The disclosure provides materials in the form of nanoparticles containing in vitro-transcribed mRNAs encoding antigenic peptides and personalized medicine methods for treating or preventing diseases such as cancer, an autoimmune disease, an infectious disease, or inflammation, wherein cells of a subject receiving prophylactic or therapeutic treatment are brought into contact with the nanoparticles ex vivo, and the subject's cells process the mRNA. expressing and presenting the encoded product to activate the subject's own T cells. thereby mounting an immune response to the diseased cells. such as cancer cells.