I: Defining the idiotype
The immune system has the capacity to develop antibodies that can specifically bind to almost any biological or chemical structure imaginable. This remarkable repertoire of antibody specificity is defined by the variable region domains of the heavy (VH) and light (VL) chains of the heterodimeric antigenic determinant region, which generates an estimated 1-6 X 106 combinations in mice and humans through the rearrangement of VH and VL genes. Additional modifications to the VH and VL genes, through somatic mutation and junctional diversity, increase the estimated capacity to produce approximately 1011 to 1013 unique antibodies (Eichmann, 2008; Mak and Saunders, 2005). To put this in perspective: there are about 3 X 1011 stars in our galaxy; so when you look up at the stars consider you have approximately 1020 antibodies circulating in your bloodstream that have the potential to recognize a diversity of structures as vast as the galaxy.
Antibodies have bispecific functionality with the antigenic determinant defining the binding specificity to the antigen, and the heavy chain constant (CH) region defining its immunological functionality, through the interactions of the CH region with specific receptors or complement. Two identical heavy and light chain pairs combine (through disulfide bonds) to form a complete antibody structure, which contains two antigen binding domains and a single constant region, known as the FC composed of two CH regions (Figure 1).
The CH region defines the isotype, or class, of antibody (Figure 1): IgM, IgG, IgA, IgE, and IgD. Therefore the isotype of the antibody defines the role the antibody will play in a specific immunological response. The variable regions that define the antigen specificity is referred to as the idiotype (Figure 1), each unique combination of VH and VL generates a unique idiotype. Therefore, the specific antigen or target that is recognized by antibodies in the immune response are determined by the idiotype. A specific idiotype can be represented as multiple isotypes: i.e. an antibody that is specific for a unique antigen or epitope of an antigen (Figure 1), can be represented by multiple isotypes: IgM and IgG for example. Conversely, a single isotype can be represented with many different idiotypes━which would define an individual’s antibody repertoire.
II: Idiotype network theory
Antibodies that recognize the unique structures of the VH and VL regions are referred to as anti-idiotypes (or anti-Ids). The principle of idiotypy, the determination of the the unique structure of one antibody by another antibody, was characterized by Henry Kunkel’s laboratory and Jacques Oudin’s laboratory in the 1950’s (Eichmann, 2008). Through the work of Kunkel, Oudin and others it was discovered that immunization of an animal with its own antibody produced anti-Ids, which was an indication that idiotypes and anti-idiotypes were present together in an immunological repertoire. Niels Jerne used these findings to develop his “network” theory (Jerne, 1974), which described antibody idiotypes and anti-idiotypes generating a functional “network” that regulated immune responses and contributed to antibody diversity. While it was controversial at the time, and remains so today, it has guided the discovery of mechanisms of antibody development and regulation of the immune system for decades, leading to current models which favor a more cellular regulatory network model (Santori, 2015). The principles of the idiotype network theory have been used to explain the development and expansion of autoimmune responses (Kieber-Emmons et al., 2012; Menshikov et al., 2015), autoantibody development (Pelsue and Agris, 1994), and the development of animal models of autoimmunity (Vangone et al., 2014).
One of the principle concepts of the idiotype network theory was the definition of anti-id antibodies (Figure 2). Experimentally anti-ids were demonstrated to interact with idiotypic regions in several ways: i) binding to the variable region outside of the antigen binding site(Ab2⍺); ii) binding to the antigen binding site(Ab2ᵦ), and iii) binding near the antigen binding site (Ab2ᵧ). Ab2ᵦ, and possibly Ab2ᵧ, would mimic the structure of the antigen as it is structurally complementary to the antigen binding site of the original antibody (Ab1). An anti-id to Ab2ᵦ would be structurally similar (if not identical) to Ab1.
III. Anti-Idiotypes as therapeutics & diagnostics
As demonstrated by Jerne’s idiotype network theory, antibodies themselves can be antigens and used to drive and/or regulate immune responses. It is this feature that allows anti-ids to be useful as analytical tools and potentially important as therapeutics (Kieber-Emmons et al., 2012). As antibodies have become important biologic therapeutics it is critical both for quality control of the antibodies as well as the evaluation of the pharmacokinetics (PK) of the antibody therapeutic to have the ability to specifically detect the antibody drug. This is frequently accomplished by the development of an anti-id to the antibody drug, which then can be used in immunoassays for the characterization of the antibody drug. In addition as the Ab2ᵦ, and possibly Ab2ᵧ, antigen determinant region would represent structures mimicking the antigen, these anti-ids have been used to induce immune responses for vaccinations as well as the treatment of cancer (Kieber-Emmons et al., 2012).
IV. Development of anti-idiotypes.
Anti-ids are simply antibodies that bind to the unique structure of another antibody (idiotype). Therefore the development of anti-idiotypes follows the same paradigm as developing any monoclonal antibody targeted towards a protein antigen. The key is to understand what class (Figure 2) of anti-id antibody is necessary for the intended applications. Does it need to functionally block or mimic the target antigen (Ab2ᵦ) or prevent the association of the antibody with its target (Ab2ᵦ or Ab2ᵧ), or does it simply need to uniquely identify the antibody (Ab⍺, Ab2ᵦ, or Ab2ᵧ)? The key to developing the appropriate anti-idiotype is to have a screening strategy that will specifically determine the class of anti-id, generally defined by immunoassay.
As the Constant regions will also be recognized, particularly if it is from a different species (i.e. human antibody immunogen for a mouse monoclonal antibody), negative selections are equally important to insure that the monoclonal antibodies selected are in fact anti-ids and not anti-Fc antibodies. Using only the antigen binding fragments or the mouse version of a humanized antibody, are also good strategies to minimize the response towards the Fc region.
Eichmann, K. (2008). The network collective: rise and fall of a scientific paradigm (Basel ; Boston: Birkhäuser).
Jerne, N.K. (1974). Towards a network theory of the immune system. Ann. Immunol. 125C, 373–389.
Kieber-Emmons, T., Monzavi-Karbassi, B., Pashov, A., Saha, S., Murali, R., and Kohler, H. (2012). The promise of the anti-idiotype concept. Front. Oncol. 2.
Mak, T.W., and Saunders, M.E. (2005). The immune response: basic and clinical principles (Academic Press).
Menshikov, I., Beduleva, L., Frolov, M., Abisheva, N., Khramova, T., Stolyarova, E., and Fomina, K. (2015). The idiotypic network in the regulation of autoimmunity: Theoretical and experimental studies. J. Theor. Biol. 375, 32–39.
Pelsue, S., and Agris, P.F. (1994). Gene usage by an anti-U1 snRNP 70K monoclonal autoantibody derived from a lupus-prone mouse. J. Autoimmun. 7, 165–177.
Santori, F.R. (2015). The immune system as a self-centered network of lymphocytes. Immunol. Lett. 166, 109–116.
Vangone, A., Abdel-Azeim, S., Caputo, I., Sblattero, D., Di Niro, R., Cavallo, L., and Oliva, R. (2014). Structural Basis for the Recognition in an Idiotype-Anti-Idiotype Antibody Complex Related to Celiac Disease. PLoS ONE 9, e102839.