The Adaptive Immune System: Monoclonal Antibodies

THE ADAPTIVE IMMUNE SYSTEM

II. HUMORAL IMMUNITY

D. Monoclonal Antibodies

The overall purpose of this Learning Object is:
1) to introduce the procedures used in producing monoclonal antibodies; and
2) to introduce some of the potential benefits from monoclonal antibody technique.

LEARNING OBJECTIVES FOR THIS SECTION

Adaptive (acquired) immunity refers to antigen-specific defense mechanisms that take several days to become protective and are designed to remove a specific antigen (def). This is the immunity one develops throughout life. There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.

1. humoral immunity (def): humoral immunity involves the production of antibody molecules in response to an antigen (def) and is mediated by B-lymphocytes.

2. cell-mediated immunity (def): Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigen (def) and is mediated by T-lymphocytes.

In this section we will look at monoclonal antibodies.

D. Monoclonal Antibodies (def)

One of the problems of using antibodies prepared in animals (by injecting the animal with a specific antigen and collecting the serum after antibodies are produced) is that up to 90% of the antibodies in the animal's serum may be antibodies the animal has made "on its own" against environmental antigens, rather than those made against the injected antigen.

One of the major breakthroughs in immunology occurred when monoclonal antibody technique was developed. Monoclonal antibodies are antibodies of a single specific type coming from a clone of identical B-lymphocytes.

In this technique, an animal is injected with the specific antigen for the antibody desired (see Fig. 1). After an appropriate time for antibody production, the animal's spleen is removed. The spleen is rich in B-lymphocytes and plasma cells and each plasma cell is genetically programmed to produce only one specificity of antibody. Plasma cells (def), however, will not grow artificially in cell culture. Therefore, a plasma cell producing the desired antibody is fused with a myeloma cell (def) - a cancer cell from a type of cell normally found in the bone marrow which will grow rapidly in cell culture - to produce a hybridoma cell (def) (see Fig. 2). The hybridoma cell has the characteristics of both parent cells; it will produce the specific antibody of the plasma cell and will grow readily in cell culture like the myeloma cell. The hybridoma cells are grown artificially in huge vats where they produce and secrete large quantities of the pure antibody (see Fig. 3).

Monoclonal antibodies are now used routinely in medical research and diagnostic serology (def) (see Labs 17 and 18). They are also being used for modulating immune responses to produce immunosuppression for treatment of autoimmune diseases (def) and graft versus host disease (def), as well as for the prevention of allograft (def) rejection. Monoclonal antibodies against normal antigens or receptors on human B-lymphocytes and/or T-lymphocytes are used in order to destroy the lymphocytes. Monoclonal antibodies have also been tried clinically against a number of viruses and bacteria.

Most monoclonal antibodies used to date have been produced using rodent cells but these antibodies will not activate the human complement pathways (def) or participate in opsonization (def). They also have a short survival time in humans. Genetically engineered antibodies are now being produced whereby the Fab portion (def) from rodent antibodies is attached to the Fc portion (def) of human antibodies.

Monoclonal antibodies are also being produced by recombinant DNA techniques by inserting random human genes coding for the variable Fab portion of human antibodies into the genome of filamentous bacteriophages (def). As the bacteriophages replicate, they display the Fab portion of the antibody on their surface. The bacteriophages are subsequently mixed with an antigen to select for those producing complementary Fabs. Those bacteriophage genomes are then converted into plasmids that can subsequently produce soluble specific Fabs in bacteria.

Immunotoxins (def) have also been produced recently for experimental use. Radioactive substances or cytotoxins (def), such as Pseudomonas exotoxin, have been attached to monoclonal antibodies made against antigens found only on specific types of cancer cells (See Fig. 4, step 1). Since the antibody will only bind to the cancer cell displaying that antigen, it will deliver the toxic substance to the cancer cells but not to normal cells (See Fig. 4, step 2. and Fig. 4, step 3). Alternately, enzymes capable of converting drug precursors injected into the bloodstream into antineoplastic drugs have been attached to these monoclonal antibodies. Results so far have been somewhat encouraging.

Attempts are also being made to design monoclonal antibodies to neutralize the growth factors cancer cells and their blood supply need in order to expand, or monoclonal antibodies against the stroma, the connective tissue between tumor cells that make up the bulk of the tumor.

Monoclonal antibodies are being tried as a means of delivering materials not related to cancers or infectious diseases. For example, monoclonal antibodies made against fibrin and with the enzyme urokinase attached are being tried to dissolve blood clots. The antibody binds to the fibrin in the clot and the urokinase attached to the antibody then converts plasminogen to plasmin, a chemical that destroys the fibrin and dissolves the clot.

In addition, monoclonal antibodies against the Fc portion of IgE (def) are being tried experimentally to prevent allergies, as will be discussed later in Unit 5 under Hypersensitivities.

Flash animation showing the use of monoclonal antibodies to block the attachment of IgE to mast cells.

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