Whitehead Institute for Biomedical Research, MIT, Boston

An immunologist marvels at dendritic cells.

Dendritic cells ingest and process all manner of bacteria and viruses, and display the invaders’ distinguishing structures so that other cells of the immune system ‘know’ what to ‘look for’. No adaptive immune response can begin without them. For years I have been fascinated by the internal details of dendritic cells that enable them to handle this task.

One set of details concerns how these cells manage protein production and disposal, given that breaking down and presenting sections of foreign proteins are the cells’ primary jobs. Last month, Hugues Lelouard and his colleagues at the University of the Mediterranean in Marseille, France, discovered that dendritic cells fine-tune the translation of messenger RNAs to proteins when they are activated by inflammatory stimuli (H. Lelouard et al. J. Cell Biol. 179, 1427–1439; 2007).

The authors’ stimuli of choice were lipopolysaccharides, signature molecules that indicate the presence of certain bacteria. Dendritic cells exposed to them showed a close correlation between the extent to which translation became more efficient and the increased formation of lumpy bodies similar to aggresomes, which is a prelude to the destruction of proteins. The authors then elucidated the biochemical steps that lead to enhanced translation of certain mRNAs when a dendritic cell becomes activated.

I consider it likely that the mRNAs in question are not randomly distributed throughout a dendritic cell’s cytoplasm. If this is so, these cells may contain a ‘translational hotspot’ of requisite proteins and enzymes around each pathogen containing vesicle, required for the orderly handling of the newly ingested microbe. The result might be the creation of an intracellular solid-state ‘device’ specifically for the processing and presentation of that microbe’s antigens.