New defense mechanism against viruses and cancer identified

"T killer cells" (CD8 T cells) represent an important element of our body's defense system. They have the capacity to specifically identify and kill cells, which harbor viruses and bacteria or form a cancer. T killer cells would therefore represent an important component of yet unavailable vaccines against infections like HIV/AIDS, hepatitis C virus and malaria, and also for the treatment of cancer.

It has been a longstanding observation that there is no match to the overwhelming T killer cell armada, which is triggered when a viral infection invades our body. Scientists had generally accredited this observation to "pathogen-associated molecular patterns" (PAMPs) on viruses and other microbes. PAMPs, i.e. the "foreign look" of viruses, alert so-called "dendritic cells," which serve as policemen coordinating the T killer cell response.

In a report now published in the journal Science, researchers led by Prof. Max Löhning (Charité-University Medicine & DRFZ Berlin) and Prof. Daniel Pinschewer (University of Geneva) describe an additional general mechanism by which viral infection triggers potent T killer cells: "Dying virus-infected cells themselves ring the alarm bells to T killer cells.," Löhning says. Viruses cause infected cells to die, resulting in the release of cell components, which normally are not be visible to the outside -- analogous to an injured individual loosing blood. Such substances, heralding injury when released, are referred to as "alarmins." The scientists found that T killer cells can sense an alarmin called "interleukin 33" (IL-33). IL-33 is contained in cells, which form the scaffold of the T killer cells' home, the spleen and lymph nodes, and is released when such scaffold cells die.

Mice lacking the gene encoding IL-33 failed to form a large T killer cell army upon viral infection. The few remaining cells had very poor fighting skills. Such mice were therefore exquisitely sensitive to several types of viral infections. Conversely, IL-33 could be used to artificially increase the T killer cell army, which was generated in response to vaccination. As Max Löhning and Daniel Pinschewer explain, PAMPs and alarmins apparently have complementary and non-redundant functions in shaping our T killer cell defense: "The "foreign look" of viruses (PAMPs) activates the "dendritic cell" policemen to engage T killer cells. T killer cells, however, remain lousy fighters unless alerted by a cell death in their neighborhood (alarmins)." These new findings could provide a key to effective vaccination against infectious diseases and cancer.