Described herein are methods for suppressing an immune response in a subject, e.g., a subject with an autoimmune disease, by administering to the subject a therapeutically effective amount of recombinant CD5L, CD5L homodimers and/or CD5L:p40 heterodimers, or nucleic acids encoding any of these. Also described are methods for enhancing an immune response in a subject, e.g., a subject with cancer, infection, or an immune deficiency, by administering to the subject a therapeutically effective amount of an antibody or antigen-binding fragment thereof that binds specifically to CD5L, D5L homodimers and/or CD5L:p40 heterodimers, and inhibits their binding to the IL-23 receptor, or inhibits formation of the CD5L homodimer and/or CD5L:p40 heterodimer, or inhibitory nucleic acids that target CD5L and/or p40.