A Killer Memory

Natural Killer Cells Never Forget is a HUM-MOLGEN article citing a study in Nature with the title Adaptive immune features of natural killer cells. Joseph C. Sun, Joshua N. Beilke and Lewis L. Lanier are the authors (Nature, | doi:10.1038/nature07665). The study revealed that some innate immune system cells (not to be confused with the adaptive immune system) have a "memory" of infectious pathogens. Adaptive immune system cells include B cells and T cells, known to have immunological memory.

There is also what is called the innate immune system and there are cells known as natural killer (NK) cells which are part of this system. Natural killer cells help do away with virus-infected cells. The study in question involved viral infected mouse cells. The physical marker of "memory" is that some NK cells become memory cells for previously encountered viruses and this enables a better and faster immunological response to another infection by the same virus.

Quoting from the abstract of the Nature article:

In an adaptive immune response, naive T cells proliferate during infection and generate long-lived memory cells that undergo secondary expansion after a repeat encounter with the same pathogen. Although natural killer (NK) cells have traditionally been classified as cells of the innate immune system, they share many similarities with cytotoxic T lymphocytes. We use a mouse model of cytomegalovirus infection to show that, like T cells, NK cells bearing the virus-specific Ly49H receptor proliferate 100-fold in the spleen and 1,000-fold in the liver after infection. After a contraction phase, Ly49H-positive NK cells reside in lymphoid and non-lymphoid organs for several months. These self-renewing 'memory' NK cells rapidly degranulate and produce cytokines on reactivation. Adoptive transfer of these NK cells into naive animals followed by viral challenge results in a robust secondary expansion and protective immunity. These findings reveal properties of NK cells that were previously attributed only to cells of the adaptive immune system.