The present invention provides methods and compositions for the use of IL-22 to promote thymic growth following thymic insult. In particularly preferred embodiments, the present invention provides methods of using therapeutic IL-22 compositions for treating patients with thymic atrophy and alterations in bone marrow derived white blood cells, including cancer patients undergoing chemotherapy, patients exposed to radiation (i.e. cancer therapy, nuclear disaster, terrorist attack, etc.), patients with HIV infections/AIDS, patients with organ transplantation, aging patients, and the like. In a further embodiment, therapeutic IL-22 compositions are contemplated as a prophylactic to boost immune response when additional T-cell function is needed, i.e. to boost immune response during vaccination.