L#15 & #16: T Cell Development, Activation, and Differentiation and The Complement System
L#15 & #16: T Cell Development, Activation, and Differentiation and The Complement System 0530
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This 4 page Class Notes was uploaded by Denise Croote on Saturday January 23, 2016. The Class Notes belongs to 0530 at Brown University taught by Dr. Richard Bungiro in Fall 2013. Since its upload, it has received 20 views. For similar materials see Principles of Immunology in Biology at Brown University.
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Date Created: 01/23/16
Lecture Fifteen: T Cell Development, Activation, and Differentiation T cells must have the following properties: they must be tolerant to self but be able to recognize non-self and they must be able to distinguish between antigens synthesized within in the cell (endogenous) and antigens acquired from the external environment (exogenous antigens) Lifecycle: o T cell precursors exit the bone marrow o Migrate to the thymus o Commit to the T cell lineage by engaging the notch receptors o TCR gene rearrangement o Develop into mature CD4+ / CD8+ double positive cells and then they become single positive o Positive selection for MHC reactivity and then negative selection kills off 60-70% more o Exit the thymus o Migrate to the secondary lymphoid organs where they may be activated by foreign antigen The PreTCR- has an alpha and a beta chain and a CD3 unit for signaling o the alpha chain doesn’t always get allelically excluded o the alpha chain is like the light chain and the beta chain is like the heavy chain o TCR Beta rearrangement with allelically exclude The MHC of the thymus determines the restriction of the thymus – give a rat a thymectomy and they will only be able to kill cells with the same MHC as the thymus that was put in Positive Selection: negatively select against those T cells that have too high an affinity for the class I or class II MHC. Positively select for those with the right amount of affinity, and most (90%) die by neglect (do not bind to an MHC) Negative Selection: negative selection of those that bind to self MHC Experiments: o If you block class I MHC or class II MHC you fail to get CD4 or CD8 T cell maturation o Transgenic T cells expressing a TCR specific for a viral peptide plus H2k haplotype only mature in H2k MHC mice o Transgenic T cells expressing a TCR that is self reactive in males will only mature in females Process to become CD4 and CD8 o CD4 and CD8 is down-regulated o CD4 is up-regulated o If the cell is capable of interacting with class II MHC, signaling through the TCR leads to continued CD4 expression o If the TR does NOT react with class II MHC CD4 is down-regulated and CD8 is up- regulated, allowing interaction with class I MHC AIRE – transcription factor regulates the expression of may tissue specific antigens that are not present in the thymus, this is key in inducing central tolerance to tissue specific antigens and reducing the chances of autoimmunity The basic transition: Naïve CD8+/CD4+ cells get activated clonally expand, and differentiate into effector and memory T cells (that then go on to activate macrophages and other B cells) and into effector and memory CTLs (that kill infected cells) Activation of Naïve T cells Requires: o 1.) Signal 1 is generated when the TCR-CD3 complex binds to the peptide/MHC complexes on the APC o 2.) Signal 2 is generated by costimulation delivered by the engagement of the CD28 molecule on the T cells by the B7 molecule on the APC o 3.) Cytokine signaling from the APC to the TCR Signal 2: CD28 binds to B7 on the APC (B cell, macrophages, and DC) but CTLA4 an inhibitory molecule also competes to bind to B7 on the T cell. CTLA – 4 is not expressed on naïve T cells but is expressed on activated T cells and has a higher affinity for the B7 receptor During T cell activation cytokines, chemokines, CAMS, and metabolic genes are up-regulated while apoptosis associated genes are down-regulated B Cell and T Cell Co-Activation: o B cells takes up Ag and presents in on MHC (B cell signal 1) o T cell recognizes MHC and peptide (T cell signal 1) o B cell starts up-regulating B7 to bind with CD28 on the T cell (T cell signal 2) o T cell starts expressing CD40L to bind to CD40 on B cell (B cell signal 2) Superantigens- bind to a specific TCR Vbeta region which may be expressed in up to 5% of all T cells, bypassing the need for antigen specific recognition, this can lead to massive T cell activation and over production of cytokines and systemic toxicity that can be fatal Dendritic Cells- T cells cannot migrate into the tissue, so the antigen must be brought to them in the lymphoid organs (these organs allow a lot of antigen to be presented to a lot of T cells in a small area o Dendritic Cells are especially effective at transporting antigen from the tissue to the secondary organs o DCs also express high levels of B7 Antigen processing – once the DCs have ingested antigen they are induced to migrate to the secondary lymph organs Antigen Presentation – DCs are able to present processed antigen on MHC II and express high levels of costimulatory molecules (like cytokines) When a Th cell is activated IL2 is up-regulated, autocrine stimulation encourages repeated cell division until there is differentiation into memory and effector cells If you can’t get signal two, where CD28 and B7 bind, the cell will be induced into anergy T Regulatory Mechanisms o Negative selection of self reactive T cells in the thymus o Up-regulation of CTLA (negative signal that binds to B7) o Induction of apoptosis FasL/Fas pathway o Induction of anergy o Counter regulation by cytokines o Regulatory T cell populations Types of T cell populations: o Effector Cells – short lived, carry out various functions, mediate the clearance of antigen CD4 secrete cytokines and activate CTLs and B cells CD8 kill other cells o Memory T Cells – long lived and mediate a secondary response, are easier and faster to activate upon a secondary attack o Tregs – T cells that suppress other T cells in a contact dependent manner o Thought to down-regulate self reactive T cells Lecture Sixteen: The Complement System The complement system is a way to help protect against the invasion of bacteria and viruses It is a part of the INNATE immune system But can have features of adaptive immunity as well. It is capable of binding to the Fc region of an antibody, causing the bacterial membrane to lyse, improving the affinity of the Ag and Ab binding complex, and improving immune memory The complement system functions in o Lysis of the target membrane o Opsonization of Antigen for phagocytosis o Internalization (removal from the extracellular space) Discovery: heat treated antiserum was not able to lyse any target cells but the bacteria could still agglutinate. This meant that the Ab were heat resistant. Found that the lytic activity could be restored by adding back fresh normal serum without a specific antibody. This meant that the lytic factor is heat sensitive and does not require a specific antibody, is present in the normal serum We now know that the lytic factor is the complement system and it consists of more than 30 distinct serum and cell membrane proteins Pathways: 1.) Classical Pathway – (activated by Ag-Ab complex) a. Activated by Ag-Ab complexes, form soluble complexes on the surface of the target cell. Can also be activated by IgM and IgG b. C1qr2s2 binds to at least two Fc regions of Abs bound to target cell surface c. IgM has 5 Fc regions so there can be C activation at a much lower concentration of IgM than of IgG. IgG only has one Fc region so you need two IgG, each bound to a different epitope on the target bacterium, to activate complement d. C4 and C2 cleave and then re-associate to form C3 convertase. e. C3 convertase hydrolyzes many C3 molecules to C3b and C3b combines with C3 convertase to generate C5 convertase f. C5 convertase binds C5, permitting C5 to cleave. C5b forms with C6 to initiate the formation of MAC – the membrane attack complex 2.) Alternmative Pathway – (activated by microbial surfaces) a. C3 spontaneously dissociates into C3a and C3b b. C3b binds to target cell microbial surfaces and initiates the activation of the alternative pathway c. C3b binds factor B. Factor D cleaves Factor B generating C3 convertase. d. C3 convertase generates more C3b, leading to the formation of C5 convertase e. MAC is generated (if a host cell is accidently targeted by the MAC protein then the regulatory proteins (factor H is an example) step in and they inactivate C3b 3.) Lectin Pathway – a. Cell bound MBL associates with MASP, which becomes activated and cleaves C4 and C2 to generate C3 convertase which then functions just like it would in the classical pathway Regulation of the Complement System: Can regulate before the assembly of the convertases o Can inhibit C1 o Can block the association of C4 and C2 which will block the formation of C3 convertase Can regulate after the assembly of the convertase o Can prevent the insertion of MAC into the membrane o Can dissociate C3 convertase by factor H and decay accelerating factor Effects of the Complement System: o Cell lysis by MAC o Inflammatory response by C3a and C5a Includes degranulation of mast cells, smooth muscle contraction and increased vascular permeability, extravasation and chemotaxis o Opsonication and phagocytosis (C3b) – making the soluble immune complex or the bacterium more likely to bind to a phagocytic cell receptor o Viral lysis and neutralization (C3b and MAC) o Clearance of immune complexes (C3b) o Immonogenicity – bind to a B cell co-receptor to increase the likelihood of activation Microbial Envasion o Resistance to complement is often correlated with virulence of microorganisms o Most gram negative bacteria are sensitive to complement mediated lysis but gram positive bacteria are resistant o Certain bacterial cells walls (like LPS) are resistant to the insertion of MAC o Some bacterial capsules interfere with C3b o Enzymes can inactivate complement proteins o Can produce mimics of complement proteins that may offer competition at the binding sites o Complement Deficiencies: o Noticed after frequent bacterial infection o C3 deficiencies tend to be the most severe because C3 is involved in everything o Deficiencies in MAC result in frequent bacterial infections o Deficiencies in C regulatory proteins result in symptoms of autoimmunity END OF THE CHAPTER QUESTIONS: 1.) A single molecule bound IgM can activate C1q in the classical pathway 2.) The enzymes that cleave C3 and C4 are referred to as convertases 3.) C3a and C3b are fragments of C3 4.) Nucleated cells are more resistant to complement lysis than red blood cells 5.) MBL is an activator in the Lectin pathway when it interacts with MASP 6.) IgM has a binding site for C1q on its Fc region but this binding site is unaccessible in the absence of antigen binding 7.) If the C3 protein were eliminated the classical pathway would still be able to form C3 convertase, but the C3 convertase would have nothing to cleave. The alternative pathway would not be initiated at all because C3 spontaneously decays in the first step 8.) C3 is needed for the clearance of pathogen because C3b increases opsonization so phagocytic cells will come and bind 9.) If C3 were absent phagocytosis would diminish, but if some Ab are present for the specific Ag it can still occur