Question

We spent a lot of time comparing B cell and T cell surface molecules and activation – how they’re similar and different. For each of the following, compare similarities and differences. Make sketches where indicated. Feel free to use additional sketches if they help you learn the material.

A. Compare the TH-Macrophage interaction (which activates TH cells) with the TH-B cell interaction (which activates B cells). Make a sketch of each interaction that shows the important cell surface proteins involved. Also state in words the sequence of events involved in each activation.

B. Compare T cell receptors (TCR) with B cell receptors (BCR). How do they form? What do they look like? To what do they bind? What is the difference between somatic recombination and class switching (a.k.a. “isotype switching”), and which of these happens only for BCRs?

C. Compare antigen display on MHC-I with display on MHC-II. Where do displayed antigens come from? How do they get attached to MHC-I/II? What happens when the corresponding T cell binds to each MHC? Which display is more important for your immune system to recognize viruses? To recognize bacteria?

Answer 1

A. The TH cells or Helper T cells become activated when they are presented with peptide antigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide rapidly and secrete small proteins called cytokines that regulate or assist in the active immune response.

An antigen-presenting cell (APC) or accessory cell is a cell that displays antigen complexed with major histocompatibility complexes (MHCs) on their surfaces; this process is known as antigen presentation. T cells may recognize these complexes using their T cell receptors (TCRs). These cells process antigens and present them to T-cells. Almost all cell types can serve as some form of APC. They are found in a variety of tissue types. Professional antigen-presenting cells, including macrophages, B cells and dendritic cells, present foreign antigens to helper T cells, while other cell types can present antigens originating inside the cell to cytotoxic T cells. In addition to the MHC family of proteins, antigen presentation relies on other specialized signaling molecules on the surfaces of both APCs and T cells.

Macrophages phagocytose the external agent/ pathogen and present it via the MHC-Class II molecules. After their activation. macrophages are able to express MHC class II and co-stimulatory molecules, including the B7 complex and can present phagocytosed peptide fragments to helper T cells.

B-cells can internalize antigen that binds to their B cell receptor and present it to helper T cells.Unlike T cells, B cells can recognize soluble antigen for which their B cell receptor is specific. They can then process the antigen and present peptides using MHC class II molecules. When a T helper cell with a TCR specific for that peptide binds, the B cell marker CD40 binds to CD40L on the T cell surface. When activated by a T cell, a B cell can undergo antibody isotype switching, affinity maturation, as well as formation of memory cells.

Note- Sketch is in end of the answer.

B. The T-Cell Receptor (TCR) is composed of two different protein chains (that is, it is a heterodimer). In humans, in 95% of T cells the TCR consists of an alpha (?) chain and a beta (?) chain (encoded by TRA and TRB, respectively), whereas in 5% of T cells the TCR consists of gamma and delta (?/?) chains (encoded by TRG and TRD, respectively). This ratio changes during ontogeny and in diseased states (such as leukemia). It also differs between species.

The generation of TCR diversity is similar to that for antibodies and B cell antigen receptors. It arises mainly from genetic recombination of the DNA encoded segments in individual somatic T cells by somatic V(D)J recombination using RAG1and RAG2 recombinases. Unlike immunoglobulins, however, TCR genes do not undergo somatic hypermutation, and T cells do not express Activation-Induced (Cytidine) Deaminase (AID). The recombination process that creates diversity in BCR (antibodies) and TCR is unique to lymphocytes (T and B cells) during the early stages of their development in primary lymphoid organs (thymus for T cells, bone marrow for B cells).

Each recombined TCR possess unique antigen specificity, determined by the structure of the antigen-binding site formed by the ? and ? chains in case of ?? T cells or ? and ? chains on case of ?? T cells.

• The TCR alpha chain is generated by VJ recombination, whereas the beta chain is generated by VDJ recombination (both involving a somewhat random joining of gene segments to generate the complete TCR chain).
• Likewise, generation of the TCR gamma chain involves VJ recombination, whereas generation of the TCR delta chain occurs by VDJ recombination.

B-Cell Receptor, is composed of two surrogate light chains and two immunoglobulin heavy chains, which are normally linked to Ig-? and Ig-?signaling molecules. Each B-cell, produced in the bone marrow, is highly specific to an antigen.The BCR can be found in a number of identical copies of membrane proteins that are exposed at the cell surface.The general structure of the B-cellreceptor includes a membrane-boundimmunoglobulin molecule and a signal transduction region. Disulfide bridgesconnect the immunoglobulin isotype and the signal transduction region.

The B-cell receptor is composed of two parts:

1. A membrane-bound immunoglobulin molecule of one isotype (IgD, IgM, IgA, IgG, or IgE). With the exception of the presence of an integral membrane domain, these are identical to a monomeric version of their secreted forms.
2. Signal transduction moiety: A heterodimer called Ig-?/Ig-? (CD79), bound together by disulfide bridges. Each member of the dimer spans the plasma membrane and has a cytoplasmic tail bearing an immunoreceptor tyrosine-based activation motif (ITAM).

Somatic recombination is an alteration of the DNA of a somatic cell that is inherited by its daughter cells. The term is usually reserved for large-scale alterations of DNA such as chromosomal translocations and deletions and not applied to point mutations. Somatic recombination occurs physiologically in the assembly of the B cell receptor and T-cell receptor genes (V(D)J recombination), as well as in the class switching of immunoglobulins.Somatic recombination is also important in the process of carcinogenesis.

Immunoglobulin class switching, also known as isotype switching, isotypic commutation, is a biological mechanism that changes a B cell's production of immunoglobulin (antibodies) from one type to another, such as from the isotype IgM to the isotypeIgG. This happens only for BCRs.

C.Class I MHC molecules bind peptides generated mainly from degradation of cytosolic proteins by the proteasome. The MHC I:peptide complex is then inserted via endoplasmic reticulum into the external plasma membrane of the cell. The epitope peptide is bound on extracellular parts of the class I MHC molecule. Thus, the function of the class I MHC is to display intracellular proteins to cytotoxic T cells (CTLs). However, class I MHC can also present peptides generated from exogenous proteins, in a process known as cross-presentation.

The endogenous pathway is used to present cellular peptide fragments on the cell surface on MHC class I molecules. Worn out proteins within the cell become ubiquitinated, marking them for proteasome degradation. Proteasomes break the protein up into peptides that include some around nine amino acids long (suitable for fitting within the peptide binding cleft of MHC class I molecules). Transporter associated with antigen processing (TAP), a protein that spans the membrane of the rough endoplasmic reticulum, transports the peptides into the lumen of the rough endoplasmic reticulum (ER). Also within the rough ER, a series of chaperone proteins, including calnexin, calreticulin, ERp57, and Binding immunoglobulin protein (BiP) facilitates the proper folding of class I MHC and its association with ?2 microglobulin. The partially folded MHC class I molecule then interacts with TAP via tapasin (the complete complex also contains calreticulin and Erp57 and, in mice, calnexin). Once the peptide is transported into the ER lumen it binds to the cleft of the awaiting MHC class I molecule, stabilizing the MHC and allowing it to be transported to the cell surface by the golgi apparatus.

Antigens bind in the peptide binding groove of MHC ClassII molecules. Because class II MHC is loaded with extracellular proteins, it is mainly concerned with presentation of extracellular pathogens (for example, bacteria that might be infecting a wound or the blood). Class II molecules interact mainly with immune cells, like the T helper cell (TCD4+). The peptide presented regulates how T cells respond to an infection.

The exogenous pathway is utilized by specialized antigen presenting cells to present peptides derived from proteins that the cell has endocytosed. The peptides are presented on MHC class II molecules. Proteins are endocytosed and degraded by acid-dependent proteases in endosomes; this process takes about an hour.The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by Ii (the invariant chain; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway. The invariant chain also facilitates MHC class II's export from the ER in a vesicle. This fuses with a late endosome containing the endocytosed, degraded proteins. The invariant chain is then broken down in stages, leaving only a small fragment called "Class II-associated invariant chain peptide" (CLIP) which still blocks the peptide binding cleft. An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome. The stable MHC class-II is then presented on the cell surface..

So, both the pathways are important when a virus infects the cells