An in vitro process of preparing virus-like particles (VLPs) from recombinant papaya mosaic virus coat protein and ssRNA, which allows for large scale production of VLPs in high yields, is provided. Also provided are VLPs comprising ssRNA prepared by the in vitro process. The VLPs can be used as adjuvants and when fused to an antigen, as vaccines. The use of the VLPs for stimulation of the innate immune response is also provided.