T Cell Immunology Notes PDF

CD4+ T cells – Part I Brian Rudd Dept of Microbiology and Immunology VMC C5-134 [email protected] Office hours: anytime, by appt Text covered in the next 3 lectures: Callahan & Yates (chapter 10) Parham #3 (pp. 75-83, 212-245, 305-307, 311-315) - optional Abbas #7 (chapter 9, 10, 11) - optional ...

CD4+ T cells – Part I Brian Rudd Dept of Microbiology and Immunology VMC C5-134 [email protected] Office hours: anytime, by appt Text covered in the next 3 lectures: Callahan & Yates (chapter 10) Parham #3 (pp. 75-83, 212-245, 305-307, 311-315) - optional Abbas #7 (chapter 9, 10, 11) - optional Overview of the Immune System Avery August, Ph.D. Forest and Trees Individual cell level Population level CD4+ T cell response  Learning Objectives 1. Describe where activation of CD4+ T cells occurs and what cells are required. 2. Define the molecular interactions required to fully activate a CD4+ T cell. 3. Define the role of IL-2 in the development of a CD4+ T cell response. 4. Explain what clonal expansion is and why it is critical to an effective host response. 5. Describe molecules and signals required to turn off T cell signaling. *Note: Learning objectives generally describe the minimum knowledge needed to pass the course. Big picture view of T cell activation  Back to the basics… Remember: 1. Every T cell is monoclonal and expresses one T cell receptor (TCR) 2. The TCR recognizes one peptide presented by a MHC molecule on the surface of another cell How does the host know which TCRs it is going to need?  The immune system generates a diverse repertoire of T cells TCRa chain TCRb chain - 71 Va gene segments - 35 Vb gene segments - 51 Ja gene segments - 2 Db gene segments - 12 Jb gene segments 1. Recombination of V-(D)-J germline segments by Rag 1 and Rag2 2. Addition of N-nucleotides between germline segments by TdT 3. Pairing of different alpha and beta chains V- variable D- diversity J- joining  T cells – by the numbers ~1015 TCRs possible in mice Limited by size Limited by thymic selection ~8x107 total T cells/mouse 80M in lymph nodes 5M in blood 20-200 CD4+ T cells/peptide Individuals respond to some but not all peptides for each pathogen Flu WNV peptide peptide Very few T cells are specific for any one microbe CD4+ T cells: 1 in 2e5 to 2e6 bind to the same peptide:MHCII molecule Key question: How do these few T cells patrol all tissues where antigen may be present?  Antigen and lymphocytes must be brought together in lymph nodes Dendritic cells T cells The dendritic cell odyssey  What initiates migration of dendritic cells to lymph nodes during infection? Immature Mature Sites of Lymph infection nodes Optimal for antigen processing Optimal for antigen presenting High Ag uptake Low Ag uptake PAMPs Low MHCII High MHCII (peptide on surface) Low B7 molecules High B7 molecules (costim molecules) Low CCR7 High CCR7 (trafficking to a lymph node) O’Neill, Blood, 2004  T cell migration Mature DCs enter lymph nodes via afferent lymphatics Naive T cells enter the lymph node via the blood (and stay for ~2-24hrs) T cells that DON’T encounter specific antigen return to the circulation via efferent lymphatics and thoracic duct (~30 mins before entering another LN) T cells that DO encounter specific antigen become activated and begin to proliferate and differentiate into DCs in the LN can make contact with effectors (~3 days). up to 5,000 naïve T cells/hr  Live Cell Imaging of T Cells and DC in a Lymph Node Dendritic cells Dendritic cells that have taken up antigen Timeline = 4 hrs Dendritic cells Immunological synapse Antigen-specific T cells Key question: How do T cells know that it is an appropriate time to be activated?  T cell activation requires interactions with other cells T cells only see antigens displayed by molecules (MHC) on the surface of cells (not in the circulation). CD4+ T cells recognize peptide in the context of MHC class II (this ensures cellular communications). Cells that express MHC class II include: – Dendritic cells – B cells – Macrophages  Activation of T cells requires 2 signals Activation of naïve T cells requires: – Signal 1: TCR/peptide – Signal 2: CD28/B7 Expression of as CD80 and CD86 is upregulated during DC maturation ‘Professional antigen presenting cells’ express both MHCII and B7 – Dendritic cells (most potent) – Macrophages – B cells 2 signal system ensures that naïve T cells respond to microbes (inducers of costimulators) and not harmless antigen co-stimulation is the ‘safety’ on the TCR trigger  Possible outcomes Signal 1 + + - Signal 2 + - + Anergic = unreactive to further antigenic stimulation  How is the TCR signal transduced? T cell receptor complex T cell receptor CD28 complex CD4 (co-stimulatory (co-receptor) molecule) V-J V-D-J Signaling via the TCR complex: 1. TCRs recognize Ag but can’t pass on the signal by themselves 2. CD3 proteins associated with alpha/beta chains of the TCR 3. Antigen binding > Tyr phosphorylation in ITAM 4.ITAM: immunoreceptor tyrosine-based activation motif → triggers intracellular signaling cascade → biochemical intermediates → enzymes → activation of transcription factors (NF-Kb, NFAT, AP-1) that regulate gene expression.  What does the co-receptor do? T cell receptor CD28 complex CD4 (co-stimulatory (co-receptor) molecule) CD4 co-receptor binds to a conserved portion of the MHC II molecule Function: to increase affinity of TCR for MHC and lower threshold of activation Co-receptor binding decreases the number of MHC molecules required to activate naïve T cells from ~10,000 pMHC molecules (~100% of total MHC on surface of APC) to ~100 pMHC (0.1-1% of total MHC on surface of APC)  The immunological synapse Outer ring: adhesion molecules Inner ring: TCR/MHC, CD28/B7 Immune synapse: area of contact between two cells http://jcs.biologists.org/content/126/5/1049 The immunological synapse Key question: What signals are required for T cells to start proliferating? Proliferation is driven by IL-2 Signal 1 and 2 Antigen presenting cell CD4 T cell IL-2 released by T cells binds to IL-2 receptor on T cells (autocrine growth factor) Clonal expansion Activated T cells produce and respond to IL-2 Naïve T cells – express the low affinity IL-2 receptor (β, γ chains) Activated T cells – upregulate the α chain, enabling them to express the high affinity IL-2 receptor (α, β , γ chains) IL-2 signaling is a target for many immunosuppressive drugs 1. Cyclosporine and Tacrolimus (FK506) inhibit IL-2 production 2. Rapamycin (sirolimus) inhibit the IL-2 receptor  Activated T cells undergo massive clonal expansion T cells proliferate 2-3 times per day One T cell gives rise to thousands of daughter cells One precursor pool (~100 cells) gives rise to 1M effector cells in ~1 week. 50% of effector cells may be specific for the same peptide:MHC complex  How do we turn T cell proliferation off? CTLA4 is a co-receptor on activated T cells CTLA4 transmits an inhibitory signal (downregulates IL-2 signaling) Cells stop proliferating and die from apoptosis (programmed cell death) Why do I need to learn this stuff? Ipilimumab: CTLA-4 inhibitor CTLA-4 is conserved in dogs and humans Success! Ipilimumab = “plateau” 46% 24% Ipilimumab 25% 14% Control NEJM, Hodi, 2010 And the Nobel Prize goes to… NEJM, Hodi, 2010  Summary How do T cells know which TCRs are needed? How do T cells come in contact with antigen during infection? How do T cells know when it is an appropriate time to be activated? How do T cells know when to start and stop proliferating? Why do I need to learn this stuff?