B. Bajoghli et al., “A thymus candidate in lampreys,” Nature, 470:90-94, 2011. Free F1000 evaluation
Jawless vertebrates such as lampreys and hagfish have an adaptive immune system, with two lymphocyte cell types that resemble T and B cells, despite lacking other hallmarks of adaptive immunity, such as immunoglobulins and MHC. Now, Thomas Boehm at the Max Planck Institute in Freiburg and colleagues, think they have identified a new component of the adaptive immune system: the lamprey thymus.
The lack of a thymus in lampreys was puzzling, as the organ is essential for the final step of T-cell maturation in humans. Do human and lamprey immune systems share a common ancestor, or was the evolution convergent—hitting on a similar solution by chance? Unlike B cells, “T cells are always linked to a thymus,” says Bruno Kyewski of the German Cancer Research Center in Heidelberg, and its lack suggested convergence. Boehm’s finding changes all that.
After Max Cooper at Emory University in Atlanta showed lampreys had T-like cells, Boehm’s group discovered the expression of a lamprey equivalent of FOXN1—a transcription factor required for the proper development of a thymus. Boehm and Cooper worked together to show that the lamprey T-like cells, but not the B-like cells, were processed in the same place where FOXN1 is expressed—the lamprey’s gill basket—as would be expected if it were a genuine thymus.
The MHC question
MHC molecules, which interact with T-cell receptors to initiate an immune response, are “the hallmark of the cellular immune system,” says Boehm. Searching for the lamprey MHC molecules is next on Boehm’s plate.