Protochordates lack adaptive immune receptors and rely instead on the expression of innate recognition receptors (VCBPs) composed of tandem variable (V) domains linked to a chitin-binding domain. In the Proceedings of the National Academy of Science, Dishaw et al. describe the genomic structure and function of VCBP found in Ciona, a sessile urochordate organism. Unlike those in amphioxus, another protochordate, genes encoding VCBPs in Ciona do not show extensive polymorphism in their exons, although there is allelic diversity in intronic regions, and they are not generated by somatic rearrangement. VCBP is expressed in gut and stomach epithelial cells and in vacuoles of granular amebocytes, which have phagocytic activity. Soluble VCBPs are released into the gut lumen, where they recognize and opsonize bacteria via their V domains. Surprisingly, the chitin-binding domain does not contribute to these activities, which raises the issue of the function of this conserved domain in host-microbe interactions.

Proc. Natl. Acad. Sci. USA (23 September 2011) doi:10.1073/pnas.1109687108