Characterization of a monoclonal antibody recognizing an epitope designated as canine leukocyte-associated antigen

An IgG1 monoclonal antibody (mAb), designated as 15F1.5, was generated against surface determinants of a dog peripheral blood-derived PHA-induced IL-2-dependent T cell line. It reacted with 65-80% of peripheral blood mononuclear cells (PBMCs), 90-95% of polymorphonuclear cells (PMNs), 65-70% of thymocytes, 85-95% of Thy-1 positive cells and 85-95% of IL-2-dependent T lymphoid cells in flow cytometry. It was nonreactive with peripheral blood red cells and platelets. It immunoprecipitated 95 and 150 Kd proteins derived from detergent solubilized lymphocyte membranes. Indirect immunofluorescent and immunoperoxidase staining of frozen tissue sections demonstrated positive reactivity to cells in lymphoid but not nonlymphoid tissues. The 15F1.5 antibody was not directly mitogenic for PBMC's. It caused significant decrease (P < or = 0.05) in the lymphoproliferative response to T-dependent B cell mitogens, such as pokeweed mitogen (PWM) and staphage lysate (SPL), without significant effects on responses to the T cell mitogens, phytohemagglutinin (PHA), and concanavalin A (Con A). The mixed lymphocyte culture (MLC) response and both the proliferative and effector arms of the cell-mediated cytotoxicity reactions (CMC) were inhibited in a dose-dependent manner. The mAb also inhibited the auto- and allolymphoproliferative reactivity of mixed lymphocyte kidney or islet cell cultures (MLKC and MLIC), and the adhesion of T lymphoblasts and PMA-treated PMNs to endothelial cells. In vivo administration of the 15F1.5 (20 mg/day for 5 days) caused an immediate and prolonged reduction in MLC responses, associated with cell binding of the mAb to PBMC and epitope modulation during the course of treatment, as indicated by flow cytometry. These results suggest that 15F1.5 is an immunomodulating antibody reacting with canine LFA-1. Thus, this mAb would be useful in studying the role of LFA-1/ICAM-1 in graft rejection as well as other inflammatory responses. It would also allow the use of an animal model to investigate the immunoregulatory effects of in vivo administration of anti-CD11/CD18 antibodies in organ/tissue transplants.