United we stand, divided we fall: using monoclonal antibody combinations to improve drug effectiveness
What is this research about?
The immune system fights infection by identifying and destroying foreign material such as viruses and bacteria. Antibodies recognize these “invaders” through interactions with molecules called antigens on the surface of the virus or bacteria. Human cells also have antigens; for example, antigens on red blood cells determine blood types (ABO and Rh factor). If someone with type A blood receives a transfusion of type B blood, their antibodies will attack the unfamiliar “B” antigens on the red blood cells, causing a dangerous transfusion reaction.
This study, recently published in a top hematology journal, shows that a blend of two monoclonal antibodies is more effective at preventing an immune response than either antibody alone.
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.
What did the researchers do?
Dr. Lazarus’ research group used a mouse model of red blood cell immunization in which red blood cells displaying a test protein are isolated from transgenic mice and transfused into wild type recipient mice. This test protein includes a foreign antigen, hen egg lysozyme (HEL), which induces an immune response. To determine the ability of polyclonal and monoclonal antibodies to suppress this immune reaction, recipient mice were transfused with HEL-displaying red blood cells mixed with various antibodies targeting HEL. Blood was collected from the mice for three weeks and examined for an immune response by measuring HEL-specific antibodies.
What did the researchers find?
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.
How can you use this research?
These findings help clarify the reasons for the disappointing clinical failure of monoclonal antibodies as a replacement for polyclonal Rho(D) immune globulin. The polyclonal antibody used in this study effectively prevented an immune response in a mouse model system whereas the monoclonal antibodies caused only partial immune suppression. Significantly, Dr. Lazarus’ research group has shown that the efficacy of monoclonal antibodies can be greatly improved by combining antibodies. A mixture of two monoclonal antibodies suppressed the immune response in this model system as effectively as the polyclonal antibody. Notably, this additive effect was seen only when antibodies targeting different epitopes were combined; a blend of antibodies targeting the same epitope was no more effective than either antibody alone.
This work was done in a mouse model system, which allows the study of complex whole-body processes such as immune suppression but also has many important differences from human physiology. Before this research can be translated into potential human treatments, similar findings must be confirmed in human blood. The significance of targeting different epitopes should also be validated by examining additional monoclonal antibody combinations. Furthermore, the antigen used in this study was an artificial test protein which may behave differently from natural antigens. Additional research is needed to extrapolate these findings to clinically relevant antigens such as the Rh D antigen targeted by Rho(D) immune globulin.
In light of this study, clinical research should prioritize the evaluation of various monoclonal antibody combinations as potential replacements for the current Rho(D) immune globulin treatment. A new drug based on monoclonal antibody blends would increase safety by eliminating the potential risk of pathogen transmission, making drug activity more predictable, and reducing the likelihood of a drug shortage.
About the research team
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.
This research unit is derived from the following publication(s)
Acknowledgements: This research was funded by a grant from Health Canada as part of the Canadian Blood Services/Canadian Institutes of Health Research partnership fund. Dr. Bernardo received postdoctoral scholarships from Canadian Blood Services, funded by the federal government (Health Canada) and provincial and territorial ministries of health. The views herein do not necessarily reflect the views of the federal government of Canada. Dr. James Zimring of Bloodworks NW Research Institute (Seattle, USA) provided the HOD transgenic mice, the monoclonal anti-HEL antibodies and scientific advice.
Keywords: anti-D, Rho(D) immune globulin, Rh D antigen, monoclonal, polyclonal, antibody, red blood cells, immune suppression