Monoclonal antibodies against porcine LFA-1: Species cross-reactivity and functional effects of β-subunit-specific antibodies

As a first step in developing a porcine model system to study the effect of LFA-1-specific monoclonal antibodies on allograft rejection, we have identified anti-human LFA-1 monoclonal antibodies that cross-react with porcine leukocytes and have used one of these antibodies against the LFA-1 beta-subunit to purify porcine LFA-1. Immunization of Balb/c mice with the purified antigen yielded eight monoclonal antibodies directed against the beta-subunit of porcine LFA-1. These antibodies immunoprecipitated pig proteins corresponding to the human alpha-subunits of LFA-1 (CD11a), Mac-1 (CD11b) and LeuM5 (CD11c), and their common beta-subunit (CD18). Under both reducing and non-reducing conditions, the porcine beta-subunit showed a different electrophoretic mobility from that of the human beta-subunit. In addition, results of peptide mapping studies revealed structural differences between the beta-subunits of human and porcine LFA-1. The anti-LFA-1 beta-chain antibodies were found to cross-react with antigens expressed on rat, mouse, hamster, rabbit, dog, cow and human leukocytes, indicating that the beta-subunit contains highly conserved epitopes. Competitive binding studies showed that these antibodies and three anti-human LFA-1 antibodies defined at least four distinct epitopes on the porcine beta-subunit, three of which were also found on human beta-subunits. Whereas the cross-reactive anti-human beta-subunit antibody H52 profoundly inhibited mitogenic and allogenic stimulation of porcine T lymphocytes as well as cytotoxic T cell and natural killer (NK) cell function, the anti-porcine LFA-1 monoclonal antibodies had very limited effect on porcine lymphocyte function. Our results demonstrate that porcine leukocytes express the LFA-1 molecule and that this antigen is involved in T cell and NK cell functions, as has previously been shown in human and murine systems. We conclude that the pig may serve as a useful model in which to test the effect of anti-LFA-1 antibodies on allograft rejection.