User:Echall.su/Peripheral tolerance

LEAD

In immunology, peripheral tolerance is the second branch of immunological tolerance, after central tolerance. It takes place in the immune periphery (after T and B cells egress from primary lymphoid organs). Its main purpose is to ensure that self-reactive T and B cells which escaped central tolerance do not cause autoimmune disease. Peripheral tolerance can also serve a purpose in preventing an immune response to harmless food antigens and allergens.

'''Self reactive cells are subject to clonal deletion or clonal diversion. Both processes of peripheral tolerance control the presence and production of self reactive immune cells'''. Deletion of self-reactive T cells in the thymus is only 60-70% efficient, and naive T cell repertoire contains a significant portion of low-avidity self-reactive T cells. These cells can trigger an autoimmune response, and there are several mechanisms of peripheral tolerance to prevent their activation. Antigen-specific mechanisms of peripheral tolerance include persistent of T cell in quiescence, ignorance of antigen and direct inactivation of effector T cells by either clonal deletion, conversion to regulatory T cells (Tregs) or induction of anergy. Tregs, which are also generated during thymic T cell development, further suppress the effector functions of conventional lymphocytes in the periphery. Dendritic cells (DCs) participate in the negative selection of auto-reactive T cells in the thymus, but they also mediate peripheral immune tolerance through several mechanisms.

Dependence of a particular antigen on either central or peripheral tolerance is determined by its abundance in the organism. '''B Cells have a lower probability that they will express cell surface markers to pose the threat of causing an autoimmune attack.  Peripheral tolerance of B cells''' is largely mediated by B cell dependence on T cell help. However, B cell peripheral tolerance is much less studied.

Quiescence[edit]
When naive T cells exit the thymus, they are in a quiescent state. That means they are in the non-proliferative, G0 stage of the cell cycle and they have low metabolic, transcriptional and translational activities, but still retain the capacity to enter the cell cycle. Quiescence can prevent naive T cell activation after tonic signaling, meaning that T cells may be constitutively activated when not in the presence of a ligand. After antigen exposure and costimulation, naive T cells start the process called quiescence exit, which results in proliferation and effector differentiation.

'''Naive cells must enter and exit a quiescent state at the proper timing in their life cycle. If T cells exit a quiescence prematurely there is a lack of tolerance to potential self-reactive cells. T cells rely on negative regulators to keep them in a quiescence state until they are ready for exit, the down regulation of negative regulators increases T cell activation. Premature and over activation of T cells can lead to harmful down stream responses and possibly trigger an autoimmune response.'''

'''As cells exit a quiescent state they will up regulate enzymes that are responsible for production of essential pathways (nucleic acids, proteins, carbohydrates, etc.). At this stage the T cell will enter the cell cycle and continue to be metabolically active.'''

Ignorance[edit]
When self-reactive T cells escape thymic deletion they ay enter an ignorant state. Self-reactive T cells can fail to initiate an immune response after recognition of self-antigen. These T cells are not classified as dysfunctional members of the immune response, rather they are antigen-inexperienced naive cells that will remain in circulation . These cells remain the ability to become activated if in the presence of the correct stimuli.

Ignorance can be seen in situations where there is not a high enough concentration of antigen to trigger activation. The intrinsic mechanism of ignorance is when the affinity of self TCR to antigen is at too low of concentration to elicit T cell activation. There is also an extrinsic mechanism. Antigens, which are present in generally low numbers, can´t stimulate T cells sufficiently. '''Additionally, there are anatomical barriers that prohibit the activation of these T cells. These''' specialized mechanisms ensure ignorance by the immune system have developed in so-called immune privileged organs.

T cells can overcome ignorance through a sufficient signal from signaling molecules (cytokines, infection, inflammatory stimuli, etc.) and induce an autoimmune response .

Peripheral deletion[edit]
'''Before release into the periphery T cells are subjected to thymic deletion if they prove to have the capacity to react with self. Peripheral deletion is the disposal of potential self reactive T cells that escaped thymic deletion'''.

After T cell response to co-stimulation-deficient antigen, a minor population of T cells develop anergy and a large proportion of T cells are rapidly lost by apoptosis. This cell death can be mediated by intrinsic pro-apoptotic family member BIM. The balance between proapoptotic BIM and the antiapoptotic mediator BCL-2 determine the eventual fate of the tolerized T cell. There are also extrinsic mechanisms of deletion mediated by the cytotoxic activity of Fas/FasL or TRAIL/TRAILR interaction.

'''Cell death can be mediated by a collection of mechanisms. In most instances there is an up regulation of death markers or the presence of Bcl-2 proteins, which are proteins that are essential in facilitating programmed cell death .'''

Immunoprivileged organs

'''immunopriveiledged organs evolved mechanisms in which specialized tissue cells and immune cells can mount an appropriate response without disturbing the specialized tissue. Immunopathogenic disturbances are not present in a variety of specialized organs such as; the eyes and reproductive organs.'''

Potentially self-reactive T-cells are not activated at immunoprivileged sites, where antigens are expressed in non-surveillanced areas. Naive T-cells are not present in high numbers in peripheral tissue but stay mainly in the circulation and lymphoid tissue.

SPLIT TOLERANCE

Split tolerance describes how some antigens can trigger an immune response in one aspect of the immune system and the same antigen could not trigger a response in another set of immune cells. Since many pathways of immunity are interdependent, they do not all need to be tolerized. For example, tolerized T cells will not activate autoreactive B cells. Without this help from CD4 T cells, the B cells will not be activated.