Allorecognition

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Movie generated from tracings of gastrovascular canals (red) in two colonies of Hydractinia symbiolongicarpus that come into contact and then separate. The green structures are polyps.

Allorecognition is the ability of an individual organism to distinguish its own tissues from those of another. It manifests itself in the recognition of antigens expressed on the surface of cells of non-self origin. Allorecognition has been described in nearly all multicellular phyla.

Contents

This article focuses on allorecognition from the standpoint of its significance in the evolution of multicellular organisms. For other articles which focus on its importance in medicine, molecular biology, and so forth, the following topics are recommended as well as those in the Categories links at the bottom of this page.

The ability to discriminate between self and non-self is a fundamental requirement for life. At the most basic level, even single-celled organisms need to be able to distinguish between food and non-food, to respond appropriately to invading pathogens, and to avoid cannibalism. In sexually reproducing organisms, self/non-self discrimination is essential to ensuring species-specific egg/sperm interaction during fertilization. Hermaphroditic organisms, such as annelids and certain plants, require recognition mechanisms to prevent self-fertilization. Such functions are all carried out by the innate immune system, which employs evolutionarily conserved pattern recognition receptors to eliminate cells displaying "nonself markers." [1]

Evolution of multicellularity

The evolution of multicellularity brought about various challenges, many of which could be met by increasingly sophisticated innate immune systems, but which also served as an evolutionary driving force for the development of adaptive immune systems. The adaptive or "specific" immune system in its fully qualified form (i.e. based on major histocompatibility complex (MHC), T-cell receptors (TCR), and antibodies) exists only in jawed vertebrates, but an independently evolved adaptive immune system has been identified in hagfish and lampreys (non-jawed vertebrates). [2]

Multicellularity has arisen independently dozens of times in the history of life, in plants, animals, fungi, and prokaryotes, [3] appearing first several billion years ago in cyanobacteria. Two categories of advantages have been attributed to the early development of multicellular existence: advantages related to size, and advantages related to functional specialization and division of labor. [4] Size advantages may include greater feeding efficiency or increased robustness. For example, myxobacteria, moving in swarms, are able to maintain a high concentration of extracellular enzymes used to digest food, from which all the bacteria in the swarm benefit. Under various conditions, many microorganisms form biofilms which provide them with a protected environment. In organisms that have evolved functional specialization, an important division of labor may exist over reproduction: only a small fraction of cells contribute to the next generation. Somatic growth represents a form of altruism, where somatic cells give up reproduction helping germline cells reproduce. [5]

Free rider problem

The extracellular enzymes secreted by swarming bacteria, the slime of a biofilm, or the soma cells in a differentiated organism represent public goods which are vulnerable to exploitation by cheaters. [6] This issue is well known in economics and evolutionary biology as the "free rider problem" or the "tragedy of the commons." A free rider (or freeloader) is an individual that consumes a resource without paying for it, or pays less than the full cost. In multicellular organisms, cheaters may arise from mutations in somatic cells that no longer contribute to the common good, or ignore controls on their reproduction. [7] Another possibility may arise from somatic fusion: there are multicellular life-styles where there are few if any physical barriers to the intermingling of cells (for example: sponges, fungal mycelia) and even among organisms that have evolved physical integuments representing a first line of defense against invasion, opportunities for cellular exchange occur. Witness, for example, the spread of devil facial tumour disease among Tasmanian devils and transmissible venereal tumor in dogs. [8]

In metazoans, defense against disruption of the multicellular life style by such cheaters takes two major forms. First, a consistent feature of the multicellular life cycle is the interposition of a unicellular phase, even among organisms whose major mode of propagation may be via many-celled vegetative propagules. [9] This unicellular phase usually takes the form of a sexually produced zygote. Passage through a unicellular bottleneck assures that each representative of the next generation of organisms represents a distinct clone. Some offspring will carry a large number of deleterious mutations and will die off, while other offspring will carry few. In this manner, the organism bypasses "Muller's ratchet," the process by which the genomes of an asexual population accumulate deleterious mutations in an irreversible manner. The second defense against cheaters is the development of allorecognition mechanisms that guard against invasion by parasitic replicators. [10] Allorecognition acts as an agent of kin selection by restricting fusion and community acceptance to related individuals. If related individuals fuse, the benefits of fusion will still apply, while the costs of competition for shared resources or reproductive opportunities will be reduced by a fraction proportional to the degree of relatedness between the fusing partners. [6] If unrelated individuals fuse, or if a mutated cell arises within an organism that is distinguishable from self by the allorecognition system, a rejection response will be activated. As a general rule, rejection is mediated by the gene products of highly variable loci, which must match (or nearly match) between organisms for fusion to be successful. [11]

Allorecognition phenomena

Allorecognition phenomena have been recognized in bacterial self-identity and social recognition systems, [12] kin discrimination in social amoebae, [13] [14] fungal mating types, [15] fungal vegetative incompatibility, [16] plant self-incompatibility systems, [17] colonial marine invertebrates (such as corals, sponges, hydroids, bryozoans, and ascidians), [18] and of course, vertebrates. The manner in which allorecognition manifests itself in these different systems varies greatly. Bacteria, for instance, secrete bacteriocins, proteinaceous toxins specifically targeted against members of their own species. Colonies of marine invertebrates, each representing a single genotype, expand across the ocean floor by asexual reproduction. Where colonies meet, they may, if compatible, fuse to form a single unit, or if incompatible, they may aggressively attempt to overgrow, poison, sting, or consume each other. [18]

Innate and adaptive immune systems

Vertebrate immunity is dependent on both adaptive and innate immune systems. In vertebrates, the innate immune system is composed of cells such as neutrophils and macrophages (which also have a role in the adaptive immune system as antigen presenting cells), as well as molecular pathways such as the complement system which react to microbial non-self. The innate immune system enables a rapid inflammatory response that contains the infection, and it activates the adaptive immune system, which eliminates the pathogen and, through immunological memory, provides long term protection against reinfection. [19]

Comprehensive sequence searches across multiple taxonomic groups have failed to identify MHC and TCRs outside of the jawed vertebrates. Allorecognition in these animals rely on molecular mechanisms distinct from those of the jawed vertebrates. In sponges, various receptors (sponge adhesion molecules, receptor tyrosine kinase) with domains similar to those found in immunoglobulins have been identified. Sequence variability in "hot spots" have been identified in these receptors. [2] It would appear that molecules which, later in evolution, were exploited in the adaptive immune response, had an earlier role in innate recognition. Lampreys and hagfish appear to have evolved, by convergent evolution, an adaptive immune response that is independent and distinct from the adaptive immune systems of higher vertebrates. Lymphocyte-like cells in these fish express highly variable lymphocyte receptor genes, which undergo somatic rearrangements reminiscent of the manner in which mammalian immunoglobulin genes are rearranged during development. [20]

Summary

In summary, allorecognition, the ability to distinguish self from non-self, is basic to all life, unicellular as well as multicellular. The earliest recognition systems were innate, and were based on the recognition of self molecules. The evolution of multicellular forms brought about selective pressures for ever-increasing sophistication to innate immune systems. Adaptive immune systems, based on the recognition of non-self, have arisen independently in two lines of chordates, and exploit molecules and cellular systems which had a previous role in innate immune responses. Allorecognition as it currently exists in mammals can be traced back as the result of sequential modification to immunity mechanisms dating back to some of the earliest multicellular organisms.

See also

Related Research Articles

<span class="mw-page-title-main">Immune system</span> Biological system protecting an organism against disease

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<span class="mw-page-title-main">Autoimmunity</span> Immune response against an organisms own healthy cells

In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an "autoimmune disease". Prominent examples include celiac disease, diabetes mellitus type 1, Henoch–Schönlein purpura, systemic lupus erythematosus, Sjögren syndrome, eosinophilic granulomatosis with polyangiitis, Hashimoto's thyroiditis, Graves' disease, idiopathic thrombocytopenic purpura, Addison's disease, rheumatoid arthritis, ankylosing spondylitis, polymyositis, dermatomyositis, and multiple sclerosis. Autoimmune diseases are very often treated with steroids.

An immune response is a physiological reaction which occurs within an organism in the context of inflammation for the purpose of defending against exogenous factors. These include a wide variety of different toxins, viruses, intra- and extracellular bacteria, protozoa, helminths, and fungi which could cause serious problems to the health of the host organism if not cleared from the body.

<span class="mw-page-title-main">B cell</span> Type of white blood cell

B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasma membrane where they serve as a part of B-cell receptors. When a naïve or memory B cell is activated by an antigen, it proliferates and differentiates into an antibody-secreting effector cell, known as a plasmablast or plasma cell. In addition, B cells present antigens and secrete cytokines. In mammals B cells mature in the bone marrow, which is at the core of most bones. In birds, B cells mature in the bursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick, which is why the B stands for bursa and not bone marrow, as commonly believed.

<span class="mw-page-title-main">Major histocompatibility complex</span> Cell surface proteins, part of the acquired immune system

The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are called MHC molecules.

<span class="mw-page-title-main">Multicellular organism</span> Organism that consists of more than one cell

A multicellular organism is an organism that consists of more than one cell, unlike unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

<span class="mw-page-title-main">Toll-like receptor</span> Class of immune system proteins

Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have reached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses. The TLRs include TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10, TLR11, TLR12, and TLR13. Humans lack genes for TLR11, TLR12 and TLR13 and mice lack a functional gene for TLR10. The receptors TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the cell membrane, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles.

<span class="mw-page-title-main">Adaptive immune system</span> Subsystem of the immune system

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Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed mainly by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils, as well as by epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.

<span class="mw-page-title-main">Phagosome</span> Vesicle formed around a particle engulfed by a phagocyte via phagocytosis

In cell biology, a phagosome is a vesicle formed around a particle engulfed by a phagocyte via phagocytosis. Professional phagocytes include macrophages, neutrophils, and dendritic cells (DCs).

<span class="mw-page-title-main">Innate immune system</span> Immunity strategy in living beings

The innate immune system or nonspecific immune system is one of the two main immunity strategies in vertebrates. The innate immune system is an alternate defense strategy and is the dominant immune system response found in plants, fungi, prokaryotes, and invertebrates.

The gene-for-gene relationship is a concept in plant pathology that plants and their diseases each have single genes that interact with each other during an infection. It was proposed by Harold Henry Flor who was working with rust (Melampsora lini) of flax (Linum usitatissimum). Flor showed that the inheritance of both resistance in the host and parasite ability to cause disease is controlled by pairs of matching genes. One is a plant gene called the resistance (R) gene. The other is a parasite gene called the avirulence (Avr) gene. Plants producing a specific R gene product are resistant towards a pathogen that produces the corresponding Avr gene product. Gene-for-gene relationships are a widespread and very important aspect of plant disease resistance. Another example can be seen with Lactuca serriola versus Bremia lactucae.

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Gamma delta T cells are T cells that have a γδ T-cell receptor (TCR) on their surface. Most T cells are αβ T cells with TCR composed of two glycoprotein chains called α (alpha) and β (beta) TCR chains. In contrast, γδ T cells have a TCR that is made up of one γ (gamma) chain and one δ (delta) chain. This group of T cells is usually less common than αβ T cells. Their highest abundance is in the gut mucosa, within a population of lymphocytes known as intraepithelial lymphocytes (IELs).

Somatic hypermutation is a cellular mechanism by which the immune system adapts to the new foreign elements that confront it. A major component of the process of affinity maturation, SHM diversifies B cell receptors used to recognize foreign elements (antigens) and allows the immune system to adapt its response to new threats during the lifetime of an organism. Somatic hypermutation involves a programmed process of mutation affecting the variable regions of immunoglobulin genes. Unlike germline mutation, SHM affects only an organism's individual immune cells, and the mutations are not transmitted to the organism's offspring. Because this mechanism is merely selective and not precisely targeted, somatic hypermutation has been strongly implicated in the development of B-cell lymphomas and many other cancers.

Jawless vertebrates, which today consist entirely of lampreys and hagfish, have an adaptive immune system similar to that found in jawed vertebrates. The cells of the agnathan AIS have roles roughly equivalent to those of B-cells and T-cells, with three lymphocyte lineages identified so far:

Immunological memory is the ability of the immune system to quickly and specifically recognize an antigen that the body has previously encountered and initiate a corresponding immune response. Generally, they are secondary, tertiary and other subsequent immune responses to the same antigen. The adaptive immune system and antigen-specific receptor generation are responsible for adaptive immune memory.

A somatic mutation is a change in the DNA sequence of a somatic cell of a multicellular organism with dedicated reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. Unlike germline mutations, which can be passed on to the descendants of an organism, somatic mutations are not usually transmitted to descendants. This distinction is blurred in plants, which lack a dedicated germline, and in those animals that can reproduce asexually through mechanisms such as budding, as in members of the cnidarian genus Hydra.

Germ-Soma Differentiation is the process by which organisms develop distinct germline and somatic cells. The development of cell differentiation has been one of the critical aspects of the evolution of multicellularity and sexual reproduction in organisms. Multicellularity has evolved upwards of 25 times, and due to this there is great possibility that multiple factors have shaped the differentiation of cells. There are three general types of cells: germ cells, somatic cells, and stem cells. Germ cells lead to the production of gametes, while somatic cells perform all other functions within the body. Within the broad category of somatic cells, there is further specialization as cells become specified to certain tissues and functions. In addition, stem cell are undifferentiated cells which can develop into a specialized cell and are the earliest type of cell in a cell lineage. Due to the differentiation in function, somatic cells are found only in multicellular organisms, as in unicellular ones the purposes of somatic and germ cells are consolidated in one cell.

References

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