Transcomplementation can result from the combination in trans of and chains encoded by MHC class II genes on different chromosomes.

Inter-isotypic molecules also can be formed by and chains of two different loci (for example: DR - DQ ).

HLA-DP, -DQ, and –DR (MHC class II) are expressed on APC (classical MHC molecules).

HLA-DM and HLA-DN/DO are not, but are involved in the regulation of class II expression:

From Dr. Robert Busch’s web site: http://www.stanford.edu/~rbusch/research1.htm

“… frequency of naive lymphocytes specific for any given antigen is estimated to be between 1 in 10,000 and 1 in 1,000,000 …”

In unimmunized mice:

1 in 26,300 B cells could make anti-SRC IgM

no detectable (<1 in a million) B cells that could make anti-SRC IgG(Martínez-Maza, et al. Scandinavian J. Immunol 17:251, 1983)

In immunized mice:

1 in 219 B cells could make anti-SRC IgM (5d post-immunization)

1 in 112 B cells could make anti-SRC IgG (12d)

1 in 3,030 B cells could make anti-SRC IgG (180d)(Martínez-Maza, et al. Scandinavian J. Immunol 17:345, 1983)

• Ag-activated B cells give rise to germinal centers (GC), zones of proliferating activated B cells:

 

Calame, K. 2001. Plasma cells: finding new light at the end of B cell development . Nature Immunology 2:1103.

T CELL DEVELOPMENT AND ACTIVATION

• There are a lot of similarities between T and B cells, in their development:

– arise from hematopoietic precursors that are generated in the bone marrow

– undergo similar DNA rearrangements to generate the genes for their antigen receptor molecules

– have the capacity to respond to nearly any antigen

– the initial stages of development are antigen-independent, with final differentiation occurring after exposure to antigen

– cells that express antigen-receptors that react with self are eliminated

• However, there are some significant differences:

– since the T cell receptor can interact with antigen only when it is presented in association with self-MHC molecules, T cells need to learn how to bind to a complex of self MHC + Ag peptide

– in addition to this (perhaps because of this) T cells do not develop in the bone marrow, they undergo development in a specialized organ, the thymus.

• T lymphocytes or T cells got their name from original observations that indicated that they were thymus-derived lymphocytes.

• T cell precursors travel from the bone marrow to the thymus:

• Following development into mature, antigen-responsive T cells, these T cells emerge from the thymus and migrate to secondary lymphoid tissues, where they interact with antigen, antigen-presenting cells, and other lymphocytes:  

• The importance of the thymus in T cell development is demonstrated by inherited immune deficiencies: people that do not have a thymus (DiGeorge’s syndrome, aka Thymic Aplasia) do not develop functional T cells.

• DiGeorge’s syndrome results from a developmental defect – the failure of the third and fourth pharyngeal pouches to develop, which results not just in thymic defects, but also in absent parathyroids and in aortic arch defects.

• Thymectomy early in life reduces the ability to produce T cells.

• Thymectomy later in life does not markedly impair T cell number.

• In fact, the thymus decreases in size with age.

• However, the thymus can still produce new T cells up to middle-age, especially in situations where there is loss of T cells (HIV/AIDS).  

• The thymus is composed of several lobes, each of which has cortical and medullary regions:

• The cortex contains immature thymocytes in close contact with thymic epithelial cells.

• Medullary areas contain more mature thymocytes, epithelial cells, and dendritic cells and macrophages

• While in the thymus, immature T cells, or thymocytes, undergo several changes that allow them to develop into mature T cells, ready for contact with antigen.

• Thymocytes interact with thymic epithelial cells and various other cells while in the thymus.

• During thymic differentiation,

the great majority of thymocytes die by apoptosis, and are ingested by macrophages.

• Only a small minority of these T cell progenitors make it out as mature T cells

• Thymic development occurs in two phases:

1) production of T cell receptors for antigen, by rearrangement of the TCR genes

2) selection of T cells that can interact effectively with self-MHC

 

• Changes in the expression of cell-surface molecules accompany the thymic differentiation of T cells:

– entering thymocytes are TCR, CD3, CD4, and CD8-negative

– as thymocytes mature, and undergo rearrangement of their TCR genes to generate a functional TCR, they begin to express CD3, CD4, and CD8

– mature T cells ready to go to the periphery are TCR/CD3+, and either CD4 or CD8 positive

Phase 1 of thymic development: rearrangement of TCR genes to

produce a functional TCR

• Progenitor T cells enter the thymus (sub-capsular region of the outer cortex).

• These cells do not have rearranged TCR genes and lack expression of characteristic T cell surface molecules.

• Interaction with thymic stromal cells induces these progenitor T cells to proliferate.

• These immature thymocytes do not yet express CD4 or CD8, molecules that are expressed by mature T cells: double-negative thymocytes.

  

• There are two types of T cell receptors: and

  TCR T cells are the most

abundant, by far:

 

(or & chain)

Unlike B cells, in which the genes that encode the BCR rearrange in a set order, the TCR , , and genes start to rearrange at about the same time.  

If a productive and rearrangement occurs first, the T cell is committed to that lineage, and stops further rearrangement of the TCR gene.

 

However, if is rearranged first, then the T cell continues to proliferate, and undergoes further rearrangements.

This results either in rearranged TCR gene, yielding an TCR lineage cell, or rearranging and genes, resulting in a TCR cell.

 

• Rearrangements that lead to an T cell begin the rearrangement of the TCR gene.

• The first step is D-J joining, followed by VDJ rearrangement.

• Expression of chain

stops further chain rearrangements.

 

chain is then expressed on the surface of the thymocyte in association with a surrogate chain (pT).

 

• Following this, there is rearrangement of the TCR gene, resulting in a functional chain, and in the expression of surface TCR, in association with other T cell-associated cell surface molecules.  

• During this process, a cell that makes an unproductive chain rearrangement can try again until gets a good chain, or it exhausts its possibilities:

• Thymocytes that have a functional rearrangement, and express or + the surrogate chain (pT) are induced to express both CD4 and CD8 simultaneously – these are called double-positive cells.

 • Immature T cells that do not undergo a productive

rearrangement die by apoptosis.

Phase 2 of thymic development: selection of T cells that can interact with

self MHC and antigen

• This applies only to TCR-bearing cells (>95% of T cells).

T cells are not restricted to interactions with MHC class I or class II molecules

• This phase of T cell development consists of two steps:

– positive selection (TCR that can interact with self-MHC)

– negative selection (eliminate self-reactive cells that are stimulated by MHC + self)

 

• In positive selection, developing thymocytes continue to live if they receive a signal through their TCR.

• This signal is mediated by the interactions of these cells with MHC-expressing thymic cortical epithelial cells.

• The ~95% of thymocytes that do not receive this signal undergo apoptosis.

Positive Selection

Positive selection takes place in the cortex of the thymus lobules:

• These CD4+ CD8+ TCR+ thymocytes interact with thymic epithelial cells that express both MHC class I and MHC class II molecules, complexed with self-peptides.

• Thymocytes that bind MHC survive; those that don’t die.  • TCR chain rearrangements can continue during positive selection, allowing

cells to explore alternative chains for MHC binding.

• Once a T cell is positively selected, TCR rearrangement stops. 

 

• The expression of either CD4 or CD8 by a given T cell is determined during positive selection, leading to single-positive cells (CD4 or CD8-positive).

• Those cells that have a TCR that binds to MHC class II end up as CD4 single-positive cells

• Those that bind MHC class I as CD8 positive cells:  

• Thymocytes undergo negative selection in the medullary region:

Negative Selection

• There, they interact with antigen-presenting cells (dendritic cells, macrophages) that express self-antigens + MHC class I or MHC class II molecules.

• Thymocytes that bind to self + MHC too strongly are eliminated as possibly self-reactive cells, and undergo apoptosis.

• If self-reactive T cells were allowed to exit the thymus, such cells would mediate autoimmune disease.

• Some T cells are reactive with self molecules that are not expressed in the thymus:

– such cells can be eliminated in peripheral lymphoid tissues by the induction of anergy

– (incomplete stimulation via their TCR)

• Some T cells are reactive with self molecules that are not expressed in the thymus:

– such cells can be eliminated in peripheral lymphoid tissues by the induction of anergy

– (incomplete stimulation via their TCR)

X

anergy or apoptosis

• T cells that exit the thymus have undergone a series of changes that allow them to:

– develop a functional TCR

– interact with self-MHC

– while eliminating self-reactive T cells

Antigen-driven T cell Differentiation in Secondary Lymphoid Organs

• Mature T cells leave the thymus and migrate to secondary lymphoid tissues (lymph nodes, spleen, mucosa-associated lymphoid tissue), recirculating via the blood and lymph, just like mature B cells do.

• Mature T cells are longer lived than mature B cells, and can survive for years without antigenic stimulation.

 

 

• Unlike B cells, which have just one type of terminally-differentiated cell (plasma cell), there are various types of effector T cells:

– CD8 T cells, which can differentiate into cytotoxic T cells

– CD4 T cells, which can become either TH1 or TH2 helper cells.