Antibodies, also known as immunoglobulins (Ig), are supramolecular protein complexes that belong to the immune system. Antibodies are designed and produce by leucocytes (precisely, B cells) when foreign agents, such as bacteria, parasytes or viruses, invade our organism. Antibodies have the particular ability for recognizing and binding with great affinity these exogenous agents, so that the immune system can kill or neutralize them. The function and structure of immunoglobulins has been extensively studied and there is a broad knowledge; as a consequence, antibodies have been acquired for biotechnological applications and pharmacology. For this purpose antibodies have been engineered and tuned to satisfy the required properties. In the following article, I am going to review the main features of immunoglobulins and how they have been engineered in the biotechnological field. Finally, I am going to present some pharmacological antibodies that are been used for the treatment of some diseases.
Antibody classification and structure
Antibodies are usually depicted as Y-shaped complexes, consisting on four different subunits or polypeptides. These subunits are readily known as the heavy chain (H) and the light chain (L). As shown in figure 1, antibodies are simetrically identical: one half consists on the heavy chain associated with the light chain at the N-terminal region; two of this units associate to form the whole antibody. These associations are mainly mediated by disulfide bridges (strong covalent bonds), although non-covalent interactions also play an important role.
Besides the four different polypeptides, two different segment are identified in the Y-shaped antibody. Namely, the tips of the antibody (upper edges of the Y) are the variable segments, while the rest of the antibody are the constant segments. These segments are defined based on the amino acid sequence; specifically, in the variable segments the amino acid sequence varies greatly among different antibodies, whereas the constant segment remains the same among the same type of antibodies. This variability provides the structural adaptability to recognize with great especificity the exogenous agent; in other words, because the antibodies vary their tips they are able to recognize a wide range of molecules, with distinct structures. The variable segment contains a particular region, known as the hypervariable region, and this is primarily responsible for the binding of the molecule.
In contrast, the constant segments of antibodies remain unchangeable and determine the different isotypes or classes. Human antibodies are classified into 5 different isotypes (IgG, IgM, IgE, IgD and IgA) and the differences of the heavy chains’ constant segment confer them particular properties and fuctions. For instance, IgG, IgM, IgE and IgD type antibodies are primarily found in blood, whereas IgA antibodies are secreted into the mucoses, breast milk and saliva. Moreover, different isotypes produce a different physiological response; for instance, IgM antibodies are produced as a first response against infections by activating the immune system, whereas IgE are primarily involved in allergy.
How are antibodies used in therapy and treatment?
As antibody structure and function were understood, we have envisaged the great variety of applications of these macromolecules. Certainly, these molecules are able to recognize and neutralize a very specific molecule, or produce the activation of the immune system against another specific molecule. As a result, antibodies are really interesting tools for disease treatment and therapy. However, in order to fullfil the requirements of pharmacology, these macromolecules have been extensively modified and adjusted; as a result, comercially available antibodies are known as engineered antibodies. Depending on the modification and origin of the antibody sequence (i.e. amino acid sequence) these are named with distinctive sufixes: -mumab, -ximab or -zumab.
- -ximab: these type of engineered antibodies are chimeric, meaning that contains a foreign variable segment (originating from one species other than human), linked to a constant segment of human origin.
- -zumab: these type of engineered antibodies are also chimeric; however, the content of foreign segments is much smaller and are also known as humanized antibodies. These type of antibodies are likely to be more compatible with the recipient.
- -mumab: these type of antibodies are from human origin, with more than 90% of sequence identity.
Once explained the characteristic and properties of antibodies, I am going to review some therapeutic antibodies that have been succesfully introduced in the clinic.
Tocilizumab is a humanized monoclonal antibody which has been proved to be effective in the treatment of rheumatoid arthritis (RA). RA is an autoinflammatory disease of the joints; this means that the organism recognizes joints as foreing and destroys them. Tocilizubam prevents the destruction of the joints because it acts as an antagonist (or blocker) of the interleukin 6 (IL-6) receptor. As a result, IL-6 signalling is inhibited and the inflammatory equilibrium is restored. In fact, IL-6 is an inflammatory molecule mediating many symptoms in rheumatoid arthritis.
On the other hand, IL-6 is not only implicated in rheumatoid arthritis and it has been shown to be involved in many inflammatory disease, such as systemic sclerosis, inflammatory diseases of the eye, Still’s disease and myocardial infarction. For this reason, tocilizumab has been assayed in many clinical trials and it appears to be highly effective in many autoinflammatory diseases.
Rituximab is a human/murine chimeric monoclonal antibody that recognizes and binds the molecule CD20; rituximab is currently used in hematological malignancies (cancer of white blood cells or lymphomas) and some autoimmune diseases (for instance, rheumatoid arthritis). CD20 molecule is specifically presented by B cells (immature and mature B cells but not on plasma cells). Thus, rituximab deplets peripheral B cells; the effect of rituximab results from its regulatory effect on the cell cycle, including apoptosis stimulation, complement-dependent B-cell lysis, and antibody-dependent cell-mediated cytotoxicity.
Ibalizumab is a monoclonal antibody against the molecule CD4, which prevents the entry of the HIV virus into human cells. In order to understand the mechanism of ibalizumab, it should be noted that CD4 acts as the entry receptor for the virus in immune T cells. Ibalizumab binds and alters the conformation (i.e. structure) of CD4, and thus, prevents the entry of the virus.
Although there are many other anti-CD4 recombinant antibodies, ibalizumab is the monoclonal antibody to have passed most trials as an antiviral drug.
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