The Generation of Diversity (GOD): How to Ensure Nonreactivity with Self

The Immunological Orchestra Richard Gershon, Maestro

Goal: Develop TCR That Recognizes and Binds Foreign Antigens The receptor for antigens on the T cell surface comprises eight proteins.  (a) Two disulfide-bonded chains of the T cell receptor which form a heterodimer. These recognize peptides associated with MHC molecules.  (b) Four chains, collectively termed CD3, that associate with the T cell receptor dimer and participate in its transport to the surface of the cell. The CD3 complex together with the zeta chains, which form a homodimer, transduce the signal after antigen has bound. SOURCE: http://pathmicro.med.sc.edu/  

Generating Functional T cells Prethymic T cells enter the thymus rudiment and proliferate as large lymphoblasts in the sub-capsular region of the thymus; result is a pool of cells ready to differentiate. Here the cells become CD8 and CD4 positive but expression is low. TCR genes are also rearranged in these cells and the products may also be expressed on the cell surface at low levels.

The T cell receptor heterodimer comprises two transmembrane glycoproteins, the alpha and beta chains. There are two domains in the external part of each chain and these resemble immunoglobulin variable and constant regions.  There are sugar chains on each domain. There is a short sequence similar to the immunoglobulin hinge region that connects the immunoglobulin-like domains to the transmembrane sequence. This contains cysteines that form a disulfide bridge. The hydrophobic transmembrane helical structures are unusual in that they contain positively charged amino acids (basic amino acids). The alpha chain has two positively charged residues while the beta chain has one. SOURCE: http://pathmicro.med.sc.edu/

Generating Functional T cells As the cells mature, they move into the cortex where they adhere to cortical epithelial cells which are long and branched, providing a large surface area to interact with other cells. TCR required. TCRs on the surfaces of thymocytes interact with the MHC molecules on the epithelial cells—these thymocytes have been positively selected to survive. The cells that are not selected are subject to apoptosis and are phagocytosed by macrophages.

Generating Functional T cells As the thymocytes migrate further into the cortex of the thymus, the expression of CD3, CD4, CD8 and TCR increases. TCRs with self-reactivity are deleted because of contact with autoantigens presented by dendritic cells and macrophages. This is negative selection. Cells that express CD4 or CD8 appear and migrate to the periphery by specialized vessels in the cortico-medullar region.

Goals of Thymic Education 1. Functional T cells in the periphery have to recognize foreign antigens associated with self MHC, because APC or target cells present foreign antigen associated with self MHC (Class 2 for APC; Class 1 for targets). 2. Therefore, an individual does not need functional T cells in the periphery that cannot recognize self MHC. 3. An individual also does not want functional T cells in the periphery that can recognize self antigens associated with self MHC because they could lead to damage of healthy, normal tissues.

The Process of Thymic Education 1. T cells with the ability to bind to self MHC molecules expressed by cortical thymic epithelial cells are retained. This is known as positive selection. Those that do not bind, die. Thus, T cells having a TCR that recognizes self MHC survive. 2. T cells with the ability to bind with high affinity to self MHC molecules associated with self molecules expressed by dendritic cells and macrophages are killed. This is known as negative selection. Those that do not bind are retained. As a result of these two steps, T cells having a TCR that recognizes self MHC and foreign antigen survive. 3. Each T cell that survives positive and negative selection in the thymus and is released into the periphery retains its specific T cell receptor (TCR).

Do we need to run through this again? Animation has several positive features: attracting and directing attention; fostering mental model construction; representing information that involves movement; and explaining complex phenomena. M.S. Chan and J.B. Black. Proceedings of EdMedia2005, AACE. http://www.bio.davidson.edu/Courses/Immunology/Flash/Main.html

But the system isn’t perfect….

Autoimmunity Breakdown of mechanisms responsible for self tolerance and induction of an immune response against components of the self. Activation of the immune system often leads to tissue damage; disease results. Disease may be organ specific or generalized. Evidence for genetic predisposition.

Possible mechanisms Sequestered or late-developing antigen; not present during “education” Escape of auto-reactive clones Cross-reactive antigens, including exogenous antigens (pathogens) and altered self antigens (chemical and viral infections) Loss of suppressor cells

The Search for the TS T cells that suppress the immune response were discovered in the early 1970s. Richard Gershon of Yale University coined the term "suppressor T cell" after experiments suggested that a certain type of T cell suppressed immune responses through a soluble factor and not through direct interaction. Many prominent groups tried—without success- to characterize these suppressing factors. Plus, the genes for the postulated suppressing factors couldn't be found. Many doubted that they were real. A different line of study lead to an understanding of negative regulation by T cells.

The Search for the TS In 1995, Shimon Sakaguchi showed that removing a subset of CD4+ T cells that express a marker called CD25 resulted in severe autoimmune disease in mice (J. Immunol. 155, 1151, 1995). The condition resembled the one described in the 1969 study. Sakaguchi called the cells regulatory T cells. It is clear that these regulatory T cells—or Tregs—are crucial to keeping the immune system in check, although the exact mechanism as to how they suppress their target cells has not yet been resolved. Tregs are important in suppressing autoimmune disease in that they suppress T cells directed against self antigens.

Tregs in Humans Tregs shown to suppress immune responses, including the activation of CD4+ and CD8+ T cells and also dendritic cells. They are important because, for example, not all T cells against self-antigens are deleted in the thymus, and Tregs are thought to suppress the activity of those that escape. They can also suppress T-cell activation in response to pathogens. In 2001, several groups identified a similar population of Tregs in cultured human cells. Defined as being high expressers of CD25 Several soon identified one more marker: the transcription factor Foxp3.

Generation of Tregs International AIDS Vaccine Initiative Report Vol. 11 (4), July - Aug. 2007.

Not-so-innocent bystander: Regulatory T cells suppress the immune reactions of neighboring T cells. This “bystander suppression” happens when a Treg that recognizes a single peptide from one particular antigen also suppresses the responses of T cells that recognize different peptides from that or any other antigen presented by the same antigen-presenting cell. Tregs specific for a given antigen can thereby imbue a tolerance to several antigens. It is not yet known whether the Tregs directly restrain the effector T cells or alter the antigen-presenting cell’s ability to activate them. SOURCE: Harvard Focus March 21, 2008

Do we need to run through this again? ab-direct.com - Regulatory T cell animation