Defence Mechanisms and Innate Immunity

The following points highlight the top six defence mechanisms involved in innate immunity. The defence mechanisms are: 1. Physical (or Mechanical) and Chemical Barriers 2. Inflammation 3. Phagocytosis 4. The Complement System 5. Antibacterial Substances 6. Antiviral Substances.

Mechanism # 1. Physical (or Mechanical) and Chemical Barriers:

Physical (or mechanical) barriers of the host in cooperation with chemical barriers (secretions) act as the first line of defence against pathogenic microorganisms and foreign materials. These barriers include skin, mucous membranes, respiratory system, gastrointestinal tract, genitourinary tract, eye, bacteriocins, and beta-lysin and other polypeptides.

Skin, mucous membranes, respiratory system, gastrointestinal tract, genitourinary tract, and eyes are the barriers that provide both physical and chemical defence (e.g., gastric juices, lysozyme, lactoferrin, glycoproteins, urea etc.) in cooperation. In addition, bacteriocins and beta-lysin and other polypeptides are the defensive chemicals against microorganisms.

1. Skin:

Intact skin is a very effective physical or mechanical barrier to block the entry of microbial pathogens into the body. With few exceptions the microorganisms fail to penetrate the skin because its outer layer consists of thick, closely packed cells called keratinocytes that produce keratins.

Keratins are scleroproteins comprising the main components of hair, nails, and outer skin cells. These scleroproteins are not easily degradable enzymatically by microorganisms. They resist the entry of microbe-containing water and thus function as physical barrier to microorganisms.

In addition to direct prevention of penetration, continuous shedding of the outer epithelial cells of skin removes many of those microbial pathogens that manage to adhere on the surface of the skin.

2. Mucous membranes:

Mucous membranes of various body systems such as respiratory, gastrointestinal, genitourinary, and eye prevent invasion by microorganisms with the help of their intact stratified squamous epithelium and mucous secretions, which form a protective covering that resists penetration and traps many microorganisms.

3. Respiratory system:

An average person inhales about 10,000 microorganisms per day usually at the rate of eight microorganisms per minute. These microorganisms are deposited on the moist, sticky mucosal surfaces of the respiratory tract. The mucociliary blanket of the respiratory epithelium traps the microorganism less than 10 μm in diameter and transports them by ciliary action away from the lungs.

Microorganisms larger than 10 μm normally are trapped by hairs and cilia lining the nasal cavity which beat towards the pharynx so that the mucus with its trapped microorganisms is moved towards the mouth and expelled. Coughing and sneezing also help removal of microorganisms from the respiratory tract.

They make clear the respiratory system of microorganisms by expelling air forcefully from the lungs through the mouth and nose, respectively. Salivation also washes microorganisms from the mouth and nasopharyngeal areas into the stomach.

4. Gastrointestinal system:

Microorganisms may manage to reach the stomach. Many of them are destroyed by the gastric juice of the stomach. The gastric juice is a mixture of hydrochloric acid, proteolytic enzymes, and mucus, and is very acidic with a pH 2 to 3. This juice is normally sufficient to kill most microorganisms and destroy their toxins.

Furthermore, the normal microbial population of the large intestine is extremely significant in not allowing the establishment of pathogenic microorganisms in it.

For convenience, many commensalistic microorganisms in the intestinal tract secrete metabolic products (e.g., fatty acids) that prevent “unwanted” microorganisms from becoming established in the tract. In small intestine, however, the microbial pathogens are often killed by various pancreatic enzymes, bile, and enzymes in intestinal secretions.

5. Genitourinary system:

Kidneys, ureters, and urinary bladder are sterile under normal conditions. Kidney medulla is so hypertonic that it allows only few microorganisms to survive.

Urine destroys some microorganisms due to its low pH and the presence of urea and other metabolic end-products like uric acid, hippuric acid, mucin, fatty acids, enzymes, etc. The lower urinary tract is flushed with urine eliminating potential microbial pathogens. The acidic environment (pH 3 to 5) of vagina also confers defence as it is unfavourable to most microorganisms to establish.

6. Eye:

The conjunctiva of eye lines the interior surface of each eyelid and the exposed surface of the eyeball. It is a specialised mucus-secreting epithelial membrane and is kept moist by continuous flushing action of tears secreted by the lacrimal glands. Tears contain lysozyme and lactoferrin and thus act as physical as well as chemical barriers.

7. Bacteriocins:

The surfaces of skin and mucous membranes are inhabited by normal microbial flora. Of this, many bacteria synthesize and release toxic proteins (e.g., colicin, staphylococcin) under the direction of their plasmids. These toxic proteins are called bacteriocins, which kill other related species thus provide an adaptive advantage against other bacteria.

8. Beta-lysin and other polypeptides:

Blood platelets release a cationic polypeptide called beta-lysin, which disrupts the plasma membrane of certain gram-positive bacteria and kills them. Leukin, cecropins, plakins, and phagocytin are some other cationic polypeptides that kill specific gram-positive bacteria. Prostatic antibacterial factor, a zinc-containing polypeptide, is an important antimicrobial chemical secreted by the prostrate glands in males.

Mechanism # 2. Inflammation (Inflammatory Response):

Inflammation (L. inflammatio = to set on fire) is an innate (nonspecific) defence response of the body to pathogenic infection or tissue injury and helps localizing the infection or injury in its local area. Many of the classic features of inflammation were described as early as 1600 BC in Egyptian papyrus writings.

In the first century AD, the Roman physician Celsus described the four cardinal signs of inflammation as redness (rubor), swelling (tumor), heat (color) and pain (dolor). In the second century AD, another physician, Galen added a fifth sign: altered function (functio laesa).

1. Major events that result in cardinal signs:

Following are the major events that result in the cardinal signs of inflammation:

(i) The redness and heat (rise in temperature) of the localized area is due to vasodilation (an increase in the diameter of blood vessels) of nearby capillaries that occurs as the vessel that carry blood away from the affected area constrict resulting in engorgement of the capillary network.

(ii) Tissue swelling occurs due to accumulation of exudates in the area of infection or injury. An increase in capillary permeability facilitates an influx of fluid and cells from engorged capillaries into the tissue. The fluid that accumulates (exudate) possesses a much higher protein content than fluid normally released from the vascular system.

(iii) Pain is due to lysis of blood cells. The lysis triggers the production of prostaglandins and bradykinin, the chemical substances that alter the threshold and intensity of the nervous system response to pain. Pain probably serves a protective role as it normally causes individual to protect the infected or injured area.

2. Mechanism of defence:

Inflammatory response is a collective term representing the complex sequence of events during inflammation. It initiates when injured tissue cells release inflammatory mediators
(chemicals). Among the inflammatory mediators are various serum proteins called acute-phase proteins; the principal acute-phase proteins are histamine and kinins.

The acute-phase proteins bind to receptors on nearby capillaries and venules causing vasodilation and increased permeability which results in influx of phagocytes (e.g., neutrophils, lymphocytes monocytes and macrophages) from the blood into the tissues.

The emigration of phagocytes is a multistep process (Fig. 44.14) that includes adherence of the cells to the endothelial wall of the blood vessels (margination), followed by their emigration between endothelial cells in to the tissues (diapedesis or extravasation), and finally, their migration through the tissue to the site of the invasion (chemotaxis).

As the phagocytic cells accumulate in the site of injury and begin to phagocytose microbial pathogens, during this process they release lytic enzymes that normally damage the nearby healthy cells. Dead host cells, dead phagocytic cells, dead microbial pathogens, and the body fluid collectively form a substance called pus (the inflammatory exudate).

When the acute-phase proteins bind to receptors on nearby capillaries and venules and cause vasodilation and increased permeability, the latter enable enzymes of the blood-clotting system to enter the tissue. These enzymes activate an enzyme cascade that results in the deposition of insoluble strands of fibrin, a main constituent of a blood clot.

The fibrin strands wall off the injured area from the rest of the body and serve to prevent the spread of infection. Once the inflammatory response is subsided and the pus is removed, the infected or injured area is filled with new tissues that start normal function.

Mechanism # 3. Phagocytosis:

Phagocytosis (Gk. Phagein = to eat; cyte = cell; and osis = a process) is a process during which large particles and microbial cells are enclosed in a phagocytic vacuole or phagosome and ingulfed. It acts a highly efficient cellular barrier against the pathogenic microorganisms and is met out by uptake and digestion of microorganisms by a variety of cells of the body’s defence system.

Besides its contribution in defence, phagocytosis helps certain cells and even organisms (e.g., protozoa) to obtain their nutrients. However, phagocytosis was a chance discovery by E. Metchnikoff (a native of Ukraine) in 1884 who suggested that the motile cells of larvae of starfish actively sought out and engulfed foreign particles present in their environment.

The following lines are devoted in the context of the role of phagocytosis in innate (nonspecific) host defence:

1. Recognition and adherence of microorganisms:

Phagocytic cells (neutrophils, monocytes macrophages, and dendritic cells) employ two fundamental molecular mechanisms for the recognition o microbial pathogens and their adherence on phgocyte’s plasma membrane:

(i) Opsonin-dependent (opsonic) recognition (called opsonization) and

(ii) Opsonin-independent (nonopsonic) recognition.

Opsonin-dependent recognition or opsonization (Gk. opson = to prepare victim for) is a process in which the phagocytic cells readily recognize the microbial pathogens that are coated by serum components (antibodies especially lgG1 and lgG3, complement C3b, and both antibody and complement C3b) called opsonins.

The opsonins function as a bridge between the microorganism and the phagocyte by binding to he surface of microorganism at one end and to specific receptors on the phagocyte surface at the other (Fig. 44.15) and enhance phagocytosis multifold. In one study for convenience, the rate of phagocytosis of a microorganism was 4000-fold higher in the presence of opsonin than in its absence.

Opsonin-independent recognition involves the mechanism which does not involve opsonins and employs other receptors on phagocytic cells that recognise structures (adhesins) expressed on the surface of different microbial pathogens (Fig. 44.16). Important ones of such receptors are lectins, polysaccharides, glycolipids, proteolycans, lypopolysaccharides (LPS), flagellin, etc.,.

It is important to note that during opsonin-independent recognition a particular microbial species may display multiple adhesins, each recognised by a distinct receptor present on phagocytic cells.

2. Ingestion and digestion of microorganisms:

Adherence of microorganisms on phagocyte’s plasma membrane is followed by their ingestion and digestion. Adherence induces plasma membrane protrusions, called pseudopodia, 10 extent around the adhered microorganisms.

Fusion of the pseudopodia encloses the microorganisms within a membrane-bounded structure called a phagosome, which moves towards the cell interior and fuses with a lysosome to form a phagolysosome (Fig. 44.17) Lysomes contribute to the phagolysosome a variety of hydrolytic enzymes such as lysozyme, phospholipase A₂, ribonuclease deoxyri- bonuclease, and proteases.

An acidic vacuolar pH favours the activity of hydrolytic enzymes. Hydrolytic enzymes digest the entrapped microorganisms. The residual contents after digestion inside the phagolysosome are then eliminated through a process called exocytosis.

Mechanism # 4. The Complement System:

The serum of the blood contains a large number (over 30) of serum proteins that circulate in an inactive state and following their initial activation by specific (adaptive) and nonspecific (innate) immunogenic mechanisms, interact in a highly regulated cascade-fashion in which the activation of one component results in the activation of next in the cascade. This cascade of scrum proteins is collectively called the complement system and the serum protein of the complement system are called complement proteins.

When the inactive forms of complement proteins are converted into active forms by various specific (adaptive) and nonspecific (innate) immunologic mechanisms, they damage the membranes of microbial pathogens either destroying them or facilitating their clearance.

Complement system may act as an effector system that is triggered by binding if antibodies to certain cell surfaces, or it may be activated by reactions between complement proteins and receptors of microbial cell walls. Reactions between complement proteins and cellular receptors trigger activation of cells of the innate or adaptive immunity.

There are three pathways of complement activation:

(i) Classical complement pathway,

(ii) Alternate complement pathway, and

(iii) Lectin complement pathway.

Although these pathways employ similar mechanisms, specific proteins are unique to the first part of each pathway. Classical pathway is involved in specific or acquire (adaptive) immunity, whereas both the alternate and lectin pathways play important role in innate (nonspecific) immunity.

Mechanism # 5. Antibacterial Substances:

Human hosts possess antibacterial substances with which they combat the continuous onslaught of bacterial pathogens. These antibacterial substances are produced either by the host itself or by certain indigenous bacteria. The important antibacterial substances are the lysozyme, bacteriocins, and beta-lysin, and other polypeptides.

Lysozyme:

Lysozyme is the enzyme that breaks the β-1, 4-glycosidic bonds between N-acetylglucosamine and N- acetylmuramic acid in peptidoglycan, the signature molecule of bacterial cell wall. This bond breakage weakens the bacterial cell wall.

Water then enters the cell, and the cell swells and eventually bursts, a process called lysis (Fig. 44.18). Lysozyme occurs in body secretions including tears, saliva, and other body fluids, and presumably functions as a major line of non-specific defence against bacterial infections.

Bacteriocins:

Many of the normal bacterial flora of the host body synthesize and release plasmid-encoded toxic proteins (e.g., colicins, staphylococcin) collectively called bacteriosins that inhibit or kill closely related bacterial species or even different and may give their producers and adaptive advantage against other bacteria.

These toxic proteins are called bacteriocins to distinguish them from the antibiotics because possess a more narrow spectrum of activity than antibiotics. Bacteriocins producing genes are often present on plasmid or a transposon.

Most bacteriocins are produced by gram-negative bacteria, and are generally named after the species of the bacterial genera that produce them; the bacteriocin produced by E. coli is colicin, by Bacillus subtilis is substilicin.

E. coli synthesizes colicins. Some colicins bind to specific receptors on the surface of susceptible cells and kill them by disrupting some critical cell function. For example, many colicins form channels in the plasma membrane that allows potassium ions and protons to leak out, leading to a loss of the cell’s energy forming ability. Colicin E2 (encoded by plasmid col E2) is a DNA endonuclease and cleaves DNA. Colicin E3 (encoded by plasmid Col E3) is a nuclease that cuts at a specific site in 16S rRNA and inactivates ribosomes.

Recently it has been discovered that some grain-positive bacteria produce bacteriocin-like peptides. For example, lactic acid bacteria produce Nisin A, which strongly inhibits the growth of a wide range of gram- positive bacteria.

Beta-lysin and other polypeptides:

Beta-lysin is a cationic polypeptide synthesized and released by blood platelets, and kills some gram-positive bacteria by disrupting their plasma membranes. Other cationic polypetides produced in host body include leukins, plakins, cecropins, and phagocytin. A zinc-containing polypeptide named ‘prostatic antibacterial factor’ is secreted by the prostate gland in males, and acts as an important antibacterial substance.

Mechanism # 6. Antiviral Substances:

The outcome of a virus infection is influenced by the virulence of the infecting strain and the resistance conferred by the host. Mechanisims of host resistance may be immunological or non-specific. The latter include various genetic and physiological factors such as interferons, reactive nitrogen intermediates (RNIs), defensins, and fever.

Interferons:

Interferons are a family of host coded proteins produced by cells on induction by viral inducers, and are considered to be the first line of defence against viral attacks. Interferon by itself has no direct effect on viruses but it acts on other cells of the same species rendering them refractory to viral infection.

On exposure to interferon, cells produce a protein (translation inhibiting protein, TIP) which selectively inhibits translation of viral mRNA without affecting cellular mRNA. Translation inhibiting protein (TIP) is actually a mixture of at least three different enzymes, namely, protein kinase, oligonucleotide synthetase, and ribonuclease (RNAse).

These enzymes together block translation of viral mRNA into viral proteins. It has also been suggested that inhibition of viral transcription may also be responsible for the antiviral activity of interferon.

Reactive nitrogen intermediates:

Macrophages (also neutrophils and mast cells) have been found recently producing reactive nitrogen intermediates (RNIs). These molecules include nitric oxide (NO) and its oxidized forms, nitrite (NO₂^–) and nitrite (NO₃^–), and are very potent cytotoxic agents.

RNIs may be either released from cells or generated within cell vacuoles. Macrophages produce RNIs from the amino acid arginine. Macrophages have been found to destruct the herpes simplex virus with the help of RNIs produced by them.

Definsins:

Definsins are broad-spectrum antimicrobial peptides synthesized by myeloid precursor cells during their sojourn in the bone marrow, and are then stored in the cytoplasmic granules of mature neutrophils.

Besides gram-positive and gram-negative bacteria and yeasts and moulds, defensins target some viruses. Antiviral activity of defensins involves direct neutralization of enveloped viruses; non-enveloped viruses are not affected by defensins.

Fever (Elevated Body Temperature):

Fever is a physiological factor and results from disturbance in hypothalamic thermoregulatory activity leading to an increase in normal body temperature. In adult humans fever is defined as an oral temperature above 98°F (37°C) or a rectal temperature above 99.5°F (37.5°C).

In almost every instance there is a specific constituent called ‘endogenous pyrogen’ that directly triggers fever production. These pyrogens include interleukin 1 (IL-1), interleukin (IL-6), and tissue necrosis factor that are synthesized and released by host macrophages in response to pathogenic factors that include viruses, bacteria, and bacterial toxins. It has been found that fever may act as natural defence mechanism against viral infections because most viruses are inhibited by temperatures above 39°C.