Species extinction and Processes responsible for species extinction

Species extinction and Processes responsible for species extinction

Extinction is irreversible natural process and about 99.9% of the species that ever lived on Earth are now extinct. Extinctions are caused by long- term alterations of the environment, such as climate change, or by catastrophic events, such as asteroid impacts or by human activity such as overhunting, habitat destruction, introduction of invasive species, etc.

Like the evolution of new species, extinction is also important in the history of life.  Typically, a species becomes extinct about 10 million years after it evolves and some well adapted species such as sharks and cockroaches remain unchanged for hundreds of millions of years.

Species become extinct for two reasons. Some species die off when their numbers reduce so that they become unable to maintain a successful breeding population. Some species become extinct when they successfully evolve into new species.

Many extinctions in Earth have occurred as mass extinctions, during which 25% or more of all species are wiped out and there have been at least five major mass extinction events in earth’s history. The sixth extinction is currently under way.

Some natural causes of mass extinctions are

Plate tectonics: Continents join together and break apart and land masses also drift which affect environmental conditions including climate, the availability of coastal habitat, and the circulation of ocean currents, etc.

Climate change: Due to sudden climate changes organisms that are adapted to one climate may not be able to evolve fast enough to adapt to the new conditions and may be driven to extinction.

Volcanic eruptions: Volcanic eruptions blow ash, gases and particles into the atmosphere that cause global warming (CO2 and sulfur dioxide), global cooling (sulfuric acid aerosols and dust particles), and acid rain (sulfur and nitrogen oxides) which could bring about extinctions.  Flood basalts, voluminous eruptions of fluid lava cover thousands of square miles. The eruptions also result in changes in ocean chemistry and ocean circulation.

Asteroid impact: An asteroid is supposed to be the reason for the extinctions of the dinosaurs and

many other species on land and in the oceans 65 million years ago.

Humans and Extinction: Now Earth is entering into the sixth mass extinction event and this is due to the destructive effects of one single species, humans.

During the Pleistocene period, wooly mammoths and saber toothed cats were among the large mammals that dominated terrestrial ecosystems. The rising temperatures after the Pleistocene period resulted in the decline of these populations and many went extinct. Humans are at least partially responsible for these and other relatively recently extinctions and this hypothesis is known as human overkill hypothesis. The extinctions were caused because of over hunting, animal habitat destroyed, and alien species and disease introduction. There is a great deal of evidence for the human overkill hypothesis:

·         The post-Pleistocene extinctions did not occur simultaneously around the world. The extinctions closely follow the first appearance of humans on a continent or island.

·         Only large mammals, which are easily hunted, became extinct.

·         Climate change is another factor.

An example of human overkill. Before the arrival of the paleo-Indians, the prairies east of the Rocky Mountains were home to the greatest diversity of large mammals such as mammoths, mastodons, giant ground sloths, buffalo, lions, tigers, and enormous birds of prey. Around 13,400 years ago with the arrival of humans in North America, all of these grand creatures were extinct within 1,000 years.  In Europe and Asia, half the species of large animals, wooly mammoths, elephants, rhinos, giant deer, hyenas, lions, panthers, bison, hippos, and bears were eliminated between 30,000 and 15,000 years ago.

Human caused large numbers of extinctions on oceanic islands where due to the lack of predators, many bird species evolved to be flightless and naive. When humans arrived there the birds became easy prey for people and their cats and rats. Within 500 years of the arrival of the Maori people in New Zealand during AD 1000, the island’s 12 species of giant flightless birds called moas and many species of frogs, lizards, and other birds disappeared. Hawaii lost more than 50 species of birds after the arrival of the Polynesians.

In Africa large animals and flightless birds did not die out in a rapid rate and this is assumed to be due to the fact that, humans did not invade Africa but evolved there.  Because they evolved with humans, the animals are favored by natural selection and many were supremely adapted to running.

The millennium ecosystem assessment by the United Nations Environment Programme (UNEP) found that the current global extinction rate is between 100 and 1,000 times higher than the average over geologic time. Entire ecosystems are being lost or altered beyond recognition. Worldwide, 30,000 species are lost per year, or about 3 per hour.

An endangered species is any plant or animal species whose ability to survive and reproduce has been jeopardized by human activities.

A threatened species is one that is likely to become endangered.

The root of the problem is human population growth, which increased from 6 million when agriculture began 10,000 years ago, to around 900 million at the beginning of the nineteenth century and by the end of the twentieth century the population increased to around 6 billion. All of these people need food, access to clean water and secure shelter and a place for their wastes.

The World Conservation Union (ICUN) projected in 2004 that about 1 million land organisms will disappear in half a century. Harvard University biologist E.O. Wilson predict that one-half of all species on Earth will be extinct by 2100.

 

 

References

 

Emerging Consequences of Biotechnology - Biodiversity Loss and IPR Issues, Krishna Dronamraju, World Scientific Publishing Co. Pte. Ltd.

Biosphere - Ecosystems and Biodiversity Loss, Dana Desonie, Chelsea House

Sterilization in Fermentation

Sterilization in Fermentation

If the fermentation is invaded by a foreign microorganism, then the following consequences may occur:

1.      The medium would have to support the growth of both the production organism and the contaminant, resulting in a loss of productivity.

2.      If the fermentation is a continuous one then the contaminant may 'outgrow' the production organism and displace it from the fermentation.

3.      The foreign organism may contaminate the final product

4.      The contaminant may produce compounds which make extraction of the final product difficult.

5.      The contaminant may degrade the desired product, e.g. the degradation of Beta lactam antibiotics by Beta lactamase-producing bacteria.

6.      Contamination of a bacterial fermentation with phage result in the lysis of the culture.

The problem of contamination may be prevented by:

1.      Using a pure inoculum to start the fermentation

2.      Sterilizing the medium to be employed.

3.      Sterilizing the fermenter vessel.

4.      Sterilizing all materials to be added to the fermentation during the process.

5.      Maintaining aseptic conditions during the fermentation.

Medium Sterilization

Media may be sterilized by filtration, radiation, ultrasonic treatment, chemical treatment or heat. Heat sterilization is generally employed. Media may be sterilized either through the batch mode or through the continuous mode.

Batch Sterilization

A batch sterilization process results in the destruction of nutrients.  So it will be designed such that the required sterility is achieved with minimum loss of nutritive quality.  The highest temperature feasible for batch sterilization is 121°C.  Exposure of medium to this temperature is kept to a minimum required time.

The batch sterilization of the medium for a fermentation may be achieved either in the fermentation vessel or in a separate mash cooker.

The major advantages of a separate medium sterilization vessel are

·         One cooker may be used to serve several fermenters and the medium may be sterilized as the fermenters are being cleaned and prepared for the next fermentation, thus saving time between fermentations.

·         The medium may be sterilized in a cooker in a more concentrated form than would be used in the fermentation and then diluted in the fermenter with sterile water prior to inoculation.

·         In some fermentations, the medium is at its most viscous during sterilization.  So the sterilization vessel will be equipped with a powerful motor and the fermenter could be equipped with a less powerful motor.

·         The fermenter would be spared the corrosion which may occur with medium at high temperature.

The major disadvantages of a separate medium sterilization vessel are

·         The cost of constructing a batch medium sterilizer is almost the same as that for the fermenter.

·         If a cooker serves a large number of fermenters complex pipework would be necessary to transport the sterile medium and the risk of contamination is high.

·         Mechanical failure in a cooker supplying medium to several fermenters will result in the whole system temporarily out of work.

Continuous sterilization

The continuous system includes a time period during which the medium is heated to the sterilization temperature, a holding time at the temperature and a cooling period to restore the medium to the fermentation temperature. The temperature of the medium is elevated in a continuous heat exchanger and is then maintained in an insulated serpentine holding coil for the holding period. The length of holding period depend on the length of the coil and the flow rate of the medium. The hot medium is cooled to the fermentation temperature using two sequential heat exchangers - the first utilizing the coming medium as the cooling source and then using cooling water.

There are two types of continuous sterilizer, the indirect heat exchanger and the direct heat exchanger (steam injector).

The indirect heat exchanger: The double-spiral type and the alternating plate type.

The Indirect double spiral heat exchanger consists of two sheets of high-grade stainless steel which is curved around a central axis to form a double spiral. The ends of the spiral are sealed by covers. Steam is passed through one spiral and medium through the other in countercurrent streams.

Advantages of the spiral heat exchanger are:

(i) The two streams of medium and cooling liquid, or medium and steam, are separated by a continuous stainless steel barrier and thus cross contamination between the two streams does not occur.

(ii) Suitable for media with suspended solids. The exchanger is self-cleaning with low risk of sedimentation.

Indirect plate heat exchangers consist of alternating plates through which countercurrent streams are circulated. The plates are separated by gaskets and there are chances of cross-contamination between the two streams. Also suspended solids in the medium may block the exchanger, so the system is useful for sterilizing completely soluble media only.

The direct heat exchanger – here the continuous steam injector injects steam directly into the unsterile media followed by a flash cooling, where the sterilized medium is cooled by passing it through an expansion valve into a vacuum chamber. The advantages of this method are

(i) Very short heating up times

(ii) suitable for media containing suspended solids

(iii) Low capital cost

(iv) Easy cleaning and maintenance

(v) High steam utilization efficiency

The disadvantages are:

(i) Foaming may occur during heating

(ii) Since the medium comes into direct contact with steam, the steam should be clean and free from any additives and there should be a condense dilution step.

Continuous sterilization -direct heat exchanger 

(Principles of Fermentaion technology, PF Stanburry, A Whittaker, SJ Hall, 1995, Butterworth Heinemann Publications)

Sterilization of the fermenter

If the medium is sterilized in a separate batch cooker, or is sterilized continuously, then the fermenter is sterilized separately. This is done by heating the jacket or coils of the fermenter with steam and sparging steam into the vessel through all entries, and allowing steam to exit slowly through the air outlet. Steam pressure is held at 15 psi for about 20 minutes.

Sterilization of the feeds

A variety of additives which are to be added to the fermentation during the process are sterilized either using the batch or the continuous sterilization process.

Sterilization of wastes

Waste biomass of microorganisms must be sterilized before disposal. Sterilization may be achieved by either batch or continuous system. After sterilization, the effluent must be cooled to below 60°C discharging it to waste.

Filter sterilization

Suspended solids are separated from a fluid during filtration by the following mechanisms,

(i) Inertial impaction - The fluid will flow through the filter through the route of least resistance while due to momentum, suspended particles tend to travel in straight lines and therefore become impacted upon the filter fibres.

(ii) Diffusion - Extremely small particles in a fluid due to Brownian motion deviate from the fluid flow pattern and become impacted upon the filter fibres.

(iii) Electrostatic attraction - Charged particles may be attracted by opposite charges on the surface of the filtration medium.

(iv) Interception - Particles larger than the filter pores are removed by direct interception.

Filters are of two types – those in which the pores in the filter are smaller than the particles which are to be removed and those in which the pores are larger than the particles which are to be removed. The first type is the absolute filter or 'fixed pore filters, which are supposed to be 100% efficient in removing micro-organisms. Second type are depth filters or non-fixed pore filters and are composed of felts, woven yarns, asbestos pads and loosely packed fibre glass. In fixed pore filters, interception is the major mechanism of filtration and non-fixed pore filters remove particles by inertial impaction, diffusion and electrostatic attraction rather than interception.

Filters should be steam sterilized before and after operation and thus the materials must be stable at high temperatures and the steam used for sterilization must be free of particulate matter. Thus the steam is prior filtered through stainless steel mesh filters rated at 1µm.

Filter sterilization of fermentation media

Media for animal-cell culture cannot be sterilized by heat since it contains heat-labile proteins. Thus, filtration is used. An ideal filtration system for the sterilization should have the following criteria:

·         The filtered medium must be free of fungal, bacterial and mycoplasma contamination.

·         There should be minimal adsorption of protein to the filter surface.

·         The filtered medium should be free of viruses and endotoxins.

Absolute filtration systems for sterilization of animal cell culture medium are available as membrane cartridges constructed from steam sterilizable hydrophilic material and they are fitted into steam sterilizable stainless steel modules.

Filter sterilization of air

Aerobic fermentations require large quantities of sterile air.  Air can be sterilized by heat treatment, but the most commonly used method is filtration.

 

References

• Principles of Fermentaion technology, PF Stanburry, A Whittaker, SJ Hall, 1995, Butterworth Heinemann Publications
• Industrial Microbiology, Second Edition, AH Patel, Trinity press

Single Cell Protein

 Single Cell Protein

Single Cell Protein (SCP) are protein derived from microorganisms.  The biomass or protein extract from pure or mixed cultures of algae, yeasts, fungi or bacteria is used as an ingredient or a substitute for protein-rich foods.  This is suitable for human consumption and as animal feeds.

It emerged in the 1950s and 1960s as an alternate and unconventional source of food to bridge the ‘food gap’ between the industrialized and the less industrialized parts of the world, especially as a protein source.

SCP has a number of attractive features:

·         It is not subject to weather variations and can be produced throughout year

·         Microorganisms have a much more rapid growth than plants or animals.

·         Waste products can be turned into SCP.

Disadvantages of SCP

·         Lack of expertise and/or the financial resources to develop fermentation industries in developing countries, where protein malnutrition exists

·         Microorganisms contain high levels of RNA, consumption of which lead to uric acid accumulation, kidney stone formation and gout.

During the First World War, Saccharomyces cerevisiae, were grown on a molasses-ammonium medium. Geotrichum lactis, Endomyces vernalis, and Candida utilis were grown for food.

A wide variety of substrates are used for SCP production such as hydrocarbons, alcohols, and wastes from various sources.

Hydrocarbons

a. Aliphatic hydrocarbons are assimilated by strains of yeasts while other classes of hydrocarbons, including aromatics are not efficiently assimilated.

b. n-Alkanes of chain length shorter than n-nonane are not assimilated and Yield factors increase with increasing chain length.

c. Unsaturated compounds are degraded less readily than saturated ones and branched chain compounds are degraded less readily than straight chain compounds.

Among the gaseous hydrocarbons, methane has been most widely studied as a source of SCP and propane and butane are also studied.   Single cell protein production from methane use continuous cultures and a mixed population of microorganisms and the advantages are higher growth rate, higher yield coefficient, greater resistance to contaminations and a reduction in foaming.

The four-organism mixture is a fast growing mixture - the unnamed methane bacterium utilizes methane and produces methanol, Hyphomicrobium utilizes the methanol and Flavobacterium and Acinetobacter remove waste products.

The major source of liquid hydrocarbons is crude petroleum which is highly variable in composition. The petroleum hydrocarbons which are used to grow SCP are diesel oil, gas oil, fuel oil, n-alkanes (C10 - C30 and C14 – C18, C11 – C18, C10 - C18) n-hexadecane n-dodecane.

Due to the crude oil price rise, the use of crude oil as a substrate for SCP is on a decline.

Methanol is suitable as a substrate for SCP for the following reasons:

(a) it is highly soluble in water

(b) the explosion hazard of methanol is less

(c) it is readily available in a wide range of hydrocarbon sources

(d) it can be readily purified

(e) it requires less oxygen than methane for metabolism by micro-organisms

(f) it is not utilized by many organisms.

Several companies in Italy, West Germany, Norway, Sweden, Israel, the United Kingdom, and the United use methanol as a SCP substrate. Example is Imperial Chemical Industries (ICI) in UK which use the bacterium, Methylophilus methylotropha to produce ‘Pruteen’ using the loop fermentor.

Hansenula, Pichia, Torulopsis and Candida grow on methanol.

Ethanol can be utilized by many bacteria and yeasts and as a substrate for SCP, it is used by yeasts. Ethanol has the following advantages:

(a) It is like methanol, highly miscible with water

(b) Can be more safely stored and transported

(c) Unlike methanol, it is non-toxic it can be more easily handled

(d) Ethanol is partially oxidized, requires less oxygen

The major disadvantage in using ethanol for SCP production is that it is expensive

Candida utilis. Hansenula anomala, Acinetobacter caloaceticum grow using ethanol

Waste Products

Due to the hike in petroleum prices, substrates derived from plants which are renewable are used as substrate for SCP.

(i) Plant/wood wastes: corn cobs, plant stems, leaves, stalks, husks, etc  are cellulose containing materials. Pretreatment such as ball-milling, acid, alkali, sodium chlorate or liquid ammonia treatment is needed to make cellulose susceptible to fermentation and lignin must be broken down. 

(ii) Starch-wastes: Starch-containing wastes from rice, potatoes, or cassava industry can be used for SCP production. Starch hydrolysis is relatively easy.  In Symba Process developed by the Swedish Sugar Corporation, two yeasts are used symbiotically - Endomycopsis fibuligera hydrolyses starch to glucose and maltose and Candida utilis utilizes these sugars.

(iii) Dairy wastes: Whey, by-product of diary industry is liquid rich in lactose.  Saccharomyces fragilis is grown in it to produce either SCP or alcohol.

(iv) Wastes from chemical industries: C. lipolytica or Trichosporon cutaneum can be used for SCP production in oxanone water, a waste mixture of organic acids from the copralactam used for the manufacture of nylon.

(v) Miscellaneous substrates: Molasses, the by-product of the sugar industry is used for production of SCP.  Coffee wastes, coconut wastes, palm-oil wastes, citrus waste, etc can also be used.

Microorganisms used in SCP production

Organisms to be used in SCP production should have the following properties:

(a) Absence of pathogenicity and toxicity

(b) Protein quality and content should be high

(c) Digestibility and organoleptic qualities

(d) Must grow rapidly in a cheap, easily available medium.

(e) Adaptability to unusual environmental conditions such as low pH conditions or high temperature

The heterotrophic microorganisms currently used are bacteria and fungi.  Protozoa are not used in SCP production. The gaseous hydrocarbons are used by bacteria and liquid hydrocarbons and alcohols are utilized by both bacteria and yeasts. Cellulose in peanut shells, carob beans, spoiled fruits, corn and pea wastes, sugarcane bagasse, palm, cassava wastes are used to make SCP using Trichoderma sp., Glicladium sp., Geotrichum sp., Fusarium, and Aspergilus.  Fungi are lower in RNA content and are easily harvested.

Autotrophic organisms such as photosynthetic bacteria and algae are used as SCP. The disadvantage with photosynthetic bacteria is that they require anaerobic conditions for photosynthesis which is difficult to provide and maintain.

Algae have high protein concentration, greater than soya bean and dietary energy from algae is higher than that of sugar beet, corn and potato.  For high algal yields carbon dioxide is supplied to algae growing in day light.  Where saline water rich in bicarbonates is available, supplementation with CO2 is not necessary.  Effluents from sewage treatment are ideal for growing algae for animal feed, where the algae should be heat-treated to avoid any possibility of pathogen transmission.

Algal cultivation is easier and it is highly digestible by ruminants and other animals.

Microbes employed include:

Yeast - Saccharomyces cerevisiae, Pichia pastoris, Candida utilis, Torulopsis coralline, Geotrichum candidum

Fungi - Aspergillus oryzae, Fusarium venenatum, Sclerotium rolfsii, Polyporus, Trichoderma

Bacteria - Methanomones sp., Methylococius capsulatus, Pseudononas sp., Flavobacterium sp. Arthrobacter simplex, Nocardia paraffinica, Nocardia paraffinica, Rhodobacter capsulatus, Rhodopseudomonas glatinosa

Algae – Spirulina, Chlorella

Concerns of using SCP

Due to the novelty of SCP as food receives strong opposition especially in Japan and Italy where the government is concerned with the possibility of the presence of carcinogenic compounds in petroleum-grown SCP, content of nucleic acid in SCP, the polycyclic aromatic hydrocarbons and the presence of n-paraffins, etc

Protein Advisory Group (PAG) formed by WHO in 1955 concluded that low levels of residual alkanes, the presence of odd-number fatty acids, or polycyclic hydrocarbons derived from petroleum do not possess a danger in terms of carcinogenicity or toxicity. They also developed guidelines for the production and nutritional and safety standards of SCP for human consumption.  These include microbiological examination for pathogens and toxin producers, chemical analyses for heavy metals, nucleic acid content, presence of hydrocarbons, safety tests on animals and protein quality studies.

Another problem associated with SCP is the nucleic acid content.  When nucleic acid is eaten by man, it is broken up by nucleases present in the pancreatic juice, and converted into nucleosides by intestinal juices. Guanine and adenine are converted to uric acid. As a result, when foods rich in nucleic acid are consumed in large amounts, uric acid level increases in blood plasma resulting in its deposition in various tissues in the body including the kidneys and the joints and kidney stones and gout may result.

Various methods for the removal of nucleic acids from SCP are

(a) Growth and cell physiology method: The RNA content of cell is dependent on growth rate.  The growth rate is reduced to reduce nucleic acid.

(b) Extraction with chemicals: Dilute bases such as NaOH or KOH will hydrolyze RNA easily. Hot 10% sodium chloride may also be used to extract RNA and the protein may then be extracted, purified and concentrated.

(c) Use of pancreatic juice: RNAase from bovine pancreatic juice, which is heat-stable, can be used to hydrolyze yeast RNA at 80°C.   At this temperature the cells are more permeable.

(d) Activation of endogenous RNAase: The RNAase of the organism is activated by heat-shock or by chemicals to reduce the RNA content of yeasts.

The nutritional value of SCP depends on the composition of the microbial cells used, especially their protein, amino acid, vitamin, and mineral contents. SCP derived from bacteria and yeasts is deficient in methionine. Glycine and methionine are sometimes deficient in molds. These can be improved by supplementation with small amounts of animal proteins.

 

References

1. Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers

 

Autoimmune diseases

Autoimmune diseases

Autoimmune diseases result when the body's immune system turns against itself and mistakenly attacks healthy cells.  An autoimmune disease is a pathological state due to an abnormal immune response of the body to substances and tissues that are normally present in the body.  Structural or functional damage is produced by the action of immunologically competent cells or antibody directed against the normal components of the body

Autoimmunity is the presence of self-reactive immune response (e.g., auto-antibodies, self-reactive T-cells), with or without damage or pathology resulting from it.

For a disease to be regarded as an autoimmune disease it needs to follow Witebsky's postulates

·         Direct evidence from transfer of disease-causing antibody or disease-causing T lymphocyte

·         Indirect evidence based on reproduction of the autoimmune disease in experimental animals

·         Circumstantial evidence from clinical clues

Features of autoimmune diseases

·         Increased levels of immunoglobulin

·         Presence of auto antibodies

·         Deposition of antibodies or their derivatives at site of disease

·         Accumulation of lymphocytes and plasma cells at the site of disease

·         Benefit from immunosuppressive therapy

·         Occurrence of more than one type of auto immune lesions

·         Genetic predisposition

·         Chronic and generally irreversible

·         More incidence among females

Human autoimmune diseases can be divided into organ specific and systemic diseases. The organ-specific diseases involve an autoimmune response directed primarily against a single organ or gland. The systemic diseases are directed against a broad spectrum of tissues and have manifestations in a variety of organs.  Sometimes the damage to self-cells or organs is caused by antibodies; in other cases, T cells are the culprit.

Organ-Specific Autoimmune Diseases: Here the immune response is directed to a target antigen unique to a single organ, so that the manifestations are limited to that organ. The cells of the target organs may be damaged directly by humoral or cell-mediated mechanisms. In some cases, the antibodies may overstimulate or block the normal function of the target organ.

Systemic Autoimmune Diseases: Here, the response is directed toward a broad range of target antigens and involves a number of organs and tissues. These diseases reflect a general defect in immune regulation that results in hyperactive T cells and B cells. Tissue damage is widespread, from cell mediated immune responses and auto-antibodies or by accumulation of immune complexes.

Organ-Specific Autoimmune Diseases

Disease	Self-antigen	Immune response
Goodpasture’s syndrome	Renal and lung basement membranes	Auto-antibodies
Graves’ disease	Thyroid-stimulating hormone receptor	Auto-antibody (stimulating)
Hashimoto’s thyroiditis	Thyroid proteins and cells	TDTH cells, auto-antibodies
Idiopathic thrombocytopenia purpura	Platelet membrane proteins	Auto-antibodies
Insulin-dependent diabetes mellitus	Pancreatic beta cells	TDTH cells, auto-antibodies
Myasthenia gravis	Acetylcholine receptors	Auto-antibody (blocking)
Myocardial infarction	Heart	Auto-antibodies
Pernicious anemia	Gastric parietal cells; intrinsic factor	Auto-antibody
Poststreptococcal glomerulonephritis	Kidney	Antigen-antibody complexes
Spontaneous infertility	Sperm	Auto-antibodies
Addison’s disease	Adrenal cells	Auto-antibodies
Autoimmune hemolytic anemia	RBC membrane proteins	Auto-antibodies

 

Systemic Autoimmune Diseases

Disease	Self-antigen	Immune response
Ankylosing spondylitis	Vertebrae	Immune complexes
Multiple sclerosis	Brain or white matter	TH1 cells and TC cells, auto-antibodies
Rheumatoid arthritis	Connective tissue, IgG	Auto-antibodies, immune complexes
Scleroderma	Nuclei, heart, lungs, gastrointestinal tract, kidney	Auto-antibodies
Sjogren’s syndrome	Salivary gland, liver, kidney, thyroid	Auto-antibodies
Systemic lupus erythematosus (SLE)	DNA, nuclear protein, RBC and platelet membranes	Auto-antibodies, immune complexes

 

Mechanisms for Induction of Autoimmunity

A variety of mechanisms have been proposed and it is likely that autoimmunity does not develop from a single event but rather from a number of different events.

In addition, susceptibility to many autoimmune diseases differs between the two sexes.

1.      Release of Sequestered Antigens

The induction of self-tolerance in T cells results from exposure of immature thymocytes to self-antigens and the subsequent clonal deletion of self-reactive clones. Any tissue antigens that are sequestered from the circulation, and are therefore not seen by the developing T cells in the thymus, will not induce self-tolerance. Exposure of mature T cells to such normally sequestered antigens at a later time might result in their activation.

For example, sperm arise late in development and are sequestered from the circulation. When some sperm antigens are released into the circulation they can induce auto-antibody formation in men. Similarly, the release of lens protein after eye damage or of heart-muscle antigens after myocardial infarction has been shown to lead to the formation of auto-antibodies.

2.      Molecular Mimicry or cross reacting antigens

A pathogen may express a region of protein that resembles a particular self-component in conformation or primary sequence.  Such molecular mimicry appears in a wide variety of organisms.  Molecular mimicry has been suggested as one mechanism of autoimmunity.  One of the best example is post-rabies encephalitis, which develop in some individuals who had received the rabies vaccine if the preparations of the vaccine included antigens derived from the rabbit brain cells. In a vaccinated person, these rabbit brain-cell antigens could induce formation of antibodies and activated T cells, which could cross-react with the recipient’s own brain cells, leading to encephalitis.

Cross-reacting antibodies are also thought to be the cause of heart damage in rheumatic fever, which can sometimes develop after a Streptococcus infection. In this case, the antibodies are to streptococcal antigens, but they cross-react with the heart muscle.

3.      Inappropriate Expression of Class II MHC Molecules

The pancreatic beta cells of individuals with insulin-dependent diabetes mellitus (IDDM) express high levels of both class I and class II MHC molecules, whereas healthy beta cells express lower levels of class I and do not express class II at all.

Similarly, thyroid acinar cells from those with Graves’ disease have been shown to express class II MHC molecules on their membranes.

4.      Polyclonal B-Cell Activation

A number of viruses and bacteria can induce nonspecific polyclonal B-cell activation. Gram-negative bacteria, cytomegalovirus, and Epstein-Barr virus (EBV) are examples.  If B cells reactive to self-antigens are activated by this mechanism, auto-antibodies can appear. For instance, during infectious mononucleosis, which is caused by EBV, a variety of auto-antibodies are produced, including autoantibodies reactive to T and B cells, rheumatoid factors, and antinuclear antibodies.

5.      Antigenic alteration or formation of neo antigens

This may be due to any physical, chemical or biological influence.  Physical influence includes radiation, photosensitization or cold which may modify the antigen to be immunogenic in the individual while chemical agents include drugs.  Antigenic change due tomicrobial infection, mutation, etc comes under biological alteration.

6.      Defects in the idiotype-anti idiyotype network

Autoimmune Diseases Mediated by Direct Cellular Damage or by Stimulating or Blocking Auto-Antibodies.  Autoimmune diseases involving direct cellular damage occur when lymphocytes or antibodies bind to cell-membrane antigens and cause cellular lysis and/or an inflammatory response in the affected organ. Gradually, the function of the organ declines.  Examples of this type of autoimmune disease are Hashimoto’s Thyroiditis, Goodpasture’s Syndrome. 

In some autoimmune diseases, antibodies act as agonists, binding to hormone receptors in stead of the normal ligand and stimulating inappropriate activity. This usually leads to an overproduction of mediators or an increase in cell growth.  Conversely, auto-antibodies may act as antagonists, binding hormone receptors but blocking receptor function. This generally causes impaired secretion of mediators and gradual atrophy of the affected organ.  Examples are Graves’ Disease and Myasthenia Gravis.

Autoimmune diseases are classified into four types

        I.            Hemocytolytic autoimmune diseases

     II.            Localized or organ specific autoimmune diseases

  III.            Systemic autoimmune diseases

  IV.            Transitory autoimmune diseases

I. Hemocytolytic autoimmune diseases

1.      Autoimmune hemolytic anemia: An individual with autoimmune hemolytic anemia will be having auto-antibody to RBC antigens.  This results in complement mediated lysis or antibody-mediated opsonization and phagocytosis of the red blood cells.

2.      Auto immune thrombocytopenia: antibodies formed against platelets. This condition can be seen in idiopathic thrombocytopenic purpura. Symptoms include epistaxis (bleeding from the nose), bleeding gums, poor clotting, hematuria etc.

3.      Autoimmune leucopenia: antibodies are formed against leucocytes and results in decreased leucocyte count

II. Localized or organ specific autoimmune diseases

1.      Hashimoto’s Thyroiditis (lymphadenoid goitre)

In Hashimoto’s thyroiditis, an individual produces auto-antibodies and sensitized TH1 cells specific against thyroid antigens. There will be intense infiltration of the thyroid gland by lymphocytes, macrophages, and plasma cells, which form lymphocytic follicles and germinal centers, causes a goiter, or visible enlargement of the thyroid gland.

Antibodies are formed to thyroid proteins, such as thyroglobulin and thyroid peroxidase and binding of the auto-antibodies to these proteins interferes with iodine uptake and leads to decreased production of thyroid hormones (hypothyroidism).

2.      Pernicious anemia

This is caused by auto-antibodies to intrinsic factor, a membrane-bound intestinal protein on gastric parietal cells. Binding of the auto-antibody to intrinsic factor blocks the intrinsic factor–mediated absorption of vitamin B12. In the absence of sufficient vitamin B12, hematopoiesis decreases and the number of mature red blood cells decrease.  Pernicious anemia is treated with injections of vitamin B12.

3.      Goodpasture’s Syndrome

In Goodpasture’s syndrome, auto-antibodies specific for basement-membrane antigens bind to the basement membranes of the kidney glomeruli and the alveoli of the lungs. Subsequent complement activation leads to direct cellular damage and inflammatory response. This leads to progressive kidney damage and pulmonary hemorrhage. Death may ensue within several months of the onset of symptoms. 

4.      Insulin-Dependent Diabetes Mellitus

IDDM is caused by an autoimmune attack on the pancreas against insulin-producing cells (beta cells) that are located in the islets of Langerhans. The autoimmune attack through CTL, autoantibodies, lytic enzymes from macrophages, destroys beta cells, resulting in decreased production of insulin and consequently increased levels of blood glucose.

The most common therapy for diabetes is daily administration of insulin.

5.      Graves’ Disease or Thyrotoxicosis

Thyroid-stimulating hormone (TSH), produced by the pituitary gland regulate the production of thyroid hormones. Binding of TSH to a receptor on thyroid cells stimulates the synthesis of two thyroid hormones, thyroxine and triiodothyronine.

A patient with Graves’ disease produces auto-antibodies that bind the receptor for TSH and mimic the normal action of TSH, resulting in production of the thyroid hormones. Unlike TSH, however, the autoantibodies are not regulated, and consequently they overstimulate the thyroid and are called long-acting thyroid-stimulating (LATS) antibodies.

6.      Myasthenia Gravis

Here, the patient produces auto-antibodies that bind the acetylcholine receptors on the muscles, blocking the normal binding of acetylcholine.  This also induces complement mediated lysis of the cells. So there will be progressive weakening of the skeletal muscles. The early signs of this disease include drooping eyelids and inability to retract the corners of the mouth, which gives the appearance of snarling.  Further it lead to severe impairment of eating as well as problems with movement.

7.      Addison’s disease

Affected organ is adrenal glands.  There will be lymphocytic infilteration of adrenal gland and circulating antibody against zona glomerulosa.  Also called adrenal insufficiency / hypocortisolism. Symptoms include weight loss, muscle weakness, darkness of skin.

8.      Autoimmune diseases of eye

Phacoanaphylaxis is intraoccular inflammation after cataract surgery which is due to formation of antibody against lens proteins

Sympathetic ophthalmia is inflammation of eye following Perforating injuries of eye. 

9.      Auto immune diseases of nervous system

Rabies vaccinanation sometimes leads to injury of nervous system. This is due to formation of antibodies against sheep nervous tissue, which cross reacts with human nervous tissue.

10.  Autoimmune diseases of skin

Mainly three types, 1. Pemphigus vulgaris 2. Bullous pemphigoid 3. Dermatitis herpatiformis.

Pemphigus vulgaris: It is characterized by blister formation on skin & mucosa. This disease is due to formation of Antibodies against intercellular cement substance.

Bullous pemphigoid: This disease is due to formation of Antibodies against dermal epithelia junction. Symptoms are eczema, rashes, hemorrhagic blisters, increased skin pigmentation & inflammation.

Dermatitis herpetiformis:  It is a rare auto immune disease of skin characterized by papules & vesicles.  Antibody not known

11.  Autoimmune orchitis

This disease is followed by mumps orchitis.  There will be lymphocytic infiltration of testes and circulating antibody against sperm and germinal cells.  The autoimmune reaction results in infertility. 

III. Systemic Autoimmune Diseases

Here, the autoimmune response is directed toward a broad range of target antigens and involves a number of organs and tissues. Tissue damage is from cell mediated immune responses and from direct cellular damage caused by auto-antibodies or by accumulation of immune complexes.

1.      Systemic Lupus Erythematosus

One of the best examples of a systemic autoimmune disease is systemic lupus erythematosus (SLE), which typically appears in women between 20 and 40 years of age. SLE is characterized by fever, weakness, arthritis, skin rashes, pleurisy, and kidney dysfunction. a rash on the cheeks and nose, which is called a “butterfly rash” will be seen.  There will be autoantibodies to several tissue antigens, such as DNA, histones, RBCs, platelets, leukocytes, and clotting factors. 

Auto-antibody specific for RBCs and platelets lead to complement-mediated lysis, resulting in hemolytic anemia and thrombocytopenia, respectively.

Immune complexes of auto-antibodies with various nuclear antigens cause a type III hypersensitive reaction in blood vessels resulting in vasculitis and glomerulonephritis.

Laboratory diagnosis of SLE is through detection of antinuclear antibodies, which are directed against double stranded or single-stranded DNA, nucleoprotein, histones, and nucleolar RNA.

An LE cell is generally observed.  This is a neutrophil or macrophage that has a large pale homogenous body known as LE body.  LE body is immunologically damaged nucleus of a leucocyte. 

2.      Multiple Sclerosis

Multiple sclerosis (MS) is a neurologic disability.  The symptoms may be mild, such as numbness in the limbs, or severe, such as paralysis or loss of vision.  Most people with MS are diagnosed between the ages of 20 and 40. Individuals with this disease produce autoreactive T cells that cause inflammatory lesions along the myelin sheath of nerve fibers. The cerebrospinal fluid of patients contains activated T lymphocytes, which infiltrate the brain tissue and cause characteristic inflammatory lesions, destroying the myelin. Breakdown in the myelin sheath leads to numerous neurologic dysfunctions.

3.      Rheumatoid Arthritis

Rheumatoid arthritis often affects women from 40 to 60 years old. The major symptom is chronic inflammation of the joints, although the hematologic, cardiovascular, and respiratory systems are also frequently affected. Individuals with rheumatoid arthritis produce a group of auto-antibodies called rheumatoid factors (RF).  This is generally an IgM antibody.  These are reactive with the Fc region of IgG.  IgM-IgG complexes are formed and deposited in the joints. These immune complexes can activate the complement cascade, resulting in a type III hypersensitive reaction, which leads to chronic inflammation of the joints.

4.      Scleroderma

This is also known as systemic sclerosis.  It is a chronic systemic autoimmune disease characterised by hardening (sclero) of the skin (derma) and may also affects internal organs.

Limited scleroderma - mainly affect the hands, arms and face. It is also called CREST syndrome as an acronym of the following manifestations

·         Calcinosis (the deposition of calcium nodules in skin)

·         Raynaud's phenomenon (vasoconstriction in hand)

·         Esophageal dysfunction (difficulty swallowing)

·         Sclerodactyly (skin thickening on fingers)

·         Telangiectasias (dilated capillaries on the face, hands and mucous membranes).

Diffuse scleroderma is rapidly progressing and affects a large area of the skin and one or more internal organs, frequently the kidneys, esophagus, heart and/or lungs.

5.      Ankylosing spondylitis

This is a type of arthritis with long term inflammation of the joints of the spine and where the spine joins the pelvis. Occasionally other joints such as the shoulders or hips are involved. Eye and bowel problems may also occur.

6.      Polyarteritis nodosa

There will be necrotizing angitis of medium sized arteries resulting in coronary thrombosis, cerebral hemorrhage and gastrointestinal bleeding. 

7.      Sjogren's syndrome

This is a long-term autoimmune disease in which the moisture-producing glands of the body are affected.  This cause dry mouth and dry eyes, dry skin, a chronic cough, vaginal dryness, numbness in the arms and legs, muscle and joint pains, etc. 

IV. Transitory autoimmune diseases

This includes anemia, thrombocytopenia and nephritis that follow microbial infections or drug therapy. This is a transient disease and undergo spontaneous cure when the infection subsides or the drug is withdrawn