Innate Immunity

1.            Categories of Innate immunity

a.            Physical barriers: intact skin, mucous membranes

b.            Phagocytic cells: neutrophils, macrophages and natural killer cells

c.            Proteins: complement, lysozyme, and interferon.

2.         Skin

a.         First line of defense against many organisms

b.            Mechanical:

i.            epithelial cells joined by tight junctions.

ii.            longitudinal flow of air across epithelium.

c.            Chemical:

i.          fatty acids secreted by sebaceous glands in the skin have antibacterial and antifungal activity.

ii.          low pH of the skin, which is due to these fatty acids, also has an antimicrobial effect.

d.            Microbiological:

i.          normal flora compete for nutrients and attachment to epithelium.

ii.          can produce antibacterial substances.

3.            Mucous membranes

a.            Mucous membrane of the respiratory tract, which is lined with cilia and covered with mucus.

b.            Coordinated beating of the cilia drives the mucus up to the nose and mouth, where the trapped bacteria can be expelled.

c.            Secretions contain lysozyme, an enzyme that attacks components of unencapsulated bacterial cell walls and breaks them down.

d.         Other protective mechanisms: alveolar macrophages, hairs in nose, cough reflex.

4.            Nonspecific protection in GI tract

a.            Hydrolytic enzymes in saliva

b.         Acid in stomach

c.         Various degradative enzymes and macrophages in the small intestine

d.         Vagina of adult women protected by low pH generated by lactobacilli that are part of normal flora.

5.            Colonial Resistance

a.         The bacteria of normal flora of skin, nasopharynx, colon and vagina occupy these ecological niches, preventing pathogens from multiplying in these sites.

b.            Antimicrobial therapy suppresses these beneficial organisms, thereby allowing Clostridium difficile and Candida albicans to cause diseases such as pseudomembranous colitis and vaginitis, respectively.

6.            Alternative pathway of Complement activation

a.         The alternative pathway of complement activation can proceed on many microbial surfaces in the absence of specific antibody.

b.         The complement component C3 is cleaved in plasma to produce C3b, which can attach to host or pathogen where it binds factor B.

c.         Factor B is in turn cleaved by a plasma protease, factor D, to Bb, which remains bound to C3b to form a C3 convertase.

d.            Inactivation of C3 convertase on host cells:

i.          if C3b forms on the surface of host cells it is rapidly inactivated by complement regulatory proteins.

ii.          host cells express the membrane proteins complement receptor 1 (CR1), decay accelerating factor (DAF) and membrane cofactor of proteolysis (MCP).

e.            Bacterial surfaces do not express complementary regulatory proteins and favor binding of factor P which stabilizes the C3 convertase.

f.          The cleavage of further molecules of C3 leads to opsonization by C3b, generation of C5 convertase, leading to activation of terminal complement components.

7.            Inflammatory Response

a.         The presence of foreign bodies within the body provokes a protective inflammatory response.

b.         This response is characterized by redness, swelling, warmth and pain at the site of the infection.

c.         These signs are due to increased blood flow, increased capillary permeability, and the escape of fluid and cells into the tissue spaces.

d.         Of the cells that appear at the site, neutrophils and macrophages, both of which perform phagocytic functions, arrive early.

e.            Neutrophils predominate in acute pyogenic infections, whereas macrophages are more prevalent in chronic or granulomatous infections.

f.          Role of Inflammation in control of local infections:

i.          recruit more phagocytic cells and effector molecules to the site of infection through release of cytokines and other inflammatory mediators.

ii.          local accumulation of fluid leads to accumulation of immunoglobulins, complement and other blood proteins in the tissue.

iii.            complement plays a role in initial immunity.

iv.         effect of mediators on endothelium induce expression of adhesion molecules that bind to circulating monocytes and neutrophils.

7.            Chemical mediators of Inflammatory response

a.         Acute phase proteins:

i.            increase in production of plasma proteins by the liver (named acute phase proteins) following an injury or pathogen invasion.

ii.          include molecules that act as opsonins, antiprotease molecules to help prevent tissue damage.

iii.         play a role in activating the alternative pathway of complement by binding to surface of bacteria.

b.            Histamine:

i.          an amine derived from the amino acid histidine.

ii.          found primarily in the granules of mast cells and basophils.

iii.         open pores in capillaries, allowing plasma proteins to escape into the interstitial space, causing edema.

iv.         dilates blood vessels, increasing blood flow to the area.

v.         result of histamine release is a hot, red, swollen area around a wound or infection site to bring more leukocytes to the injury site to kill bacteria and remove cellular debris.

vi.         mast cell degranulation is triggered by various cytokines in the immune response.

c.            Interleukins (IL-1):

i.          alter blood vessel endothelium to allow easier passage of white cells and proteins during the inflammatory response.

ii.            stimulate production of acute-phase proteins by the liver.

iii.         induce fever by acting on the hypothalamus.

iv.            stimulate cytokine and endocrine secretion by other cells.

d.         Kinins:

i.          a group of inactive plasma proteins that participate in a cascade to form bradykinin.

ii.            bradykinin causes vasodilation and stimulates pain receptors, creating the tenderness associated with inflammation.

e.            Complement:

i.          a collective term for a group of more than 25 plasma and cell membrane proteins.

ii.            complement pathway is a cascade of events, generating various intermediates which act as opsonins, chemical attractants for leukocytes, and agents to cause degranulation of mast cells.

iii.         terminal step in cascade is the formation of membrane attack complex, lipid-soluble molecules that insert themselves into the cell membranes of pathogens and form pores.

iv.         these pores allow sodium ions to enter the cell, and water follows osmotically, leading to cell swelling and lysis.

f.            Containing local infections by TNF-a:

i.          tumor necrosis factor (TNF) induce expression of molecules on endothelial cells that trigger blood clotting in the local small vessels, occluding them and cutting off blood flow.

ii.          this may be important in preventing the pathogen from entering the bloodstream and spreading via blood to organs all over the body.

iii.         instead, the fluid that has leaked into the tissue from the plasma early on carries the pathogen, either directly or enclosed in phagocytic cells, via the lymph to regional lymph nodes, where an adaptive immune response can be initiated.

g.         Sepsis and Shock:

i.          once an infection spreads to the bloodstream, the same mechanisms where TNF-a so effectively contains local infection becomes catastrophic.

ii.          this condition, called sepsis, is accompanied by the release of TNF-a by macrophages in the liver, spleen, and other sites.

iii.         the systemic release of TNF-a causes vasodilation and loss of plasma volume owing to increased vascular permeability leading to shock.

iv.         In septic shock, disseminated intravascular coagulation is also triggered by TNF-a, leading to generation of blood clots in small vessels and massive consumption of clotting proteins.

v.         this condition frequently leads to failure of vital organs such as the kidneys, liver, heart and lungs, which are quickly compromised by failure of normal perfusion.

vi.            consequently, septic shock has a high mortality rate.

8.            Phagocytosis by Neutrophils

a.         As part of the inflammatory response, bacteria are engulfed by polymorphonuclear neutrophils (PMNs) and macrophages.

b.         The increase in PMNs is caused by production of granulocyte-stimulating factors (G-CSF and GM-CSF) by macrophages soon after infection.

c.         The process of phagocytosis is divided into 3 steps: migration, ingestion and killing.

d.            Migration:

i.            migration of PMNs to the site of the organisms is due to chemotactic factors, such as complement component C5a and kallikrein.

ii.            adhesion of PMNs to the endothelium at site of infection is mediated first by interaction of the PMNs with selectin proteins on the endothelium and

iii.         then by the interaction of integrin proteins called LFA proteins, located on the PMN surface, with ICAM proteins on the endothelial cell surface.

iv.         ICAM proteins on the endothelium are increased by interleukin-1 and tumor necrosis factor which are produced by macrophages in response to bacteria.

e.            Ingestion:

i.            invagination of PMN cell membrane around bacteria, mediated by actin filaments, forming a phagosome.

ii.            engulfment aided by binding of IgG antibodies to surface of bacteria – opsonization.

iii.         C3b of complement system enhances opsonization.

f.            Respiratory burst:

i.          at time of engulfment, respiratory burst is triggered, producing superoxide radical and hydrogen peroxide.

ii.            hydrogen peroxide further reacts with chloride to yield hypochlorite ion in the presence of myeloperoxidase.

iii.         these reactive oxygen species destroy cell walls and membrane lipids.

g.         Killing:

i.          consists of degranulation followed by production of hypochlorite ions.

ii.          in degranulation, lysosomes in the neutrophil’s cytoplasm fuse with phagosome, emptying their contents in the process.

iii.         larger granules (15%) contain myeloperoxidase, lysozyme and other degradative enzymes.

iv.         small granules (85%) contain lactoferrin, proteases, nucleases and lipases.

h.            Oxygen-independent mechanisms are important under aerobic conditions:

i.            lactoferrin: chelates iron from bacteria.

ii.            lysozyme: degrades peptidoglycan in bacterial cell wall.

iii.         cationic proteins: damage bacterial membranes.

iv.         low pH

9.            Phagocytosis by Macrophages

a.         Does not make hypochlorite ion though they do produce hydrogen peroxide and superoxide by respiratory burst.

b.         Certain organisms such as agents of tuberculosis, brucellosis and toxoplasmosis are preferentially ingested by macrophages.

c.         Secrete plasminogen activator that forms active plasmin, dissolving fibrin clot.

10.            Interferons

a.            Interferons are a heterogenous group of glycoproteins produced by human and other animal cells after viral infection or after exposure to other inducers.

b.         They inhibit the growth of viruses by blocking the translation of viral proteins.

c.         Alpha and beta interferons are induced by viruses, whereas gamma interferon is induced by antigens and is one of the effectors of cell-mediated immunity.

d.         The strong inducers of these interferons are viruses and double-stranded RNAs.

e.         The induction and inhibitory action of interferons are not specific.

f.            Interferons have no direct effect on extracellular virus particles.

g.         Role of Interferons in Immunity:

i.          they induce resistance to viral replication by activating cellular genes that destroy mRNA and inhibit the translation of viral and some host proteins.

ii.          they induce MHC class 1 expression in most uninfected cells in the body, and increasing the susceptibility of cells newly-infected by virus to killing by CD8 cytotoxic T cells.

iii.         they activate natural killer cells, which then kill virus-infected cells selectively.

11.            Natural Killer cells

a.         Natural killer cells play an important role in the innate host defenses.

b.         They specialize in killing virus-infected cells and tumor cells by secreting cytotoxins (perforins).

c.         They are called ‘natural’ killer cells because they are active without prior exposure to the virus, are not enhanced by exposure, and are not specific for any virus.

d.         They can kill without antibody, but antibody enhances their effectiveness, a process called antibody-dependent cellular cytotoxicity.

e.         They have no immunologic memory and unlike cytotoxic T cells, have no T cell receptor and therefore killing does not require recognition of MHC proteins.

f.            Proposed mechanism of action:

i.          normal cells respond to interferons by increasing levels of MHC class I expression, making them resistant to activated NK killing.

ii.          infected cells may fail to increase MHC class I expression, making them targets for activated NK cells.