The influence of dietary protein insufficiency on the murine thymus. Evidence for an intrathymic pool of progenitor cells capable of thymus regeneration after severe atrophy.

Low protein diets initiated at wearning in Balb/c mice cause a rapid and profound reduction in thymus weight and cellularity. Thymus weight falls to less than that of involuted thymus of adult mice and remains depressed for as long as diets are fed. Although most peripheral T cell functions do not appear to be depressed, suppressor cell activity was not as vigorous in deprived animals despite the presence of functional suppressor populations. Thymus growth was reinitiated promptly when high protein diets were fed to deprived animals. Thymus regeneration appeared to be due to both a resident population of stem cells which persisted in the thymus through the period of deprivation and a second, probably bone-marrow derived, population of stem cells. It is suggested that in normal mice the synchronized growth of the first population produces the characteristic innate growth pattern of the thymus. This is superimposed on the growth of the second population which continuously seeds the thymus and is constantly replaced. Protein deprivation severely restricts the growth of the first and second population, but both maintain their capacity for growth during long periods of protein restriction.