An international team of researchers has discovered that the so-called "winter vomiting bug," norovirus, can escape destruction by hiding in a rare type of cell found in the gut. Norovirus, also known as the "cruise ship" virus for its propensity to wreak havoc in the self-contained environments of leisure ships, is the leading cause of non-bacterial gastroenteritis – a condition that causes 267 million infections, and 200,000 deaths worldwide each year.
Symptoms of norovirus include abdominal pain, vomiting and diarrhoea. The disease, which causes massive dehydration, is particularly deadly to vulnerable populations, such as the young, the elderly, and those with other complicating health conditions.
Norovirus is known to spread very easily in virulent outbreaks, and whilst the immune systems of many people are able to deal with the virus in the space of two to four days, others continue to spread the bug for months after contracting the malady.
"Current vaccines against norovirus have been ineffective despite eliciting strong antibody responses," said E. John Wherry, PhD, a professor of Microbiology and director of the Penn Institute for Immunology. "Understanding the unique norovirus characteristic of hiding from the host immune system may explain its biology and present opportunities to improve vaccines and therapeutics."
A new study carried out by the international team sought to discover why the immune systems of some individuals took longer than others to eradicate the virus.
To achieve this, the team infected laboratory mice with an acute or chronic strain of norovirus, and observed how the body's immune system, or more specifically its T cells, reacted to the foreign bodies.
The researchers had previously believed that the T cells were unable to eradicate the virus because they became exhausted, and so rendered inactive by the laborious process of hunting the malicious cells. This was proven not to be the case, as the study revealed that the T cells remained active and functional months after the virus was introduced to the rodent subjects.
However, stool samples collected from the mice were still exhibiting the norovirus. Since the T cells were still functional, yet the virus persisted, the team hypothesized that they must somehow be rendering themselves undetectable to the rodent immune system.
The researchers discovered evidence that the norovirus cells were hiding in a type of extremely rare cell located in the lining of the animals' guts. These cells failed to communicate the presence of the norovirus to the immune system, allowing the norovirus a safe harbor from the T cells, as they proceeded to shed large amounts of the virus from the lining of the intestines.
The study could inform future treatments for non-bacterial gastroenteritis, for example encouraging the targeting of the norovirus cells in the three days following the initial infection – the phase before the cells are believed to seek safety in the rare gut cells.
If such targeted treatments could be developed to reduce the period in which a patient is infected with the virus, it would also decrease the amount of people exposed to the virus, possibly lessening the danger of an outbreak.
A paper detailing the discovery has been published in the journal Immunity.