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Peter J. Delves

, PhD, University College London, London, UK

Medically Reviewed Sep 2021 | Modified Sep 2022
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Immunotherapeutic agents use or modify immune mechanisms Overview of the Immune System The immune system distinguishes self from nonself and eliminates potentially harmful nonself molecules and cells from the body. The immune system also has the capacity to recognize and destroy... read more . Use of these agents is rapidly evolving; new classes, new agents, and new uses of current agents are certain to be developed. A number of different classes of immunotherapeutic agents have been developed (see also table Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use  ):

  • Monoclonal antibodies

  • Fusion proteins

  • Soluble cytokine receptors

  • Recombinant cytokines

  • Small-molecule mimetics

  • Cellular therapies


Monoclonal antibodies

Monoclonal antibodies (mAbs) are manufactured in vitro to recognize specific targeted antigens (Ags); they are used to treat solid and hematopoietic tumors, inflammatory disorders, and infections. Most mAbs in clinical use target a single Ag, but a few are engineered to be bispecific. The monoclonal antibodies that are currently in clinical use include

  • Murine

  • Chimeric

  • Humanized

  • Fully human

Murine monoclonal antibodies are produced by injecting a mouse with an antigen, harvesting its spleen to obtain B cells that are producing antibody specific to that antigen, fusing those cells with immortal mouse myeloma cells, growing these hybridoma cells (eg, in cell culture), and harvesting the antibody. Although mouse antibodies are similar to human antibodies, clinical use of murine monoclonal antibodies is limited because they induce human anti-mouse antibody production, can cause immune complex serum sickness (a type III hypersensitivity reaction Classification of Hypersensitivity Reactions Allergic (including atopic) and other hypersensitivity disorders are inappropriate or exaggerated immune reactions to foreign antigens. Inappropriate immune reactions include those that are... read more ), and are rapidly cleared.

To minimize the problems due to use of pure mouse antibody, researchers have used recombinant DNA techniques to create monoclonal antibodies that are part human and part mouse. Depending on the proportion of the antibody molecule that is human, the resultant product is termed one of the following:

  • Chimeric

  • Humanized

In both cases, the process usually begins as above with production of mouse hybridoma cells that make antibody to the desired antigen. Then the DNA for some or all of the variable portion of the mouse antibody is merged with DNA for human immunoglobulin. The resultant DNA is placed in a mammalian cell culture, which then expresses the resultant gene, producing the desired antibody. If the mouse gene for the whole variable region is spliced next to the human constant region, the product is termed "chimeric." If the mouse gene for only the antigen-binding hypervariable regions of the variable region is used, the product is termed "humanized."

Chimeric monoclonal antibodies activate antigen-presenting cells (APCs) and T cells more effectively than murine monoclonal antibodies but can still induce production of human anti-chimeric antibodies.

Fully human monoclonal antibodies are produced using transgenic mice that contain human immunoglobulin genes or using phage display (ie, a bacteriophage-based cloning method) of immunoglobulin genes isolated from human B cells. Fully human monoclonal antibodies have decreased immunogenicity and therefore may have fewer adverse effects in patients.

Monoclonal antibodies that target checkpoint molecules on either T cells or tumor cells (termed checkpoint inhibitors—see table Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use  ) are used to prevent downregulation of antitumor responses and effectively treat some heretofore resistant cancers. However, because checkpoint molecules are also involved in other types of immune response, checkpoint inhibitors can cause severe immune-related inflammatory and autoimmune reactions (both systemic and organ specific).

Fusion proteins

These hybrid proteins are created by linking together the gene sequences encoding all or part of 2 different proteins to generate a chimeric polypeptide that incorporates desirable attributes from the parent molecules (eg, a cell-targeting component combined with a cell toxin). The circulating half-life of therapeutic proteins can also often be improved by fusing them to another protein that naturally has a longer serum half-life (eg, the Fc region of IgG).

Soluble cytokine receptors

Soluble versions of cytokine receptors are used as therapeutic reagents. They can block the action of cytokines by binding with them before they attach to their normal cell surface receptor.

Soluble interleukin (IL) receptors (eg, those for IL-1, IL-2, IL-4, IL-5, and IL-6) are being developed for treatment of inflammatory and allergic disorders and cancer.

Recombinant cytokines

Colony-stimulating factors (CSF), such as erythropoietin, granulocyte CSF (G-CSF), and granulocyte-macrophage CSF (GM-CSF), are used in patients undergoing chemotherapy or transplantation for hematologic disorders and cancers and in patients with severe chronic neutropenia (see table Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use Some Immunotherapeutic Agents in Clinical Use  ). Interferon-alpha (IFN-alpha) and IFN-gamma are used to treat cancer, immunodeficiency disorders, and viral infections; IFN-beta is used to treat relapsing multiple sclerosis Multiple Sclerosis (MS) Multiple sclerosis (MS) is characterized by disseminated patches of demyelination in the brain and spinal cord. Common symptoms include visual and oculomotor abnormalities, paresthesias, weakness... read more Multiple Sclerosis (MS) . Many other cytokines are being studied.

Anakinra, used to treat rheumatoid arthritis, is a recombinant, slightly modified form of the naturally occurring IL-1R antagonist; this drug attaches to the IL-1 receptor and thus prevents binding of IL-1, but unlike IL-1, it does not activate the receptor.

Cells expressing cytokine receptors can be targeted by modified versions of the relevant cytokine (eg, denileukin diftitox, which is a fusion protein containing sequences from IL-2 and from diphtheria toxin). Denileukin is used in cutaneous T-cell lymphoma to target the toxin to cells expressing the CD25 component of the IL-2 receptor.

Small-molecule mimetics

Small linear peptides, cyclicized peptides, and small organic molecules are being developed as agonists or antagonists for various applications. Screening libraries of peptides and organic compounds can identify potential mimetics (eg, agonists for receptors for erythropoietin, thrombopoietin, and G-CSF).

Cellular therapies

Immune system cells are harvested (eg, by leukapheresis) and activated in vitro before they are returned to the patient. The aim is to amplify the normally inadequate natural immune response to cancer. Methods of activating immune cells include using cytokines to stimulate and increase numbers of antitumor cytotoxic T cells and using pulsed exposure to antigen-presenting cells such as dendritic cells with tumor antigens. Before being returned to the patient, T cells can be genetically engineered to express chimeric antigen receptors (CAR) or T cell receptors (TCR) capable of recognizing tumor antigens, an approach that has shown efficacy in patients with leukemia Overview of Leukemia Leukemia is a malignant condition involving the excess production of immature or abnormal leukocytes, which eventually suppresses the production of normal blood cells and results in symptoms... read more and lymphoma Overview of Lymphoma Lymphomas are a heterogeneous group of tumors arising in the reticuloendothelial and lymphatic systems. The major types are Hodgkin lymphoma Non-Hodgkin lymphoma See table Comparison of Hodgkin... read more .

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