Toxoplasma gondii Cathepsin C1 inhibits NF-κB signalling through the positive regulation of the HIF-1α/EPO axis.

Toxoplasma gondii has evolved many successful strategies for immune evasion. However, the parasite-derived effectors involved in modulating NF-κB signalling pathway are largely unknown. T. gondii Cathepsin C1 (CPC1) is widely conserved among T. gondii strains and is important for T. gondii intracellular growth and proliferation. Our study showed that CPC1 protein could abrogate NF-κB activation after screening dense granule proteins. CPC1 suppressed NF-κB activation at or downstream of p65 and decreased the production of IL-1, IL-8, IL-6, IL-12, and TNF-α. Western blot analysis revealed that CPC1 inhibited phospho-p65 and CPC1 proteins primarily settled in cytoplasm. RNA sequencing analysis revealed that overexpression of CPC1 significantly upregulated erythropoietin (EPO), which can be induced by the hypoxia-inducible factor -1α (HIF-1α) during hypoxia. Furthermore, dual-luciferase reporter assays confirmed that CPC1 upregulated HIF-1α. Finally, both the knockdown of EPO and restriction of HIF-1α partially eliminated the suppression impact of CPC1 on the NF-κB signalling pathway. Our study identified a previously unrecognized role of CPC1 in the negative regulation of NF-κB activation through positive regulation of the HIF-1α/EPO axis. For the first time, CPC1 was shown to play an important role in immune evasion during T. gondii infection.