Later, antitoxins were obtained from blood serum. These were found to be proteins belonging to the class of gamma globulins1, which were called antibodies. Antibodies are produced by B-lymphocytes, the specific cells of the immune system, against potentially dangerous agents, such as viruses and bacteria or abnormal endogenous cells (for example, cancer cells). These agents that can stimulate the immune system to produce antibodies are called antigens (“antibody generators”). The response of the organism is always “polyclonal”, i.e. antibodies are produced by different B cell clones to different sites of an antigen.
Polyclonal antibodies produced by different clones of B cells bind to the different site of the same antigen. Whereas monoclonal antibodies are produced by the same clone of B cells, and they bind to a single unique site of the antigen.
That is why these antibodies are referred to as polyclonal antibodies (pABs)1-4. Thus, the organism produces a kind of “cocktail” of antibodies against the same bacterium, and these antibodies slightly differ in their structure, but recognize the same target. This increases the probability of elimination of a malicious agent, as multiple antibodies can bind to it.
Comparison of polyclonal and monoclonal antibodies. Polyclonal antibodies may bind to the different sites of the same antigen, while monoclonal antibodies only bind to a single unique site of the antigen.
A routine procedure for production of pABs is quite simple: a donor animal (horse, sheep or rabbit) is injected with a serum of a particular pathogen, and after the immunization the animal generates specific antibodies. The blood is then obtained from the animal to isolate antibodies.
A conventional method for producing polyclonal antibodies. 1) Immunization of an animal – injection of a specific pathogen. 2) Immune response activated in the animal. 3) B cells produce specific antibodies against the injected antigen. 4) Blood serum collection, isolation of the required antibodies and their purification.
Although the improved methods for the isolation and purification of pABs have made it possible to overcome the initial problems of donor-to-recipient disease transmission2,3,5, only a limited amount of serum can be obtained. The composition of an antibody “cocktail” is always different, since it is impossible to obtain identical antibodies from two different donors. This impedes the use of drug products for clinical purposes, when the uniform composition of medications is a crucial condition. Moreover, animal antibodies are immunogenic, i.e. they can induce an immune response in humans, from a mild allergy to anaphylaxis2,3,5, and the collection of specific antibodies from humans, who have these antibodies for any reason, requires a large number of donors1.
A technique for generating large amounts of specific monoclonal antibodies (mAbs) that was developed in the 1970s put their polyclonal predecessors into the shadow. MAbs are antibodies produced by a single B lymphocyte clone, they are highly specific and only recognize a unique site of an antigen1-5. A method of their in vitro production eliminates the possibility of donor-to-recipient disease transmission and allows for various genetic engineering modifications, for example, those reducing the immunogenicity2-4. There was a boom in the use of mAbs, it seemed that they could become a good solution in the treatment of previously hopeless cases, such as cancer and autoimmune diseases. The use of mAbs is still one of the most rapidly developing areas of biomedicine. Dozens of drugs contaning mAbs have already been approved for therapy, and hundreds are going through different stages of clinical trials1,3.
However, there is no universal remedy. Tumour cells were found to alter in a way that they were no longer affected by previously effective mAbs6. High specificity of these antibodies became their weakness: mAbs may only bind to a single site of an antigen, and a small change in this site results in a loss of the antibody ability to recognize the site2-6. Then researchers gave a chance to pAbs: at least some of the antibodies from a “cocktail” may still bind to the pathogen, because the probability of a simultaneous change in multiple sites of an antigen is exceedingly small.
Over the years of practical application, it has become clear that pAbs are more effective in the treatment of various infectious diseases and when used as antidotes and antitoxins, compared to mAbs1-3. This makes sense, as the inactivation of a pathogen or toxin is more effective when the larger number of neutralizing agents binds to it. To date, pAbs of animal origin against the rabies virus have been approved and successfully used as an antivenom to treat bites from spiders and snakes as well as drug overdose (e.g. with colchicine and digoxin)2. Investigation of the use of pAbs against dangerous pathogens, such as avian influenza, Ebola, Marburg, West Nile viruses, etc., is also promising.
The ability of pAbs to bind to different sites of an antigen and their lower sensitivity to its changes may have potential to control seasonal influenza viruses that show rapid evolutionary changes2,7. PAbs that block virus attachment and/or penetration in host cells have also demonstrated efficacy in protection from influenza in animal models7.
The potential for use of pAbs is not limited to infectious diseases. American researchers have discovered that menopausal osteoporosis can be effectively treated with pAbs to FSH that stimulates bone destruction8,9. PAbs can also be used to counteract the host immune rejection1, for example, to prevent graft rejection in transplantation10. PAbs are useful in preventing Rh incompatibility in pregnant women. This condition is quite common and caused by D-antigen sensitization of a pregnant woman via exposure to fetal RBCs, which results in a maternal immune response, as with any foreign agent in the organism, leading to hemolytic disease of the newborn or even fetal death. Timely administration of RhIG (Rho(D) immune globulin) eliminates these consequences by preventing maternal Rh alloimmunization10.
The structural diversity of pAbs makes their application more beneficial compared to mAbs. Apart from the biological benefits mentioned above, polyclonal antibodies also demonstrate a number of economic advantages: they are easier to scale-up, cost-effective and more stable2-4,7. In addition, science is in constant progress, and new technologies for the production of recombinant pAbs, which possess the advantages of polyclonal antibodies and at the same time have overcome their disadvantages, will allow achieving a completely new level of immunotherapy effectiveness 1,3,5.
1. Wootla B, Denic A, Rodriguez M. Polyclonal and monoclonal antibodies in clinic. Methods Mol Biol. 2014. 1060: 79-110.
2. Dixit R, Herz J, Dalton R, Booy R. Benefits of using heterologous polyclonal antibodies and potential applications to new and undertreated infectious pathogens. Vaccine. 2016. 34(9): 1152-61.
3. Tolstrup AB, Frandsen TP, Bregenholt S. Development of recombinant human polyclonal antibodies for the treatment of complex human diseases. Expert Opin Biol Ther. 2006. 6(9): 905-12.
4. Lipman NS, Jackson LR, Trudel LJ, Weis-Garcia F. Monoclonal versus polyclonal antibodies: distinguishing characteristics, applications, and information resources. ILAR J. 2005. 46(3):258-68.
5. Waltz E. Polyclonal antibodies step out of the shadows. Nat Biotechnol. 2006. 24(10): 1181.
6. Sharon J, Liebman MA, Williams BR. Recombinant polyclonal antibodies for cancer therapy. J Cell Biochem. 2005. 96(2): 305-13.
7. Berry CM. Antibody immunoprophylaxis and immunotherapy for influenza virus infection: Utilization of monoclonal or polyclonal antibodies? Hum Vaccin Immunother. 2018. 14(3): 796-799.
9. Zhu LL, Blair H, Cao J, Yuen T, Latif R, Guo L, Tourkova IL, Li J, Davies TF, Sun L, Bian Z, Rosen C, Zallone A, New MI, Zaidi M. Blocking antibody to the β-subunit of FSH prevents bone loss by inhibiting bone resorption and stimulating bone synthesis. Proc Natl Acad Sci USA. 2012. 109(36): 14574-9.
10. Yeung MY, Gabardi S, Sayegh MH. Use of polyclonal/monoclonal antibody therapies in transplantation. Expert Opin Biol Ther. 2017. 17(3): 339-352.
11. Aitken SL, Tichy EM. Rh(O)D immune globulin products for prevention of alloimmunization during pregnancy. Am J Health Syst Pharm. 2015. 72(4): 267-76.