The potential role of xenogeneic antigen-presenting cells in T-cell co-stimulation

CTLA4-Fc is a chimeric murine construct consisting of the CD28 homologue CTLA4 and the constant portion of the heavy chain of mouse IgG2a, with a potential to suppress cellular xeno-immune responses. The aim of this study was to determine the degree of binding of CTLA4 to B7 ligands on cells of different species and to use CTLA4-Fc as a tool for the study of cross-species CD28-B7 interactions. As assessed by flow cytometry, CTLA4-Fc bound to mouse L-cells and human Epstein Barr virus (EBV) transformed lymphoblastoid cells and concanavalin A (Con A) or LPS-stimulated peripheral blood mononuclear cells (PBMC) or splenocytes from rat, dog, and pig. CTLA4-Fc inhibited the proliferation of Con A-stimulated PBMC or splenocytes from mouse, rat, dog, and pig, in a dose-dependent fashion with approximately 80% inhibition at a concentration of 10 mu g/ml. It did not inhibit the proliferation of Con A-stimulated human PBMC, although it did inhibit the human versus human, and human versus pig primed mixed lymphocyte culture (MLC) in a dose-dependent fashion. At submitogenic concentrations, purified human T-cells did not proliferate after incubation with Con A alone. However, proliferation occurred with the addition of B7 positive L-cells or pig PBMC, but not B7-negative OKT4 cells, Furthermore, CTLA4-Fc inhibited proliferation in a dose-dependent fashion. CTLA4-Fc bound to all species tested and resulted in inhibition of Con A-stimulated proliferation in these species, except for humans, Human T-cells proliferated in response to co-stimulation with xenogeneic B7, and this could be inhibited by CTLA4-Fc, suggesting that xenogeneic B7 was capable of providing a functionally significant co-stimulatory signal necessary for human T cell activation in vitro.