Allorecognition (histocompability) in the solitary tunicate,Styela plicata

The tunicate, S. plicata.

http://www.sms.si.edu/irlspec/ images/splicata1.jpg

In a series of classic experiments, David Raftos showed that the solitary tunicate, Styela plicata, rejects tissues grafted from genetically unrelated individuals. This is sometime referred to as histocompatibility (tissue compatibility). Autografts (transplanting tissues from one area of an animal to other regions of the same individual) are tolerated well. If a tunicate is grafted with tissues of another animal, the grafted tissues are slowly rejected. If the same recipient is later provided with a second graft from the same donor, that (second) graft is quickly rejected. However, if the second graft is from a different donor (also called third party grafts), then the graft from the second donor is reject slowly. In other words, the kinetics of rejection is dependent on prior exposure of the host's immune system to the graft. Although Raftos' work did not discover the underlying mechanisms, he suggested that a there may be a genetic basis to graft rejection reactions in S. plicata.

The conclusions of David Raftos' work on S. plicata were significant from another perspective: tunicates are closely related to the vertebrates from an evolutionary standpoint. Tunicates possess a notochord during their larval stages, a feature that is shared by all members of the chordate clade. Hence, tunicates belong to a group of animals known as invertebrate chordates (Urochordates). One implication of Raftos' data was that there were commonalities between vertebrate and tunicate histocompatibility responses. However, as with all discussions regarding evolutionary relationships, apparent similarities do not automatically imply common evolutionary descent. To determine the relationship between vertebrate and tunicate histocompatibility responses, the molecular basis of those reactions had to be established. Indeed, it was also known at that time that tunicates did not possess immunoglobulin genes that are homologous to the vertebrate immunoglobulin loci.

Allorecognition (histocompability)in the colonial tunicate,Botryllus schlosseri

The colonial tunicate, B. schlosseri.


The answer to the question of the the genetic basis of histocompatibility reactions in tunicates came from another laboratory.Anthony Di Tomasso, then working at Stanford University withIrvingWeissman, investigated the genetics of 'fusability' amongst the colonial tunicate,Botryllus schlosseri. UnlikeS. plicata,B. schlosseriforms colonies on rock surfaces in the shoreline. When colonies are established, they grow outward from the founder populations. If two colonies come into contact with each other, immunological histocompatibility determines the outcome of that interaction: if the two colonies are of similar genetic constitution, then the colonies will fuse and will share a common vasculature. On the other hand, contact between 'incompatible' colonies will result in immunological rejection, wherein scar tissues will form at the boundaries of the colonies and their vasculatures remain separate. Solitary animals are not expected to 'share' tissues and thus immunologically reject non-self. Colonial animals, on the other hand, have to deal with non-self tissue recognition in the normal course of their lifecycle.

Through an elegant series of experiments, Di Tomasso and his colleagues demonstrated that the outcomes of colony interactions are dependent on the genotypes of the colonies. One particular locus (region on an organism's genome), called FuHC (Fusion/HistoCompatibility), appears to be important. This locus is highly polymorphic and may be represented by different alleles. In genetic terms, the alleles of theFuHClocus inBotryllusare said to be co-dominant.The genetic basis of allorecognition that is mediated by this locus is relatively simple: if the two colonies that come into contact share at least one allele, then fusion occurs. The absence of a common allele between adjacent colonies prevents fusion and the colonies remain distinct.

The FuHC gene encodes a complex polypeptide that consists of several well-known protein domains. These are two EGF (epidermal growth factor), three immunoglobulin (Ig) and one transmembrane domains. This means that the FuHC polypeptide in embedded in the membranes of cells that express this gene, with the EGF and Ig domains projecting into the extracellular region. Indeed, this polypeptide has been identified in the ampullae, blood cells and larval cells. A secreted form of the FuHC polypeptide has also been identified: this is likely to be the product of an alternatively spliced FuHC mRNA, which produces a truncated (smaller) polypeptide with only the EGF domains. When the FuHC gene from many different Botryllus zooids were sequenced, their sequences displayed high levels of variability. variability, or diversity, is a key feature of vertebrate MHC-based histocompatibility system.

Although the details of the Botryllus allorecognition system is not known, it is presumed that when two genetically dissimilar (at theFuHClocus) colonies come into contact, cells of the ampullae and the blood cells at the zone of contact will interact with each other. Since these cells express the FuHC polypeptide, the polypeptides from the two colonies will interact with each other. Now, if the two colonies share at least oneFuHCallele, then there will be one identical fuhc polypeptide expressed by the cells of those two colonies. It is assumed that two identical fuhc polypepties interact with each other very strongly (this type of interaction is referred to as homophillic binding). If strong homophillic binding occurs, then rejection reactions are suppressed and the two colonies will fuse permanently. However, if the two colonies do not share at least one commonFuHCallele, then strong homophillic interactions cannot form between theFuHCpolypeptides from the two colonies and, consequently, fusion is prevented at the boundaries. In support of this model is the observation that when blood cells from two incompatible colonies are mixed together, the cells release enzymes that trigger the 'rejection reactions'.

FuHc is not the only genetic determinant of allorecognition and fusion/rejection in Botryllus. Another gene, called fester, also appears to be important in this regard. The fester gene is linked to the FuHC gene (they are part of the same allorecognition locus). Fester encodes a membrane-bound polypeptide that consists of SCR (Short Consensus Repeat) domains. As with FuHC,alternativesplicing of festertranscriptsproduces a variety of differently sized polypeptides, including those that are secreted into the the blood. Also, festersequencesare highly polymorphic (diverse). The tissue expression patterns of fester are also similar to those of FuHC.

One approach that is often used to determine the biological functions of specific genes is RNA inhibition (RNAi). When double-stranded RNA (dsRNA) to a specific gene sequence is injected into a multicellular organism, the mRNA from that gene will be destroyed. This prevents the expression of that gene, since no corresponding polypeptides can be synthesised. In an experiment wherein Botryllus were injected with dsRNA to the fester gene, both fusion and rejection reactions were abrogated. In another experiment, Botryllus that were injected with antibodies to the fester protein were unable to reject incompatible colonies (instead, the researchers observed that fusion occurred). Finally, other studies have shown that when blood cells from incompatible colonies are mixed together, they will degranulate (release enzymes contained within their cytoplasm). It is thought that these enzymes are involved in the immunological reactions associated with histocompatibility.

In both vertebrates and Botryllus, specific genomically-encoded protein systems mediate allorecognition and subsequent fusion (tolerance)/rejection reactions. However, the systems are not homologous and are mechanistically different. The following table shows this difference at a glance:

This table summarises the interaction of Botryllus colonies with hypothetical genotypes. + indicates that the colonies will reject the contact. In this example with 4 alleles, 75% of the combinations will fuse, while 25% will reject. Fusion requires that the colonies in contact share at least one allele.

Parents: AB x CD
Progeny: AC, AD, BC, BD
(Adapted from Manning, M and Turner, R. (1976). Comparative Immunobiology)

In the vertebrate system, tolerance will occur only if both alleles (at the major histocompatibility complex loci) match. In this example, 25% of the combinations will be tolerated, while 75% of the combinations will be rejected.

Parents: AB x CD
Progeny: AC, AD, BC, BD
(Adapted from Manning, M and Turner, R. (1976). Comparative Immunobiology)

The cellular and molecular basis of the rejection reaction that follows allorecognition is still not clearly established. It appears that all colonies, regardless of their genotypic similarities (or differences), release nematocysts into the neighbouring colonies when they come into contact with each other. Hence, the immediate response to physical contact appears to be a rejection response. If the two colonies that come into contact are compatible, then the nematocysts simply disperse (i.e., remain undischarged), without causing damage. In incompatible colonies, the nematocysts will discharge, causing the physiological and morphological symptoms associated with allorecognition.

Allorecognition (histocompability)in the cnidarian,Hydractinia

Hydractinia growing on a hermit crab.

http://www.seawater.no/fauna/ cnidaria/images/CRW_8451.jpg

At the molecular level, the genetics of allorecognition in Hydractinia is more complex. A single genomic region (called the Allorecognition Complex), which contains two linked loci, appears to mediate allorecognition and rejection. The two loci are called alr1 and alr2. alr1 and alr 2 proteins are both transmembrane proteins. At their extracellular regions, they possess immunoglobulin-like domains. In their cytoplasmic regions are domains known as ITAM (alr1) and ITIM (alr2). Thus, the alr proteins are similar to immunoglobulin receptors in vertebrate lymphocytes (although the Hydractinia and vertebrate molecules are not homologous). Population-level studies have indicated extensive diversity in the alr sequences. Indeed, the different alr alleles may be divide into two distinct classes (types I and II), although the significance of this classification is not clear (note: the fester alleles are divided into many classes). One model of alr diversity suggests that the diversity in this gene system is generated during germ cell formation and not somatically.

Allorecognition (histocompability)in the annelid worm (earthworm),Lumbricus

The annelid worm (earthworm), Lumbricus.


Sham Nair 2014