A NOVEL CELL-SURFACE ANTIGEN, IMMATURE THYMOCYTE ANTIGEN-1, IS INVOLVED IN THE DIFFERENTIATION OF MURINE THYMOCYTES

A mAb, 1-23, which recognizes a novel cell surface Ag on immature murine thymocytes (designated as IMT-1 for immature thymocyte antigen-1) was prepared. IMT-1 was found to be expressed on 40 to 50% of CD4(-)8(-) double negative (DN), 5 to 10% of CD4(-)8(+), and 5 to 20% of CD4(+)8(+) double positive (DP) thymocytes in adult mice, but not expressed on CD4(+)8(-) thymocytes or peripheral T lymphocytes. It was not expressed on either nonlymphoid cell lines or thymic stromal cells. in subsets of DN thymocytes, IMT-1 was detected on 40 to 50% of the heat-stable Ag+, 15% of CD44(+)25(+), 70% of CD44(-)25(+), and 70% of CD44(-)25(-) populations, whereas it was not detected on heat-stable Ag- and CD44(+)25(-) populations. IMT-1(+) thymocytes expressed low levels of or no CD3 molecules. In fetal thymuses, IMT-1 was expressed on a minor population of thymocytes at day 14.5 and 15.5 of gestation. However, at day 16.5 of gestation, a majority of DN as well as DP thymocytes became IMT-1-positive. Addition of the 1-23 Ab to the fetal thymus organ culture relatively increased CD8(+) SP thymocytes. These results show that IMT-1 is expressed during the late DN stage as well as the early DP stage of thymocyte maturation and suggest the possible involvement of IMT-1 in thymocyte differentiation.