Study reveals expansion of NK cells after acute infection

The natural killer cells of our immune defense are activated for an extended period after the acute infection, which challenges the prevailing view that the elevation and activation of cells quickly pass. This is shown in a study regarding vole fever that was recently published by researchers at Umeå University, Sweden in Journal of Experimental Medicine.

These are findings of a years-long project where patients with vole fever, a northern Swedish hemorrhagic fever that has been studied with regard to natural killer (NK) cells. Vole fever is a common infection in northern Sweden that is caused by a hantavirus, Puumala virus, which is prevalent in bank voles and infects humans primarily via inhalation of virus-contaminated dust.

The symptoms are primarily high fever, head and muscle pain, abdominal pain, and generally impacted condition. Involvement of the kidneys and lungs are common. There is no dedicated and effective treatment today. As vole fever is a hemorrhagic fever, there is often a pronounced reduction in blood platelets, and bleeding complications occur. Some 30 % of the diagnosed cases are hospitalized. Mortality is 0.5 % owing to bleeding, shock, and multiple organ failure. 

The Umeå researchers, led by assistant professor Clas Ahlm, have used a unique patient cohort to study the expansion of NK cells and their activity in the course of the infection in collaboration with scientists at the Karolinska institute. The material was gathered during and following the major outbreak of vole fever in 2007. There have been few previous studies of NK cells in acute viral infections in humans, even though they are regarded as part of our so-called innate immunity. The Puumala virus itself is not cytopatogenic, i.e. doesn't kill the infected cells. The Umeå scientists' hypothesis is therefore that part of the pathological mechanism in vole fever involves the immune defense against the virus infection, which is further supported by these findings.

The study revealed an expansion of NK cells. This expansion persisted for an extended period after the acute infection, which surprised the researchers. This finding challenges to some extent the previous view that the elevation and activation of NK cells quickly subsides in acute viral infections. The results of the study indicate that some NK cells may have memory-like functions.