Monoclonal Antibodies Mimic Action of IVIG and Can Inhibit Immune Thrombocytopenia

TORONTO—Monoclonal antibodies directed against red blood cells can be used to inhibit immune forms of thrombocytopenia, according to the results of studies with mice. These results were reported by Alan H. Lazarus, PhD, assistant professor of medicine, Department of Hematology, St. Michael’s Hospital, University of Toronto.

Currently, intravenous immuoglobulin (IVIG) prepared from large pools of healthy donors is widely used to treat autoimmune diseases, especially immune thrombocytopenic purpura (ITP). Human polyclonal antierythrocyte antibodies, such as anti-D, can also be effective in treating ITP in individuals expressing the appropriate antigen. The demand for IVIG and anti-D, however, exceeds the supply. The development of a recombinant product mimicking the action of these human-derived blood products could help meet the demand.

Competing Complex

"We know that ITP is an immune-mediated disease where you have platelets that are bound by a pathologic autoantibody. This platelet-antibody complex binds to complement receptor monocytes in the spleen and causes clearance of platelets." One of the proposed theories on how the human-derived blood products work to ameliorate ITP is that IVIG or anti-D binds to an antigen in the host (most likely a red cell in the case of anti-D) to form an immune complex that becomes a competitor with the platelet complex to bind to the receptor.

"If this is true," Dr. Lazarus said, "there is no reason why a monoclonal antibody binding to, for example, a red cell or some other antigen should not be able to reverse or prevent thrombocytopenia." He cautioned, however, that some monoclonal antibodies may be shown to work while others may not.

Works With Lower Dose

Mice were individually injected with 50 mg of human IVIG (2 g IVIG/kg body weight) vs 50 µg, 5 µg, or 0.5 µg of an antibody specific for the TER-119 erythrocyte antigen as well as two antibodies against the heat-stable antigen CD-24 found on erythrocytes. After 24 hours, the mice were treated with antiplatelet (anti-GPIIb) antibody.

After another 24 hours, thrombocytopenia was assessed by flow cytometry. As expected, standard human IVIG protected against thrombocytopenia, but so did the monoclonal antibody specific for the TER-119 antigen. "At the 50-µg dose, we get protection against thrombocytopenia," Dr. Lazarus said. One of the anti-CD24 specific antibodies also inhibited thrombocytopenia almost as well as the IVIG. The dose for these monoclonal antibodies was 1,000-fold lower than IVIG.

The antibodies that did inhibit thrombocytopenia significantly blocked reticuloendothelial system (RES) function. "If you therapeutically treat with IVIG, this blocks the RES, blocking clearance of red blood cells. And antibodies 1 and 2, which were therapeutically effective, also blocked the RES and red cell clearance."

Potential Substitute

"To conclude, what we’ve found is that treatment with monoclonal antierythrocyte antibodies was able to prevent as well as reverse thrombocytopenia." The treatment is effective whether it is given before or after the thrombocytopenia develops.

"Antibodies that block FcyRII/III, but not those that bind or block complementary receptors 1, 2, and 3, were able to inhibit thrombocytopenia as well. Antibodies that are therapeutically effective also induced RES blockage," Dr. Lazarus continued. "And last, monoclonal antibodies that mimic the action of IVIG, which we are calling MIVIG, may be a potential substitute for IVIG or anti-D in treating thrombocytopenia."

Questioned about durability, Dr. Lazarus replied, "The monoclonal antibodies that we are using don’t work for as long as IVIG does, so one would have to give them on an every-second-day basis, whereas the IVIG covers the animals for up to a week. So it does appear that the monoclonal antibodies that we are using are being cleared very rapidly. They are not quite as durable as IVIG, but they do work at a 3 log lower dose."