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:

“… 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 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.