During T-cell maturation, there is a precise order in which TCR genes are rearranged and in which the TCR chains and CD4 and CD8 coreceptors are expressed (Fig. 1; see also Fig. 2). In the mouse fetal thymus, surface expression of the γδ TCR occurs first, 3 to 4 days after precursor cells first arrive, and the αβ TCR is expressed 2 or 3 days later. In human fetal thy muses, γδ TCR expression begins at about 9 weeks of gestation, followed by expression of the αβ TCR at 10 weeks.

Fig1. An overview of T-cell development in the thymus. Precursors of T cells travel from the bone mar row through the blood to the thymus. The progenitors of αβ T cells are double-negative T cells. In the thymic cortex, these cells begin to express TCRs and CD4 and CD8 coreceptors. Selection processes eliminate self reactive T cells in the cortex at the double-positive (DP) stage and also eliminate single-positive (SP) medullary thymocytes. They promote survival of thymocytes whose TCRs bind self major histocompatibility complex (MHC) molecules with low affinity. Functional and phenotypic differentiation into CD4+CD8− or CD8+CD4− SP T cells occurs in the medulla, and mature T cells are released into the circulation. Some DP cells differentiate into CD4+CD8− regulatory T cells (Treg, see Chapter 15). The development of γδ T cells is not shown.

Fig2. Stages of T-cell maturation. Events corresponding to each stage of T-cell maturation from a bone marrow stem cell to a mature T lymphocyte are illustrated. Several surface markers in addition to those shown have been used to define distinct stages of T-cell maturation. Thymocyte in the cortex lacking CD4 and CD8 expression are called double-negative or DN T cells. The CD44+CD25− cells in the cortex that have recently arrived from the the bone marrow are sometimes called DN1 T cells. CD44+CD25+ thymocytes are often referred to as DN2 T cells. CD44−CD25+ thymocytes are also known as DN3 T cells. D, Diversity; J, joining; RAG, recombination-activating gene; TCR, T-cell receptor; TdT, terminal deoxynucleotidyl transferase; V, variable.

Double-Negative Thymocytes

The most immature cortical thymocytes, which are recent arrivals from the bone marrow, contain TCR genes in their germline configuration and do not express TCR, CD3, ζ chains, CD4, or CD8; these cells are called double-negative (DN) thymocytes (based on the lack of expression of CD4 and CD8). The earliest largely undifferentiated DN thymocytes are not yet committed to the T lineage and are phenotypically CD44+CD25− cells also known as DN1 cells. It is at the next stage of cortical thymocyte differentiation that DN thymocytes that are CD44+CD25+ (DN2 cells; see Fig. 2) actually commit to the T lineage. Thymocytes at this stage are considered to be at the pro–T-cell stage of maturation. It is at this and the subsequent CD44−CD25+ DN3 (or pre-T) stage (see Fig.2) in the cortex that thymocytes rearrange their T-cell receptor β, γ, and δ genes. The majority (> 90%) of the double-negative thymocytes that survive thymic selection processes will ultimately give rise to αβ TCR–expressing, MHC-restricted CD4+ and CD8+ double-positive T cells; some double-negative thymocytes give rise to γδ T cells. The RAG1 and RAG2 proteins are first expressed at the double-negative stage of T-cell development and are required for the rearrangement of TCR genes. In αβ T cells, Dβ-to-Jβ rearrangements at the TCR β chain locus occur first; these involve either joining of the Dβ1 gene segment to one of the six Jβ1 segments or joining of the Dβ2 segment to one of the six Jβ2 segments (Fig.3A). Vβ-to-DJβ rearrangements occur at the DN3 stage in the cortex during αβ T-cell development. The DNA sequences between the segments undergoing rearrangement, including D, J, and possibly Cβ1 genes (if Dβ2 and Jβ2 segments are used), are deleted during this rearrangement process. The primary nuclear transcripts of the TCR β genes contain the intron between the recombined VDJβ exon and the relevant Cβ gene (as well as the three additional introns between the four exons that make up each Cβ gene). Poly-A tails are added after cleavage of the primary transcript downstream of consensus polyadenylation sites located 3′ of the Cβ region, and the sequences between the VDJ exon and Cβ are spliced out to form a mature mRNA in which VDJ segments are juxtaposed to the first exon of either of the two Cβ genes (depending on which J segment was selected during the rearrangement process). Translation of this mRNA gives rise to a full-length TCR β protein. The two Cβ genes appear to be functionally interchangeable and the use of either Cβ gene does not influence the specificity of the TCR. Furthermore, an individual T cell never switches from one C gene to the other. Once a rearranged functional V gene is brought close to the C gene by VDJ recombination, the promoter in the 5′ flanking region of the Vβ gene can function together with a powerful enhancer that is located 3′ of the Cβ2 gene. This proximity of the promoter to the enhancer is responsible for high-level T cell–specific transcription of the rearranged TCR β chain gene. After the addition and removal of nucleotides during gene rearrangement, the number of new nucleotides in the TCR β chain gene are a multiple of three (in one of the two inherited TCR β loci) in only about half of all developing pre–T cells, and therefore only approximately half of all developing pre–T cells express a TCR β protein. The next step in T-cell development is the selection of cells that express the first chain of the antigen receptor and can pass this checkpoint.

Fig3. TCR α and β chain gene recombination and expression. The sequence of recombination and gene expression events is shown for the TCR β chain (A) and the TCR α chain (B). In the example shown in (A), the variable (V) region of the rearranged TCR β chain includes the Vβ1 and Dβ1 gene segments and the third J segment in the Jβ1 cluster. The constant (C) region in this example is encoded by the exons of the Cβ1 gene, depicted for convenience as a single exon (though it is actually made up of four exons with three intervening introns). Note that at the TCR β chain locus, rearrangement begins with D-to-J joining followed by V-to-DJ joining. In humans, 14 Jβ segments have been identified and not all are shown in the Figure. In the example shown in (B), the V region of the TCR α chain includes the Vα1 gene and the second J segment in the Jα cluster. (This cluster is made up of at least 61 Jα segments in humans; not all are shown here.) C, Constant; D, diversity; enh, enhancer; J, joining; L, leader; mRNA, messenger RNA; V, variable.

Pre–T-Cell Receptor

 If a productive (i.e., in-frame) rearrangement of the TCR β chain gene occurs in a given double-negative T cell, the TCR β chain is expressed on the cell surface in association with an invariant protein called pre-Tα, which, along with CD3 and ζ proteins, forms the pre-TCR complex. The pre-TCR mediates the selection of the developing pre–T cells that have successfully rearranged the β chain of the TCR. The function of the pre-TCR complex in T-cell development is similar to that of the surrogate light chain–containing pre-BCR complex in B-cell development. Signals from the pre-TCR select cells that have productively rear ranged the TCR β chain gene at the DN3 stage and stimulates a proliferative burst of pre-TCR expressing CD25−CD44− DN4 thymocytes that is the largest proliferative expansion during T-cell development. The pre-TCR drives the transition from the double negative to the double-positive stage of thymocyte development. In addition to providing signals for survival and proliferation, the pre-TCR also signals to inhibit further rearrangement of the TCR β chain locus on the unrearranged allele. This results in β chain allelic exclusion (i.e., mature T cells express an antigen receptor chain from only one of the two inherited β chain loci). As is the case for the pre-BCR, pre-TCRs lack known ligands. Pre-TCR signaling, like pre-BCR signaling, may be initiated in a ligand-independent manner, after the successful assembly of the pre-TCR complex. This signaling is mediated by a number of cytosolic kinases and adaptor proteins that are also linked to TCR signaling. The essential function of the pre-TCR complex in T-cell maturation has been demonstrated by numerous studies with genetically mutated mice, in which lack of any component of the pre-TCR complex or signaling molecules it depends on (i.e., the TCR β chain, pre-Tα, CD3, ζ, or LCK) results in a block in the maturation of T cells at the double-negative stage. CD3ε mutations in humans result in SCID, while mutations in LCK in humans result in the near absence of CD4+ T cells. CD4+ cells are affected more than CD8+ cells because stronger LCK signals are likely required for CD4+ T-cell development during positive selection.

Double-Positive Thymocytes

 At the next stage of T-cell maturation, thymocytes express both CD4 and CD8 and are called double-positive thymocytes. The expression of CD4 and CD8 is essential for subsequent selection events. The rearrangement of the TCR α chain genes and the expression of TCR αβ heterodimers occur in the CD4+CD8+ double-positive population soon after cells cross the pre-TCR checkpoint (see Figs. 1 and 2). A second wave of RAG gene expression late in the pre–T stage promotes TCR α gene recombination. Because there are no D segments in the TCR α locus, rearrangement consists of the joining of only V and J segments (see Fig. 3B). The large number of Jα segments permits multiple attempts at productive V-J joining on each chromosome, thereby increasing the probability that a functional αβ TCR will be produced. In contrast to the TCR β chain locus, where production of the protein and formation of the pre-TCR suppress further rearrangement, there is little or no allelic exclusion in the α chain locus. Therefore, productive TCR α rearrangements may occur on both chromosomes, and if this happens, the T cell will express two α chains. In fact, many mature peripheral T cells express two different TCRs, with different α chains but the same β chain in each cell. It is possible that only one of the two different TCRs participates in self MHC–driven positive selection, described later. Transcriptional regulation of the α chain gene occurs in a manner similar to that of the β chain. There are promoters 5′ of each Vα gene that have low-level activity and are responsible for high-level T cell–specific transcription when brought close to an α chain enhancer located 3′ of the Cα gene. Unsuccessful rearrangements of the TCR α gene on both chromosomes lead to a failure of positive selection. Thymocytes of the αβ T-cell lineage that fail to make a productive rearrangement of the TCR α chain gene die by apoptosis.

TCR α gene expression at the double-positive stage leads to the formation of the complete αβ TCR, which is expressed on the cell surface in association with CD3 and ζ proteins. The coordinate expression of CD3 and ζ proteins and the assembly of intact TCR complexes are required for surface expression. Rearrangement of the TCR α gene results in deletion of the TCR δ locus that lies between V segments (common to both α and δ loci) and Jα segments. As a result, this T cell is no longer capable of becoming a γδ T cell and is committed to the αβ T-cell lineage. The expression of RAG genes and further TCR gene recombination cease after this stage of maturation.

Double-positive cells that successfully undergo selection processes go on to mature into CD4+ or CD8+ T cells, which are called single-positive thymocytes. Thus, the stages of T-cell maturation in the thymus can readily be distinguished by the expression of CD4 and CD8 (Fig.4). This phenotypic maturation is accompanied by commitment to different functional programs upon activation in secondary lymphoid organs. CD4+ and CD8+ T cells acquire unique properties during their maturation: CD4+ cells are able to produce different cytokines in response to antigen stimulation and to express effector molecules (such as CD40 ligand) that activate B lymphocytes, dendritic cells, and macrophages; and CD8+ cells are able to produce molecules (such as perforin and granzymes) that kill other cells. Mature single-positive thymocytes enter the thymic medulla and then leave the thymus to populate peripheral lymphoid tissues.

Fig4. CD4 and CD8 expression on thymocytes and maturation of T cells in the thymus. The maturation of thymocytes can be followed by changes in expression of the CD4 and CD8 coreceptors. A two-color flow cytometric analysis of thymocytes using anti-CD4 and anti-CD8 antibodies, each tagged with a different fluorochrome, is illustrated. The percentages of all thymocytes contributed by each major population are shown in the four quadrants. The least mature subset is the CD4−CD8− (double-negative) cells. Arrows indicate the sequence of maturation. TCR, T-cell receptor.

اخر الاخبار

اخبار العتبة العباسية المقدسة

أربعة محاور معرفية شهدتها منافساتُ الجولة الثانية من مسابقة (بصائر العلم)

مدارس الكفيل النسوية: مسابقة (بصائر العلم) مشروع تربوي حوزوي متكامل

بمشاركة 8 فرق.. انطلاق الجولة الثانية من مسابقة (بصائر العلم)

عبر وحدة الخدمة الإنسانية.. قسم حفظ النظام يقدّم خدماته لكبار السنّ من الزائرين

المزيد

الآخبار الصحية

طريقة مبتكرة للكشف المبكر عن أمراض القلب والشرايين

منظمة الصحة تحذر من خطر إدمان الشباب لأكياس النيكوتين

خبراء يحذرون من عادة شائعة تهدد صحة الملايين

خبير يكشف 5 حقائق طبية صادمة وغير معروفة

المزيد

الآخبار التكنلوجية

الأولى في العالم.. الصين تجري تجربة رائدة في الفضاء باستخدام "جنين اصطناعي"

دراسة حديثة تفند ربط التستوستيرون بالمخاطرة لدى الرجال

ميتا تضيف وضع "المحادثة المخفية" إلى تطبيق "واتساب"

فوبيا المرتفعات ليست من الدماغ.. دراسة تكشف السبب

المزيد

مواضيع ذات صلة

Mechanisms that Limit Innate Immune Responses

Stimulation of Adaptive Immunity

Systemic and Pathologic Consequences of Inflammation

Immuno microscopy

V(D)J Recombination

Role of Macrophages in Tissue Repair

Ingestion and Killing of Microbes by Activated Phagocytes

Sequence of Events in Inflammation: Vascular Changes and Leukocyte Migration Into Tissues

Double Antibody Sandwich ELISA (DAS ELISA)

Enzyme Immunosorbent Assays

Antibody Structure and Function

Apoptosis in the Immune System