IMMUNE CELL SURFACE RECEPTORS AND THEIR FUNCTIONS

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1 LECTURE: 07 Title: IMMUNE CELL SURFACE RECEPTORS AND THEIR FUNCTIONS LEARNING OBJECTIVES: The student should be able to: The chemical nature of the cellular surface receptors. Define the location of the cellular receptors. Enumerate the different given terms for the word "receptor". Define the term "CD" marker. Describe the location of the immune receptors. Classify the cellular receptors according to the information they offer. Classify the cellular receptors according to their families. Enumerate some common immunoglobulin receptors. Enumerate some common complement receptors. Enumerate some adhesion molecules receptors. Enumerate the main functions of the immune cell receptors. Realize the importance of the immune receptors. LECTURE REFRENCE: 1. TEXTBOOK: ROITT, BROSTOFF, MALE IMMUNOLOGY. 6 th edition. pp. 29, 143, 76, 77, 78, 221, TEXTBOOK: ABUL K. ABBAS. ANDREW H. LICHTMAN. CELLULAR AND MOLECULAR IMMUNOLOGY. 5 TH EDITION. Chapter. 6.pg 105. Chapter 4. pg

2 Lymphocytes can be identified by characteristic markers Lymphocytes and other leukocytes express different surface molecules which can be used for distinguishing cell population. A systemic nomenclature called the CD system has been developed, in which the term CD refers to (cluster of designation). These different cell surface molecules are detected with specific monoclonal antibodies, each of these molecules are given CD number. Molecular markers can be further defined according to the information they offer about the cell. For example; A. Lineage markers are exclusive to a particular cell line, e.g., CD3, which is found only on T cells. B. Maturation markers are transiently expressed during cell differentiation, e.g., CD1, which is only found on cells developing in thymus and is not present on peripheral T cells. C. Activation markers. e.g., low affinity T-cell growth factor receptor ((IL-2 receptor, CD 25), which is only expressed when the cell is stimulated with antigen. This way to define markers is useful but is not always used, because, maturation markers for one lineage is sometime an activation marker for another. For example; B cells express MHC class II antigens during most of their life time, but human T cells only express these marker when activated with antigen. Cell markers can be grouped into families The cell surface molecules (markers) exist as a number of different families. A. The immunoglobulin superfamily contains molecules whose structural characteristic are similar to those of the immunoglobulins. This family includes CD2, CD3, CD4, CD8, murine Thy-1, and many more. B. The integrin family consists of heterodimeric molecules containing α and β chains. There are a number of integrin subfamilies; all members of a particular subfamily share a common β chain, but each has a unique α chain. One subfamily the β 2 integrins uses CD18 as the β chain. This chain can be associated with CD11a, CD11b, or CD11c; these combinations make up the lymphocyte function antigen LFA-1, Mac-1 (CR3) and p150, 95 (CR4) surface molecules respectively and are commonly found on leukocytes. The second subfamily, the β 1 integrins has CD29 as the β chain, again associated with various other polypeptides, and includes the VLA (very late activation) markers. C. Selectin (E, L, and P), expressed on leukocytes (L), or activated endothelial cells (E and P). They have lectin like specificity for a variety of sugars expressed on heavily glycosylated membrane glycoproteins. D. Proteoglycans typically CD44, which can bind to components of extracellular matrix. The cell surface molecules (markers) can be demonstrated by using fluorescent antibodies as probes. This exploited by the technique of flow cytometry, which can enumerate and separate cells on the basis of their size and fluorescence intensity. 2

3 T Lymphocytes T cells can be distinguished by their different antigen receptors There are two defined Tcell receptors. TCR-2 is a heterodimer of two disulphide-linked polypeptides (α,β); and TCR-1 is structurally similar but consists of γ and δ polypeptides. Both receptors are associated with a set of five polypeptides (δ, γ, ε, ζ, and η), the CD3 complex, to give the T-cell receptor complex (TCR-CD3 complex). Approximately % of blood T cells express TCR-2 (α,β), and the remaining 5-10 % are TCR- 1 + (γ,δ) Figure 1, and 2. Figure 3 shows the three dimensional structure of the TCR. TCR-2 + cells are further distinguished by their expression of CD4 or CD8. TCR-2 + T cells can be subdivided into two distinct nonoverlapping populations: a subset which carries the CD4 marker (MHC class II receptor) and mainly help or induce immune responses (TH), and the other subset which express the CD8 marker (MHC class I receptor) and it is predominantly cytotoxic (TC). The expression of either CD4 or CD8 determine the type of cells that the T cell will react to, because T cells carries CD4 will recognize the antigens associated with MHC class II, and T cells which carries the CD8 marker will recognize the antigens associated with the MHC class I. Most TCR-1 + cells do not express either the CD4 or the CD8 marker; although a few of them may be CD8 +. Figure 1 The TCR α and β (or γ and δ) chains each comprise an external V and C domain, a transmembrane segment containing positively charged amino acids and a short cytoplasmic tail. The two chains are disulphide linked on the membrane side of their C domains. The CD3 γ and δ and ε chains comprise an external 3

4 immunoglobulin-like C domain, a transmembrane segment containing a negatively charged amino acid and a longer cytoplasmic tail. A dimer of ζζ, ηη, or ζη is also associated with the complex. Several lines of evidence support the notion that the TCR-CD3 complex exists at the cell surface as a dimer. The transmembrane charges are important for the assembly of the complex. A; plausible arrangement which neutralize opposite charges is shown. Figure 2 Schematic diagram shows the T cell receptors and CD3 molecules. 4

5 Figure 3 Three-dimensional structure of an αβ TCR. The chain is colored blue (residues 1-213), and the β chain is colored green (residues 3-247). The β strands are represented as arrows and labeled according to the standard convention used for the Ig fold. The disulphide bonds (yellow balls for sulphur atoms) are shown within each domain and for the COOH-terminal interchain disulphide. The CDRs lying at the top of the diagram are numerically labeled (1-4) for each chain. These form the binding site for antigen/mhc. There are functional subsets of TCR-2 + CD4 + cells On the basis of function, CD4 + cells can be subdivided into two sets of cells. A. CD4 + cells which positively influence the response of T cells and B cells (the helper cell function) are CD29 +. All the cells of this population also express the CD45R0 (leukocyte common antigen). Thus this population will carry the CD4 + CD29 + CD45R0 markers and they are helper T cells. B. CD4 + cells that induce the suppressor/cytotoxic functions of CD8 + cells (the suppressor/inducer function), these cells also carry CD45RA marker. The CD45RA and CD45R0 TH cells have been found to be naive and memory cells respectively. But a current opinion holds that the expression of CD45R0/CD29 by CD4 + cells is more relevant to the state of activation of the cell. TCR-2 + CD4 + lymphocytes can also be classified on the basis of the cytokine secretion The TH cells can be divided into two sets of cells depending on the cytokine secretions. TH1 subset secretes IL-2 and IFN γ and the TH2 subset produces IL-4, IL-5, IL-6, and IL-10. TH1 cells mediate several functions associated with the cytotoxicity and local inflammatory reactions. Generally these cells are important for combating intracellular pathogens including viruses, bacteria and parasites. TH2 cells are more effective at stimulating the B cells to proliferate and produce antibodies, the there fore function primarily to protect against free-living microorganisms (humoral immunity). Other criteria for subdividing CD4 + cells, a rare subset of CD4 + cells expresses NK cell markers, does not produce the lymphokine IL-2, and does not proliferate in response to antigen and mitogen. There are subsets of CD8 + cells (expresses MHC class I receptors) CD8 + (cytotoxic) T cells can be subdivided into specific functional subsets according to a number of criteria and using a variety of monoclonal antibodies. One subset expresses CD28 molecules and produces IL-2 in response to activation signals. Another subsets respond to (but does not produce) IL-2 and expresses the heterodimeric CD11b /CD18 molecule (CR3). Cells of the first subset show a distinct Gall body, whereas cells of the second subset show LGL morphology. TCR-1 + T cells are relatively enriched in epidermal and mucosal surfaces ( has LGL morphology) and consists of heterodimer of γ and δ. It represents about 5-10 % of blood T cells. It has been shown that, TCR-1 + T cells are most abundant in the mucosal epithelia, and form only a minor subpopulation of circulating T cells. It has been recently found that, TCR-1 + CD8 + cells have a repertoire of T- cell receptors biased towards bacterial antigens, and these cells play an important role in protecting the mucosal surfaces of the body. T cells share some markers with other cell lineages 5

6 CD2 which is found on all T cells and bind the sheep red blood cells is also found on the surface of 75 % of NK cells. Another molecule which also found of all T cells is CD5; this marker is also found on the subpopulation of B cells. CD7 is expressed on T cells, but it is also found on the majority of NK cells. Suppressor T cells Although there is a clear evidence for the presence of the antigen specific suppressor T cells (Ts), it is unlikely that they represent a functionally separate subset. Both CD4 + and CD8 + Tells can suppress immune responses. B Lymphocytes B cells are characterized by their surface immunoglobulins B lymphocytes represent about 5-15 % of the circulating lymphoid pool, and are classically defined by the presence of surface immunoglobulins. These immunoglobulin markers are made by the B cells themselves, and are inserted into the surface membrane where they act as specific antigen receptors. B-cell receptor complex The majority of human B cells in peripheral blood express two immunoglobulin isotypes on their surface, IgM and IgD. On any given cells the binding sites of these isotypes are identical. Very few cells in the circulation express IgG, IgA, or IgE, although these are present in larger numbers in specific locations in the body, for example; IgA bearing cells in the intestinal mucosa. The surface IgM is associated with other molecules on the B cell surface to form the B-cell antigen receptor complex (BCR). These accessory molecules consist of disulphide-bonded heterodimers of Ig α (CD79a, a 34kDa molecule and product of the mb-1 gene) and Ig β (CD79b, a 39 kda molecule and product of the B29 gene) Figure 4. Figure-4 B cell receptor is the primary immunoglobulin receptor consists of two identical heavy (H) chains and two identical light (L) chains. In addition, secondary components (Ig-α, and Ig-β) are closely associated with the primary receptor and are thought to couple it to intracellular signaling pathways that activate IL-2 production. Circulating antibodies are structurally identical to the primary B cell antigen receptors, except that they lack the transmembrane and intracytoplasmic sections. Other B-cell markers and subsets 6

7 The majority of B cells carry MHC class II antigens, which are important for cooperative interactions with T cells. These class II molecules consist of HLA-DP, DQ, and DR antigens in man. Complement receptors for C3b (CR1, CD35) and C3d (CR2, CD21) are commonly found on B cells and are associated with activation and possibly homing of the cells. Fc receptors for exogenous IgG (FcγRII, CD35) are also present on B cell surface. CD19, CD20, and CD22 are the main markers currently used to identify human B cells. Other molecules are used to identify human B cells includes CD CD40 is an important marker on the B cell. A marker originally found on only T cells has now been shown to be present on the B cells; it identifies a subset of B cells that is predisposed to autoantibody production. These cells termed (B1a cells), while the conventional B cells are (B2 cells). Some B cells bind to (and form a rosette with) mouse erythrocytes (ME-R). NATURAL KILLER CELLS Natural killer cell represents about 15% of blood lymphocytes and expresses neither TCR nor BCR antigen receptors. Phenotypic markers of NK cells Most the cell surface markers of the NK cell are sheered with T cells or monocytes/macrophages. The major markers of human NK cell are: Marker CD16 (Fcγ RIII) CD11b CD 38* CD 2* CD 7 CD 8* CD 56 CD 57 IL-2R (β chain, P70) p58 family CD 94 Shared specificities minority of T cells, granulocytes, some of the macrophage granulocytes, monocytes, some T cells activated T cells, plasma cells, haemopoietic precursor All T cells All T cells some of T cells minority of T cells Some of T cells activated T cells Some T cells Some T cells * expressed in % of NK cells 7

8 CD 16 (Fcγ RIII) is used to identify the NK cells in the purified lymphocytes population. CD 16 is found on some macrophages, neutrophils, and small population of TCR-1 + T cells. CD 56 molecule is homophilic adhesion molecule of the immunoglobulin superfamily receptor. The absence of CD3 and the presence of CD56 and /or CD16 is currently in use as a definitive marker for NK cells in man. Resting NK cell express the α chain of the IL-2 receptor (70 kda). All T cells that show LGL morphology (TCR-1 + cells and proportion of TCR-2 + cells) also express this 70 kda receptor. Activation markers on lymphocytes These activation markers include; * Adhesion molecules (allow efficient interaction with other cells). * Receptors for growth and differentiation factors (required for continued proliferation and maturation). For example, IL-2 receptor which expressed following T cell activation is composed of three subunits. Resting T cells have the β unit (a low-affinity receptor of 55 kda (CD25), and the γ unit. On activation the α subunit (70 kda) is induced resulting in heterotrimeric high affinity IL-2 receptor. The gp39 (CD71) is a receptor for transferrin, it transiently expressed on the activated T cell, and it is important for proliferation. Class II MHC molecules are present on human T cell as a late activation marker. CD29 is expressed on the T cells and the memory cells as a very late activation marker. Activation markers on B cells include the high affinity IL-2R, and other receptors for growth and differentiation factors such as IL-3, IL-4, IL-5, and IL-6. CD71 (transferrin receptor), and elevated level of class II MHC molecules are also expressed. CD23 (FcεRII, a low-affinity IgE receptor) drives B cells into proliferation. CD38 is present on plasma cell. PCA-1 molecules are only found at the plasma cell stage of human B cell differentiation. Activation markers on the NK cells include MHC class II molecules. Dr. Mustafa Hasan Linjawi 8

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