Objective Roles of the programmed cell death protein 1 (PD-1) and its ligands (PD-Ls) in suppressing the immune response and the soluble porcine PD-1 (sPD-1) protein in inhibiting the porcine circovirus type II (PCV2) infection on pigs were investigated in search for a new approach to control the disease in animals.

Method The recombinant host, pET32a-PD1/Rosetta, constructed in the laboratory was used for the extensive induction and purification to determine the sPD-1 expression conditions. Under which, a large amount of sPD-1 was purified to react with the peripheral blood mononuclear cells (PBMCs) infected by PCV2. The effects of PD-1 on the cells were measured by the indirect immunofluorescence and RT-qPCR, on the PBMC proliferation by CCK-8 detection and flow cytometry, and on the transcriptions and secretions of IL-2, IL-12, IL-21, IL-17A, and IFN-γby RT-qPCR and ELISA.

Results Highly purified recombinant sPD-1 showing high activity was successfully obtained. The viral load in the PBMCs induced with sPD-1 at 10 μg·mL⁻¹ were significantly lower than that in those infected by PCV2 in the absence of sPD-1 to below 1 000 copies·μL⁻¹. Meanwhile, the PBMC cell proliferation index shown by CCK-8 significantly increased (P＜0.05). The flow cytometric measurement on fluorescence intensity of (68.6±10.14)% on the PBMCs was significantly higher with the presence of sPD-1 than (28.7±3.18)% on the counterparts without the soluble protein (P＜0.05). The RT-qPCR and ELISA also showed the cytokine transcriptions and secretions of IL-2, IL-12, and IFN-γ to be significantly higher under the same condition (P＜0.05).

Conclusion The presentation of sPD-1 significantly inhibited PCV2 infection on PBMCs through the related metabolic pathways. At the same time, it promoted the PBMC cell proliferation, cytokine transcriptions, and secretion. Therefore, the protein could potentially be used in mitigating PCV2 infection on animals.