T-cells &Immunological Tolerance

Chapter 10.

Tolerance• Our own bodies produce some 100,000

different proteins and one of the longstanding conundrums of immunology has been to understand how the immune system produces a virtual repertoire against pathogens while at the same time avoiding reacting to self.

• The strict definition of immunological tolerance occurs when an immunocompetent host fails to respond to an immunogenic challenge with a specific antigen.

Tolerance

• The mechanisms the immune system uses to ensure the absence of self-reactivity (autoimmunity) include:

• Central Tolerance - this occurs during lymphocyte development.

• Peripheral Tolerance - occurs after lymphocytes leave the primary lymphoid organs.

Central Tolerance

Peripheral Tolerance

TOLERANCE 

Introduction• Tolerance refers to the specific

immunological non-reactivity to an antigen resulting from a previous exposure to the same antigen.

• While the most important form of tolerance is non-reactivity to self antigens, it is possible to induce tolerance to non-self antigens. When an antigen induces tolerance, it is termed tolerogen.

TOLERANCE 

• Tolerance is different from non-specific immunosuppression and immunodeficiency. It is an active antigen-dependent process in response to the antigen.

• Like immune response, tolerance is specific and like immunological memory, it can exist in T-cells, B cells or both and like immunological memory, tolerance at the T cell level is longer lasting than tolerance at the B cell level.

Tolerance to tissues and cells

• Tolerance to tissue and cell antigens can be induced by injection of hemopoietic (stem) cells in neonatal or severely immunocompromised (by lethal irradiation or drug treatment) animals.

• Also, grafting of allogeneic bone marrow or thymus in early life results in tolerance to the donor type cells and tissues. Such animals are known as chimeras. These findings are of significant practical application in bone marrow grafting.

Tolerance to soluble antigens

• A state of tolerance to a variety of T-dependent and T-independent antigens has been achieved in various experimental models.

• Based on these observations it is clear that a number of factors determine whether an antigen will stimulate an immune response or tolerance

Tolerance

Also see Table 10-1 of text

Tolerance• Induction of tolerance in T cells is easier

and requires relatively smaller amounts of tolerogen than tolerance in B cells.

• Maintenance of immunological tolerance requires persistence of antigen.

• Tolerance can be broken naturally (as in autoimmune diseases) or artificially (as shown in experimental animals, by x-irradiation, certain drug treatments and by exposure to cross reactive antigens).

Ignorance

• It can be shown that there are T cells and B cells specific for auto-antigens present in circulation.

• These cells are quite capable of making a response but are unaware of the presence of their auto-antigen. This arises for 2 reasons.

Ignorance

• The first is that the antigen may simply be present in too low concentration. Since all lymphocytes have a threshold for receptor occupancy which is required to trigger a response then very low concentrations of antigen (in the case of T cells these are very low, see below) will not be sensed.

Ignorance

• The second possibility is a more interesting one. Some antigens are sequestered from the immune system in locations which are not freely exposed to surveillance.

• These are termed immunologically privileged sites. Examples of such sites are the eye, CNS and testis.

• Pathologically mediated disruption of these privileged sites may expose the sequestered antigens leading to an autoimmune response.

Mechanism of tolerance induction

Clonal deletion: • Functionally immature cells of a clone

encountering antigen undergo a programmed cell death, as auto-reactive T-cells are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection).

Mechanism of tolerance induction

Clonal deletion: • Likewise, differentiating early B cells become

tolerant when they encounter cell-associated or soluble self antigen.

• Clonal deletion has been shown to occur also in the periphery.

Mechanism of tolerance induction

Clonal anergy:

• Auto-reactive T cells, when exposed to antigenic peptides which do not possess co-stimulatory molecules (B7-1 or B7-2), become anergic to the antigen.

Mechanism of tolerance induction

Clonal anergy:

• Also, B cells when exposed to large amounts of soluble antigen down regulate their surface IgM and become anergic. These cells also up-regulate the Fas molecules on their surface. An interaction of these B cells with Fas-ligand-bearing cells results in their death via apoptosis.

Mechanism of tolerance induction Receptor editing:

• B cells which encounter large amounts of soluble antigen, as they do in the body, and bind to this antigen with very low affinity become activated to re-express their RAG-1 and RAG-2 genes.

• These genes cause them to undergo DNA recombination and change their antigen specificity.

What are RAG-1 RAG-2?

• Recombination signal sequences (RAG).

• RAG-1 is a specific endonuclease and is only active when complexed with RAG-2.

• Specific DNA sequences (heptamers) found adjacent to the V, D, and J segments in the antigen receptor loci and recognized by the RAG-1/RAG-2 component of the V(D)J recombinase. (see figure 7-11)

RAG-1/RAG-2

Mechanism of tolerance induction

Anti-idiotype antibody: • Anti-idiotype antibodies produced experimentally

have been demonstrated to inhibit immune response to specific antigens.

• Anti-idiotype antibodies are produced during the process of tolerization.

• Such antibodies may respond to the unique receptors of other lymphocytes and serve to shut off antigen specific responses.

• Therefore, these antibodies prevent the receptor from combining with antigen.

Mechanism of tolerance induction

Termination of tolerance

• Experimentally induced tolerance can be terminated by prolonged absence of exposure to the tolerogen, by treatments which severely damage the immune system (x-irradiation) or by immunization with cross reactive antigens.

• These observations are of significance in the conceptualization of autoimmune diseases.

Dendritic cells: regulators of alloimmunity and

opportunities for tolerance induction

• Dendritic cells (DCs) are uniquely well-equipped antigen-presenting cells (APCs) regarded classically as sentinels of the immune response, which induce and regulate T-cell reactivity.

• They play critical roles in central tolerance and in the maintenance of peripheral tolerance in the normal steady state.

Regulatory T cells

Mechanism of tolerance induction

Suppressor cells:

• Both low and high doses of antigen may induce suppressor T cells (Regulatory T cells) which can specifically suppress immune responses of both B and T cells, either directly or by production of cytokines, most importantly, TGF- and IL-10.

Regulatory T cells

• CD4+ T lymphocytes that express high levels of IL-2r chain (CD25) but not other markers of activation.

• Regulatory T cells may be generated by self antigen recognition in the thymus or in the periphery.

• These cells induce immunosuppression by secreting TGF- and IL-10 and thereby inhibit M function and IFN- activity.

Fig 10-10

Oral tolerance

• The gastrointestinal tract is the largest immunologic organ in the body.

• It is constantly bombarded by a myriad of dietary proteins.

• Despite the extent of protein exposure, very few patients have food allergies because of development of oral tolerance to these antigens.

• Once proteins contact the intestinal surface, they are sampled by different cells and, depending on their characteristics, result in different responses.

Oral tolerance• Antigens might be taken up by Microfold cells

overlying Peyer's patches, dendritic cells, or epithelial cells.

• Different cells of the immune system participate in oral tolerance induction, with regulatory T cells being the most important.

• Several factors can influence tolerance induction.

• Some are antigen related, and others are inherent to the host. Disturbances at different steps in the path to oral tolerance have been described in food hypersensitivity.

Oral tolerance

T regulatory cell family• The idea of specific suppressor T cell

populations that counteract harmful autoaggressive immune responses in the periphery was first described in the 1970s by Gershon et al.

• However, at that time neither the cells nor the hypothetical soluble suppressor factors responsible for the observed effects could be identified.

T regulatory cell family

• In 1995 Sakaguchi et al. described for the first time a subpopulation of CD4+ T helper cells, characterized by a constitutive expression of the IL-2 receptor -chain (CD25), that is essential to control autoaggressive immune responses in mice.

T regulatory cell family• After subsequent in vitro studies by several

groups, this population is now referred to as CD4+CD25+ T regulatory cells (Tregs).

• This distinct T cell population was originally described in mice. However, comparable T cell suppressor populations, with identical phenotype and functional activities have been defined more recently in rats and humans.

T regulatory cell family• They represent 5–10% of all peripheral

CD4+ T cells.

• Freshly isolated CD25+ Tregs do not proliferate after allogeneic or polyclonal activation in vitro, but Tregs suppress the activation and cytokine release of CD4+ and CD8+ T cells in an antigen-nonspecific and cell contact-dependent manner.

T regulatory cell family• However, it should be mentioned that the

activation of Tregs is also antigen-specific.

• The main mechanism of suppression seems to be the inhibition of IL-2 transcription in the responder T cell population.

• Nevertheless, the molecules involved in this cell contact-dependent suppression are still largely unknown.

T regulatory cell family• In the human immune system, two distinct

subsets of resident CD25+ Tregs can be distinguished based on the expression of distinct integrins.

• Tregs expressing the 4 7 integrin can convert CD4+ T cells into IL-10-producing Tr1-like cells, whereas 4 1+ Tregs induce TGF- -producing Th3-like cells.

• The integrins 4 1 and 47 are homing receptors for cellular migration of T lymphocytes to inflamed tissues and to mucosal sites, respectively.

T regulatory cell family• The 41-integrin binds to VCAM1 (vascular cell

adhesion molecule-1), which is induced on the endothelium of inflamed tissues, whereas the 47-integrin binds to vascular addressins, selectively expressed by venules in mucosal tissues.

• Therefore, it can be postulated that 41+CD25+

Tregs migrate in vivo to inflamed tissues where they can inhibit effector T cell responses.

47+CD25+ Tregs are specialized to migrate to

mucosal tissues, to counteract autoreactive T cells, thereby preventing chronic mucosal inflammations.

T regulatory cell family

History of Tolerance

History of Tolerance

Timing

• Some 50 years ago Owen observed two types of non-identical twin cattle, those that had shared a hemopoietic system in utero were tolerant of blood cells from each other and those who had not, were not cross-tolerant.

History

• Burnet postulated that there was a temporal window of tolerance such that antigens encountered while the immune system was immature tolerized the relevant lymphocytes.

• Medewar subsequently investigated the effects of transferring hemopoietic cells from histoincompatible mice at different times after birth. He found that if the cells were transferred in the first few days of life (but not later) the recipient mouse acquired lifelong tolerance to the antigens of the donor.

History

HistoryThe Danger Hypothesis

• Matzinger versus Medewar?

• Matzinger has proposed that there is not a special window for tolerance during neonatal life but that whether encounter with an antigen results in tolerance or an immune response is determined by whether the prevailing host environment promotes a response via nonspecific cues 'sensing' danger.

History

• Polly has further suggested that the controlled death process of apoptosis is critical in preventing autoimmunity when old or surplus cells are disposed.

• The notion that the normal, default pathway of the immune system is tolerance rather than response is not a new idea to immunologists - antigens usually fail to elicit a response unless given with adjuvants, whose purpose is probably to generate stimulatory cues (cytokines).

History

• Polly has further suggested that the controlled death process of apoptosis is critical in preventing autoimmunity when old or surplus cells are disposed.

• The notion that the normal, default pathway of the immune system is tolerance rather than response is not a new idea to immunologists - antigens usually fail to elicit a response unless given with adjuvants, whose purpose is probably to generate stimulatory cues (cytokines).

History

• Recent experiments have shown that not only can adults be tolerize under certain circumstances, but that neonates can make effective immune responses if the antigen is presented in sufficiently immunogenic form.

History

• I believe that the supposed conflict between Matzinger and Medewar is rather 'hyped up' and essentially a matter of detail.

• Neonatal T cells are not intrinsically tolerizable but the systemic neonatal environment does predispose to tolerance.

• Nevertheless, I think that her hypothesis has drawn the attention of a wider audience to current ideas about tolerance induction and the factors determining immune responsiveness.