Cancer biomarkers – autoantibodies

How to recognize cancer using biomarkers?

Apollinaria Bogolyubova, "Biomolecule"Article for the contest "bio/mol/text"
Mortality from malignant tumors is a relatively young problem that arose when the average life expectancy of a person increased significantly due to numerous victories in the fight against infections.

Almost all tumors can be cured at the early stages of their occurrence, however, it is usually possible to detect a neoplasm only with the appearance of symptoms, when it is no longer possible to help the patient. This article describes one of the types of biomarkers that have recently been actively developed for the early diagnosis of cancer – biomarkers based on autoantibodies.

Antibodies can serve not only as an effective system of protecting the body from uninvited guests-pathogens, but also allow detecting the presence of a tumor in the early stages of its development.

Like an army armed to the teeth, the immune system protects the body from the penetration of alien pathogens. And, as befits an active army, it includes various specialized units. Scouts – dendritic cells – are the first to detect the antigen; border guards – tissue macrophages – monitor the order in the barrier tissues of the body; infantry – first of all, neutrophils, followed by T-, NK- and NKT cells - are the first to rise to fight the enemy; finally, snipers – B-lymphocytes – they "shoot" antibodies at a potential invader, and T- and B-memory cells, like wise strategists, remember information about the infection for effective and immediate fight against it during a repeated attack [1]. There is no peacetime for them – you always need to be ready to fight again without delay, for example, against the "traitors" of the body – tumor cells.

Hello, the antibody is young, unfamiliar!..Antibodies are soluble proteins-immunoglobulins produced by so-called plasma cells, which are the final stage of differentiation of B-lymphocytes and one of the key players in the adaptive (acquired) immunity system.

Antigenic stimulation of a "naive" (not encountered with an antigen) B-lymphocyte through a B-cell receptor (which is a membrane-bound form of an antibody) activates a sequence of events, as a result of which the plasma cell ends up in the bone marrow and begins to continuously produce large amounts of antibodies specific to a specific antigen. The participation of antibodies in the implementation of the immune response can be carried out both by direct action on antigen-carrying molecules and organisms, and indirectly, i.e. by attracting additional mechanisms (activation of the complement system, attraction of phagocytes, etc.; Fig. 1).

Figure 1. The main mechanisms of action of antibodies. The neutralizing effect of antibodies (option 1) is illustrated by the example of IgM antibodies blocking a microorganism and IgG antibodies forming an immune complex with a toxin. Variants 2 and 3 (opsonization, which enhances phagocytosis, and stimulation of cytotoxic activity of NK cells) are realized through the interaction of immunoglobulins with Fcy receptors of cells. Interaction with the antigen (option 4) opens complement-binding sites of IgG antibodies, which leads to the launch of complement along the classical pathway and lysis or opsonization of target cells [2].The process of forming a repertoire of B-cells necessarily includes a "negative selection" occurring in the bone marrow, during which cells carrying a B-cell receptor capable of binding the body's own antigens (autoantigens) are removed from the entire set of immature B-lymphocytes.

This process is extremely important because it serves as a guarantee that the host cells will not become targets for antibodies. Nevertheless, the effectiveness of negative selection is not one hundred percent, and in reality there remains a large number of "autoreactive" (i.e., able to recognize autoantigens) B-cells.

Normally, this phenomenon does not have a negative effect on the body, since a whole range of additional regulatory barriers lies on the way to the activation of an autoreactive B-cell. However, sometimes autoreactive cells manage to overcome all obstacles, and this can lead to the initiation of the autoimmune process underlying all autoimmune diseases. The same scenario can develop in the case of tumor formation.

Tumor cells are usually characterized by a whole spectrum of protein molecule disorders that can be detected by the adaptive immunity system and manifest themselves, including the presence of circulating autoantibodies to normal or somehow modified tumor proteins (called in this case tumor-associated autoantigens). The latter include mutant, misfolded or glycosylated, overexpressed proteins or proteins with ectopic expression, i.e. expressed by a tumor, but not by normal tissue of the same origin/anatomical localization. The sources of antigens in tumor tissue can also be proteins with hidden epitopes exposed during tumor-specific proteolysis.

Cancer and you: who is who?Malignant neoplasms are the scourge of modern humanity.

Antibiotics, vaccines [3], medicines, immune serums, etc. have made it possible to treat and prevent a huge number of diseases whose pandemics in the past claimed millions of lives (plague [4], cholera, smallpox, anthrax, etc.). The average life expectancy of a person has increased, and cardiovascular diseases and tumors have become the most common cause of death (25 and 23% of the total number of deaths per year, respectively, according to data for 2008 [5]). The average five-year survival rate of patients with diagnosed lung tumors (first in frequency in men and second after breast cancer in women) is 16.3%. However, if we analyze the survival data separately for each stage of the diagnosed tumor, it turns out that the detection of a neoplasm at the stage of a localized tumor gives 52% survival, and at the stage of regional and distant metastases – 24% and 4%, respectively [6] (Fig. 2).

Figure 2. Lung cancer diagnosis and patient survival by stage, 2001-2007 [6].Thus, the most promising way to reduce mortality from malignant neoplasms is the development and introduction of new methods for their early diagnosis, the most promising of which is the non-invasive or minimally invasive detection of tumor biomarkers (oncomarkers) in the blood, urine, saliva, bronchoalveolar fluid, sputum and tumor tissue of the patient.

Cancer markers (which can act as a "molecular witness" of the presence of a tumor in the body of almost any nature, be it protein, nucleic acid or lipid) are used to diagnose malignant neoplasms, classify and stage them, monitor relapses, as well as to assess the prognosis and choice of treatment methods (Fig. 3).

Figure 3. Schematic representation of the use of biomarkers at different stages of tumor progression [7].Autoantibodies specific to tumor-associated autoantigens can be used in almost any of the above cases and, unlike traditional biomarkers, are characterized by a number of important advantages, such as:

• low invasiveness during biomaterial sampling;
• absence of specific requirements for material sampling and sample preparation;
• simplicity and uniformity of definition (different types of enzyme immunoassay are used);
• high stability in blood serum and plasma samples;
• high half-life time;
• minimal concentration fluctuations depending on the time of day, food intake, medications, physical activity, menstrual cycle phase, certain medical interventions, etc.;
• high specificity.

Autoantibodies for diagnostics – a new solution to old problems!An example of the successful use of autoantibodies in the diagnosis of malignant neoplasms can be autoantibodies against the tumor suppressor p53 [8], which is often mutated in various types of human neoplasms.

Mutations lead to a violation of the tertiary structure of the protein, and as a result, many hidden epitopes become available for presentation to B and T cells of the immune system.

Anti-p53 antibodies in the blood serum of workers exposed to vinyl chloride are detected before the clinical diagnosis of liver angiosarcoma [9], and in patients with chronic obstructive pulmonary disease – before the clinical diagnosis of lung cancer [10]. Some studies have demonstrated a correlation between the presence of anti-p53 autoantibodies in patients and survival (ovarian cancer, colorectal cancer and lung cancer); in most cases, the registration of autoantibodies against p53 was associated with an unfavorable prognosis.

It is impossible not to mention onconeural antibodies – autoantibodies specific to neuronal antigens – proteins that are normally expressed only in nervous tissue, but due to various genetic disorders begin to be produced in tumor tissue, causing the so-called paraneoplastic syndrome (a tumor-associated pathological condition characterized by autoimmune lesions of various parts of the nervous system).

As an example of such antigens, the proteins HuB, HuC and HuD (human antigens B, C, D), which are normally expressed in all neurons, as well as in small cell lung cancer, prostate cancer and neuroblastoma, and CDR2, normally expressed only in Purkinje neurons and associated with such types of cancer, such as breast cancer, ovarian cancer and small cell lung cancer.

Onconeural antibodies are found in most patients with paraneoplastic syndrome, and it has been shown that their appearance clearly and independently correlates with the development of a malignant neoplasm in the patient's body [11]. Currently, by the presence of certain onconeural antibodies, it is possible to judge the predominant localization of the tumor focus, which, due to its small size, was not detected during the initial examination of the patient [12].

Another example of biomarkers of an antibody nature are autoantibodies against cancer-testicular (cancer-gamete) antigens [13]. They are expressed in a wide range of neoplasms (melanoma, ovarian cancer, lung cancer), but not in normal tissues of the body, with the exception of male testes. Aberrant expression of these proteins leads to the development of a T-cell and/or B-cell response to them, i.e., to the appearance of autoantibodies in the patient's bloodstream. The most well-known representatives of this family of tumor-associated autoantigens are MAGE-1 (he was the first found antigen of this group), NY-ESO1, MAGE-3 and SSX2.

Perhaps the most striking success in the development of autoantitive biomarkers is the EarlyCDT-Lung diagnosticum, based on antibodies against seven tumor-associated antigens, created in 2007 and already successfully used to detect lung cancer in its early stages. This panel contains cancer-testicular antigens (NY-ESO-1, CAGE, MAGE A4), tumor suppressor p53, transcription factor SOX2, onconeural antigen HuD and GBU4-5 – a protein whose function in the cell is still poorly understood. Interestingly, the composition of the antigenic panel changed during the development process towards higher sensitivity and specificity, which eventually managed to be raised to 41% and 91%, respectively.

A routine test for detecting lung carcinomas in the West is computed tomography (CT), however, it has been shown that EarlyCDT-Lung diagnosticum copes with this task better. CT, due to the resolution of the method, may not give reliable results if a patient has a tumor with small linear dimensions, whereas an adaptive immune response formed to tumor-associated autoantigens can be detected at the earliest stages of neoplasm development.

The search for antibodies against tumor-associated autoantigens and the development of diagnostics based on them are currently one of the promising branches of modern diagnostic oncoimmunology. Simplicity of execution and cheapness, and, consequently, accessibility to a wide range of people, in the future makes it possible to use such diagnostics as part of a planned medical examination of citizens, which is the key to timely detection of a tumor and full recovery of the patient.

That's how the immune system not only stands guard over the body, preventing the development of pathogens and tumors, but also helps researchers find more and more new methods for diagnosing various diseases, in particular, tumors. Antibody arrows aimed by B cells at the rebels allow doctors to find and neutralize the latter, thereby prolonging the patient's life for many, many years.

LiteratureBiomolecule: "Immunological Nobel Prize (2011)";

2. A.A. Yarilin. (2010). Immunology, p. 462;
3. biomolecule: "Vaccines in questions and answers";
4. Biomolecule: "It's the plague";
5. Minino A.M., Xu J., Kochanek, K.D. (2010). Deaths: preliminary data for 2008. National Vital Statistics Reports 59, 1-52;
6. American Lung Association: Lung cancer fact sheet;
7. Ludwig J., Weinstein J. (2005). Biomarkers in cancer staging, prognosis and treatment selection. Nat. Rev. Cancer 5, 845-866;
8. Nature web focus: p53 – 30 years on;
9. Trivers et al. (1995). Anti-p53 antibodies in sera of workers occupationally exposed to vinyl chloride. J. Natl Cancer Inst. 87, 1400-1407;
10. Trivers et al. (1996). Anti-p53 antibodies in sera from patients with chronic obstructive pulmonary disease can predate a diagnosis of cancer. Clin. Cancer Res. 2, 1767-1775;
11. Pittock S.J., Kryzer T.J., Lennon V.A. (2004). Paraneoplastic antibodies coexist and predict cancer, not neurological syndrome. Ann. Neurol. 56, 709-715;
12. Vedeler et al., Paraneoplastic Neurological Syndrome Euronetwork. (2006). Management of paraneoplastic neurological syndromes: report of an EFNS Task Fosce. Eur. J. Neurol. 13, 682-690;
13. Chiriva-Internati et al. (2006). Cancer testis antigens: a novel target in lung cancer. Int. Rev. Immunol. 31, 321-343.

Portal "Eternal youth" http://vechnayamolodost.ru09.09.2013