Ativação e diferenciação de Linfócitos T na periferia Flashcards
Papel dos linfócitos T na erradicação de infecções
A, CD4+ T cells (Th1, Th17) recognize antigens of phagocytosed and extracellular microbes and produce cytokines that recruit
and activate the phagocytes to kill the microbes. CD8+ T cells can also secrete some cytokines and participate in
similar reactions.
B, CD8+ cytotoxic T lymphocytes (CTLs) recognize antigens of microbes residing in the cytoplasm of infected cells and kill the cells.
Sequência de eventos para o estabelecimento da resposta imunológica mediada por linfócitos T CD4+
CD4+ T cells recognize peptides that are derived from protein antigens and presented by dendritic cells in peripheral lymphoid organs.
The T lymphocytes are stimulated to proliferate and differentiate into effector (and memory) cells, which enter the circulation and
migrate to sites of infection in peripheral tissues.
In the tissues, effector T cells
recognize the antigen and respond by secreting cytokines that recruit more
leukocytes and activate phagocytes to eradicate the infection.
Reconhecimento de antigénio e ativação de linfócitos T - Iniciação de uma resposta imunológica adquirida
Interação entre a célula T naïve e a APC:
- Sinal 1: Fornecido pela sinalização via TCR (interação do TCR com complexo MHC/péptido), co-recetor CD4 ou CD8 com MHC, e moléculas de adesão. Relevante para a iniciação da ativação de uma célula T naïve e ativação de células T efetoras.
- Sinal 2: Fornecido pela interações do recetor CD28 na célula T com moléculas coestimuladoras expressas pela APC (B7.1/B7.2). Aumenta a sobrevivência e a proliferação de células que receberam o sinal 1.
ICOS: recetor na célula T e
ICOSL: expresso por APCs =Sinal 2
• Sinal 3: Citocinas. Sinal que dita qual o tipo de diferenciação será adotado pelas células T ativadas (e.g. Th1, Th2, Th17).
The TCR/MHC-peptide interaction, along with CD4
and CD8 coreceptors and adhesion molecules, provide Signal 1. Costimulation by a separate set of molecules, including CD28 (or ICOS, not shown) provides Signal 2. Together, Signal 1 and Signal 2 initiate a signal transduction cascade that results in
activation of transcription factors and cytokines
(Signal 3) that direct T-cell proliferation (IL-2) and
differentiation (polarizing cytokines). Cytokines can act in an autocrine manner, by stimulating the same cells that produce them, or in a paracrine manner, by stimulating neighbouring cells.
Interações responsáveis pela ativação de células T
A schematic of the interactions between a CD4 T cell (left) or CD8 T cell (right) and its activating dendritic cell.
Dendritic cells process antigens and present peptides associated with MHC class II to CD4 T cells and present peptides associated with MHC class I to CD8 T cells.
Binding of TCR to MHC-peptide is enhanced by the binding of coreceptors CD4 and CD8 to MHC class II and class I, respectively. CD4 and CD8 also associate with Lck, which helps initiate signaling.
CD28 interactions with CD80/86 provide the required
costimulatory signals.
Adhesion molecule interactions, two of which (LFA-1/ICAM-1, CD2/LFA-3) are depicted, markedly strengthen the connection between the T cell and APC or target cell so that signals can be sustained.
(a) A successful T-cell/DC interaction results in the organization of signaling molecules into an immune
synapse. A scanning electron micrograph (left) shows the binding of a T cell (artificially colored yellow) and dendritic cell (artificially colored blue). A fluorescence micrograph (right) shows a cross-section of the immune synapse, where the TCR is stained with fluorescein (green) and adhesion molecules (specifically LFA-1) are
stained with phycoerythrin (red). Other molecules that can be found in the central part of the synapse (central supramolecular activation complex [cSMAC]) and the peripheral part of the synapse (pSMAC) are listed:
cSMAC: TCR/CD3 Coreceptors CD4 or CD8 Costimulatory receptors (CD28)
pSMAC:
Adhesion molecules
LFA-1/ICAM-1
LFA-3/CD2
Ativação de Linfócitos T
--> Iniciada pela interação do complexo TCR-CD3 com péptidos apresentados por moléculas de MHC: • CD8+ - MHC classe I • CD4+ - MHC classe II \+ co-estimulação \+citocinas
–> Inicia uma cascata de eventos bioquímicos que induz uma célula a entrar
em fase ativa do ciclo celular, proliferar e diferenciar-se em célula efetoras e de memória
reconhecimento do Ag + ativação –> expansão clonal + diferenciação –> contração –> memória
Sinalização via TCR
Cinases LcK & ZAP-70 –> Fosforilação de ITAMs no
complexo CD3 e adaptadores (LAT e SLP-76)
Antigen recognition by T cells induces early signaling events, which include tyrosine phosphorylation of molecules of the T cell receptor (TCR) complex and the recruitment of adaptor proteins to the site of T cell antigen recognition.
These early events lead to the activation of several biochemical intermediates, which in turn activate transcription factors that stimulate transcription of genes whose products mediate the responses of the T cells.:
PLCgama1 activation –> increased cyt Ca2+ –> calcineurin –> NFAT
AND Diacylglycerol –> PKC –> NFkB, AP-1
GTP/GDP exchange on Ras, Ras –> Ras/Rac-GTP –> ERK, JNK –> NFkB, AP-1
Activation of Pl3 kinase –> PIP3 –> Akt, mTOR –> NFkB, AP-1; +protein synthesis
The possible effects of costimulation on these
signaling pathways are not shown. These signaling pathways are illustrated as independent of one another, for simplicity, but may be interconnected in more complex networks.
Co-estimulação é necessária para ativação de linfócitos T?
- Co-estimulação é necessária para ativação de linfócitos T!!:
Experimental setup used anti-CD3 monoclonal antibodies or a mitogen, PMA, to provide Signal 1, and anti-CD28 antibodies to provide Signal 2.
(b) [3H]uridine incorporation an indicator of RNA synthesis was measured, in response to various treatments. Addition of stimulating anti-CD28 antibody increased RNA synthesis in response to activation by PMA or anti-CD3.
Papel da co-estimulação na ativação de linfócitos T
Resting antigen-presenting cells (APCs), which have not been exposed to microbes or adjuvants, may present peptide antigens, but they do not express costimulators and are unable to activate naive T cells. T cells that recognize antigen without costimulation may become unresponsive (tolerant) to subsequent exposure to antigen.
Microbes, as well as cytokines produced during innate immune responses to microbes, induce the expression of costimulators, such as B7 molecules, on the APCs. The B7 costimulators are recognized by the CD28 receptor on naive T cells, providing signal 2. In conjunction with antigen recognition (signal 1), this recognition initiates T cell
responses. Activated APCs also produce cytokines that stimulate the differentiation of naive T cells into effector
cells. IL, Interleukin.
Recetores de coestimulação e inibição
Recetores de co-estimulação:
• CD28
• ICOS
Recetores de inibição:
• CTLA4
• PD1
Ligands on APCs that are homologous to B7 bind to receptors on T cells that are homologous to CD28. Different ligand-receptor pairs serve distinct roles in immune responses. CD28 and ICOS are stimulatory receptors on T
cells, and CTLA-4 and PD-1 are inhibitory receptors.
Exemplo expressão/função CTL4?
• Células T naïve expressam CD28 que, por interação com moléculas coestimuladoras
presentes em APCs (B7.1
e B7.2) promove a sobrevivência e expansão das células T.
[cross-linking of CD28 delivers the co-stimulatory signal during activation of naive T cells and induces the expression of CTLA-4 (CD152)]
- Células T ativadas expressam níveis aumentados de CTLA4 (CD152) que possue uma maior afinidade para moléculas de B7.1 e B7.2, fornecendo um sinal inibitório à célula T ativada.
- CTLA4 serve assim como um regulador da fase proliferativa das células T
[CTLA-4 binds B7 (CD80 or CD86) more avidly than does CD28 and delivers inhibitory signals to activated T cells]
Células T efectoras podem ser ativadas na ausência de sinal 2!!
Sinal 1 (via TCR) é sempre necessário para ativação
naive T cell stimulated by virus-infected DC –> T cell recognizes same antigen on infected epithelial cell –> activated T cell kills infected epithelial cells
Papel dos Recetores de “inibição”
• Papel importante na manutenção da tolerância na periferia
• Ajudam a reduzir a inflamação no curso natural de uma infeção e durante
infeções crónicas
ver a tabela?? específica para CTLA-4 e PD-1
Naïve T cells are activated when they engage both TCR and CD28 costimulatory receptors on antigen-presenting cells. Activated T cells up-regulate the coinhibitory molecule CTLA-4, which binds CD28 ligands even more strongly. This interaction inactivates the T cell. However, when the CTLA-4 engagement is blocked with the antibody ipilimumab, the T cell is reactivated.
costimulation via CD28: tcell activation
CTLA- 4 blocks costimulation: no tcell activation
ipilimumab blocks CTLA-4: t-cell activation
Sinais que induzem anergia
Resting T cells that engage antigen and costimulatory ligands CD80/86 are activated and expand.
(b) T cells that engage antigen on the surface of a non-antigen-presenting cell (such as a pancreatic islet beta cell) will not receive costimulatory signals and, instead, are inactivated or anergized.
(c) Anergy can also be induced when a T cell engages antigen and receives inhibitory rather than costimulatory signals through coinhibitory receptors such as CTLA-4 and PD-1.
Células apresentadoras de antigénio (APCs) profissionais
3 major classes of professional APCs.
- Dendritic cells are the best activators of naïve T cells. This may be due, in part, to their relatively high levels of expression of MHC and costimulatory molecules
when they are mature and activated.
-Activated B cells interact most efficiently with differentiated T cells that are specific for the same antigen that activated them.
-Macrophages play several different roles, processing and distributing antigen in
secondary lymphoid tissues as well as interacting with effector cells in the periphery.
!It is important to recognize that the distinctions shown are rules of thumb only. Functions among the APC classes overlap, and investigators now recognize
many different subsets within each major group of APCs, each of which may act independently on different T-cell subsets.
This diversity may be a consequence of activation by different innate immune receptors or may reflect the existence of independent cell lineages. Note that activation of effector and memory T cells is not as dependent on costimulatory interactions.
Key features - DCs and macrophages
- phagocytic
- express receptors for apoptotic cells, DAMPs, and PAMPs
- localize to tissues
- localize to T-cell zone of lymph nodes following activation (DCS)
- constitutively express high levels of MHC class II molecules and antigen-processing machinery
- express costimulatory molecules following activation
- variety of subtypes, some migrating, some resident, some better at cross-presenting
- specific subtypes may specialize in activating different T cells
