Plant viruses are natural, self-assembling nanostructures with versatile and genetically programmable shells,making them useful in diverse applications ranging from the development of new materials to diagnostics andtherapeutics. Here, we describe the design and synthesis of plant virus nanoparticles displaying peptides associatedwith two different autoimmune diseases. Using animal models, we show that the recombinant nanoparticles canprevent autoimmune diabetes and ameliorate rheumatoid arthritis. In both cases, this is based on a strictlypeptide-related mechanism in which the virus nanoparticle acts both as a peptide scaffold and as an adjuvant,showing an overlapping mechanism of action. This successful preclinical testing could pave the way for the developmentof plant viruses for the clinical treatment of human autoimmune diseases.