Invertebrate immune diversity.

The arms race between hosts and pathogens (and other non-self) drives the molecular diversification of immune response genes in the host. Over long periods of evolutionary time, many different defense strategies have been employed by a wide variety of invertebrates. We review here penaeidins and crustins in crustaceans, the allorecognition system encoded by fuhc, fester and Uncle fester in a colonial tunicate, Dscam and PGRPs in arthropods, FREPs in snails, VCBPs in protochordates, and the Sp185/333 system in the purple sea urchin. Comparisons among immune systems, including those reviewed here have not identified an immune specific regulatory "genetic toolkit", however, repeatedly identified sequences (or "building materials" on which the tools act) are present in a broad range of immune systems. These include a Toll/TLR system, a primitive complement system, an LPS binding protein, and a RAG core/Transib element. Repeatedly identified domains and motifs that function in immune proteins include NACHT, LRR, Ig, death, TIR, lectin domains, and a thioester motif. In addition, there are repeatedly identified mechanisms (or "construction methods") that generate sequence diversity in genes with immune function. These include genomic instability, duplications and/or deletions of sequences and the generation of clusters of similar genes or exons that appear as families, gene recombination, gene conversion, retrotransposition, alternative splicing, multiple alleles for single copy genes, and RNA editing. These commonly employed "materials and methods" for building and maintaining an effective immune system that might have been part of that ancestral system appear now as a fragmented and likely incomplete set, likely due to the rapid evolutionary change (or loss) of host genes that are under pressure to keep pace with pathogen diversity.

Echinoderm immunity.

A survey for immune genes in the genome for the purple sea urchin has shown that the immune system is complex and sophisticated. By inference, immune responses of all echinoderms maybe similar. The immune system is mediated by several types of coelomocytes that are also useful as sensors of environmental stresses. There are a number of large gene families in the purple sea urchin genome that function in immunity and of which at least one appears to employ novel approaches for sequence diversification. Echinoderms have a simpler complement system, a large set of lectin genes and a number of antimicrobial peptides. Profiling the immune genes expressed by coelomocytes and the proteins in the coelomic fluid provide detailed information about immune functions in the sea urchin. The importance of echinoderms in maintaining marine ecosystem stability and the disastrous effects of their removal due to disease will require future collaborations between ecologists and immunologists working towards understanding and preserving marine habitats.

Sp185/333: a novel family of genes and proteins involved in the purple sea urchin immune response.

The Sp185/333 system of genes, messages and proteins are expressed in the coelomocytes of the purple sea urchin, Strongylocentrotus purpuratus, and is an extraordinary example of diversification of a putative innate immune response system in an invertebrate. Reviewed here, is the current understanding of this complex system as illustrated by sequence comparisons of the genes, messages and deduced proteins with descriptions of diversity, including preliminary results on genomic organization and descriptions of 185/333 in other echinoids. Sp185/333 gene expression in adults and embryos occurs in response to immune challenge and includes changes in the frequencies of Sp185/333-positive coelomocytes in the adults. The diversity of the Sp185/333 protein repertoire in coelomocytes is far greater than the sequence diversity encoded in the genes, which may be the result of rapid gene recombination, RNA editing and/or low-fidelity transcription, plus post-translational modifications. This review concludes with preliminary results and speculations on protein function.