Specificity and Nature of the Antigen-Combining Sites on Fetal and Mature Thymus Lymphocytes

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Abstract
The thymus of several species was examined for antigen-binding lymphocytes (ABL), by using autoradiography and 125I-labeled antigens, flagellin from Salmonella adelaide and hemocyanin from Jasus lalandii. Counts of ABL in thymic cell suspensions, expressed per 104 lymphocytes, varied according to the species and antigen used. For flagellin the count of thymic ABL was considerably greater in the human fetus (182 ± 50) than the mouse fetus (2.9 ± 0.5), but for hemocyanin was greater in the mouse (36 ± 8.5) than the human (10 ± 2.5). In man and mouse counts of thymic ABL fell steadily with increasing age, from fetal to late adult life. The property of antigen binding could not be attributed to cytophilic antibody because this was not passively conferred on to lymphocytes in vitro or in vivo by antibody to flagellin in high titer. The in vivo experiment was on rats immunized with flagellin before pregnancy; antibody crossed the placenta yet failed to confer antigen-binding properties on lymphocytes of various fetal tissues. The binding of flagellin and hemocyanin to thymic lymphocytes in man and mouse was blocked by monospecific antisera to light chains and µ chains, even in high dilution. Most of the ABL in thymus cell suspensions would be expected to be thymus-derived (T) lymphocytes rather than immigrant B lymphocytes and the data were therefore interpreted as showing that such lymphocytes carry antigen-binding receptors of IgM character.

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