A human thymoma with prothymocyte-like infiltration.

We have examined a human thymoma, round-oval epithelial-type cell with moderate lymphocytic infiltration, whose major lymphocytic component (67%), unlike the minor one (33%), did not form rosettes with sheep red blood cells (E rosettes). However, these cells were of T-cell nature as indicated by the positive staining for both acid phosphatase and beta-glucuronidase, two well-accepted cytochemical markers for T cells. The non-E-rosetting lymphocytes expressed T 10, but not T 3 and T 6 antigens. Moreover, they lacked both peanut agglutinin and Fc-IgG receptors. On the contrary, 71 and 19% of the E-rosetting cells were PNA- and Fc-IgM-receptor positive, respectively. Furthermore, the non-E-rosetting cells were phytohemagglutinin unresponsive. The non-E-rosetting lymphocytes were larger (greater than 7 micron) than the E-rosetting cells and showed a different nuclear chromatin pattern. These immunological, cytochemical, and morphological features strikingly resemble those exhibited by cells (prothymocytes) normally found only in fetal thymus. On the basis of these findings we hypothesize the existence in this thymoma of a prothymocyte-like infiltration.