An investigation of a possible humoral factor produced by the thymus in terms of its effect on immunological competence.

Four to six week old neonatally thymectomised mice given syngeneic homografts of neonatal thymus gained weight normally and produced a normal number of spleen cells capable of haemolysing sheep erythrocytes. Syngeneic newborn thymus grown in tissue culture for up to 9 days and then grafted similarly prevented neonatally thymectomised mice developing wasting disease and tended to restore the number of immunological competent spleen cells to normal. Repeated injections of fluid in which such thymuses had been grown in tissue culture failed, however, to affect neonatally thymectomised mice. They lost weight, suffered from wasting disease, and their antibody producing spleen cells were as diminished in number as those of untreated thymectomised controls. Most thymectomised mice given repeated injections of an aqueous extract of mouse thymus were similarly unaffected but a few of them gained weight and a few developed an increased number of spleen cells producing sheep haemolysins. Neonatal thymus enclosed in “Millipore” diffusion chambers implanted into neonatally thymectomised mice underwent early necrosis. The seals of most of the chambers broke down, and then on some occasions there was weight gain or an increase in number of immunologically competent spleen cells. It was possible that some thymus tissue may have escaped from the chamber and become a true homograft in the peritoneal cavity. It was concluded from these experiments that the part played by the thymus in developing the immune response in newborn animals was not mediated by an obvious humoral factor but the interaction between thymus and immature lymphocyte takes place at a more intimate level.