Unicellular or multicellular origin of human T-lymphocyte colonies in soft agar?

Human peripheral lymphocytes were cultivated in soft agar or agarose, containing 3H-thymidine or bromodeoxyuridine (BrdU), which is an analogue of thymidine readily incorporated into deoxyribonucleic acid by lymphocytes. Colonies were isolated and analysed by autoradiography or stained with the fluorescence-plus-Giemsa technique, which identifies cells that have passed one, two, three, or more replication cycles with incorporation of BrdU. Colonies contained cells in their first and second division cycle together with later generations. The labelling index was very high. Consequently, it was concluded that, at least without special precautions being taken, lymphocyte colonies in semi-liquid medium are likely to grow through recruitment of mobile cells as well as through divisions within the colonies. Thus, T-cell growth in gels seems to be a matter of recognition between activated blast cells rather than monoclonal growth.