Viruses might provide mucosal immunity

T4 HocThe mucosal membranes that line our respiratory, alimentary, and urogenital tracts and the outer surface of the eyes are portals of entry for microbes. The cells at these surfaces have functions that require that they are exposed to the environment – for example, gaseous exchange in the lung between inspired air and the blood. Mucus, pH extremes, enzymes, and immune cells are some of the antimicrobial defenses that are present at various mucosal surfaces. It now appears that bacteriophages – viruses that infect bacteria – might also be part of the mucosal antimicrobial defense system.

A sampling of the ratio of bacteriophages to bacteria in a variety of mucosal surfaces (sea anemone, hard and soft coral, polychaete, teleost, human gum, and mouse intestine) revealed higher ratios when compared to non-mucosal samples (e.g. neighboring sea water or saliva). A model bacteriophage of E. coli, T4, was used to show that phage specifically attach to mucus: adherence to cultured cells was reduced when mucus was not produced or removed by chemical treatment.

The principal macromolecules in mucus are mucin glycoproteins, which consist of a polypeptide chain linked to hundreds of variable, branched sugar molecules. Mucins are continuously produced at mucosal surfaces which gives rise to a thick protective layer. Phage T4 was found to attach specifically to mucins and not other components of mucus such as protein or DNA. The attachment of phage T4 to mucus-producing cultured cells reduced the number of bacteria that could attach to and kill the cells. This antimicrobial effect was substantially reduced when a strain of phage T4 was used that cannot lyse its bacterial host. Therefore phages bound in mucus protect cells by infecting and lysing bacterial invaders.

Phage T4 attaches to mucins through immunoglobulin-like (Ig-like) proteins present in the viral capsid. First discovered in antibody molecules, the Ig domain has since been found in hundreds of different proteins with various functions and appear to be encoded in ~25% of dsDNA phages. The Ig domain, typically 80 amino acids in length, is often involved in interactions with other proteins or ligands. For example, the Ig domains of antibodies interact with antigens, and the poliovirus receptor interacts with poliovirus via an Ig domain on the receptor molecule. The capsid of phage T4 contains 155 copies of an Ig-like protein called Hoc (colored yellow in the image). Deletion of the phage T4 hoc gene reduced binding of the virus to mucin, showing that adherence to mucin requires Ig-like protein domains.

These results demonstrate that a model bacteriophage, T4, attaches to mucus via an interaction between viral Ig-like capsid proteins and mucins. The ability of phages to attach to mucin clearly helps protect cultured cells from bacterial attachment and killing. Bacteriophages may be part of a previously unrecognized mucosal immune defense system. This suggests a symbiotic relationship between phages and metazoan hosts: the phages provide protection to mucosal surfaces, and in turn are provided hosts (bacteria) in which to reproduce. However, additional experiments are required to prove the authors’ conclusion of a “key role of the world’s most abundant biological entities in the metazoan immune system”. It will be necessary to directly demonstrate that phages attaching to mucins in mucus can protect an animal (e.g. mice) from bacterial invasion. This will not be an easy experiment because the phage and bacteria composition of mucus is likely to be complex and continuously changing as mucus is sloughed from cells and new mucins are produced.

The finding that phages play roles in mucosal immunity would have far-reaching consequences for human health. Some fascinating questions that come to mind include: do phage populations play roles in human diseases? Are they altered in human diseases and can we correct these diseases by restoring phage populations? Are the phage populations altered by antimicrobial therapy that alters bacterial populations? Do phages contribute to development of the immune system by modulating bacterial populations? Might mucus-bound phages stabilize the microbiome?

Update: Michael Schmidt and I discussed these remarkable findings on episode #59 of the science show This Week in Microbiology.

TWiV 179: Was ist ein Virus?

On episode #179 of the science show This Week in Virology, Gertrud joins the TWiVoners to review how dengue virus infection of mosquitoes alters blood feeding behavior, and gene therapy as practiced by parasitoid wasps.

You can find TWiV #179 at www.microbe.tv/twiv.

TWiV 164: Six steps forward, four steps back

xmrvHosts: Vincent RacanielloRich Condit, and Alan Dove

Vincent, Alan, and Rich review ten compelling virology stories of 2011.

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Click the arrow above to play, or right-click to download TWiV 164 (60 MB .mp3, 99 minutes).

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Ten virology stories of 2011:

  1. XMRV, CFS, and prostate cancer (TWiV 119, 123, 136, 150)
  2. Influenza H5N1, ferrets, and the NSABB (TWiV 159)
  3. The Panic Virus (TWiV 117)
  4. Polio eradication (TWiV 127, 149)
  5. Viral oncotherapy (TWiV 124, 131, 142, 156)
  6. Hepatitis C virus (TWiV 130, 137, 141)
  7. Zinc finger nuclease and HIV therapy (TWiV 144)
  8. Bacteria help viruses (TWiV 154)
  9. Human papillomaviruses (TWiV 126)
  10. Combating dengue with Wolbachia (TWiV 115, 147)

Links for this episode:

Weekly Science Picks

Rich – Fundamentals of Molecular Virology by Nicholas H. Acheson
AlanFetch, with Ruff Ruffman
Vincent – Year end reviews at Rule of 6ix and Contagions

Listener Pick of the Week

GarrenTrillion-frame-per-second video
Judi – iBioMagazine
Ricardo –
Brain Picking’s 11 best science books of 2011

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TWiM 10: A symbiotic cloaking device

euprymna scolopesHosts: Vincent RacanielloMichael SchmidtDickson DespommierMargaret McFall-Ngai, and Elio Schaecter

On episode #10 of the podcast This Week in Microbiology, Vincent, Margaret, Elio, Michael and Dickson discuss the symbiosis between the Hawaiian bobtail squid and the luminous, gram-negative bacterium Vibrio fischeri.

Click the arrow above to play, or right click to download TWiM #10 (47 MB, .mp3, 68 minutes).

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Send your microbiology questions and comments (email or mp3 file) to twim@microbe.tv, or call them in to 908-312-0760. You can also post articles that you would like us to discuss at microbeworld.org and tag them with twim.

TWiV 92: Live at ASV in Bozeman

Hosts: Vincent Racaniello, Rich Condit, Karla Kirkegaard, and Marilyn Roosinck

On episode #92 of the podcast This Week in Virology, Vincent, Rich, Karla, and Marilyn recorded TWiV at the 29th Annual Meeting of the American Society for Virology in Bozeman, where they discussed plant viruses and how they make plants resistant to adverse conditions, and identification of dominant negative drug targets.

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Links for this episode:

Weekly Science Picks

Marilyn – Viruses in the faecal microbiota of monozygotic twins and their mothers (Nature)
Rich –
The Known Universe by the American Museum of Natural History
Vincent – The Red Queen by Matt Ridley (thanks, Jesper!)

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TWiV #21: Viruses of bacteria

twiv_aa_2001In episode #21 of This Week in Virology, Vincent, Dick, and Alan are joined by Max Gottesman, who has researched viruses of bacteria – bacteriophages – for many years. They discuss an unusual wasp-virus symbiosis, influenza transmission and absolute humidity, how mosquitoes survive Dengue virus infection, and bacteriophages.

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