A historical perspective of immunology

Transcription

1 Dendritic Cells and the adaptive immune response A historical perspective of immunology A portion of immunity involves both humoral and cellular components Humoral immunity combats pathogens via antibodies Antibodies are produced by B cells Antibodies can be transferred between individuals to provide passive immunity Cell-mediated immunity involves primarily T lymphocytes These can eradicate pathogens, clear infected self-cells, or aid other cells in inducing immunity 1

2 A historical perspective of immunology Clonal selection Individual B and T cells each have an individual specificity for a single antigen This is due to each cell having many copies of a receptor on their surface that only bind to one type of antigen When a B or T cell interacts with its specific antigen, it is selected and becomes activated Activation results in a proliferation, producing a large number of clones Each clone is reactive against the antigen that initially stimulated the original lymphocyte 2

3 Important concepts for understanding the mammalian immune response Pathogens fall into four major categories Immune responses are quickly tailored to the type of organism involved 3

4 Important concepts for understanding the mammalian immune response Immune responses rely on recognition molecules Germ-line encoded (pattern recognition receptors, PRRs) These bind to pathogen-associated molecular patterns (PAMPs) generic molecules found on many different types of pathogens (e.g., peptidoglycan) Randomly generated (B and T cell receptors) These bind to very specific antigens, rather than generic molecules found on many pathogens Important concepts for understanding the mammalian immune response Humoral and cell-mediated immunity relies on surface receptors (B and T cell receptors) These are randomly generated by DNA rearrangements in B and T cells Many of these are nonviable and are deleted during development 4

5 Important concepts for understanding the mammalian immune response Tolerance ensures that the immune system avoids destroying host tissue Many of the random rearrangements used to create B and T cell receptors could be antiself Tolerance helps to keep these anti-self recognition molecules/cells from circulating in the bloodstream 5

6 The good, bad, and ugly of the immune system Dysfunctions of immunity two broad categories Overly active or misdirected immune responses Allergies/asthma Autoimmune disease (e.g., multiple sclerosis, Crohn s disease) Immunodeficiency Primary (genetic) loss of immune function Secondary (acquired) loss of immune function Opportunistic infections (e.g., oral thrush) can occur in people with impaired immune responses The good, bad, and ugly of the immune system Transplanted tissues A rare case where we want to AVOID an immune response (rejection) The body s natural response to foreign tissue is to attack it and destroy it Cancer A situation where the dangerous cells we want to target are our own self cells Generally tolerated and hard to generate immunity against 6

7 Summary Immunity is a complex subject, broken down into many different layers and areas Understanding how immunity works allows us to: Exploit it to prevent infections (vaccination) Exploit it to treat illness (shutting down autoimmune disease or ramping up anti-cancer responses) Provide safer organ and tissue transplants Coordination of the immune response innate immunity recognition of pattern generation of inflammatory signals Info 1: Inflammation 7

8 Coordination of the immune response innate immunity recognition of pattern generation of inflammatory signals dendritic cells recognise pattern & inflammation & antigen uptake transport this information to the lymphatic organs Info 1: Inflammation Info 1 + 2: inflammation plus antigen identity Coordination of the immune response innate immunity adaptive immunity recognition of pattern generation of inflammatory signals dendritic cells recognise pattern & inflammation & antigen uptake transport this information to the lymphatic organs initiation of cellular immunity T cells initiation of humoral immunity B cells Info 1: Inflammation Info 1 + 2: inflammation plus antigen identity 16 8

9 Coordination of the immune response adaptive immunity initiation of cellular immunity T cells initiation of humoral immunity B cells 17 overview innate immunity adaptive immunity recognition of pattern generation of inflammatory signals dendritic cells recognise pattern & inflammation & antigen uptake transport this information to the lymphatic organs initiation of cellular immunity T cells initiation of humoral immunity B cells Info 1: Inflammation Info 1 + 2: inflammation plus antigen identity 18 9

10 Dendritic cells (DC) connect innate and adaptive immunity Dendritic cells (DC) first of description 1868 as Langerhans cells (LC) in the skin characterization started 25 years ago initial identification as accessory cell with function during antibody production dendritic cell research was complicated due to rare frequency of DC (approx. 1% DC per spleen) since 20 years: generation of large amounts of dendritic cells in cell culture possible (bone marrow derived dendritic cells = BMDC; bone marrow + Granulocyte macrophage colonystimulating factor, GM-CSF) 10

11 12/18/17 DCs: Most potent APC for T-cell activation RM Steinman Nobelpreis 2011 ZA Cohn A: DCs in der Zellkultur B: Raster-Elektronenmikroskopie C & D: Langerhans-Zellen der Epidermis E & F: DCs in T-Zell-Zonen eine Lymphknoten (grün: B-Zell-Follikel) Microbes cover our body surfaces 11

12 DC express pattern recognition receptors (PRRs) PRRs detect and bind Pathogen-associated molecular pattern (PAMPs) Dendritic Cells are Antigen Presenting Cells (APC) 12

13 simplified immune response Establishment of an infection Induction of adaptive immunity Effectormechanisms clearance of infection Immunological memory Only DCs transport Ag to LN macrophages are mostly not migratory 13

14 Establishment of an infection Induction of adaptive immunity Effectormechanisms clearance of infection Immunological memory Antigen presentation dendritic cell migration 14

15 12/18/17 migratory DC = DC in peripheral non-lymphoid tissues detection of migrating DC Randolph et al Nat Rev Immunol 5:617 15

16 Schematic drawing of a primary (peripheral) LN in a mouse Trends in Immunology, June 2012, Vol. 33, No. 6 Highly organized distribution of different APC in lymph nodes 16

17 12/18/17 CCR7+-DCs migrate to lymph nodes CC-Chemokine Receptor 7 (CCR7) -is the receptor for CC-Chemokine ligand 19 (CCL19) and 21 (CCL21) -CCR7+ DC respond to a chemotactic gradient of CCL19 and/or CCL21that originates from the lymphatic vessel CCR7-expression on DC is induced by: - DC maturation signals (Sozzani et al J Immunol :1083) -uptake of apoptotic material without maturation (Verbovetski et al J Exp Med :1553) modified from (Randolph et al Nat Rev Immunol :617) 17

18 Highly specialized APC: Dendritic Cells immature mature Hackstein et al Nat Rev Immunol 4:24 dendritic cell Ag-uptake and presentation 18

19 extra- and intracellular Ag Virus Bacterium 19

20 TCR recognizes fragments of proteins Antigenic protein Antigenprocessing Antigen- Presentation Antigenrecognition APC APC MHC MHC Peptide TCR* T-cell For example bacterial or viral proteins Fragmentation by Proteases Only few peptides per protein are presented TCR recognizes complex of MHC and peptide (=epitope) APC: Antigen-presenting cell MHC: Major Histocompatibility Complex TCR: T-cell-Receptor The role of the MHC and expression patterns Evidence suggests different antigen processing and presentation pathways Class I presentation requires cytosolic or endogenous processing Class II presentation requires exogenous processing 20

21 Cross-presentation of exogenous antigens Dendritic cells appear to be the primary cross-presenting cell type Exogenous antigens are redirected to the exogenous presentation pathway This allows for their presentation on MHC class I molecules, priming CD8+ T-cell responses Dendritic cells are the only APCs (so far) to exhibit this activity in vivo MHC/Peptide complexes 21

22 DCs interact with both, CD4 and CD8 T cells CD4 and CD8 T cells recognize only peptides bound to MHCII or MHCI molecules è T cell activation è T cell help è Effector functions dendritic cell function 22

23 A constant interplay between the two systems exists Dendritic cells are a key bridge DC effector functions are controlled by external stimuli no pathogen signals: pattern recognition and/or inflammation: DC induce tolerance DC induce immunity concept of maintenance of peripheral tolerance to self antigens by DC 23

24 DC functions: peripheral T cell tolerance Banchereau and Palucka Nat Rev Immunol 2006, 5:296 Dendritic cells upregulate CD86 after pattern recognition before after CD86 MHC class II BMDC 6 hours LPS 24

25 DC functions: T cell immunity Banchereau and Palucka Nat Rev Immunol 2006, 5:296 Ablation models Jung et al. (2002) Immunity, 17:211 25

26 Ablation models cdcs requirement for efficient defense against: viruses lymphocytic choriomeningitis virus herpes simplex 1 vesicular stomatitis virus Influenza respiratory syncytial virus herpes simplex murine hepatitis virus bacteria Listeria Mycobacteria Salmonella Jung et al. (2002) Immunity, 17:211 parasites Plasmodium Leishmania Toxoplasma gondii Nippostrongylus brasiliensis Bar-On & Jung Immunological Reviews 234/2010 dendritic cell differentiation 26

27 Regulation of DC development and homeostasis in mice Annu. Rev. Immunol : HSC, hematopoietic stem cell CMP, common myeloid progenitor CLP, common lymphoid progenitor MDP, macrophage DC progenitor CDP, common DC progenitor ETP, early thymic progenitor mono, monocyte LC, Langerhans cell) 27

28 generation of inflammatory dendritic cells Shortman & Naik Nat Rev Immunol 2007, 7:19 there is more than one type of dendritic cell! DCs can differentiate from different progenitors 28

29 Phenotype of human DC subsets Annu. Rev. Immunol : Annu. Rev. Immunol :

30 Alignment of DC across Tissues and species Guilliams et al. (2016) Immunity, 45:669 T cell priming by dendritic cells 30

31 31

32 32

33 T-cell activation and the two-signal hypothesis T cells require antigen presentation as a first signal Other molecular interactions can provide the second required activation signal Once activated, T cells differentiate into their effector forms CD8+ T cells go on to become killer T cells CD4+ T cells differentiate into several different subsets This chapter is focused primarily on activation events overall, followed by differentiation of CD4+ T cells 33

34 T-cell activation and the two-signal hypothesis T-cell activation and the two-signal hypothesis 34

35 T-cell activation and the two-signal hypothesis Successful T cell APC interactions organize signaling molecules into an immunological synapse TCR/MHC-peptide complexes and coreceptors centralize Central supramolecular activating complex, csmac Adhesion molecules/bound ligands peripherally localize Peripheral supramolecular activating complex, psmac T-cell activation and the two-signal hypothesis Costimulatory signals are required for optimal T- cell activation and proliferation Signal 1 = antigen-specific TCR engagement Signal 2 = contact with costimulatory ligands Signal 3 = cytokines directing T-cell differentiation into distinct effector cell types 35

36 T-cell activation and the two-signal hypothesis Costimulatory signals are required for optimal T-cell activation and proliferation Positive costimulatory receptors facilitate activation CD28 and ICOS Negative costimulatory receptors help turn activation off CTLA-4, PD-1, and BTLA T-cell activation and the two-signal hypothesis Costimulatory signals are required for optimal T-cell activation and proliferation Positive costimulatory receptors facilitate activation CD28 44 kda glycoprotein homodimer expressed on majority of T cells Markedly enhances TCR-induced proliferation and survival Binds to B7-1 and B7-2 expressed by APCs Generally involved in initial activation events in T cells ICOS Inducible costimulator, binds ICOS-ligand on activated APCs Expressed on memory and effector T cells» May help to maintain activity of already differentiated cells 36

37 T-cell activation and the two-signal hypothesis Costimulatory signals are required for optimal T-cell activation and proliferation Negative costimulatory receptors help turn activation off CTLA-4 (CD152) Induced within 24 hours after activation, peaks 2 3 days poststimulation Binds to B7-1/B7-2 with higher affinity than CD28, but shuts down signaling pathways ( putting the brakes on ) PD-1 (program death-1, CD279) and BTLA (B- and T-lymphocyte attenuator) PD-1 may help to mediate T-cell tolerance in nonlymphoid tissues BTLA may down-regulate inflammatory and autoimmune responses T-cell activation and the two-signal hypothesis Clonal anergy results if a costimulatory signal is absent This helps provide tolerance (especially in periphery) If only signal 1 is received, the cell is rendered nonresponsive This might happen if a T cell isn t screened against a peripheral selfantigen during development 37

38 T-cell activation and the two-signal hypothesis Cytokines provide signal 3 Depending on which cytokines are present as the T cell is becoming activated, different outcomes can occur IL-2 is an example of an autocrine type of cytokine response system T cells produce the cytokine and the receptor for it Binding of this ligand induces a very strong proliferation signal during activation stages Other cytokines can send the T cell down different subset development pathways T-cell activation and the two-signal hypothesis Antigen-presenting cells have characteristic costimulatory properties Dendritic cells, macrophages, and B cells provide the right signals to activate T cells 38

39 T-cell differentiation Initial activation signals 1 and 2 induce Upregulation of prosurvival genes Transcription of IL-2 and IL-2R genes Outcome is activation and robust proliferation Production of memory and effector clonal cell populations 39

40 T-cell differentiation Differentiation of T helper cell subsets is regulated by polarizing cytokines Signal 3 what cytokines are nearby when the T cell is activated? APCs may bind PAMPS via PRRs, inducing cytokine secretion Different PRRs engaged (via different antigens) = different cytokines produced» Viruses stimulate IL-12 to induce T H 1 subsets» Worms stimulate IL-4 to induce T H 2 subsets T-cell differentiation Helper T cells can be divided into distinct subsets At least five distinct subsets T H 1 and T H 2 T H 17 T REG T FH Each produces a distinct cytokine profile and regulates distinct activities within the body 40

41 T-cell differentiation Effector T helper subsets an overview T-cell differentiation Effector T helper subsets Differentiation and function of T H 1/T H 2 cells IL-12, IL-18, and IFN-γ induce T H 1 differentiation Facilitate induction of T-bet master regulator in positive feedback loop Characterized by strong IFN-γ production, which:» Leads to class switching to IgG classes that support phagocytosis and complement fixation» Supports differentiation of antiviral CD8+ killer T cells 41

42 T-cell differentiation Effector T helper subsets Differentiation and function of T H 1/T H 2 cells IL-4 promotes T H 2 subset differentiation Triggers master regulator GATA3 to upregulate characteristic IL-4, IL-5, and IL-13 production» IL-4 acts to promote activities of eosinophils against helminths» IL-4 induces class switching to IgE, which helps other cell types to release antiparasite inflammatory mediator molecules T-cell differentiation Effector T helper subsets TH1/TH2 cross-regulation Cytokines can achieve cross-regulation IFN-γ from TH1 responses inhibits IgG1/IgE class switching (a common TH2-induced response) IL-4 from TH2 responses inhibits production of IgG2a (a common TH1-induced response) IL-10 from TH2 responses also inhibits TH1 responses by suppressing the production of inflammatory mediators from APCs Master regulators commit T cells to one subset or the other T-Bet suppresses TH2 pathway gene expression GATA3 suppresses TH1 pathway gene expression 42

43 T-cell differentiation Effector T helper subsets TH17 cells Activation in the presence of IL-6 and TGF-β forms this subset IL-23 also plays a role in finalizing the subset commitment The master regulator RORγt, an orphan steroid receptor, becomes active and differentiates activating T cells into this subset IL-17A produced is associated with chronic inflammatory and autoimmune responses IL-17F and IL-22 produced may play a role in warding off fungal and extracellular bacterial infections T-cell differentiation Effector T helper subsets (induced) TREG cells Similar in function to natural TREG cells originating in the thymus Arise during activation of T cells in the presence of TGF-β TGF-β induces FoxP3 master regulator, shifting activating cells into this subset itreg cells secrete IL-10 and TGF-β to downregulate inflammation (by inhibiting APCs) and suppress other T- cell subsets 43

44 T-cell differentiation Effector T helper subsets TH17/TREG cross-regulation TGF-β is a key cytokine for differentiation of both subsets IL-6 is the switch, allowing RORγt to dominate and induce TH17 subset differentiation instead Thinking about the beneficial outcomes of each subset, a balance between the two is ideal Normal state could favor development of suppressive itreg population to keep inflammation down Inflammation from an infection (leading to IL-6 production) would stimulate more antibacterial TH17 differentiation T-cell differentiation Effector T helper subsets Follicular helper T (TFH) cells IL-6 and IL-21 appear to be the polarizing cytokines Leads to activation of Bcl-6 master regulator» This also inhibits T-Bet, GATA3, and RORγt expression IL-4 and IL-21 are characteristic secreted cytokines, promoting B-cell differentiation Other potential helper T-cell subsets Understanding of subtypes continues to evolve Other populations with distinct polarizing cytokine requirements have been described (e.g., TH9) However, overlap of cytokines secreted may mean these are subtle variants of other subsets 44

45 12/18/17 T-cell differentiation Helper T cells may not be irrevocably committed to a lineage Understanding still evolving Early in differentiation, TH subpopulations may be able to shift When exposed to IL-12, young TH2 cells will produce IFN-γ Young TH1 cells will produce IL-4 in TH2 polarizing conditions Neither TH1 or TH2 can adopt TH17 or itreg traits Fluidity among subsets makes definitive establishment of helper cell lineages difficult O'Shea, J. J., & Paul, W. E. (2010). Science, 327:

46 T-cell memory Naïve, effector, and memory T cells display broad differences in surface protein expression Three surface markers can differentiate the sets CD44 increases in response to activation signals CD62L an adhesion protein CCR7 a chemokine receptor T-cell memory T CM and T EM are distinguished by their locale and commitment to effector function T CM cells central memory T cells reside in/travel between secondary lymphoid tissues Live longer/divide more times than T EM cells Are rapidly reactivated by second Ag exposure Can differentiate into several subset types depending on cytokine environment T EM cells effector memory T cells Travel to/between tertiary tissues Contribute better to first-line defenses Shift right back into effector functions on second Ag exposure Generalizations may not hold up as we learn more 46

47 T-cell memory How and when do memory cells arise? Ongoing studies, but currently indicate/hypothesize that: Memory cells may arise very early in immune response (3 days) T CM cells may arise from or prior to T EM cells T EM cells may be derived from fully differentiated effector cells They may arise from asymmetrical division of activated T cells Self-renewing memory stem cell populations may be generated during T- cell activation CTL responses 47

48 CTL responses CTLs recognize and kill infected/tumor cells via TCR activation Effector CTL generation from CTL precursors Signal 1 TCR binds peptide presented by APC on MHC class I Signal 2 costimulatory signal transmitted by CD28-B7 interaction between T cell and APC APCs get help from T cells to upregulate stimulation molecules Signal 3 provided by IL-2, inducing proliferation and differentiation into CTL form Robust Anti-Viral Immunity Requires Multiple Distinct T Cell- Dendritic Cell Interactions Eickhoff et al., 2015, Cell 162,

49 Robust Anti-Viral Immunity Requires Multiple Distinct T Cell- Dendritic Cell Interactions During the course of a viral infection, CD4+ and CD8+ T lymphocyte activation is initially separated spatially, but a subset of dendritic cells acts as a platform to orchestrate their communication to optimize CD8+ T cell expansion and memory function. o o o o Initial activation of CD4+ and CD8+ T lymphocytes is spatially separated Later during infection, XCR1+ DCs present antigen to both lymphocyte subsets XCR1+ DCs are a critical platform for delivery of CD4+ T cell help to CTL Absence of XCR1+ DCs leads to aberrant memory CD8+ T cell differentiation Eickhoff et al., 2015, Cell 162, Approaches to DC-Based Therapeutic Vaccination in Cancer and Chronic Infection Immunity Volume 33, Issue