Monoclonal antibodies to B and T lymphocyte attenuator (BTLA) have no effect on in vitro B cell proliferation and act to inhibit in vitro T cell proliferation when presented in a cis, but not trans, format relative to the activating stimulus

Summary B and T lymphocyte attenuator (BTLA) is an immunoglobulin superfamily member surface protein expressed on B and T cells. Its ligand, herpesvirus entry mediator (HVEM), is believed to act as a monomeric agonist that signals via the CRD1 of HVEM to inhibit lymphocyte activation: HVEM is also the receptor for lymphotoxin‐α and LIGHT, which both bind in the CRD2 and CRD3 domains of the HVEM molecule, and for CD160 which competes with BTLA. We have shown that recombinant HVEM and a panel of different monoclonal antibodies specifically bind murine BTLA on both B and T cells and that some antibodies inhibit anti‐CD3ε‐induced T cell proliferation in vitro, but only when constrained appropriately with a putatively cross‐linking reagent. The antibodies had no significant effect on in vitro T cell proliferation in a mixed lymphocyte reaction (MLR) assay nor on in vitro DO11.10 antigen‐induced T cell proliferation. None of these antibodies, nor HVEM‐Fc, had any significant effect on in vitro B cell proliferation induced by anti‐immunoglobulin M antibodies (±anti‐CD40) or lipopolysaccharide. We further elucidated the requirements for inhibition of in vitro T cell proliferation using a beads‐based system to demonstrate that the antibodies that inhibited T cell proliferation in vitro were required to be presented to the T cell in a cis, and not trans, format relative to the anti‐CD3ε stimulus. We also found that antibodies that inhibited T cell proliferation in vitro had no significant effect on the antibody captured interleukin‐2 associated with the in vivo activation of DO11.10 T cells transferred to syngeneic recipient BALB/c mice. These data suggest that there may be specific structural requirements for the BTLA molecule to exert its effect on lymphocyte activation and proliferation.