Nature Immunology pp 1135 - 1141

Langerhans cells (LCs) act as mobile sensors, constantly surveying the skin for abnormalities such as infections. The mystery has been, where do they come from? In the December issue of Nature Immunology scientists report that these critical "eyes and ears of the immune system" are generated by two very different means. Normally, local LCs multiply just enough to replace those that die. But during a catastrophic event such as a bacterial infection of the dermis or a sunburn, the skin rapidly sends out signals into the bloodstream to recruit additional precursor cells.

To effectively act as an early warning system, LCs migrate to lymph nodes where immune responses are initiated. The mechanism by which the cells that migrate away from the skin, or who have died, are replenished is controversial. Merad and colleagues from Stanford University in California now conclusively settle the issue. Under normal conditions, the replacement cells are rarely derived from the blood. On the other hand, during inflammation, such as exposure to ultraviolet light (as would happen during a sunburn, for example), LCs are depleted beyond the capacity of the local precursors to adequately replenish the skin. In this case, skin cells send out specific signals to recruit precursor cells circulating in the bloodstream that can subsequently differentiate into LCs. A better understanding of the role of LCs in skin immunity is important if we hope to manipulate them in future immunotherapies or vaccine strategies.

Nature Immunology pp 1200 - 1207

Antibody-like molecules exist in primitive jawless sea animals. In the December issue of Nature Immunology, researchers in South Florida found a diverse family of proteins, exhibiting key features of antibody molecules, in amphioxus, a bottom-dwelling creature common to coastal marine environments. These findings may provide the earliest link to the evolution of specific recognition molecules that arise during immune responses against pathogens.

Using multiple criteria to identify even distantly related genes, Litman and colleagues show that protochordates, animals that lack a true backbone and have only a primitive immune system, nevertheless express proteins that possess highly conserved elements of antibody molecules. Rather surprisingly, these proteins all contain a module capable of binding chitin, a substance that forms the exterior of many insects and crustaceans, whereas the antibody-like modules are highly diverse, suggesting that the ability to diversify these molecules - a key component of adaptive immunity - may also be evolutionarily well-conserved. Individual amphioxus express a wide repertoire of these proteins in gut tissues, which is a site that comes into contact with many pathogens. The true function of these sea antibodies - whether immune-related or not - remains to be determined.