Unraveling immune system origins

Figure 1: Microphotograph showing the thymus (dark yellow) in the embryo of a shark — one of the first species to evolve this vital organ.

© (2009) Thomas Boehm

The thymus is an essential component of our immune systems, but mysteries remain about how it evolved. Now, Thomas Boehm at the Max-Planck Institute of Immunobiology in Freiburg, Germany and co-workers, including researchers at A*STAR in Singapore, have uncovered some of the genetic networks that established the thymus in vertebrates.

“The thymus probably arose as a consequence of selection pressures for adaptive immune systems in early vertebrates,” explains Boehm, before noting that this organ is first seen in cartilaginous fish, for example sharks (Fig. 1), which appeared around 500 million years ago.

To investigate this important evolutionary step, the researchers identified candidate genes that they thought might be associated with thymus development, and predicted how they might interact. They then tested their models with a genetic analysis of bony fish.
Their results, reported in Cell¹, show that the genetic networks associated with the throat wall expanded to include new genes. Crucially, some of these new genes encoded chemokines — small proteins that induce cells to move in certain directions.

Meanwhile, genetic networks evolved in lymphocytes to control the expression of chemokine receptors. This led to very efficient signaling between the throat wall and lymphocytes, establishing the thymus and encouraging the lymphocytes to develop more specific immune system functions.