Coengagement of CD2 with LFA‐1 or VLA‐4 by bispecific ligand fusion proteins primes T cells to respond more effectively to T cell receptor‐dependent signals

To examine the effects of ligand engagement and accessory molecule juxtaposition on T cell receptor (TCR) signaling, we prepared LFA‐3/ICAM‐1 Rg and LFA‐3/VCAM‐1 Rg bispecific immunoglobulin fusion proteins (Rg, recombinant globulin). These novel fusion proteins allowed us to examine the effects of ligand driven co‐engagement of T cell proteins CD2 and LFA‐1 or CD2 and VLA‐4 on TCR‐dependent mobilization of intracellular Ca 2+ . We observed that preincubation of resting T cells with LFA‐3/ICAM‐1 Rg or LFA‐3/VCAM‐1 Rg fusion proteins resulted in significantly enhanced mobilization of intracellular Ca 2+ following TCR‐accessory molecule cross‐linking relative to T cells preincubated with each of the monospecific Rgs alone or with combinations of the monospecific Rg fusion proteins. In addition, such coengagement stimulated TCR‐dependent activation and tyrosine phosphorylation of phospholipase C γ 1 (PLC γ 1). These results suggest that when T cells interact with antigen presenting cells the engagement of multiple cell adhesion molecules such as CD2, LFA‐1, and VLA‐4 primes the T cell to respond more effectively to signals delivered through the TCR. J. Leukoc. Biol . 56: 444–452; 1994.