Complement systems in invertebrates. The ancient alternative and lectin pathways

The complement system in higher vertebrates is composed of about thirty pro teins that function in three activation cascades and converge in a single t erminal pathway. It is believed that these cascades, as they function in th e higher vertebrates, evolved from a few ancestral genes through a combinat ion of gene duplications and divergences plus pathway duplication (perhaps as a result of genome duplication). Evidence of this evolutionary history i s based on sequence analysis of complement components from animals in the v ertebrate lineage. There are fewer components and reduced or absent pathway s in lower vertebrates compared to mammals. Modern examples of the putative ly ancestral complement system have been identified in sea urchins and tuni cates, members of the echinoderm phylum and the protochordate subphylum, wh ich are sister groups to the vertebrates. Thus far, this simpler system is composed of homologues of C3, factor B, and mannose binding protein associa ted serine protease suggesting the presence of simpler alternative and lect in pathways. Additional components are predicted to be present. A complete analysis of this invertebrate defense system, which evolved before the inve ntion of rearranging genes, will provide keys to the primitive beginnings o f innate immunity in the deuterostome lineage of animals. (C) 1999 Elsevier Science B.V. All rights reserved.