We have isolated and characterized a highly polymorphic candidate fusion/histocompatibility (cFuHC) gene from the primitive chordate Botryllus schlosseri which encodes an immunoglobulin superfamily member. The cFuHC segregates absolutely with histocompatibility in all mapping crosses, by itself predicts the outcome of interactions between wild-type colonies, and is expressed in tissues directly associated with the natural transplantation reaction: by every criterion this candidate is directly responsible for controlling histocompatibility. Other genes with these characteristics have not been identified in the FuHC locus, and together this is strong evidence that the cFuHC is the first metazoan histocompatibility ligand ever identified outside the vertebrates.
The ability to discriminate between multiple ligands is the universal principal shared by immunity and histocompatibility, leading F.M. Burnet to hypothesize that the origins of the most complex recognition system in the metazoa, the vertebrate adaptive immune system, would be found in histocompatibility systems of more primitive organisms13
Does identification of the cFuHC in a primitive chordate confirm this hypothesis? As a type I transmembrane protein with multiple extracellular Ig domains, the cFuHC is certainly structurally similar to the vertebrate MHC, however, it is clearly not a direct homolog; for example the Ig domains do not correspond to the C1 type found in the MHC. Alternatively, it has been suggested that vertebrate genes with the more ancient Ig domains, like human IgSF4, with which the cFuHC has high homology, may be descendents of the first antigen receptors2,30
. This is also based on observations of the organization of these genes in mammalian genomes2
, which may also be shared with the FuHC (not shown). Further structural and genetic analysis of the cFuHC may help elucidate the evolutionary origins of the different Ig domains, and ultimately, the molecules in adaptive immunity.
From a functional standpoint, there is another possible relationship between histocompatibility in Botryllus
to the origins of vertebrate immunity. In Botryllus
, individuals sharing only a single allele are compatible. It seems unlikely that the hundreds of FuHC alleles could evolve to clearly distinguish themselves via only homotypic interactions, suggesting that effector systems in Botryllus
work by missing self recognition, a strategy first identified in vertebrate Natural Killer (NK) cells14
. We hypothesize that a population of effector cells are educated to both FuHC alleles, and that recognition of just one allele blocks a default rejection reaction. Histocompatibility in Botryllus
evolutionarily links an innate function of vertebrate NK cells, scanning for altered MHC expression, to a highly polymorphic, Ig superfamily ligand. Thus missing-self recognition and natural killing effector systems may have originated in histocompatibility systems of non-vertebrates. In fact, the genome of the related ascidian, Ciona intestinalis
contains a number of genes highly-homologous to those involved in natural killing in the vertebrates33
. A major goal of future research will be to identify the cells, molecules and mechanisms underlying the function and education of this effector system. Interestingly, we have already found what appears to be a putative polymorphic receptor encoded within 200Kb of the cFuHC. However, these polymorphisms play no role in histocompatibility outcomes (Nyholm, et. al., in preparation).
Vertebrate MHC-based histocompatibility is thought to be an unintended consequence of peptide presentation and polymorphism, but histocompatibility and, more importantly, extraordinary polymorphism, have a well-defined function in B. schlosseri
. After compatible colonies fuse, blood-borne germline or totipotent stem cells transfer between colonies and have the ability to expand and differentiate in the newly arising, asexually derived individuals of the vascular partner. This can often result in a situation where only one of the genotypes is represented in the gametic output of the fused individuals, a situation which can remain constant for life15–17
. FuHC polymorphism functions to restrict vascular fusion, and the possibility of this germline parasitism, to kin. This stem cell parasitism is thought to be widespread, and the selective force driving the evolution of polymorphic histocompatibility systems in other phyla18,19
. Moreover, this movement of stem cells in Botryllus
is reminiscent of the transfer of hematopoietic stem cells between fetal mammals sharing a common placenta, the basis of natural tolerance20
. These same studies did not identify germline chimeras despite the fact that mammalian germline precursors are also migratory during development20,21
. Thus it may be that histocompatibility is still one function of vertebrate immunity, providing both a historical origin, and contemporary selective force to explain extraordinary MHC polymorphism. Alternatively, the FuHC may represent an undiscovered Ig-based system of allorecognition which still exists in the vertebrates; for example, NK cells have recently been found to interact with a non-MHC ligand via missing-self recognition22,23
Furthermore, CD155, a nectin family member with which the cFuHC shares structural homology, is a ligand for the NK activating receptor CD22631
. In any case, the cFuHC is the first non-vertebrate histocompatibility ligand every identified, and as an Ig superfamily member provides the first structural link between invertebrate histocompatibility and vertebrate adaptive immunity.