Studies on thymocyte subpopulations in guinea pigs. VII. Characterization of cell populations responsive to guinea pig interleukin 1 and interleukin 2.

The properties of thymocytes responding by proliferation to a mitogenic lectin (PHA), interleukin 1 (IL 1), or interleukin 2 (IL 2) were studied and compared to the properties of cells known to respond to a separate thymocyte growth factor (TGP), which has so far only been studied in guinea pigs. Thymocytes from guinea pigs were separated into subpopulations by density gradient centrifugation with Percoll and by rosette formation with rabbit erythrocytes. PHA-responsive cells were recovered exclusively in a non-rosetting, low-density population designated Ia, RFC-, constituting approximately 4 per cent of all thymocytes. Thus, according to the prevailing view of lymphocyte mitogenesis, IL 1-producing, IL 2-producing as well as IL 2-responding cells are all present in this population. The mitogen-responsive cells could be further stimulated by addition of IL 1 or IL 2, indicating that the magnitude of the mitogenic response was regulated by the production of these factors and was not restricted by the number of IL 2-responding cells. IL 1, to some extent, also enhanced the mitogen response in a non-rosetting, intermediate density population designated Ib, RFC-. None of the factors could affect the lack of mitogen responsiveness in the high-density population II, constituting approximately 85% of the total population and probably including most small cortical thymocytes. We conclude that IL 2-responding thymocytes are present above all in the quantitatively small population Ia, RFC- (shown to contain also the mature, mitogen-responding cells) and to a smaller extent in Ib, RFC- (where a deficit in IL 1-producing cells may explain the poor mitogen responsiveness), but not in the major, high-density population II. From our data, it is also evident that the mitogen, IL 1- and IL 2-reactive cells can be separated from a population of intensely proliferating thymic precursor cells which are stimulated to grow by TGP. Therefore, the growth of these immature cells does not seem to be regulated by IL 1 or IL 2 in the guinea pig.