Antibody-induced lysis of isolated rat epididymal adipocytes and complement activation in vivo

Objective: To identify human monoclonal antibodies selectively binding to human adipocytes and to evaluate their ability to induce lysis of isolated rat adipocytes in vitro and to reduce rat complement levels in vivo. Research Methods and Procedures: Using phage display technology, human monoclonal antibodies binding to human adipocyte plasma membranes were identified. Three antibodies (Fat 13, Fat 37, and Fat 41) were selected based on their additional cross-reaction with rat adipocytes and reformatted as a rat chimeric IgG2bs. The ability of these antibodies, both singly and in combination, to induce lysis of rat epididymal adipocytes in vitro and the reduction of serum complement levels in vivo in the rat was evaluated. Results: All antibodies caused similar time- and dose-dependent lysis of isolated rat adipocytes. Calculated mean EC50 values (maximum percentage of lysis in parentheses) were 0.680 μg/mL (63.2%), 0.546 μg/mL (72.4%), and 0.391 μg/mL (73.7%) for Fat 13, Fat 37, and Fat 41, respectively. Combinations were no more effective than individual antibodies in inducing lysis. Anti-adipocyte antibodies (both singly and in combination) were also similarly effective in vivo. In rats, doses of monoclonal antibody up to 10 mg/kg intraperitoneal generally caused almost complete depletion of serum complement up to 24 hours after dosing recovering to baseline values by day 5. Discussion: Individual and combinations of monoclonal anti-adipocyte antibodies produced a complement-dependent and concentration-dependent activity to lyse adipocytes in vitro and in vivo as measured by a dramatic depletion in serum complement.