Natural killer cells (NK cells) are an integral part of the innate immune system that serves as the first line of defense against infectious disease causing pathogens.  This important role lead to the assumption that the more activated NK cells present during an immune response, the better.  However, research done at the Helmholtz Centre for Infection Research (HZI) has shown that this principle does not apply to all stages of the immune response.

Natural killer cells (NK cells) are an integral part of the innate immune system that serves as the first line of defense against infectious disease causing pathogens.  This profound role lead to the assumption that the more activated NK cells present during an immune response, the better.  However, research done at the Helmholtz Centre for Infection Research (HZI) has shown that this principle does not apply to all stages of the immune response.

The research being done at the HZI is focusing on Listeria monocytogenes, which is an infection that causes fatal sepsis in mice, and immune suppression in humans.  This research has shown that mice that have fewer activated NK cells during the initial stages of the immune response have an improved survival rate. This is thought to be the result of the overproduction of interferon gamma, a chemical released to activate immune cells, by the surplus NK cells. The overproduction of interferon gamma actually blocks the recruitment of other immune cells, specifically neutrophilic granulocytes.

Neutrophils, the most common type of white blood cell, are capable of neutralizing the bacteria via phagocytosis.  By blocking their recruitment to the infection site, the bacteria can grow unhindered. Similarly, if activated NK cells are sparse or absent from the initial immune response, additional immune cells will not be recruited to fight the infection.  This will again lead to unhindered bacterial growth.

Additionally, the presence of activated NK cells in the later stages in the immune response has proved crucial to the survival of both mice and humans in the case of Listeria monocytogenes.   Therefore, there is clearly a delicate balance between the number of activated NK cells, the stage of the immune response, and the strength of the immune response.  It is still unclear as to where exactly this perfect balance falls, but this research has illuminated the importance of further examination of this topic and its implications in the treatment of infectious disease.