Engagement of the CD4 molecule influences cell surface expression of the T-cell receptor on thymocytes

The intrathymic differentiation process by which precursor cells derived from the bone marrow develop into immuno-competent T lymphocytes is poorly understood. Most thymocytes express both CD4 and CD8 accessory molecules, yet little is known about either the function of these molecules or the responsiveness of the CD4+8+ double positive thymocytes that bear them. Here, we address the possibility that CD4 engagement influences T-cell receptor (TCR) expression on developing thymocytes. We engaged CD4 molecules on murine thymocytes by in vivo injection of an anti-CD4 monoclonal antibody, which reduced the surface expression of CD4 on CD4+ thymocytes. More importantly, CD4 engagement also affected TCR expression on CD4+ thymocytes, but the effect on CD4+8+ double positive and CD4+8- single positive thymocytes was very different. CD4+8+ thymocytes responded to CD4 engagement by dramatically increasing surface expression of TCR, whereas CD4+8- thymocytes decreased surface expression of TCR. These results demonstrate that the effect of CD4 engagement on TCR expression is dependent upon the developmental state of the responding thymocyte, and, most interestingly, results in increased TCR expression by double positive thymocytes.