–CSF–macrophages, indicating that CatK may be also involved in the COVID-19 pathology. Altogether, we demonstrated the age- and sex-dependent effects of the Spike-protein on M-CSF and IL-34-macrophages using a novel in vitro mouse model of SARS-CoV-2/COVID-19.