TEMA 5. ANTICUERPOS Y SUS RECEPTORES

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1 TEMA 5. ANTICUERPOS Y SUS RECEPTORES The B-cell antigen receptor (left) 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 signalling pathways. Circulating antibodies (right) are structurally identical to the primary B- cell antigen receptors, except that they lack the transmembrane and intracytoplasmic sections. Many proteolytic enzymes cleave antibody molecules into three fragments - two identical Fab (antigen binding) fragments and one Fc (crystallizable) fragment. Electrophoresis of human serum showing the distribution of the four major immunoglobulin classes. Serum proteins are separated according to their charges in an electric field, and classified as α1, α2, β andγ, depending on their mobility. (The IgE class has a similar mobility to IgD but cannot be represented quantitatively because of its low level in serum.) IgG exhibits the most charge heterogeneity, the other classes having a more restricted mobility in theβand fastγregions. 1

2 Each immunoglobulin class has a characteristic type of heavy chain. Thus IgG posessesγchains; IgM, µ chains; IgA, α chains; IgD, δ chains; and IgE, ε chains. Variation in heavy chain structure within a class gives rise to immunoglobulin subclasses. For example, the human IgG pool consists of four subclasses reflecting four distinct types of heavy chain. The properties of the immunoglobulins vary between the different classes. Note that in secretions, IgA occurs in a dimeric form (s-iga) in association with a protein chain termed the secretory component. The serum concentration of s-iga is very low, whereas the level in intestinal secretions can be very high. The N-terminal end of IgG1 is characterized by sequence variability (V) in both the heavy and light chains, referred to as the Vh and Vl regions respectively. The rest of the molecule has a relatively constant (C) structure. The constant portion of the light chain is termed the Cl region. The constant portion of the heavy chain is further divided into three structurally discrete regions: Ch1, Ch2 and Ch3. These globular regions, which are stabilized by intrachain disulphide bonds, are referred to as 'domains'. The sites at which the antibody binds antigen are located in the variable domains. The hinge region is a segment of heavy chain between the Ch1 and Ch2 domains. Flexibility in this area permits the two antigen-binding sites to operate independently. There is close pairing of the domains except in the Ch2 region (see Fig. 4.6). Carbohydrate moieties are attached to the Ch2 domains. 2

3 In each of the structures shown the carbohydrate side-chains are shown in blue. Inter heavy chain disulphide bonds are shown in red but interchain bonds between H and L chains are omitted. A model of IgG1 indicating the globular domains of heavy (H) and light (L) chains. Note the apposition of the Ch3 domains and the separation of the Ch2 domains. The carbohydrate units lie between the Ch2 domains. Polypeptide chain structure of human IgG3. Note the elongated hinge region. IgM heavy chains have five domains with disulphide bonds crosslinking adjacent Cµ3 and Cµ4 domains. The possible location of the J chain is shown. IgM does not have extended hinge regions, but flexion can occur about the Cµ2 domains. 3

4 The secretory component of s- IgA is probably wound around the dimer and attached by two disulphide bonds to the Cα2 domain of one IgA monomer. The J chain is required to join the two subunits. This diagram of IgD shows the domain structure and a characteristically large number of oligosaccharide units. Note also the presence of a hinge region and short octapeptide tailpieces. IgE can be cleaved by enzymes to give the fragments F(ab')2, Fc and Fc'. Note the absence of a hinge region. 4

5 All immunoglobulins have the basic four-chain structure. There are three types of immunoglobulin variability: Isotypic variation is present in the germ line of all members of a species, producing the heavy (µ, δ, γ, ε, α) and light chains (Igκ, λ) and the V-region frameworks (subgroups). Allotypic variation is intraspecies allelic variability. Idiotypic variation refers to the diversity at the antigen-binding site (paratope) and in particular relates to the hypervariable segments. These effector functions are associated with different parts of the Fc region. Placental transfer of IgG in man and intestinal transport in rodents are mediated by an MHC class-i-like receptor molecule (see Fig. 4.24) A complex family of receptor molecules able to bind immunoglobulin continues to be discovered (selected examples are listed here). FcµR is expressed by activated B cells but not by T cells or monocytes. FcεRII is also expressed on eosinophils, platelets, T cells and B cells. 5

6 The human Fcγ receptor structures shown are those for FcγRI (expressed by monocytes), FcγRIIa (expressed by monocytes and neutrophils), FcγRIIIa (expressed by monocytes and attached as a normal transmembrane protein) and FcγRIIIb (expressed by neutrophils and attached by a phosphatidyl inositol glycan [GPI] membrane anchor). Each receptor belongs to the immunoglobulin superfamily and expresses two or three extracellular immunoglobulin-like domains. Several of the receptors are now known to exist as complexes with various disulphide-linked subunits. FcγRI and FcγRIIIa both associate with dimers of theγ chain originally described as part of the highaffinity FcεRI complex (see Fig. 4.23). FcγRIIIa has also been shown to associate with dimers of theζchain found in the TCR-CD3 complex. In the case of FcγRIIIa these subunits can associate as either homodimers (γ-γ or ζ-ζ) or as heterodimers (γ-ζ). They appear to be essential for surface expression and signal transduction. In FcγRI interactions, the receptor appears to bind a structural motif centred around Leu 235 in the Ch2 domain, present in IgG1, IgG3 and IgG4. The model for FcεRI proposes a tetramer consisting of one α chain with two disulphide-linked immunoglobulin-like loops. Theβchain has two extracellular portions near twoγchains which are linked by disulphide bonds (red). Theαchain is crucial for IgE binding. The model for FcεRII is hypothetical, and is based on sequence data and the homology with animal lectins. Proteolytic cleavage can release several types of IgE-binding factors, including the 25-kDa soluble CD23 molecule, which contains the lectin domain. This cleavage is inhibited by IgE, accounting for the apparent increase of FcεRII expression on lymphocytes cultured in the presence of IgE. 6

7 Grupo 1 Grupo 2 Supervivencia al suero en cepas clínicas 7