Expression of interleukin 2 receptor on murine fetal thymocytes

Rat monoclonal antibodies AMT‐13, 3C7 and 7D4 which react to the mouse interleukin 2 (IL2) receptors were used to define cell populations with the putative IL2 receptors in the mouse thymus as a part of series of investigations to elucidate the mechanism of intrathymic cell proliferation and differentiation. With freshly dissociated cells from the thymus of 15 gestational days, the anti‐IL2 receptor monoclonal antibodies (anti‐IL 2 receptor) reacted with about half of them. The proportion of the reactive cells decreased rapidly thereafter till birth as the numbers of thymus cells expanded. The antibodies reacted with only two to three percent of thymic cells from newborn mice and less than one percent of cells from adult thymus. Through the gestational period, the fetal liver cells did not react with the antibodies. When the thymus cells at early gestational days were subjected to a two‐color analysis, one for the anti‐IL2 receptor and the other for anti‐Thy‐1 or anti‐asialo GM1, by a fluorescence‐activated cell sorter, it was found that the majority (up to 80%) of anti‐IL2 receptor‐reactive cells (IL2 receptor‐positive cells) was also reactive with anti‐Thy‐1. Some of the IL2 receptor‐positive cells but Thy‐1‐negative cells reacted with anti‐asialo GM1, a marker of the immature thymocytes. Immunohistochemically, the IL2 receptor‐positive cells were found mainly in the subcapsular area and in the border region of cortex and medulla. Collectively, these results suggest that the pre‐T lymphocytes are stimulated immediately after their arrival to the thymus from the stem cell source such as fetal liver and bone marrow and are driven into the proliferation via the IL2 receptor system.