What happens to T-cells when they are presented with an antigen by a cell that lacks B7?
The T-cell become anergic-lacks functional responsiveness. Remember that B7 binding to CD28 on T-cells is required for T-cells to differentiate into effector and memory cells.
What process occurs when an anergic T-cell is presented an antigen by an APC that has B7 on its surface?
The T-cell stays anergic. Anergic T-cells are non-responsive to subsequent stimulation by antigen in the context of co-stimulation.
Most effector T-cells are eliminated by apoptosis. What are two mechanisms by which this occurs?
Most effector T-cells will be eliminated by apoptosis. This is mediated by removal of survival signals (e.g., IL-2 production ceases because limited antigen to stimulate T-cells to make IL-2) or induction of Fas ligand, which engages Fas receptors on the activated effector T-cell and trigger apoptosis.
Note that IL-2 stimulates anti-apoptotic proteins and stimulates the sequestration of apoptotic proteins into the mitochondria. Removal of IL-2 stimulation therefore induces apoptosis.
What are two accessory molecules expressed on T-cells that are involved in deactivation of T-cells? When in T-cell development are they expressed? On what T-cells are they expressed?
Another mechanism of T-cell inactivation involves the expression of two accessory molecules Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and Program Death 1 (PD1). Both of these molecules are expressed the surface of CD4+ and CD8+ T- cells following activation of T-cells.
Explain the inactivation of T-cells (and B-cells) through PD-1 signaling.
Program death (PD)-1 molecule: PD-1 is induced on the surface of activated T-cells. When PD1 engages PD ligands (PDL) on APCs the T-cells become nonresponsive. This is due to the activation of phosphatases (e.g., SHP2) when PD1 is triggered on lymphocytes. Phosphatases remove phosphate moieties within the cell. Phosphorylated proteins are important in lymphocyte activation.
Note that B-cells also express PD-1 (see attached pic).
What are the 3 ways in which CTLA-4 limits T-cell responses? Explain in detail each of the ways this occurs.
Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4): CTLA-4 interacts with B7.1 or B7.2 (CD80 & 86)molecules the same ligands for CD28. Expression of CTLA-4 works in three ways to stop activation of antigen specific T-cells.
- CTLA-4 inhibits T-cells is based on the ability of CTLA-4 to bind to B7 molecules with 20-fold greater affinity than CD28.
- If a naïve T-cell interacts with antigen in the context of MHC molecules in the absence of CD28 engagement of B7 molecules then the cell becomes “anergic”.
- Anergic cells cannot respond to subsequent exposure to peptide antigen even when B7 becomes available on the antigen presenting cell.
- Thus if CTLA-4 binds to B7 with 20-fold greater affinity, then CD28 cannot engage B7 as effectively and thus, CD28 does not signal and the T-cell becomes anergic.
- Effector T-cells triggering CTLA-4 with B7 molecules leads to the recruitment and activation of phosphatase (e.g., PP2A and SHP2).
- Phosphatases remove phosphate moieties, which are important in T-cell activation.
- Thus CTLA-4 limits TCR signaling in effector T-cells and limits the T-cells’ ability to function.
- Binding of B7 molecules by CTLA-4 also triggers the activation of Indoleamine 2,3 dioxygenase (IDO) within dendritic cells. IDO converts tryptophan into kynuernine. Thus IDO will limit tryptophan availability to T-cells. (see slide 9 of PP)
Explain how B-cells are inactivated through inhibitory receptors. Explain what receptors are involved and under what circumstances this occurs.
Typically with antibody excess as is the case of antigen decline (when a pathogen and its components are cleared) the Ig has the ability to turn off B-cell responses. B-cells possess Fcγ receptor IIB, which interact with the Fc portion of IgG.
If the IgG interacts with this receptor, it is able to trigger phosphatases, which in turn remove positive signals within the cell. In turn the cell decreases its ability to differentiate to plasma cell.
Remember that B-cells are also inactivated by PD1 ligands bound to APCs binding to PD1 on B-cells.
Note that IgG binding to FcγIIB receptors is also used as a method to inactivate IgM producing B-cells in secondary immune responses (see immune memory lecture).
What is tolerance? What is the most common form of tolerance?
Tolerance is unresponsiveness of the adaptive immune system to antigens as a result of inactivation (e.g., anergy, regulatory cytokine exposure) or death of antigen specific lymphocytes induced by exposure to antigens.
The most common means is tolerance to self antigen but this can also occur with tolerance to foreign antigens as well.
Where does tolerance occur? Where does central tolerance occur? What occurs in central tolerance?
Tolerance occurs in primary or secondary lymphoid tissues in response to self-antigen recognition.
Tolerance that occurs in generative primary lymphoid tissue (i.e., bone marrow, thymus) is known as central tolerance.
This typically involves removal of cells directed to self-antigens in the case of B- cells and T-cells and the editing of Ig in B-cells.
Where does peripheral tolerance occur? What happens to cells in peripheral tolerance? What are the mechanisms by which it occurs?
What is peripheral tolerance key for the prevention of?
Rarely, lymphocytes evade negative selection in the generative lymphoid tissues and make it to the periphery. In this case peripheral tolerance will take place in order to avoid auto immune responses.
Peripheral tolerance includes the lymphocytes becoming anergic or undergoing apoptosis when triggered by self-Ag. Recognition of self-antigen in the periphery may occur as a result to recognition of self antigen on non-professional antigen presenting cells. In this way T-cells may not get the co-stimulatory signal to induce activation and the cell but instead become anergic. Another possibility is that the cell will undergo apoptosis due to expression of Fas and Fas ligand.
Peripheral tolerance is key to preventing over-reactivity of the immune system to exposure to environmental triggers such as food, commensal bacteria and particles in the air (more to be covered in lectures on Allergies, Autoimmunity and Mucosal Immunity).
What cell is used to maintain peripheral tolerance? (just name cell)
regulatory T-cells (Tregs)
What CD molecule (CD4 or CD8) to Tregs possess? What cytokines do they secrete and what are their effects? What triggers secretion of these cytokines?
What cytokine are Tregs dependent upon for survival? What trasncription factor do they express?
Explain the type of T-cells Tregs suppress. Explain how the selection process for Tregs is different for other T-cells.
A major way in which peripheral tolerance is maintained is by regulatory T-cells (Tregs). These CD4+ T-cells typically secrete IL-10 and TGF-beta when TCR is engaged. These cytokines can be immunosuppressive.
Tregs express high affinity alpha chain of IL-2 receptor and the transcription factor FoxP3 causing, Tregs to be highly dependent on IL-2 for survival.
Unlike other T-cells in which self-antigen reactive cells are eliminated during development, Tregs directed to self-antigens are maintained. This is important since Tregs only work in suppressing T-cells directed to self antigen.
Thus, only Tregs directed to self-antigens are able to suppress T-cells directed to self-antigen. Similarly Tregs directed to foreign antigen X can only suppress T-
cells to antigen X and not T-cells directed to antigen Y. Thus, Treg activity is highly specific.
T or F: Regulatory T-cells prevent T-cell responses at the level of priming and effector function.
Why does B-cell tolerance occur? What are the possible outcomes for B-cell tolerance?
B-cell tolerance to self-antigen occurs as a consequence of high avidity antigen interacting with B-cells with antibodies to self-antigens.
If there is high avidity binding to self antigen, the two possible outcomes are for rearrangement of the light chain V and J on another allele (rearranges all possible light chain genes) or apoptosis. If the self-antigen is low avidity (e.g., monomer epitopes) than the B-cell becomes anergic.