Provided herein is a mammalian expression system for producing recombinant porcine circovirus type 2 (PCV2) virus-like particles (VLPs). The expression system includes a mammalian cell and a plasmid that comprises a PCV2 gene encoding a capsid protein. The PCV2 gene is codon optimized, and the mammalian cell is transfected with the plasmid. The expression system produces recombinant PCV2 VLPs, such as PCV2d VLPs. Also provided herein are a method for producing porcine circovirus type 2 (PCV2) virus-like particles (VLPs), as well as a PCV2 VLP generated by the method.

The present invention relates to self-replicating DNA expression vectors comprising elements from Beak and Feather Disease Virus (BFDV), including a BFDV LIR element comprising an origin of replication (Ori), genes encoding a BFDV Rep protein and a heterologous polypeptide of interest under the control of a mammalian promoter, wherein the Rep protein is capable of binding to the Ori to initiate replication of the expression vector. Pharmaceutical compositions and cells comprising the self-replicating vectors are also provided. The self-replicating vectors may be used in methods of inducing an immune response or expressing a polypeptide in a subject, or as an expression system.