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

Abstract

Abstract: 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 μg/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.