The transmembrane region of CD2‐associated signal‐transducing proteins is crucial for the outcome of CD2‐mediated T‐cell activation

Signalling through the CD2 molecule was shown previously to employ similar signalling molecules as the T‐cell receptor (TCR). Here, we show that CD2‐mediated signalling is strongly influenced by the expressed transmembrane region of the employed signal‐transducing molecule. We used TCR‐negative cells expressing chimeric fusion proteins that consist of human interleukin‐2 (IL‐2) receptor α‐chain‐derived sequences (hCD25) fused to mouse‐specific ζ‐chain segments (hCD25‐ζ). One set of TCR‐negative cell lines expressed the hCD25‐derived extracellular part fused to mouse‐specific transmembrane and cytoplasmic ζ‐protein sequences (‘TZZ’). The second type of cell lines expressed the hCD25‐derived extracellular and transmembrane portions fused to the mouse‐specific ζ‐chain cytoplasmic segment (‘TTZ’). After cross‐linking the hCD25‐ζ molecules with specific monoclonal antibodies (mAb), all TCR‐negative cell lines produced similar amounts of IL‐2. Cross‐linking with stimulating pairs of CD2‐specific mAb, however, led to IL‐2 production only in cell lines expressing the ζ‐chain‐specific transmembrane segment. Co‐cross‐linking of CD25 and CD2 molecules resulted in an effective stimulation of both TZZ‐ and TTZ‐expressing cell lines. Moreover, TTZ‐ and TZZ‐expressing cell lines differed in their pattern of tyrosine‐phosphorylated proteins after stimulation with hCD25‐specific mAb. Thus, although CD2 and TCR molecules share signalling components and pathways, the fine tuning of CD2 co‐receptor function appears to be regulated in part by transmembrane regions of signal‐transducing molecules like the TCR‐associated ζ‐chain.