United we stand, divided we fall: using monoclonal antibody combinations to improve drug effectiveness

An immune reaction against foreign red blood cells may also be triggered during pregnancy if red blood cells from the fetus enter the bloodstream of the mother and there is a blood type mismatch. For example, if the mother is Rh negative and the baby is Rh positive, a dangerous condition can occur in a subsequent pregnancy: hemolytic disease of the fetus and newborn (HDFN), where the mother’s antibodies begin to destroy the red blood cells of her child. HDFN is life-threatening but very effectively prevented by treating the mother with Rho(D) immune globulin during pregnancy.

Rho(D) immune globulin, also known as anti-D, is a polyclonal antibody which targets the Rh D antigen, an important element of the Rh blood type. Treating Rh negative mothers with Rho(D) immune globulin prevents the mother’s immune system from recognizing Rh D on the red blood cells of her fetus. This treatment has saved millions of babies worldwide since its development in the 1960s. Despite its great clinical success, Rho(D) immune globulin is not an ideal treatment. As it is derived from donated plasma, it is vulnerable to shortages and — although safe — carries a theoretical concern for pathogen transmission. Researchers are working on developing potential alternatives, but so far these have not been clinically successful.

Polyclonal antibodies such as Rho(D) immune globulin are a mixture of antibodies formed by the immune system upon exposure to a foreign antigen. Several types of immune cells in the body produce antibodies which recognize various sites (epitopes) on the antigen. Rho(D) polyclonal antibodies can be isolated from the plasma of certain donors. Another way to obtain antibodies is to isolate single immune cells and grow them in the laboratory to generate large volumes of identical antibodies. Antibodies produced by these “single clone” cells are called monoclonal antibodies.

Compared to polyclonal antibodies, monoclonal antibodies are very specific (making them unlikely to bind to antigens other than the desired target) but not as sensitive (making them less likely to find tiny amounts of the target antigen). This is because monoclonal antibodies target a single epitope whereas polyclonal antibodies target multiple areas of the antigen. These properties may contribute to the failure of monoclonal antibodies to adequately recapitulate the clinical efficacy of polyclonal antibodies. Researchers at Canadian Blood Services recently tested this theory by comparing the immune-suppressive activity of polyclonal antibodies, monoclonal antibodies, and combinations of monoclonal antibodies.

• The polyclonal HEL-targeted antibody had superior immune-suppressing ability in this mouse model relative to any of the four monoclonal antibodies tested.
• A mixture of two monoclonal antibodies targeting different epitopes was just as effective at immune suppression as the polyclonal antibody.
• Monoclonal antibodies targeting the same epitope did not show this additive effect.

This research was conducted in the laboratory of Dr. Alan Lazarus. Dr. Lazarus is a Canadian Blood Services scientist and a professor of medicine at the University of Toronto. Dr. Lazarus is a member of the Toronto platelet immunology group at the Keenan Research Centre for Biomedical Science at St. Michael’s Hospital, Toronto. Dr. Lidice Bernardo, a postdoctoral fellow at the Keenan Research Centre, was the lead author. Several experiments were also performed by Dr. Alaa Amash, a postdoctoral fellow in the Lazarus research group, and by graduate student Danielle Marjoram.