Different response of mouse thymocyte subpopulations to interleukin 2 and concanavalin A

Fractionation of C57BL/6 mouse thymocytes by Percoll density gradient centrifugation resulted in three thymocyte subpopulations of different characters. The first population (Fr 1 cells) and both the second (Fr 2 cells) and the third (Fr 3 cells) populations mainly consisted of Thy 1.2+, Ly 1+, Ly 2-, PNA- cells and Thy 1.2+, Ly 1-, Ly 2+, PNA+ cells, respectively. Although all these subpopulations expressed Thy 1.2 alloantigen, its density on the cells was different and was high in the order of Fr 3, Fr 2, and Fr 1 cells. In terms of blastogenic response, Fr 1 cells were highly responsive to either IL 2 or Con A, but Fr 2 cells did not respond to Con A, unless IL 2 was present. Fr 3 cells were not responsive to Con A even in the presence of IL 2. When fractionated cells were cultured in normal culture medium for 12 h, the spontaneous incorporation of 3H-thymidine (SIT) into the cells was lowered in Fr 3 cells to a greater extent than was observed with Fr 1 and Fr 2 cells. These results indicated that the response of thymocyte subpopulations to IL 2 and/or Con A was closely related to the maturation stages of T cells and that Ly 1-, Ly 2+, PNA+ immature thymocytes might consist of two different subpopulations as indicated by their IL 2 response and the reduction of SIT capacity by culture.