Inducing tissue damage and inflammation to eliminate disease is a long established clinical concept. Historically, Hippocrates is claimed to have said, “Give me the power to induce fever, and I cure all diseases.” At the turn of the last century, William B Coley, a surgeon at Memorial Sloan Kettering Hospital, New York (from 1892 to 1936) used a mixed bacterial vaccine (Coley’s toxin) in a systemic way to treat patients with metastatic cancer.
The Nobel Prize for Physiology or Medicine was shared in 1960 by Peter Medawar and Frank MacFarlane Burnet for their 1950’s work on the ‘self non-self’ theory of immune recognition. In 1989, Charles Janeway postulated the ‘infectious non-self’ model and later proposed that the activation of the adaptive immune response is controlled by the evolutionarily older innate immune system. He suggested the principles that might underlie this control.
In 1994, the ‘danger theory’, proposed by Polly Matzinger, brought a new insight in to adaptive and innate immunity. In 2011, the Nobel Prize for Physiology or Medicine was awarded to Ralph Steinman, “for his discovery of the dendritic cell and its role in adaptive immunity” and to Bruce Beutler and Jules Hoffmann for their “discoveries concerning the activation of innate immunity” by receptor proteins.
In a published interview in May 2012, Polly Matzinger describes the evolution of the ‘danger theory’, beginning with the questions as to why mothers’ immune systems do not kill their fetuses; why your immune system does not turn against you at puberty when you change; why newly lactating breasts do not experience severe immune reactions to the newly produced milk proteins.
In 2012, a BBC Horizon documentary, ‘Turned On By Danger’ and other appearances at international meetings brought Polly Matzinger’s ‘danger theory’ again to the fore.
The ‘danger theory’ has caused support and criticism in equal measure, particularly when it was first proposed. But 20 years on, the theory has support from some new experimental approaches. This model is of relevance in the understanding of immune recognition, innate immunity, immune tolerance, transplant immunology and with the increased interest in immune therapy to treat malignancy, to understanding more about immunity to cancer.
The use of the term ‘damage theory’ to replace the more emotive term ‘danger theory’ could benefit what is a very challenging model for the understanding of some of the complexities of our human immune response.
THE IMPORTANCE OF THE INNATE IMMUNE RESPONSE
The cellular immunologist, Charles Janeway will be remembered for his work on the innate immune system, which he believed to be the controller of immunity. At a time when most research laboratories were interested only in antigen recognition, Janeway asked the very basic question of why was the mere ‘foreignness’ of a protein not enough to elicit immunity and why did we need to add adjuvant (damaging substances like mineral oil, LPS, mycobacteria or aluminum hydroxide) in order to get a response to a vaccine.
The ‘self non-self’ model neither predicted nor explained the need for adjuvant. Janeway called this the “immunologists’ dirty little secret”. He slowly reached an important understanding that caused a major switch in immunological thinking; that immune responses could not occur unless antigen-presenting cells were first activated. Antigen-presenting cells (APC) were activated via pattern recognition receptors (PRRs) that recognized evolutionarily conserved molecules on infectious non-self organisms. Janeway believed that the immune system’s default state is ‘off’ and that it can be ‘turned on’ by bacteria.
PRRs are found on the surface of APCs, but can also be found in cell cytoplasm and incorporated in the membrane of endo-lysosomes. Stimulation of PRRs leads to activation of the APC to process antigen, up-regulate expression of co-stimulatory molecules and to present antigen to T-helper cells.
When a PRR associated with an APC binds to its corresponding PAMP, the cell begins to efficiently present antigen (Signal 1), upregulate the expression of co-stimulatory molecules (Signal 2), and elaborate cytokines (known as Signal 3) that guide the formation of adaptive cells ready to respond to the inciting pathogen.
Pattern recognition receptors (PRRs) include:
The PRRs on antigen-presenting cells (APCs) also recognize ‘alarmins’ which are released during tissue damage. These ‘alarmins’ include ‘danger-associated molecular patterns’ (DAMPs) released following tissue injury or stress and ‘pathogen-associated molecular patterns’ (PAMPs).
‘THE HIDDEN SELF MODEL’ OF DAMAGE SIGNALS: ALARMINS
The ‘danger theory’ postulates that the immune system is triggered by ‘danger signals’ or ‘alarmins’, DAMPs and PAMPs.
The release of DAMPs in the extracellular environment functions as a sign of cell death to the innate immune system; this then triggers a pro-inflammatory response. This view, which elaborates on the danger theory, has been called the ‘hidden self model’.
When cells die by necrosis, they lose the integrity of their plasma membrane and therefore they release their intracellular contents in to the extracellular matrix, ialong with DAMPs. This is different to cells that die by apoptosis and which are cleared rapidly.
‘Alarmins’ include:
CHALLENGES TO THE ‘DANGER THEORY’
‘Danger’ is difficult to define in immunological terms. Matzinger and her colleagues use several terms that they interpret as equivalent: ‘danger’, ‘damage’, ‘stress’, ‘injury’. But how does the notion of ‘danger’ explain the immune responses to innocuous antigens such as allergens?
The theory is much more precise if its central claim is that every immune response is due to damages to the organism’s cells or tissues. It is easier to define what a ‘damage’ is (for an organism, a tissue or a cell) than what a ‘danger’ is. In fact, this is the interpretation that Matzinger now proposes when she describes the molecular details of her theory.
Allergy, including asthma, has been proposed as a challenge for the danger hypothesis. However, Matzinger has suggested that the ‘allergen’ in individuals who are atopic has one or more of the following properties:
As Matzinger suggests, the claim that immune responses are due to ‘danger’ was merely a theoretical suggestion, while the idea that they are due to ‘damage’ has given rise to several experimental investigations. Therefore, in order to assess the ‘danger theory’, the most important investigations have been done to define damage signals.
‘THE DANGER MODEL’: IMPLICATIONS FOR IMMUNE THERAPY
In vitro studies have confirmed that if injected with an antigen, any endogenous activating substances can function as a natural ‘adjuvant’, which may stimulate a primary immune response, such as is seen in transplant rejection, tumour rejection and some types of autoimmunity.
Matzinger’s three rules for T-cells should be noted, because of their practical implications:
For transplant and tumour immune therapy, this means that T-cells will delete if they bind antigen but do not get co-stimulated by FDCs. Tumour ‘vaccines’ may need to be repeatedly boosted. Adoptive T-cell therapy (ACT) may need to be given repeatedly if the tumour cells are to be destroyed by immune therapy.
Initially, to explain the immune response to cancer cells, according to Matzinger, “the danger model suggests that no immune response occurs because tumours are healthy, growing cells that do not normally die necrotically or send out alarm signals.”
Even though tumour cells possess new surface antigens, according to the ‘danger theory’, the host regards the proliferating tumour as successful and not as a threat, as long as tumour cells die by apoptosis. By the time the tumour has outgrown its blood supply and cells within the tumour begin to die by hypoxic necrosis, the tumour may be too large for the immune system to destroy.
Thus cancer ‘vaccines’ do not obey the same rules as vaccines used for prevention of infectious disease; the ‘danger theory’ has the following implications for new cancer vaccines:
There are several reasons why the tumour vaccine still may not work:
Firstly, after the vaccine boost, the T-cells do what they are programmed to do, which is kill one round of tumour cells, then go into a resting ‘memory’ state. The killing done by a cytotoxic T-cell is apoptotic; it does not cause the release of alarm signals so it does not boost the response, which consequently displays typical immune response kinetics and wanes after about 2 weeks. All the killer cells that were activated by the vaccination go back into a resting ‘memory’ state. The tumour will continue to grow. So you would need to keep boosting.
Second, even if you have an activated immune system, it will only locate to certain places, so you need to do ‘damage’ to the tumour to direct the activated T-cells there. Also, if the endothelial cells in the tumour are not activated, the T-lymphocytes are not going to migrate to the tumour.
Third, tissues have ways of communicating with the immune system so that a local immune response clears a pathogen without destroying the tissue itself. A tumour will also have mechanisms to prevent immune-mediated destruction. With application of tumour vaccines, there needs to be a way to overcome the normal tissue signals that instruct the immune system to produce a non-destructive response, to give a strong strong Th1 or delayed-type hypersensitivity killer response to the tumour. The problem is that the signals that the tissues use to educate the immune system are poorly understood and as yet it is not known how to overcome them.
Fourth, if the patient has no remaining tumour-specific T-cells, the vaccine will have no effect, unless the patient is given tumour-specific T-cells with the vaccine.
To overcome some of these practical issues, adoptive T-cell therapy (ACT) has evolved as a new approach in immune therapeutics. The method is to collect naïve T-cells from peripheral blood and engineer them to express cancer antigen-specific receptors, including chimeric antigen receptors (CARs). This approach by-passes the need for APCs. This is an approach that could be applicable to all forms of cancer.
CONCLUSION
Sixty years on from the ‘self non-self’ theory of immune recognition and 20 years on from the beginning of the evolving ‘danger theory’, this is an important time for a ‘unified’ theory of immune recognition, based on scientific data. The importance of this unification is demonstrated by the increasing investment in, and hopes for, immune therapy for diseases such as cancer.
The Nobel Prize for Physiology or Medicine was shared in 1960 by Peter Medawar and Frank MacFarlane Burnet for their 1950’s work on the ‘self non-self’ theory of immune recognition. In 1989, Charles Janeway postulated the ‘infectious non-self’ model and later proposed that the activation of the adaptive immune response is controlled by the evolutionarily older innate immune system. He suggested the principles that might underlie this control.
In 1994, the ‘danger theory’, proposed by Polly Matzinger, brought a new insight in to adaptive and innate immunity. In 2011, the Nobel Prize for Physiology or Medicine was awarded to Ralph Steinman, “for his discovery of the dendritic cell and its role in adaptive immunity” and to Bruce Beutler and Jules Hoffmann for their “discoveries concerning the activation of innate immunity” by receptor proteins.
In a published interview in May 2012, Polly Matzinger describes the evolution of the ‘danger theory’, beginning with the questions as to why mothers’ immune systems do not kill their fetuses; why your immune system does not turn against you at puberty when you change; why newly lactating breasts do not experience severe immune reactions to the newly produced milk proteins.
In 2012, a BBC Horizon documentary, ‘Turned On By Danger’ and other appearances at international meetings brought Polly Matzinger’s ‘danger theory’ again to the fore.
The ‘danger theory’ has caused support and criticism in equal measure, particularly when it was first proposed. But 20 years on, the theory has support from some new experimental approaches. This model is of relevance in the understanding of immune recognition, innate immunity, immune tolerance, transplant immunology and with the increased interest in immune therapy to treat malignancy, to understanding more about immunity to cancer.
The use of the term ‘damage theory’ to replace the more emotive term ‘danger theory’ could benefit what is a very challenging model for the understanding of some of the complexities of our human immune response.
THE IMPORTANCE OF THE INNATE IMMUNE RESPONSE
The cellular immunologist, Charles Janeway will be remembered for his work on the innate immune system, which he believed to be the controller of immunity. At a time when most research laboratories were interested only in antigen recognition, Janeway asked the very basic question of why was the mere ‘foreignness’ of a protein not enough to elicit immunity and why did we need to add adjuvant (damaging substances like mineral oil, LPS, mycobacteria or aluminum hydroxide) in order to get a response to a vaccine.
The ‘self non-self’ model neither predicted nor explained the need for adjuvant. Janeway called this the “immunologists’ dirty little secret”. He slowly reached an important understanding that caused a major switch in immunological thinking; that immune responses could not occur unless antigen-presenting cells were first activated. Antigen-presenting cells (APC) were activated via pattern recognition receptors (PRRs) that recognized evolutionarily conserved molecules on infectious non-self organisms. Janeway believed that the immune system’s default state is ‘off’ and that it can be ‘turned on’ by bacteria.
PRRs are found on the surface of APCs, but can also be found in cell cytoplasm and incorporated in the membrane of endo-lysosomes. Stimulation of PRRs leads to activation of the APC to process antigen, up-regulate expression of co-stimulatory molecules and to present antigen to T-helper cells.
When a PRR associated with an APC binds to its corresponding PAMP, the cell begins to efficiently present antigen (Signal 1), upregulate the expression of co-stimulatory molecules (Signal 2), and elaborate cytokines (known as Signal 3) that guide the formation of adaptive cells ready to respond to the inciting pathogen.
Pattern recognition receptors (PRRs) include:
- Toll-like receptors (TLR);
- Nucleotide oligomerization domain (NOD)-like receptors;
- Retinoic acid inducible gene-1 (RIG-1)-like receptors;
- C-type lectin-like receptors (CTLRs).
The PRRs on antigen-presenting cells (APCs) also recognize ‘alarmins’ which are released during tissue damage. These ‘alarmins’ include ‘danger-associated molecular patterns’ (DAMPs) released following tissue injury or stress and ‘pathogen-associated molecular patterns’ (PAMPs).
‘THE HIDDEN SELF MODEL’ OF DAMAGE SIGNALS: ALARMINS
The ‘danger theory’ postulates that the immune system is triggered by ‘danger signals’ or ‘alarmins’, DAMPs and PAMPs.
The release of DAMPs in the extracellular environment functions as a sign of cell death to the innate immune system; this then triggers a pro-inflammatory response. This view, which elaborates on the danger theory, has been called the ‘hidden self model’.
When cells die by necrosis, they lose the integrity of their plasma membrane and therefore they release their intracellular contents in to the extracellular matrix, ialong with DAMPs. This is different to cells that die by apoptosis and which are cleared rapidly.
‘Alarmins’ include:
- Heat shock proteins (HSP)
- RNA
- DNA
- High-mobility group protein B1
- Reactive oxygen molecules
- Hyaluronic acid
- Serum amyloid A protein
- ATP
- Uric acid
- Cytokines including: a-interferon, interleukin-1b, CD40L.
- LPS
- Cardiolipin
- Mitochondria
- ‘Hydrophobicity’
CHALLENGES TO THE ‘DANGER THEORY’
‘Danger’ is difficult to define in immunological terms. Matzinger and her colleagues use several terms that they interpret as equivalent: ‘danger’, ‘damage’, ‘stress’, ‘injury’. But how does the notion of ‘danger’ explain the immune responses to innocuous antigens such as allergens?
The theory is much more precise if its central claim is that every immune response is due to damages to the organism’s cells or tissues. It is easier to define what a ‘damage’ is (for an organism, a tissue or a cell) than what a ‘danger’ is. In fact, this is the interpretation that Matzinger now proposes when she describes the molecular details of her theory.
Allergy, including asthma, has been proposed as a challenge for the danger hypothesis. However, Matzinger has suggested that the ‘allergen’ in individuals who are atopic has one or more of the following properties:
- it does damage, or;
- it is packaged with something that does damage, or
- it mimics an ‘alarm’ system.
As Matzinger suggests, the claim that immune responses are due to ‘danger’ was merely a theoretical suggestion, while the idea that they are due to ‘damage’ has given rise to several experimental investigations. Therefore, in order to assess the ‘danger theory’, the most important investigations have been done to define damage signals.
‘THE DANGER MODEL’: IMPLICATIONS FOR IMMUNE THERAPY
In vitro studies have confirmed that if injected with an antigen, any endogenous activating substances can function as a natural ‘adjuvant’, which may stimulate a primary immune response, such as is seen in transplant rejection, tumour rejection and some types of autoimmunity.
Matzinger’s three rules for T-cells should be noted, because of their practical implications:
- T-cells bind antigen (Signal 1) but,
- T-cells are deleted (die) if they are not activated by antigen-presenting cells (APC) (Signal 2);
- T-cells only stay activated for about 14 days, after which they die or return to a resting state, requiring both Signals 1 and Signal 2 to be re-activated.
For transplant and tumour immune therapy, this means that T-cells will delete if they bind antigen but do not get co-stimulated by FDCs. Tumour ‘vaccines’ may need to be repeatedly boosted. Adoptive T-cell therapy (ACT) may need to be given repeatedly if the tumour cells are to be destroyed by immune therapy.
Initially, to explain the immune response to cancer cells, according to Matzinger, “the danger model suggests that no immune response occurs because tumours are healthy, growing cells that do not normally die necrotically or send out alarm signals.”
Even though tumour cells possess new surface antigens, according to the ‘danger theory’, the host regards the proliferating tumour as successful and not as a threat, as long as tumour cells die by apoptosis. By the time the tumour has outgrown its blood supply and cells within the tumour begin to die by hypoxic necrosis, the tumour may be too large for the immune system to destroy.
Thus cancer ‘vaccines’ do not obey the same rules as vaccines used for prevention of infectious disease; the ‘danger theory’ has the following implications for new cancer vaccines:
- For a vaccine to be successful, according to the ‘danger theory’, there needs to be a tumour against which the host still has a few tumour-specific T-cells. Because an early growing tumour is a healthy tissue, not sending alarm signals, it is therefore constantly inducing tolerance to itself.
- Tumour vaccination may increase the number of the few remaining tumour-specific T-cells which are present. If these T-cells do exist, repeated vaccine boosts would be required to expand this population further.
There are several reasons why the tumour vaccine still may not work:
Firstly, after the vaccine boost, the T-cells do what they are programmed to do, which is kill one round of tumour cells, then go into a resting ‘memory’ state. The killing done by a cytotoxic T-cell is apoptotic; it does not cause the release of alarm signals so it does not boost the response, which consequently displays typical immune response kinetics and wanes after about 2 weeks. All the killer cells that were activated by the vaccination go back into a resting ‘memory’ state. The tumour will continue to grow. So you would need to keep boosting.
Second, even if you have an activated immune system, it will only locate to certain places, so you need to do ‘damage’ to the tumour to direct the activated T-cells there. Also, if the endothelial cells in the tumour are not activated, the T-lymphocytes are not going to migrate to the tumour.
Third, tissues have ways of communicating with the immune system so that a local immune response clears a pathogen without destroying the tissue itself. A tumour will also have mechanisms to prevent immune-mediated destruction. With application of tumour vaccines, there needs to be a way to overcome the normal tissue signals that instruct the immune system to produce a non-destructive response, to give a strong strong Th1 or delayed-type hypersensitivity killer response to the tumour. The problem is that the signals that the tissues use to educate the immune system are poorly understood and as yet it is not known how to overcome them.
Fourth, if the patient has no remaining tumour-specific T-cells, the vaccine will have no effect, unless the patient is given tumour-specific T-cells with the vaccine.
To overcome some of these practical issues, adoptive T-cell therapy (ACT) has evolved as a new approach in immune therapeutics. The method is to collect naïve T-cells from peripheral blood and engineer them to express cancer antigen-specific receptors, including chimeric antigen receptors (CARs). This approach by-passes the need for APCs. This is an approach that could be applicable to all forms of cancer.
CONCLUSION
Sixty years on from the ‘self non-self’ theory of immune recognition and 20 years on from the beginning of the evolving ‘danger theory’, this is an important time for a ‘unified’ theory of immune recognition, based on scientific data. The importance of this unification is demonstrated by the increasing investment in, and hopes for, immune therapy for diseases such as cancer.
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