What signaling proteins are present adjacent to immunoglobulins that initiate signal transduction upon antigen binding? What is the reason for the necessity of these signaling proteins?
Igs on B cells have Ig α, Ig β, CD 19, and CD 21 as signaling molecules. Note that CD 19 and 21 are present on all B-cells and only on B-cells. There are only approximately 3 aa of the Ab that extend into cytoplasmic side of B cell membrane. Need other proteins for signal transduction for this reason. When B cell binds antigen with Abs, this initaties aggregate, signal transduction--> activation of tyrosine kinases, second messengers, etc. Note: On each B cell, there are thousands of receptors but all have same specificity for antigen. need billions of B cells for this reason.
What are the components of T-cell receptors? What domains do they contain? How are the domains linked to one another?
How many antigens does one TCR recognize? How many different types of TCRs are present on one T-cell?
What signaling molecules are present in T-cells? Why are they required?
- The T-cell receptor is a two chain molecule. The TCR chains are named alpha, beta, gamma and delta and are expressed only as α,ß pairs or γ,δ pairs. The chains are linked by disulfide bonds.
- All T cell antigen receptors have a domain structure similar to antibody Fab fragments (i.e., both chains have a variable domain with hypervariable loops and one constant domain.
- Since each T cell, like each antibody-producing B cell, recognizes ONE antigenic determinant, it has ONE antigen receptor specificity, with a unique idiotype.
- Only ONE alpha chain variable region is expressed by each αß T cell, and only ONE beta chain. Similar rules exist for the γδ T cells.
- Noncovalently associated with the TCR on the surface of T cells is a molecular complex called CD3, which is composed of several membrane-bound polypeptides called gamma, delta, epsilon, zeta, and eta. The role of CD3 is to act as the signal transduction component of the antigen receptor. While CD3 does not recognize or interact with antigen, it undergoes a conformational change when the TCR binds to the MHC-antigenic peptide complex. This leads to the activation of intracellular protein tyrosine kinases that activate the effector functions of the T cell.
- Only a small portion of the TCR extends into the cytoplasm so signaling molecules are needed for signal transduction.
What are CD molecules?
• Cellular Differentiation molecules on cell surfaces.
– Differentiate one cell type from another.
– Differentiate one stage of cell differentiation from an earlier or later stage within the same cell
• Originally CD stood for “Cluster of Differentiation”
Are T-cell epitopes typically hydrophilic or hydrophobic? Why?
T-cell antigen receptors CANNOT recognize free, unprocessed antigen. They are only able to recognize peptides bound in the groove of a major histocompatibility Class I or Class II molecule. T cell epitopes therefore tend to be mainly hydrophobic, and are always linear determinants.
What is the approximate percentage of γ-δ T-cells in the body? Where are they found?
• Small percentage of total T cells (~5%). 95% have alpha and beta chains
• Found mainly near surface barriers (subepithelial regions in skin, submucosa of GI, respiratory tract) (unlike alpha-beta T-cells which are mostly found in LNs)
• Limited variability of TCR, probably directed against common pathogenstructures
Complete the attached table.
attached is slide 18 of course notes
Where are major histocompatibility complex (MHC) genes located? (what chromosome)
What is the name of their gene products?
How many classes of MHC gene products are there?
Cells of the immune system utilize surface glycoproteins to communicate (interact) with one another. MHC gene products are central in these interactions. While they were discovered and named for their role in mediating allograft rejection, this is not their main function. In humans the MHC gene products are called the HLA molecules (Human Leukocyte Antigens).
The MHC genes are found as a tightly linked group on Chromosome 6 in humans.
They are divided into three Classes, which are expressed on different tissues and have very different functions.
What are the Class I MHC molecules? What are the Class II MHC molecules?
Note that class I MHC molecules are named with one letter and class II MHC molecules are named with two letters.
What is the tissue distrubution of class I MHC molecules?
How many different types of class I MHC molecules are expressed on one cell?
ALL nucleated cells, platelets. MHC class I molecules are coexpressed so that each cell has 6 different MHC class I molecules on the cell surface-3 from mother and 3 from father (unless homozygous for any of them)
note that RBCs do not express MHC class I molecules-no nucleus
What chains are MHC class I molecules composed of? How do the chains interact with one another? Explain the structure of the protein.
At what end of the peptide (N or C) does peptide binding occur?
Class I MHC molecules consist of two chains:
1. a heavy (α) chain that has a transmembrane anchor near the carboxy-terminal end, and a peptide-binding groove near the amino-terminal end.
2. a light (β) chain called β2-microglobulin.
The alpha and beta chains interact non-covalently.
Is beta-2 microglobulin encoded for on the same chromosome as the alpha chain? What is its function?
β2-microglobulin: This chain is NOT encoded by a gene on chromosome 6, but instead, the gene is found on chromosome 15 in humans. The function of β2-microglobulin is to transport the MHC Class I molecule, with a peptide bound to the α chain, to the cell surface.
What are the functions of MHC class I molecules?
THE FUNCTION OF THE CLASS I MHC MOLECULES IS TO ACT AS THE TARGET FOR THE ELIMINATION OF ABNORMAL HOST CELLS.
An abnormal host cell is a cell that is:
(a) infected with an intracellular agent (viruses, intracellular bacteria, intracellular protozoal parasites, intracellular fungi, etc.),
(b) mutated or transformed to produce an abnormal cellular gene product (oncogenes activated in tumor cells, etc.).
Note: We need class I molecules on all nucleated cells because they can all be infected by a pathogen and can all have cancer causing mutations.
Explain the process of how MHC class I molecules present peptides to the surface.
What is the source of peptides presented by these molecules?
What kind of T-cells recognize peptides presented by class I MHC molecules? What does binding of these molecules with these T-cells trigger?
The class I MHC molecules carry out this function by binding to peptides that are actively being synthesized by the host cell. Proteins are degraded into peptides by proteosomes in the cytoplasm, and the peptides are transported into the rough endoplasmic reticulum for loading into the groove of an MHC Class I molecule. This is a very nonspecific process, so both self peptides and abnormal (for example, viral) peptides are bound to the Class I molecules, which then carry the peptides to the cell surface, where they are presented to the outside.
Cytotoxic T lymphocytes recognize the abnormal peptide bound to the Class I molecule. This recognition is an essential signal that triggers the release of cytotoxic molecules from the cytotoxic T cells, resulting in the destruction of the infected host cell, and the elimination of the infecting agent. Note that cytotoxic T-cells "check out" self peptides but do no react. If foreign antigen is presented by MHC class I, cytotoxic T-cell reacts and triggers cytotoxicity of infected cell. If have mutations of self proteins--> trigger cytotoxicity of host cell--> ridding of cancer
Note that there is a portion of the TCR that recognizes antigen peptide and a portion that recognizes MHC class I molecule
What is the structure of MHC class II molecules? How many chains are there? What are they called? What kind of peptides do they mainly bind? How long are these peptides?
Structure: MHC Class II
• Two chains ( & ), both anchored in cell membrane (looks like a roman numeral II)
• Peptide binding groove - binds mainly hydrophobic peptides 18-20 aa long
attached is slide 16 of notes
Where MHC class II molecules distributed?
Where is HLA-DM found? What is its function?
The MHC Class II molecules have a very restricted tissue distribution. They are found only on B lymphocytes, monocytes, macrophages, dendritic cells, Langerhans cells (skin), activated endothelial cells, and activated human T cells.
There are also three different Class II molecules expressed on cell surfaces: HLA-DP, HLA-
DQ, HLA-DR. A fourth MHC Class II molecule, HLA-DM, is found only within the endosomal compartment, and plays the role of transferring peptides to the other MHC class II molecules.
What is the function of MHC class II molecules? What kind of T-cells do they present peptides to?
What is the source of peptides presented by MHC class II molecules?
Explain the process of how MHC class II molecules present peptides to the cell surface.
- Proteins from outside these antigen-presenting cells are nonspecifically internalized by endocytosis. In the endocytic vacuoles in these cells, enzymes degrade (process) the proteins into small peptides. The Class II MHC molecules are synthesized by these cells, then transported into the same vacuoles, where they bind to the peptides.
- At the time they are synthesized, the α and β chains of the class II molecules are bound to a third chain, called the INVARIANT chain, which prevents any peptide binding to MHC class II molecule until it reaches the endosomal compartment, where the invariant chain is degraded.
- A remnant peptide called CLIP (CLass II-associated Invariant chain Peptide) remains in the peptide-binding groove of the MHC class II molecule until HLA-DM in the endosome catalyzes the exchange of CLIP for a processed exogenous peptide. The low pH in the endosome (~5.0) permits the proteolytic degradation of the invariant chain, and facilitates the exchange of CLIP for the processed peptide.
- The MHC Class II - peptide complex is then transported to the cell surface, where it can be recognized by Helper T cells, which have antigen receptors that ONLY recognize peptides bound to MHC Class II molecules.
What is the function of CD4 (helper T-cells) and CD8 (CTLs) as it pertains to MHC interactions?
The surface molecules, CD4 and CD8, are involved in the stabilization of the T-cell receptor-antigen-MHC interaction. The CD3 T-cell surface molecule, of course, is closely associated with the TCR and serves to transduce thesignal that is elicited upon binding of antigen to the T-cell receptor.