Detection of protein on BTLAlow cells and in vivo antibody-mediated down-modulation of BTLA on lymphoid and myeloid cells of C57BL/6 and BALB/c BTLA allelic variants

B- and T-lymphocyte attenuator (BTLA, CD272) is a polymorphic molecule belonging to the Ig superfamily (SF) that attenuates BCR and TCR-mediated signaling, and thereby functions as a negative regulator of lymphocyte activation. Herein, we report an anti-murine BTLA mAb (clone 4G12b) that remarkably detects protein expression on BTLAlow cells such as naïve CD4+ cells, CD8+ T cells, dendritic cells (DC), as well as in NKT cells and for the first time, we found BTLA expression on DX5dim and DX5bright subsets of non-T NK cells in both C57BL/6 (B6) and BALB/c BTLA allelic variants. Anti-BTLA 4G12b mAb binds to an overlapping epitope to that recognized by anti-BTLA 6A6 mAb, but in contrast to the concept widely accepted of blocking activity of 6A6 mAb, surprisingly neither 4G12b nor 6A6 mAbs impeded murine HVEM–mIgG2a.Fc recombinant fusion protein from interacting with BTLA-expressing cells. Lastly, in vivo administration of anti-BTLA 4G12b mAb induced a profound and lasting down-modulation of BTLA expression that led to BTLA receptor internalization with the potential utility of shutting down BTLA expression at any stage during the course of the immune response in both B6 and BALB/c strains of mice.