Immunological effects of insulin-like growth factor-I--enhancement of immunoglobulin synthesis.

In addition to its activity as a metabolic hormone and a regulator of somatic growth, insulin-like growth factor-I (IGF-I) has cytokine-like activities on lymphoid cells. A 14-day infusion of recombinant human (rh)IGF-I increased lymphocyte numbers in all the peripheral lymphoid organs examined. This increase was apparent for up to 3 weeks following cessation of hormone treatment. A second administration of rhIGF-I, given when the lymphocyte numbers in the rhIGF-I-treated mice had returned to control values, resulted in similar increases in the peripheral T and B cell populations. This increase in lymphocyte numbers had functional significance, since rhIGF-I-treated mice produced elevated antibody titres following primary or secondary antigen challenge compared with controls. In addition, when rhIGF-I-treated mice were immunized with a suboptimal dose of antigen they produced antibody titres which were equivalent to those generated by immunization with optimal doses of antigen. When examined in vitro, addition of rhIGF-I alone to cultures of splenocytes from antigen-primed mice stimulated immunoglobulin synthesis. These studies suggest that IGF-I produced locally by thymic and bone marrow stromal cells may be a natural component of B and T cell lymphopoiesis.