20.4: Chemical Defenses - Antimicrobial Peptides (AMP) and Plasma Protein Mediators
-
- Last updated
- Save as PDF
- Ying Liu
- City College of San Francisco
Learning Objectives
- Describe the functions of antimicrobial peptides (AMPs)
- Distinguish between defensins and bacteriocins
Antimicrobial Peptides (AMPs)
The antimicrobial peptides (AMPs) are a special class of nonspecific cell-derived mediators with broad-spectrum antimicrobial properties. Some AMPs are produced routinely by the body, whereas others are primarily produced (or produced in greater quantities) in response to the presence of an invading pathogen. Research has begun exploring how AMPs can be used in the diagnosis and treatment of disease.
Functions of AMPs
AMPs may induce cell damage in microorganisms in a variety of ways, including by inflicting damage to membranes , destroying DNA and RNA , or interfering with cell-wall synthesis . Depending on the specific antimicrobial mechanism, a particular AMP may inhibit only certain groups of microbes (e.g., gram-positive or gram-negative bacteria) or it may be more broadly effective against bacteria, fungi, protozoa, and viruses. Many AMPs are found on the skin, but they can also be found in other regions of the body.
Defensins versus Bacteriocins
A family of AMPs called defensins can be produced by epithelial cells throughout the body as well as by cellular defenses such as macrophages and neutrophils (see Cellular Defenses ). Defensins may be secreted or act inside host cells; they combat microorganisms by damaging their plasma membranes. AMPs called bacteriocins are produced exogenously by certain members of the resident microbiota within the gastrointestinal tract. The genes coding for these types of AMPs are often carried on plasmids and can be passed between different species within the resident microbiota through lateral or horizontal gene transfer.
There are numerous other AMPs throughout the body. The characteristics of a few of the more significant AMPs are summarized in Table \(\PageIndex{1}\).
| AMP | Secreted by | Body site | Pathogens inhibited | Mode of action |
|---|---|---|---|---|
| Bacteriocins | Resident microbiota | Gastrointestinal tract | Bacteria | Disrupt membrane |
| Cathelicidin | Epithelial cells, macrophages, and other cell types | Skin | Bacteria and fungi | Disrupts membrane |
| Defensins | Epithelial cells, macrophages, neutrophils | Throughout the body | Fungi, bacteria, and many viruses | Disrupt membrane |
| Dermicidin | Sweat glands | Skin | Bacteria and fungi | Disrupts membrane integrity and ion channels |
| Histatins | Salivary glands | Oral cavity | Fungi | Disrupt intracellular function |
Query \(\PageIndex{1}\)
Plasma Protein Mediators
Many nonspecific innate immune factors are found in plasma, the fluid portion of blood. Plasma contains electrolytes, sugars, lipids, and proteins, each of which helps to maintain homeostasis (i.e., stable internal body functioning), and contains the proteins involved in the clotting of blood. Additional proteins found in blood plasma, such as acute-phase proteins, complement proteins, and cytokines, are involved in the nonspecific innate immune response.
Acute-Phase Proteins
The acute-phase proteins are another class of antimicrobial mediators. Acute-phase proteins are primarily produced in the liver and secreted into the blood in response to inflammatory molecules from the immune system. Examples of acute-phase proteins include C-reactive protein, serum amyloid A, ferritin, transferrin, fibrinogen, and mannose-binding lectin. Each of these proteins has a different chemical structure and inhibits or destroys microbes in some way (Table \(\PageIndex{1}\)).
| Some Acute-Phase Proteins and Their Functions | |
|---|---|
| C-reactive protein | Coats bacteria (opsonization), preparing them for ingestion by phagocytes |
| Serum amyloid A | |
| Ferritin | Bind and sequester iron, thereby inhibiting the growth of pathogens |
| Transferrin | |
| Fibrinogen | Involved in formation of blood clots that trap bacterial pathogens |
| Mannose-binding lectin | Activates complement cascade |
Key Concepts and Summary
- Antimicrobial peptides (AMPs) found on the skin and in other areas of the body are largely produced in response to the presence of pathogens. These include dermcidin, cathelicidin, defensins, histatins, and bacteriocins.