Monoclonal antibodies in therapy

Immunogenicity problem and antibody humanization

Originally, monoclonal antibodies are produced using murine cells. However, direct administration of mouse proteins to humans triggers a strong immune response: the body recognizes them as foreign agents and seeks to destroy them.

Genetic engineering methods are used to solve this problem. Specialists create humanized monoclonal antibodies based on human Immunoglobulin G (IgG). The goal of the modification is to minimize the immunogenicity of the drug while preserving its ability to precisely recognize the target antigen.

There are two main approaches to engineering such molecules:

• Framework region replacement (Option A). In this case, all domains of the molecule are replaced with human ones, except for the variable regions. The variable regions responsible for antigen binding remain entirely murine.
• Maximum humanization (Option B). Absolutely all structures of the molecule become human, except for the hypervariable regions. This particular option provides the best biocompatibility, as only tiny fragments directly contacting the target cell antigen remain murine.

Immunotoxins: structure and conjugation principle

Monoclonal antibodies alone can bind to tumor cells, but their intrinsic cytolytic action is not always sufficient to destroy the pathology. To significantly enhance the damaging effect, immunotoxins were developed.

An immunotoxin is a hybrid molecule. It is produced by chemical conjugation of two elements:

1. A monoclonal antibody that is strictly specific to certain molecules on the surface of the pathological cell.
2. A toxic subunit derived from a plant poison (e.g., ricin).

A critically important structural element is the method of joining these two parts. The toxic chain is attached to the antibody via a disulfide bridge (-S-S-). This chemical bond is strong enough to prevent the poison from detaching in the bloodstream, but capable of cleaving once inside the target cell.

Step-by-step mechanism of action of immunotoxins

The clinical significance of immunotoxins lies in their ability to induce selective lysis. They destroy tumor tissue while minimizing damage to surrounding healthy cells. This process is strictly regulated and occurs in four stages:

1. Delivery. The monoclonal antibody introduced into the body circulates in the bloodstream until it encounters a specific antigen on the tumor cell membrane. Firm binding occurs.
2. Internalization. The target cell independently engulfs the formed antigen-immunotoxin complex. This process occurs via endocytosis, resulting in the complex ending up inside a membrane vesicle, the endosome.
3. Toxin release. Inside the endosome, conditions change, leading to the rupture of the bond (disulfide bridge). The toxic subunit exits the endosome into the cell cytosol, ending up in close proximity to its main intracellular targets.
4. Cytotoxic effect. Once free, the plant toxin blocks vital intracellular enzymes. The cell's function is paralyzed, leading to its inevitable death.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Structure of the Immunoglobulin G (IgG) molecule
• Structure of variable and hypervariable regions of antibodies
• Mechanisms of receptor-mediated endocytosis and endosome formation
• Intracellular targets for ricin and other plant toxins
• Processes of enzymatic blockade during the cytotoxic effect

Other topics in the section "Immunity in defense and damage of the organism. Pathology of immunity"

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