Purified lymphocyte function‐associated antigen‐3 and T11 target structure are active in CD2‐mediated T cell stimulation

In this study we have used cells expressing LFA‐3 or T11TS, the human and sheep forms of the ligand of CD2, as well as the purified LFA‐3 and T11TS molecules themselves to study their effects on T cell activation via the CD2‐mediated “alternative pathway”. Sheep red blood cells, which bind to CD2 via T11TS in E‐rosette formation, and human autologous monocytes, which express the LFA‐3 molecule, both induce proliferation of resting T cells in the presence of per se submitogenic concentrations of anti‐T112 plus anti‐T113 monoclonal antibodies (mAb). This effect is blocked by mAb to LFA‐3, T11TS and CD2 known to inhibit CD2‐ligand interaction. In addition, purified LFA‐3 and T11TS, when added at ng amounts to cultures containing submitogenic concentrations of anti‐T112 + 3 mAb, are also strongly mitogenic for resting human T cells. Thus, both LFA‐3 and T11TS are potent co‐stimulators of the alternative pathway of T cell activation but by themselves do not provide a mitogenic signal. This finding is discussed with regard to a physiological role of CD2‐LFA‐3 interaction in T cell activation.