Deficiency of Selected Cathepsins Does Not Affect the Inhibitory Action of ECTV on Immune Properties of Dendritic Cells.

Ectromelia virus (ECTV), an orthopoxvirus, undergoes productive replication in conventional dendritic cells (cDCs), resulting in the inhibition of their innate and adaptive immune functions. ECTV replication rate in cDCs is increased due to downregulation of the expression of cathepsins - cystein proteases that orchestrate several steps during DC maturation. Therefore, this study was aimed to determine if downregulation of cathepsins, such as B, L or S, disrupts cDC capacity to induce activating signals in T cells or whether infection of cDCs with ECTV further weakens their functions as antigen-presenting cells. Our results showed that cDCs treated with siRNA against cathepsin B, L and S synthesize similar amounts of pro-inflammatory cytokines and exhibit comparable ability to mature and stimulate alloreactive CD4⁺ T cells, as untreated wild type (WT) cells. Moreover, ECTV inhibitory effect on cDC innate and adaptive immune functions, observed especially after LPS treatment, was comparable in both cathepsin-silenced and WT cells. Taken together, the absence of cathepsins B, L and S has minimal, if any, impact on the inhibitory effect of ECTV on cDC immune functions. We assume that the virus-mediated inhibition of cathepsin expression in cDCs represents more a survival mechanism than an immune evasion strategy.