Innate Immunity

The ability of a multicellular organism to defend itself against invasion by pathogens (bacteria, fungi, viruses, etc.) depends on its ability to mount immune responses. All metazoans (probably) have inborn defense mechanisms that constitute innate immunity. Vertebrates have not only innate immunity but also are able to mount defense mechanisms that constitute adaptive immunity. This table gives some of the distinguishing features of each type of immunity.

Pathogen-Associated Molecular Patterns (PAMPs)

• are not shared with their host;
• are shared by many related pathogens;
• are relatively invariant; that is, do not evolve rapidly (in contrast, for example, to such pathogen molecules as the hemagglutinin and neuraminidase of influenza viruses).

Examples:

• the flagellin of bacterial flagella;
• the peptidoglycan of Gram-positive bacteria;
• the lipopolysaccharide (LPS, also called endotoxin) of Gram-negative bacteria;
• double-stranded RNA. (Some viruses of both plants and animals have a genome of dsRNA. And many other viruses of both plants and animals have an RNA genome that in the host cell is briefly converted into dsRNA [link to examples]).
• unmethylated DNA (many of the CpG islands in eukaryotes have methyl groups attached).

Pattern Recognition Receptors (PRRs)

1. secreted molecules that circulate in blood and lymph;
2. surface receptors on phagocytic cells like macrophages that bind the pathogen for engulfment;
3. cell-surface receptors that bind the pathogen initiating a signal leading to the release of effector molecules (cytokines).

1. Secreted PRRs

• C4 and C2 leading to the cleavage of
• C3 and the remaining steps of complement fixation.

2. Phagocytosis Receptors

3. Toll-Like Receptors (TLRs)

Macrophages, dendritic cells, and epithelial cells have a set of transmembrane receptors that recognize different types of PAMPs. These are called Toll-like receptors (TLRs) because of their homology to receptors first discovered and named in Drosophila.

Mammals have 12 different TLRs each of which specializes — often with the aid of accessory molecules — in a subset of PAMPs. In this way, the TLRs identify the nature of the pathogen and turn on an effector response appropriate for dealing with it. These signaling cascades lead to the expression of various cytokine genes.

Examples:

• TLR-1 and TLR-2.

  Form a heterodimer at the cell surface which binds to the peptidoglycan of Gram-positive bacteria like Streptococci and Staphylococci.

• TLR-3.

  Binds to the double-stranded RNA of viruses engulfed in endosomes.

• TLR-4.

  Activated by the lipopolysaccharide (endotoxin) in the outer membrane of Gram-negative bacteria like Salmonella and E. coli O157:H7

• TLR-5.

  Binds to the flagellin of motile bacteria like Listeria.

• TLR-6.

  Forms a heterodimer with TLR-2 and responds to peptidoglycan and certain lipoproteins.

• TLR-7 and TLR-8.

  Form a heterodimer that binds to the single-stranded RNA (ssRNA) genomes of such viruses as influenza, measles, and mumps that have been engulfed in endosomes.

• TLR-9.

  Binds to the unmethylated CpG of the DNA of bacteria that have been engulfed in endosomes. (CpG islands in the host tend to have methyl groups attached.)

• TLR-11.

  In mice, it binds proteins expressed by several infectious protozoans (Apicomplexa).

In all these cases, binding of the pathogen to the TLR initiates a signaling pathway leading to the activation of NF-κB. [Link to discussion]

• tumor necrosis factor-alpha (TNF-α);
• Interleukin-1 (IL-1);
• chemokines, which attract white blood cells to the site.

All of these effector molecules lead to inflammation at the site.

• Gram-positive bacteria to TLR-2 and
• Gram-negative bacteria to TLR-4

The innate immune system and commensal bacteria

The human large intestine (colon) contains an enormous (~10¹⁴) population of microorganisms. (Our bodies consist of only ~10¹³ cells!) Most of the species live there perfectly harmlessly; that is, they are commensals. Some are actually beneficial, e.g.,

• by synthesizing vitamins;
• by digesting polysaccharides for which we have no enzymes (providing an estimated 10% of the calories we acquire from our food);
• by stimulating the development of the adaptive immune system, including Th1 helper cells (which then inhibit Th2 helper cells which may provide protection against allergies like asthma). [Link]

Despite the name ("pathogen-associated"), PAMPs are found on all these nonpathogenic bacteria as well.

• Mice that are raised in a germfree environment [Link] or are treated with antibiotics to clean all the bacteria out of their colon are unhealthy and have G.I. tracts that are hypersensitive to injury. However, if they are given
  • LPS (from Gram-negative bacteria) or
  • lipoteichoic acid (from the peptidoglycan of Gram-positive bacteria)
  in their drinking water, their colons resist injury.
• However, "knockout" mice that
  • lack TLR-2 or TLR-4 or
  • cannot signal through their TLRs [Link]
  are not protected by such treatment.

• suggest that a continuous low level of activity of the innate immune system is essential to health — even in the absence of pathogens;
• explain why deliberating feeding harmless bacteria ("probiotics") has proved beneficial to some human patients with a diseased colon;
• provide another reason for avoiding the indiscriminate use of antibiotics (which kill harmless bacteria as easily as pathogenic ones).

Innate Immunity can trigger Adaptive Immunity

1.

• Digested fragments of the engulfed pathogen are returned to the cell surface nestled in the cavity of class II histocompatibility molecules. [Link to a detailed description.]
• Gene transcription turned on by the interaction of PAMPs and TLRs causes transmembrane molecules called B7 to appear at the cell surface.
• T cells have a receptor for B7 called CD28.
• Simultaneous binding of
  • CD28 to B7 and
  • the antigen/class II complex to TCRs specific for it [View]
• activates the T cell.
• This leads to repeated mitotic divisions producing clones of CD4⁺ T cells that can
  • carry out cell-mediated immune responses and/or
  • stimulate B cells to secrete antibodies of the appropriate specificity. [Discussion]

2.

• interleukin 12 (IL-12) which stimulates the production of Th1 cells [Link];
• interleukin 23 (IL-23) which stimulates the production of Th17 cells [Link];
• interleukin 6 (IL-6), which interferes with the ability of regulatory T cells to suppress the responses of effector T cells to antigen. A double-negative is a positive.

  Link to discussion of regulatory T cells.

3.

B cells are also antigen-presenting cells. They bind antigen with their BCRs and engulf it into lysosomes. They then transport the digested fragments to the cell surface incorporated in class II histocompatibility molecules just as macrophages and dendritic cells do.

B cells also have TLRs. When a PAMP such as LPS binds the TLR, it enhances the response of the B cell to the antigen.

It has been known for many years that for vaccines to be effective, the preparation must contain not only the antigen but also materials called adjuvants. Several adjuvants contain PAMPs, and their stimulus to the innate immune system enhances the response of the adaptive immune system to the antigen in the vaccine.

4.

Pathogens coated with fragments of the complement protein C3 are not only opsonized for phagocytosis but also bind more strongly to B cells that have bound the pathogen through their BCR. This synergistic effect enables antibody production to occur at doses of antigen far lower than would otherwise be needed.

Some workers feel that, in fact, adaptive immunity is not possible without the assistance of the mechanisms of innate immunity.

Antimicrobial Peptides

Vertebrates (including ourselves), invertebrates (e.g., Drosophila), even plants and fungi secrete antimicrobial peptides that protect them from invasion by bacteria and other pathogens. In fact, probably all multicellular organisms benefit from this form of innate immunity.

• defensins and the
• cathelicidins

Defensins

• skin
• lining of the GI tract
• lining of the genitourinary tracts
• lining of the nasal passages and lungs

• Some defensins are secreted by the epithelial cells themselves; others by Th17 cells and neutrophils.
• Some are secreted all the time; others only in response to attack by pathogens. (In some cases their genes are turned on by activated TLRs.)
• They are synthesized from a larger gene-encoded precursor which is
• cut to produce the active peptide.
• These range in length from 29 to 42 amino acids.
• They contain several invariant cysteines that form disulfide bonds that assist in producing
• a secondary structure that consists of a beta sheet.
• They attack the outer surface of the cell membrane surrounding the pathogen eventually punching lethal holes in it. (Unlike eukaryotes, the phospholipids in the outer membrane of bacteria carry a surplus of negative charges, and the positive charges on the defensins probably enable them to penetrate the bacterial membranes while sparing host membranes.)

Curiously, some defensins (β-defensin) also affect coat color (in dogs and mice) and in other ways mimic the effects of melanocyte-stimulating hormone (MSH).

Cathelicidins

The best known human cathelicidin is LL37, a peptide of 37 amino acids synthesized by macrophages, neutrophils, and epithelial cells (providing antimicrobial protection to our skin and the lining of our urinary tract). Unlike the defensins, its secondary structure is alpha helix.

• why people with a deficiency of vitamin D are more susceptible to tuberculosis;
• the physiological basis for the practice of exposing patients to sunlight in TB sanitariums (before the days of antibiotics).

Antimicrobial Peptides and the GI Tract: an example

• The epithelium of the mouth and tongue is protected by a layer of antimicrobial peptides as well as those secreted in the saliva.
• The stomach is also protected by antimicrobial peptides (cut by pepsin from a larger precursor) as well as by the low pH of gastric juice.
• The liquefied contents that leave the stomach are quickly neutralized by the bicarbonate ions in the pancreatic fluid. However, any bacteria that survived the trip through the stomach (e.g., E. coli has a proton pump that enables it to survive the strong acid of the gastric juice) are kept in check by the antimicrobial peptides secreted by the Paneth cells of the small intestine. So, the contents of the small intestine normally contain only a small population of microbes.
• Not so for the large intestine (colon). The colon supports an enormous population (>10¹³) of microorganisms, but these seldom invade its lining thanks to
  • a protective barrier of antimicrobial peptides as well as
  • the protective actions of continuous stimulation of
    • TLR-2s by Gram-positive commensals and
    • TLR-4s by Gram-negative commensals
• The rectum is also protected by an epithelial barrier of antimicrobial peptides.