A multimeric form of soluble recombinant sheep LFA‐3 (CD58) inhibits human T‐cell proliferation

The rosetting of T cells by sheep erythrocytes is mediated through the interaction of the CD2 molecule on T cells with T11TS, a molecule on sheep erythrocytes homologous to lymphocyte function‐associated antigen‐3 (LFA‐3, CD58). We cloned a T11TS cDNA from sheep leucocyte mRNA which encodes a soluble molecule comprising the distal D1 and the D2 extracellular domains, but not the transmembrane domain. cDNA for this soluble D1+D2 form of sheep LFA‐3 (sLFA‐3) was expressed in Escherichia coli and the properties of the purified recombinant protein were assessed by inhibition of T‐cell rosette formation. sLFA‐3 inhibited rosette formation, but its activity was low, 50% inhibition occurring at 25 μg/ml, consistent with the observed low binding avidity of fluorescein isothiocyanate (FITC)‐labelled sLFA‐3. sLFA‐3 was made multimeric to increase its affinity, by crosslinking biotinylated sLFA‐3 to streptavidin–biotinylated dextran complexes. The binding of crosslinked sLFA‐3 multimers, tested by fluorescence‐activated cell sorting (FACS) analysis, was significantly increased compared to sLFA‐3 monomers. Competition with monoclonal antibodies demonstrated that multimeric sLFA‐3 bound to the T111 epitope on CD2. The multimeric form of sLFA‐3 was significantly more potent than the monomer in inhibiting proliferation of human T cells in response to purified protein derivative (PPD), tetanus toxoid (TT) or allogeneic cells. Multimeric sLFA‐3 might, therefore, have potential as an immunotherapeutic agent to inhibit and/or anergize antigen‐specific T‐cell responses.