Simultaneous presence of receptors for complement and sheep red blood cells on human fetal thymocytes

The cells of four thymus glands, two at 12 weeks, one at 16 weeks and one at 22 weeks gestational age, were examined cytochemically for enzymic activity and for surface markers. The reactions for chloroacetate esterase, peroxidase, and alkaline phosphatase were negative and the reaction for nonspecific esterase was only weakly positive in some of the cells. In contrast, nearly all thymocytes at the 12th gestational week showed strong focal acid phosphatase (acP) activity. The number of acP-stainable cells and the staining intensity declined progressively. A high percentage of thymocytes in early fetal life express a complement receptor (CR); the proportion of CR-bearing cells decreased while cells forming rosettes with sheep erythrocytes (possessing erythrocyte receptors: ER) correspondingly increased during fetal life. Using a mixed rosette assay, up to 30% of the thymocytes at 12 weeks gestation were found to bear CR and ER simultaneously. The number of CR- and ER-positive cells also declined progressively with fetal age. These findings show that CR-positive, ER-negative thymus cells mature into CR-negative, ER-positive thymocytes via a CR-positive and ER-positive intermediate cell, indicating that mature ER-positive thymocytes do not originate from another cell line lacking CR. The changes in surface markers during early T cell maturation is relevant to certain lymphomas whose cells also show strong focal acP activity and also express both CR and ER. These lymphoma cells may correspond to fetal thymocytes or T precursor cells present in small numbers after birth.