Fighting foreigners prevents self-destruction

Children everywhere rejoice—there may be such a thing as being too clean. According to Cecile King, Nora Sarvetnick (Scripps Research Institute, La Jolla, CA), and colleagues, overprotection from exposure to germs might cause autoimmune diseases.

Autoimmune diseases, including rheumatoid arthritis and lupus, are generally thought to result from an overstimulated immune system. This assumption does not, however, explain why these diseases are more prevalent in sterile western societies or why they often correlate with depleted numbers of T cells. Sarvetnick and colleagues now show that the combination of a depleted and understimulated immune system leads to autoimmunity.

The authors find that a mouse model of autoimmune diabetes has just this combination. These mice make too much of the IL-21 cytokine, which promotes proliferation (like other cytokines)...