The Innate Immune System: Pattern-Recognition Receptors, Antigen-Nonspecific Antimicrobial Body Molecules, and Cytokines

I. THE INNATE IMMUNE SYSTEM

B. PATHOGEN-ASSOCIATED MOLECULAR PATTERNS (PAMPs), PATTERN-RECOGNITION RECEPTORS (PRRs), AND CYTOKINES IMPORTANT IN INNATE IMMUNITY

4. Harmful Effects Associated with Abnormal Pattern-Recognition Receptor Responses and/or Cytokine Production

The overall purpose of this Learning Object is:
1) to learn some of the harmful effects associated with improper pattern-recognition receptor responses and/or excessive cytokine production; and
2) to introduce potential therapeutic possibilities associated with improper pattern-recognition receptors responses.

LEARNING OBJECTIVES FOR THIS SECTION

Innate immunity is an antigen-nonspecific defense mechanisms that a host uses immediately or within several hours after exposure to almost any microbe. This is the immunity one is born with and is the initial response by the body to eliminate microbes and prevent infection.

Unlike adaptive immunity, innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells. These unique microbial molecules are called pathogen-associated molecular patterns or PAMPS and include LPS from the gram-negative cell wall, peptidoglycan and lipotechoic acids from the gram-positive cell wall, the sugar mannose (a terminal sugar common in microbial glycolipids and glycoproteins but rare in those of humans), bacterial and viral unmethylated CpG DNA, bacterial flagellin, the amino acid N-formylmethionine found in bacterial proteins, double-stranded and single-stranded RNA from viruses, and glucans from fungal cell walls. In addition, unique molecules displayed on stressed, injured, infected, or transformed human cells also act as PAMPS. (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)

Most body defense cells have pattern-recognition receptors (PRRs) for these common PAMPSand so there is an immediate response against the invading microorganism. Pathogen-associated molecular patterns can also be recognized by a series of soluble pattern-recognition receptors in the blood that function as opsonins and initiate the complement pathways. In all, the innate immune system is thought to recognize approximately 10³ of these microbial molecular patterns.

The innate immune responses do not improve with repeated exposure to a given infection and involve the following:

phagocytic cells (neutrophils, monocytes, and macrophages);
cells that release inflammatory mediators (basophils, mast cells, and eosinophils);
natural killer cells (NK cells); and
molecules such as complement proteins, acute phase proteins, and cytokines.

Examples of innate immunity include anatomical barriers, mechanical removal, bacterial antagonism, pattern-recognition receptors, antigen-nonspecific defense chemicals, the complement pathways, phagocytosis, inflammation, and fever.

We will now take a closer look at some of the harmful effects associated with abnormal pattern-recognition receptor responses and/or cytokine production.

4. Harmful Effects Associated with Abnormal Pattern-Recognition Receptor Responses and/or Cytokine Production

There are a number of harmful effects that are known to occur as a result of either an overactive or an underactive innate immune response. These include:

a. Sepsis (Systemic Inflammatory Response Syndrome or SIRS) from a severe systemic infection or an overactive innate immune response

Cytokines (def) such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), and interleukin-8 (IL-8) are known as inflammatory cytokines (def) because they promote inflammation. Some cytokines, such as IL-8, are also known as chemokines (def). Chemokines promote an inflammatory response by enabling white blood cells to leave the blood vessels and enter the surrounding tissue, by chemotactically attracting these white blood cells to the infection site, and by triggering neutrophils (def) to release killing agents for extracellular killing. In addition to promoting an inflammatory response, these same cytokines activate the complement pathways (def) as well as the coagulation pathway (def).

Inflammation (def) is the first response to infection and injury and is critical to body defense. Basically, the inflammatory response is an attempt by the body to restore and maintain homeostasis (def) after injury. Most of the body defense elements are located in the blood, and inflammation is the means by which body defense cells and defense chemicals leave the blood and enter the tissue around an injured or infected site. The release of inflammatory cytokines eventually leads to vasodilation of blood vessels.
Vasodilation (def) is a reversible opening of the junctional zones between endothelial cells (def) of the blood vessels and results in increased blood vessel permeability. This enables plasma, the liquid portion of the blood, to enter the surrounding tissue. The plasma (def) contains defense chemicals such as antibody molecules (def), complement proteins (def), lysozyme (def), and beta defensins (def). Increased capillary permeability also enables white blood cells to squeeze out of the blood vessels and enter the tissue. As can be seen, inflammation is necessary part of body defense. Excessive or prolonged inflammation can, however, cause harm as will be discussed below.

For More Information: Inflammation from Unit 4

As mentioned in a previous section, products of the complement pathways (def) lead to more inflammation (def), opsonization (def) of bacteria, chemotaxis (def) of phagocytes to the infected site, and MAC lysis (def) of gram-negative bacteria.

For More Information : The Complement Pathways from Unit 4

The products of the coagulation pathway (def) lead to the clotting of blood to stop bleeding, more inflammation, and localization of infection.

At moderate levels, inflammation, products of the complement pathways, and products of the coagulation pathway are essential to body defense. However, these same processes and products when excessive, can cause considerble harm to the body.

When there is a minor infection with few bacteria present, low levels of cell wall components are present. This leads to moderate inflammatory cytokine (def) production with the results being primarily beneficial (see Fig. 9).

However, in the case of a severe infection with very large numbers of bacteria present, high levels of cell wall components are present. This leads to excessive inflammatory cytokine (def) production with the results causing damage to the body (see Fig. 10). People with overactive TLR-4 receptors (def) may be prone to developing SIRS from gram-negative bacteria. Those with underactive TLR-4 are more prone to developing infections with gram-negative bacteria.

This excessive inflammatory response is referred to as Systemic Inflammatory Response Syndrome or SIRS. Death is a result of what is called the shock cascade (def). The sequence of events is as follows:

For More Information: Systemic Inflammatory Response Syndrome (SIRS) from Unit 2

This is seen during septicemia (def), a condition where bacteria enter the blood and cause harm. There are approximately 750,000 cases of septicemia per year in the U.S. and the mortality rate is between 20% and 50%. Over 210,000 people a year in the U.S. die from septic shock. Approximately 45% of the cases of septicemia are due to gram-positive bacteria, 45% are a result of gram-negative bacteria, and 10% are due to fungi (mainly the yeast Candida).

b. People with an underactive form of TLR-4 (def), the toll-like receptor for bacterial LPS, have been found to be five times as likely to contract a severe bacterial infection over a five year period than those with normal TLR-4. People with overactive TLR-4 receptors may be more prone to developing SIRS from gram-negative bacteria.

c. Most people that die as a result of Legionnaire's disease have been found to have a mutation in the gene coding for TLR-5 (def) that enables the body to recognize the flagella of Legionella pneumophila.

d. B-lymphocytes (def), the cells responsible for recognizing foreign antigens (def) and producing antibodies (def) against those antigens, normally don't make antibodies against the body's own DNA and RNA. The reason is that any B-lymphocytes that bind the body's own antigens normally undergo apoptisis (def), a programmed cell suicide. People with the autoimmune disease systemic lupus erythematosis have a mutation in a gene that signals the cell to undergo apoptosis. As a result, these B-cells are able to bind and engulf the body's own DNA and RNA and place them in an endosome (def) or phagolysosome (def) where the the DNA can be recognized by TLR-9 (def) and the RNA by TLR-7(def). This, in turn, triggers those B-lymphocytes to make antibody molecules against the body's own DNA and RNA. Another gene error enables these B- cells to increase the expression of TLR-7.

e. TLR-4 (def), MyD88, TLR-1 and TLR-2 (def) have been implicated in the production of artherosclerosis in mice and some humans.

f. Mutations in the gene coding for NOD-2 (def) that prevents the NOD-2 from recognizing muramyl dipeptide make a person more susceptible to Crohn's disease, an inflammatory disease of the large intestines.

g. People with chronic sinusitis that does not respond well to treatment have decreased activity of TLR-9 (def) and produce reduced levels of human beta-defensin 2 (def), as well as mannan-binding lectin (def) needed to initiate the lectin complement pathway (def).

h. Pathogenic strains of Staphylococcus aureus producing leukocydin (def) and protein A (def), including MRSA (def), cause an increased inflammatory response. Protein A, a protein that blocks opsonization (def) and functions as an adhesin (def), binds to cytokine receptors for TNF-alpha (def). It mimics the cytokine and induces a strong inflammatory response. As the inflammatory response attracts neutrophils to the infected area, the leukocydin causes lysis of the neutrophils (def). As a result, tissue is damaged and the bacteria are not phagocytosed.

4. Therapeutic Possibilities

Researchers are now looking at various ways to either artificially activate TLRs in order to enhance immune responses or inactivate TLRs to lessen inflammatory disorders. Examples of agents being evaluated in clinical studies or animal studies include:

1. TLR activators to activate immune responses

a. Both TLR-4 (def) and TLR-9 (def) activators are being tried in early clinical trials as vaccine adjuvants to improve the immune response to vaccines. TLR-9 activators are being tried as an adjuvant for the hepatitis B and anthrax vaccines and a TLR-4 activator is being tried as an adjuvant for the vaccine against the human papillomaviruses that cause most cervical cancer.

b. Both TLR-7 (def) and TLR-9 activators are being tried in early clinical trials as an antiviral against hepatitis C. Activation of these TLRs triggers the synthesis and secretion of type I interferons that block viral replication within infected host cells.

c. TLR-9 activators are being tried in early clinical trials as an adjuvant for chemotherapy in the treatment of lung cancer.

d. TLR-9 activators are being tried in early clinical trials to help in the treatment and prevention of allergies and asthma. Activation of TLR-9 in macrophages and other cells stimulates these cells to to kill T_H2 cells, the subclass of T-helper lymphocytes responsible for most allergies and asthma.

2. TLR inhibitors to suppress immune responses

a. General TLR inhibitors might onday be used to treat autoimmune disorders.

b. A TLR-4 (def) inhibitor, a mimic of the endotoxin from the gram-negative cell wall, is being tried in early clinical trials to block or reduce the death rate from gram-negative sepsis and SIRS (def).

c. TLR-4, TLR-2, and MyD88 inhibitors might possibly one day lessen atherosclerotic plaques and the risk of heart disease.

Of course using TLR activators or TLR inhibitors to turn up or turn down immune responses also carries risks. Trying to suppress harmful inflammatory responses may also result in increased susceptibility to infections; trying to activate immune responses could lead to SIRS or autoimmune disease.

A number of human cytokines produced by recombinant DNA technologies are now being used to treat various infections or immune disorders. These include:

1. recombinant interferon alfa-2a (Roferon-A): a cytokine used to treat Kaposi's sarcoma, chronic myelogenous leukemia, and hairy cell leukemia.
2. peginterferon alfa-2a (Pegasys) : used to treat hepatitis C (HCV).
3. recombinant interferon-alpha 2b (Intron A): a cytokine produced by recombinant DNA technology and used to treat Hepatitis B; malignant melanoma, Kaposi's sarcoma, follicular lymphoma, hairy cell leukemia, warts, and Hepatitis C.
4. peginterferon alfa-2b (PEG-Intron; PEG-Intron Redipen): used to treat hepatitis C (HCV).
5. recombinant Interferon alfa-2b plus the antiviral drug ribavirin (Rebetron): used to treat hepatitis C (HCV).
6. recombinant interferon-alpha n3 (Alferon N): used to treat warts.
7. recombinant iInterferon alfacon-1 (Infergen) : used to treat hepatitis C (HCV).
8. G-CSF (granulocyte colony stimulating factor): for reduction of infection in people after myelotoxic anticancer therapy for solid tumors.
9. GM-CSF (granulocyte-macrophage colony stimulating factor): for hematopoietic reconstruction after bone marrow transplant in people with lymphoid cancers.

QUIZ YOURSELF ON THIS SECTION

RETURN TO UNIT 4 TABLE OF CONTENTS