Regulation of self‐renewal of human T lymphocyte colony‐forming units (TL‐CFUs)

Formation of human T lymphocyte colonies in semisolid medium from T lymphocyte colony‐forming units (TL‐CFUs) under stimulation of phytohemagglutinin (PHA) has been reported by several authors. These TL‐CFUs were present in unsensitized lymphocyte populations. We report here that such TL‐CFUs are capable of renewing themselves. This was observed when colony cells from primary T cell colonies that developed in the presence of PHA were replated in methylcellulose medium containing irradiated autologous leukocytes and PHA. We have also been able to demonstrate serial transfer of TL‐CFU for up to six passages. At each passage, colony‐forming frequency was determined from the proportional relationship between the number of new colonies obtained and the number of colony cells plated. Examination of the number of new colonies derived from each individual T cell colony (“burst size of TL‐CFU”) showed that most colonies contained very few new TL‐CFU and only a very small number of colonies contained many new TL‐CFU. The distribution of burst sizes could be well fitted to a gamma distribution, in agreement with prediction from a stochastic model. We have identified an activity that enhanced the mean TL‐CFU burst size three to ten times. This work provides the first evidence in vitro that self‐renewal of human T lymphocyte progenitor cells can be stimulated by specific regulatory proteins.