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.