Immunology Ch 4 71-91
Antigen Recognition in Adaptive Immune System
-B and T cells express different receptors that recognize antigens: membrane-bound antibodies
on B cells and T cell receptors (TCRs) on T cells
-total number of distinct lymphocyte clones is large, and makes up immune repertoire,
recognizing different antigens
Antigen Receptors of Lymphocytes
-plasma membrane antibodies that function as antigen receptors of B lymphocytes recognize a
much broader range of chemical structures than the antigen receptors of T cells (proteins, lipids,
carbs), whereas T cells see only peptides that are displayed on APCs bound to MHC genes
-Antigen receptor molecules consist of regions involved in antigen recognition (varying between
clones of lymphocytes) and other regions required for structural integrity and effector functions
(conserved between clones)
-Variable region – antigen binding region; constant region (conserved)
-antigen receptor chains are associated with invariant membrane proteins whose function is to
deliver intracellular signals that are triggered by antigen recognition
-these signals may cause lymphocyte to divide, differentiate, or die
-thus, 2 functions of antigen receptors are antigen recognition and signal transduction
-in B cells, antigen receptor is called B cell receptor (BCR) complex
-in T cells, antigen receptor is called T cell receptor (TCR) complex
-antigen binding causes complexes to be brought together to initiate cascade
-antibodies exist in 2 forms, membrane bound antigen receptors on cells or secreted proteins
-TCRs exist ONLY as membrane receptors on T cells
-secreted antibodies are in the blood and mucosa where they neutralize microbes and toxins
-antibodies are also called immunoglobulins, and have number of functions to eliminate
antigens, such as complement binding, direct elimination, bind to phagocytes
Antibodies – composed of 4 polypeptide chains including two identical heavy chains and two
identical light chains, each chain containing a variable region and a constant region
-each light chain is attached to each heavy chain, and the two heavy chains are connected all by
disulfide bonds
-a light chain is made up of one V and one C domain; heavy chain has one V and 3 or 4 C domain
-each domain folds into an immunoglobulin domain (Ig) which is a loop held by disulfides
-antigen binding site of an antibody is composed of the V regions of both the heavy chain and
the light chain, and each core antibody structure has 2 identical antigen binding sites
-Fab fragment – 2 variable and 2 constant regions on the heavy and light chains (arms on Y)
-Fc fragment – The rest of the heavy chain C domain (body of Y shape)
-Two types of light chains (κ and λ) differing in C regions
-Five types of heavy chains (μ, δ, γ, ε, α), which differ in C regions
-Antibodies that contain different heavy chains belong to different classes (Isotypes) named
IgM, IgD, IgG, IgE, and IgA
-antigen receptors of naïve B cells are IgM and IgD; after maturation, some progeny may secrete
IgM or IgD, but others can also be secreted, known as heavy-chain or isotype switching
-antibodies are capable of binding a wide variety of antigens, including macromolecules and
small chemicals
-parts of antigens that are recognized by antibodies are called epitopes or determinants, and
they bind reversibly and non-covalently
-strength of which one antibody binds to one epitope is called affinity of the interaction
-often expressed as dissociation constant Kd, which is the molar concentration of an
antibody required to occupy half the available antibodies in solution
-in a primary immune response, antibodies have a Kd between 10^-6 and 10^-7, but after
repeated exposure, the affinity increases to a Kd of 10^-8 to 10^-11, called affinity maturation
-IgG, IgD and IgE have 2 antigen binding sites; IgA has 4 antigen binding sites; IgM has 10
-total strength of binding is much greater than affinity of a single antibody/antigen bond as is
called avidity of interaction
-binding to different epitope by same antibody is called cross-reaction
Monoclonal Antibodies – one clone of B cells produces one antibody specificity
Hybridoma – fusing two cell populations and culturing them with drug makes it possible to grow
out fused cells derived from B cells making one antibody of desired specificity
T cell Receptors for Antigens – TCR recognizes peptide antigens displayed on MHC; TCR is a
membrane-bound heterodimer of an α chain and a β chain, each containing one variable V
region and one constant C region
-both the α and the β chains of TCR participate in specific recognition of MHC bound peptides
-can recognize as little as 1-3 residues of MHC bound peptide
-5-10% of T cells contain TCRs with γ and δ chains; γδ TCR can recognize a variety of protein and
nonprotein antigens not usually displayed on MHC
-these are abundant in epithelia
-Natural Killer Cells (NK-T) cells express αβ TCRs and recognize MHC class I molecules
-TCR can recognize antigen but is unable of transmitting signal on its own
-associated with the TCR is a complex of proteins called CD3 and ζ, which make up the
TCR complex
-CD3 and ζ transmit some signals that are initiated when TCR recognizes antigen
-furthermore, T cell activation requires activation of a co-receptor CD4 or CD8, which recognize
nonpolymorphic portions of MHC molecules and also transmit activating signals
Development of Immune Repertoires – there are not enough genes in genome to produce
enough T cells for every antigen in existence
Early Lymphocyte Development – development of lymphocytes from bone marrow stem cells
involves commitment of hematopoietic progenitors to B or T lineage, the proliferation of these
progenitors, the rearrangement of expression of antigen receptor genes, and selection events to
identify and expand cells that express potentially useful antigen receptor
-commitment to B cell or T cell lineages is associated with changes in common lymphoid
progenitors in the bone marrow
-Immature lymphocytes undergo proliferation at several stages of maturation
-IL-7 stimulates growth of early lymphocytes produced by bone marrow stromal cells
and expands number of lymphocyte progenitors before they express antigen receptor
-Antigen receptors are encoded by several gene segments that are separate from one another in
the germline and that recombine during lymphocyte maturation (varies nucleotide sequence)
-lymphocytes are selected at multiple steps in maturation to preserve the useful specificities
-selection is based on expression of intact antigen receptor (die if not expressing it )
-immature T cells are also selected to recognize self MHC molecules (positive selection)
-strongly self reactive T cells are killed in a process called negative selection
Production of Diverse Antigen Receptors – formation of functional genes that encode B and T
cell antigen receptors is initiated by somatic recombination of gene segments that code for
variable regions of the receptors, and diversity is generated during this process
-between the variable and constant genes lie short sequences called diversity (D) and joining (J)
segments
-all antigen receptors have V, C, and J regions, and only Ig heavy chain has D as well
-commitment of lymphocyte progenitor to become B lymphocyte is associated with
recombination of randomly selected gene segments in Ig heavy chain locus to form VDJ exon
-VDJ exon is tanscribed and spliced to C-region exons of μ heavy chain, which is
translated and becomes the first Ig protein synthesized during B cell maturation
-recombination of VDJ segments is mediated by VDJ Recombinase enzyme composed of
recombinase activating gene 1 and 2 (RAG1 and 2)
-recombinase brings 2 Ig or TCR segments close together and cleaves at specific sites
which are later ligated by ligases
-diversity of antigen receptors is produced by the use of different combinations of V, D, and J
gene segments in different clones of lymphocytes (combinatorial diversity), and even more by
changes in nucleotide sequences introduced at the junctions of the recombining V, D, and J gene
segments (Junctional diversity)
-junctional sequences and D and J segments encode amino acids of CDR3 loop (most
variable of the CDRs, and is the MOST IMPORTANT for antigen recognition
-junctional diversity maximizes variability in antigen-binding regions of antibody and TCR
Maturation and Selection of B cells – maturation of B cells occurs mainly in bone marrow
-progenitors give rise to large number of pro-B cells which begin to rearrange Ig genes
-cells that make productive VDJ rearrangements at Ig heavy chain locus develop into pre-B cells
defined by presence of Ig μ heavy chain protein in cytoplasm
-the μ chain associates with Igα and Igβ signaling molecules to form pre-B cell receptor (preBCR) complex which delivers signals to promote survival and proliferation of that lineage (first
checkpoint in B cell development)
-Pre-BCR complex also signals to shut off recombination of Ig heavy chain genes on the 2nd chr.
Because each B cell can express Ig heavy chain from only one allele, called allelic exclusion
-pre-BCR also triggers recombination of light chain Ig κ and λ
-whichever functional light chain is produced associates with the μ chain to form complete
membrane-associated IgM antigen receptor, which signals for survival of that B cell (second
checkpoint)
-IgM expressing B lymphocyte is called immature B cell, and its final step in maturation may
occur in bone marrow or after it leaves, and it involves coexpression of IgD with IgM which
occurs because recombined VDJ exon may be spliced to the μ or the δ mRNA
-IgM+/IgD+ cell is known as the mature B cell and is able to respond to antigen
-B cell repertoire is shaped by negative selection, and is done through random V-J
recombination
Maturation and Selection of T Lymphocytes – the most immature progenitors are pro-T cells or
double-negative T cells because they do not express CD4 or CD8, and expand under influence of
IL-7 in the thymus
-TCRβ gene recombination, mediated by VDJ recombinase, occurs in some of these cells
-if VDJ recombination is successful in one of the 2 inherited loci and a TCR B-chain is synthesized,
it is expressed on the surface in association with an invariant protein called pre-Ta, to form preTCR complex of pre-T cells
-if complete TCR B chain is not produced in a pro-T cell, it dies
-pre-TCR complex delivers signals to promote survival, proliferation and TCR-a gene
recombination and inhibit VDJ recombination of second TCR-B gene
-failure to express a-chain results in death of the cell
-the surviving cells express complete aB TCR and both CD4 AND CD8 receptors, called doublepositive T cells
-different clones of double-positive T cells express different aB TCRs
-if T cell recognizes antigen in the thymus, but it is low affinity, it is selected to survive
-T cells that do not recognize an MHC molecule in the thymus die by apoptosis because they
would not be useful ( positive selection)
-during this process, T cells that recognize MHC class I preserve CD8 and lose CD4
expression
-conversely, T cells that recognize MHC class II preserve CD4 and lose CD8
-they become single-positive T cells
-immature, double-positive T cells who recognize MHC receptors undergo apoptosis in a process
known as negative selection