TWiV #356: Got viruses?

On episode #356 of the science show This Week in Virology, Stephanie joins the super professors to discuss the gut virome of children with serious malnutrition, caterpillar genes acquired from parasitic wasps, and the effect of adding chemokines to a simian immunodeficiency virus DNA vaccine.

You can find TWiV #356 at www.microbe.tv/twiv.

An unexpected benefit of inactivated poliovirus vaccine

Poliovirus by Jason Roberts

Poliovirus by Jason Roberts

The polio eradication and endgame strategic plan announced by the World Health Organization in 2014 includes at least one dose of inactivated poliovirus vaccine (IPV). Since 1988, when WHO announced the polio eradication plan, it had relied exclusively on the use of oral poliovirus vaccine (OPV). The rationale for including a dose of IPV was to avoid outbreaks of vaccine-derived type 2 poliovirus. This serotype had been eradicated in 1999 and had consequently been removed from OPV. However IPV, which is injected intramuscularly and induces highly protective humoral immunity, is less effective in producing intestinal immunity than OPV. This property was underscored by the finding that wild poliovirus circulated in Israel during 2013, a country which had high coverage with IPV. Furthermore, in countries that use only IPV, over 90% of immunized children shed poliovirus after oral challenge. I have always viewed this shortcoming of IPV as problematic, in view of the recommendation of the World Health Organization to gradually shift from OPV to IPV. Even if the shift to IPV occurs after eradication of wild type polioviruses, vaccine-derived polioviruses will continue to circulate because they cannot be eradicated by IPV. My concerns are now mitigated by new results from a study in India which indicate that IPV can boost intestinal immunity in individuals who have already received OPV.

To assess the ability of IPV to boost mucosal immunity, 954 children in three age groups (6-11 months, 5 and 10 years) were immunized with IPV, bivalent OPV (bOPV, containing types 1 and 3 only), or no vaccine. Four weeks later all children were challenged with bOPV, and virus shedding in the feces was determined 0, 3, 7, and 14 days later. The results show that 8.8, 9.1, and 13.5% of children in the 6-11 month, 5-year and 10-year old groups shed type 1 poliovirus in feces, compared with 14.4, 24.1, and 52.4% in the control group. Immunization with IPV reduced fecal shedding of poliovirus types 1 (39-74%) and 3 (53-76%). The reduction of shedding was greater after immunization with IPV compared with bOPV.

This study shows that a dose of IPV is more effective than OPV at boosting intestinal immunity in children who have previously been immunized with OPV. Both IPV and OPV should be used together in the polio eradication program. WHO therefore recommends the following vaccine regimens:

  • In all countries using OPV only, at least 1 dose of type 2 IPV should be added to the schedule.
  • In polio-endemic countries and in countries with a high risk for wild poliovirus importation and spread: one OPV birth dose, followed by 3 OPV and at least 1 IPV doses.
  • In countries with high immunization coverage (90-95%) and low wild poliovirus importation risk: an IPV-OPV sequential schedule when VAPP is a concern, comprising 1-2 doses of IPV followed by 2 or mores doses of OPV.
  • In countries with both sustained high immunization coverage and low risk of wild poliovirus importation and transmission: an IPV only schedule.

Type 2 OPV will be gradually removed from the global immunization schedules. There have been no reported cases of type 3 poliovirus since November 2012. If this wild type virus is declared eradicated later this year, presumably WHO will recommend withdrawal of type 3 OPV and replacement with type 3 IPV.

All 342 confirmed cases of poliomyelitis in 2014 were caused by type 1 poliovirus in 9 countries, mainly Pakistan and Afghanistan. Given the social and political barriers to immunization, it will likely take many years to eradicate this serotype.

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.