Circulating thymic hormone levels in severe combined immunodeficiency.

Twenty-three patients with severe combined immunodeficiency disease were studied for circulating thymic hormone levels (facteur thymique serique, FTS), 21 prior to treatment by transplantation of bone marrow, thymus or fetal liver. Thirteen showed undetectable FTS activity. Only two had normal levels of this hormone. In serial determinations of FTS activity prior to and after transplantation, patients given bone marrow transplants developed sustained increments of serum FTS activity early in the course of their immunological reconstitution. However, patients given transplants of fetal liver alone or fetal liver plus thymus from fetuses of less than 12 weeks gestation generally did not show an increment of FTS activity during the period of observation. Transplantation of irradiated thymus derived from fetuses of more than 14 weeks gestation produced sustained increases of thymic hormone activity. These observations suggest that a cell of haematopoietic origin provides a stimulus necessary for differentiation or maturation of thymic secretory activity and that this cell(s) is present in post-natal marrow, but is either undeveloped or immature in the early fetal liver or fails to migrate to the thymus of an allogeneic host.