Download Immunology Lecture 3 Feb 7 2013

Immunology Lecture 3
Antigen Recognition by
T Lymphocytes
T lymphocytes
 T lymphocytes–antigen specific cells that are related to and perform
complementary functions to B lymphocytes
 T cell receptor (TCR)–antigen receptor on T cells
 TCRs are similar to B cell receptors (BCRs) in many ways:
 Structure is similar to immunoglobulin (Ig) structure
 Are produced as a result of gene rearrangement
 Are highly variable and diverse in antigen specificity
 Express a single species of antigen receptor
Antigens recognized by TCRs are very different from
those recognized by Igs.
 BCRs recognize a broad range of full intact lipids, carbohydrates, or proteins.
 TCRs only bind peptide (small pieces of protein) antigens presented by major
histocompatibility (MHC) molecules on the surface of other cells.
 MHC molecules are so named because they vary greatly in the human
population and differences in MHC molecules between individuals are
responsible for graft rejection in transplants.
The TCR resembles a membrane-associated Fab
fragment of Ig.
 The TCR is composed of two polypeptide chains, TCRa and
TCRb.
TCRa and TCRb genes have similar germline
organization as Ig heavy (H) and light (L) chain genes

TCR genes are organized into variable (V) and constant (C)
gene segments.
TCR gene segments encode immunoglobulin fold
domains similar to those in Igs

V domains of the TCRa and TCRb chains
form the antigen-recognition site.


Like Igs, TCRa and TCRb also contain 3
hypervariable regions HV1, HV2, and
HV3 that are sometimes called
complementarity determining regions
(CDR1, CDR2, CDR3) because they form
the antigen binding site.
V and C domains of the TCR are followed by a
membrane anchoring domain.
TCRs are unlike Igs in that:
 TCRs have only one antigen binding site.
 TCRs are always surface receptors, never soluble antigen binding
molecules.
 TCRs always bind antigen in context of two opposing surfaces with
multiple antigen-MHC complexes on one surface binding TCR on the
opposing surface.
TCR diversity is generated by gene rearrangement
 In the TCR all diversity is generated before encounter with antigen.
 Occurs during T cell development in the thymus
 TCRa and TCRb are encoded on separate chromosomes.
 TCRa genes
 Contain one C region
 Contain only V and J gene segments like Ig L chains
TCR diversity is generated by gene rearrangement
 TCRb genes
 Contain two C region genes with no known functional difference.
 Contain V, D, and J gene segments like Ig H chains
TCR diversity is generated by gene rearrangement
 TCRβ genes
 Involves mechanisms similar to Ig gene rearrangement including
 First D-J rearrangement then V-DJ rearrangement
 Diversity of variable domain due to:
Mixing and matching of different V, D and J regions
TCR diversity is generated by gene rearrangement
 The pairing of different a and b chains provides diversity.
 The generation of the TCR DOES NOT include processes
analogous to those in Ig formation that occur after encounter
with antigen:
 TCRs do not under go somatic hypermutation
 TCRs do not undergo a process analogous to class
switching
Expression of the TCR on the cell surface requires
association with additional proteins.
 TCRs only reach the cell surface in
association with 4 invariant membrane
proteins that are collectively called the
CD3 complex.
 3 of these proteins are homologous
and designated CD3g, CD3d, and
CD3e.
 The 4th protein of the CD3 complex,
the z chain, is encoded on a
separate chromosome.
Expression of the TCR on the cell surface requires
association with additional proteins.
 The TCR complex is formed from TCRa and TCRb and the CD3
complex.
 TCRa and TCRb have short cytoplasmic domains that lack signaling
function.
 The CD3 complex interacts with intracellular proteins to send a signal
when the TCR receptor binds antigen.
Gamma (g and delta (d chains form a second class of
TCR expressed by a distinct population of T cells.
 Instead of the a:b TCR that is expressed on a:b T cells some T cells
called g:d T cells express a g:d TCR composed of
 A g chain that is similar to the a chain.
 A d chain that is similar to the b chain.
Gamma (g and delta (d chains form a second class of
TCR expressed by a distinct population of T cells.
 g:d T cells are rare in the circulation but common in epithelial
tissue.

Overall g:d T cells form only a small subset of T cells and
references to T cells generally are to a:b T cells.
Antigen processing and presentation
Antigen processing –degradation of pathogen proteins into short
peptide fragments that bind to MHC
Antigen presentation –binding of the peptide antigen by an MHC
molecule and display on the cell surface
Two classes of T cells are specialized to respond to
intracellular and extracellular sources of infection.
 Circulating T cells will express either the
CD4 or the CD8 glycoprotein which
confer distinctly separate functions to
the T cell:

CD8 T cells (cytotoxic T cells)–kill
cells that have become infected with
intracellular (endogenous)
pathogens.

CD4 T cells (helper T cells)–help
other cells of the immune system
respond to extracellular
(exogenous) pathogens.
Two classes of CD4 T cells
 Two major types of CD4 T cells:
 TH1 cells–secrete cytokines at the
site of infection that activate
macrophages.
 TH2 cells–secrete cytokines in
secondary lymphoid organs that
primarily stimulate B cells to make
antibodies to bind to extracellular
pathogens and virus particles.
Two classes of MHC molecule present antigen to CD8
and CD4 T cells respectively
MHC class I molecules–present antigens of
intracellular origins to CD8 T cells.
MHC class II molecules–present antigens of
extracellular origin to CD4 T cells.
CD8 and CD4 are described as co-receptors
 CD8 and CD4 are described as co-receptors:
 When peptide antigen presented by the MHC interacts with the TCR
specific to the antigen, MHC I interacts with CD8 or MHC II interacts
with CD4 on the T cell. (Remember a given T cell will express only
CD4 or CD8).
HIV selectively infects CD4 T cells by using CD4 as its primary receptor for
entering the cell.
 MHC class I
 is made up of a transmembrane heavy
chain (a chain) that is bound to a small
protein, b2 microglobulin.
 The MHC class I a chain
 Contains 3 extracellular Ig-like
domains
 Has a peptide-binding site formed by
a groove between two of the
extracellular domains. (Note b2
microglobulin does not form part of
peptide binding site)
 Is encoded by the major
histocompatibility (MHC) genes (B2
microglobulin is not)
MHC class I α genes are expressed by all cell
types except erythrocytes
MHC class II
 Is composed of 2 transmembrane
chains (a and b
 Each chain is composed of two Iglike domains
 Uses both chains to form the peptide
binding site
MHC class II α and β genes are expressed by
antigen presenting cells (APC)
Example: Dendritic, Macrophages and B cells
MHC molecules bind a variety of peptides.
 The MHC peptide binding sites
 Are formed from a deep groove on the surface of the MHC molecule.
 Have degenerate binding specificity–the ability to bind several
different amino acid sequences. (This contrasts with TCRs which
have a single specificity)
Peptides generated in the cytosol are transported into the
endoplasmic reticulum (ER) where they bind MHC class I
molecules.
Peptides generated in the cytosol are transported into the
endoplasmic reticulum (ER) where they bind MHC class I
molecules.
 Proteasome–barrel shaped protein
complex that degrades protein in the
cytosol.
 When viruses infect cells they use the cell's
protein synthesis machinery to make viral
proteins that are processed and presented
in MHC just like cell proteins.
Formation of Peptide MHC class I complex
 Transport associated with antigen processing (TAP) protein
embedded in the ER membrane transports cytosolic peptides into
the ER
 When MHC class I heavy chain
proteins first enter the ER they bind
to calnexin–a membrane protein
that maintains Ig fold structure.
 Once B2 microglobulin binds the
MHC class I protein the fully formed
MHC class I is released from
calnexin and then forms a complex
with calreticulin and tapasin.
Formation of Peptide MHC class I complex
 Calreticulin–a soluble molecule
similar to calnexin which likely
continues to maintain Ig fold structure
 Tapasin–binds the TAP protein
positioning the MHC class I to await
receipt of a suitable peptide antigen
 MHC class I does not leave ER until peptide antigen is bound.
Formation of Peptide MHC class I complex
Formation of Peptide MHC class I complex
Bare lymphocyte syndrome–a rare disease characterized by defective
TAP protein resulting in
– MHC class I retention in ER
because it never receives
peptide antigen
– Poor CD8 T cell responses to
viruses
Peptides presented by MHC class II molecules are generated
in acidified intracellular vesicles.
 Cells take up material at their surface by endocytosis.
 Phagocytosis–the uptake of large objects or dead cells by
specialized cells like macrophages
 Membrane bound structures called endocytic vesicles or
phagosomes contain the engulfed material.
 Phagolysosomes are formed when phagosomes fuse with the
acidic contents of lysosomes.
 Extracellular pathogens and proteins
 Are taken up by phagocytosis and degraded in the
phagolysosome
 Are assembled with MHC class II in the lysosome
MHC Class II molecules are prevented from binding
peptides in the ER by the invariant chain.
 MHC class II proteins in the ER associate with the invariant chain.
 The invariant chain protein
 Prevents MHC class II from binding
peptides in the ER
 Delivers the MHC class II protein to the
phagolysosome for binding to extracellular
proteins.
 Contains a segment called CLIP–Class II
associated invariant chain peptide
segment that covers the peptide binding
site.
The HLA DM protein in the phagolysosome interacts with MHC class II,
releasing the CLIP so the peptide antigen can bind MHC class II.
 Once MHC class II has bound peptide antigen it travels to the cell
surface.
The two classes of MHC molecule are expressed
differentially on cells
 Expression of MHC class I by all cells (except erythrocytes) allows them to
be under constant surveillance by CD8 T cells for infection by intracellular
pathogens.
 MHC Class II alerts the immune system to extracellular pathogens and is
expressed only on professional antigen presenting cells:
 Macrophages (take up antigens by phagocytosis)
 B cells (internalize antigens bound to surface Ig)
 Dendritic cells (DCs)–specialized antigen presenting cells found in
lymph nodes and other lymphoid tissues
 During an immune response cytokines cause cells to upregulate expression
of MHC molecules which enhances antigen presentation.
The MHC class I and class II genes occupy different
regions of the MHC
 The Human MHC is called the human leukocyte antigen (HLA) complex.
 The genes for the HLA complex are in one particular region of human
chromosome 6.
 Genes for MHC I on one end and MHC II on the other.
 MHC class III genes are located in the middle of chromosome 6.
Other proteins involved in the immune response are
encoded in the MHC region
 The MHC class III region contains genes for other
proteins of the immune system including complement.
 Other genes encoded by the MHC include
 TAP proteins
 Components of the proteasome
MHC restriction
 T cells are MHC restricted–the antigen sensitive response of a
particular T cell is restricted to a particular MHC and peptide
combination.
 MHC restriction of T cells results in an individuality of T cell responses.
MHC polymorphism triggers T-cell reactions that can
reject transplanted organs.
 Alloantigens–molecules such as MHC that vary from one individual to
another.
 Allogenic differences–differences between molecules such as MHC
from one individual to the next.
 Transplant rejection is due to an alloreaction–an immune response in
which T and B cells recognize and respond to allogenic differences
between the transplant donor and the transplant recipient.
 The greatest danger from bone marrow transplant is not graft rejection,
but graft versus host disease – an immune response mounted by
transplanted cells against the recipient
MHC polymorphism triggers T-cell reactions that can
reject transplanted organs.
 Alloantibodies–antibodies generated in one member of a
species against allotypic proteins in another member of the
same species.
Transplantation
 HLA type–the combination of the HLA alleles expressed in an
individual.
 HLA type is used to match transplantation donors and recipients.
 For tissues other than bone marrow considerable HLA mismatch
and potential transplant rejection can be overcome by
immunosuppresive drugs.
 However for bone marrow transplant the tissues transplanted in are
those that form the immune system