A number of different classes of molecules function as structural matrices for effecting innate and adaptive immunity. The most extensively characterized mediators of adaptive immunity are the immunoglobulins and T-cell antigen receptors found in jawed vertebrates. In both classes of molecules, unique receptor specificity is effected through somatic variation in the variable (V) structural domain. V region-containing chitin-binding proteins (VCBPs) consist of two tandem Ig V domains as well as a chitin-binding domain. VCBPs are encoded at four loci (i.e., VCBPA-VCBPD) in Ciona, a urochordate, and are expressed by distinct epithelial cells of the stomach and intestine, as well as by granular amoebocytes present in the lamina propria of the gut and in circulating blood. VCBPs are secreted into the gut lumen, and direct binding to bacterial surfaces can be detected by immunogold analysis. Affinity-purified native and recombinant VCBP-C, as well as a construct consisting only of the tandem V domains, enhance bacterial phagocytosis by granular amoebocytes in vitro. Various aspects of VCBP expression and function suggest an early origin for the key elements that are central to the dialogue between the immune system of the host and gut microflora.bacteria opsonization | bacteria phagocytosis | immunoglobulin variable domains D ifferent molecular and cellular mechanisms that effect "innate" or "adaptive" immune responses shape immunity to pathogens such as viruses, bacteria, and parasites in all metazoans. The innate immune system includes germline-encoded receptor molecules that recognize widely divergent molecular structures. In contrast, the gene loci that encode the receptor molecules of the adaptive immune system undergo unique rearrangements in individual somatic cells that expand clonally and account for nearlimitless functional variation of receptors. The adaptive immune response is limited to jawed vertebrate species, whereas innate immunity is characteristic of all metazoan phyla.Alternative mechanisms of innate and adaptive immunity have been described in jawless vertebrates, protochordates, and other invertebrates (1, 2). Given the absence of V domain-mediated immunity in jawless vertebrates, the protochordate lineages are particular significant for understanding the origins of V region diversity as a basic form of immune recognition. In the protochordate Branchiostoma floridae (amphioxus), variable region-containing chitin-binding proteins (VCBPs) were described that consist of two variable (V) Ig domains and a single chitin-binding domain (3). VCBPs are encoded by diverse, nonrecombining, haplotypically variable alleles (4-6). Detailed structural studies of VCBPs reveal that the hyperpolymorphic positions are localized on the β-sheet surfaces of the folded V domains (7) and not on the connecting loops, which are the sites of the highest variability in the V domains of Ig and T-cell receptor (TCR). Other innate immune functions (e.g., viral receptor and superantigen-binding sites) are associated with V regi...

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