Reconstructing immune phylogeny: new perspectives

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Abstract
The traditional view is that innate immunity has a long phylogenetic history, whereas conventional adaptive immunity is restricted to jawed vertebrates. However, recent studies have shown that there might be alternative forms of adaptive immunity in jawless vertebrates and invertebrates and that diversified immune receptors are far more broadly distributed in phylogeny than was previously thought. Mediators of adaptive and innate immunity use related mechanisms to diversify structural domains in immune receptors. Pathogens use similar methods of genetic change to diversify many of the same types of domain to evade host immune defences. Various mechanisms are used to generate diversity in immune effector molecules. Depending on the level of phylogenetic development, different mechanisms or combinations of these mechanisms might be used to carry out these processes. Lymphocyte-like cells are seen in jawless vertebrates despite the apparent absence of authentic B- and T-cell-receptor homologues in these species. A leucine-rich-repeat-encoding gene that undergoes rearrangement in single lymphocytes is a possible mediator of adaptive immunity in both extant groups of jawless vertebrates (lampreys and hagfish). Natural killer (NK)-cell immunity seems to have a long phylogenetic history. Members of a family of authentic variable-region-containing activating and inhibitory type I transmembrane proteins that is found in bony fish share several properties with killer-cell immunoglobulin-like receptors (KIRs), which are the main family of diversified NK-cell receptors in humans. The enormously complex conventional adaptive immune system, which uses segmental rearrangement of variable, diversity and joining elements, has been assembled during evolution by integrating molecules that are involved in unrelated aspects of cellular metabolism. Furthermore, many of these same molecules are used to effect other germline changes during the developmental maturation of single lymphocytes. Both the innate and adaptive immune systems use an unexpectedly large number of different solutions in terms of receptor variation to solve the similar problem of host defence against infectious challenge.