Module 7 Flashcards
Cell-Mediated Immunity
sequence of cell mediated immunity (CMI)
effector cells are responsible for the immune response in CMI
- immune response can be divided into two phases based on the effector cells present: non-specific and specific
non-specific CMI
responses performed by macrophages and NK cells, which result in an increased IFN-a and IFN-b
- in the later stages of the non-specific stage of the response, continued contribution of macrophages will activate Th1
specific CMI
includes responses performed by cytotoxic T cells, which are activated by Th1 cells
antigen non-specific effector cells
these can recognize pathogens, but are not specific to a particular antigen
- they do not distinguish one pathogen from another
antigen specific effector cells
CTLs are effector cells specific for a processed pathogen antigen in complex with MHC class I
non-specific effector cells: macrophages
the initial macrophage response is nonspecific
during later stages, antigen specific Th1 cells play a role in activating the macrophage response
macrophage functions
produce reactive oxygen intermediates, nitric oxide and lysosomal enzymes
secrete cytokines
activate NK cells during infection
non-specific effector cells: NK cells
NK cells play a role in the early response to infection
- NK response can be activated by IFN-a, IFN-b, and IL-2
NK cell functions
Fas-mediated killing
control infection during the period required for generation of specific cells
involved in ADCC
specific effector cells: CD4+ Th1 cells
antigen specific effector T cells are specialized to deal with different classes of pathogens
- Th1 activates macrophages, NK cells, and CTLs
Th1 and Th2 cell actions
Th1: results in CMI response
Th2: results in humoral immune respones
specific effector cells in CMI: CD8+ CTL
antigen specific effector T cells are specialized to deal with different classes of pathogens
- CTLs kill pathogen infected cells
CTL antigen recognition
CTLs are CD8+ cells that kill target cells that display peptide fragments of cytosolic pathogens bound to MHC class I molecules (ex: kills influenza virus)
CD4+ antigen recognition
expressed by Th2 cells, recognizes fragments of antigens degraded within intracellular vesicles displayed by MHC Class II molecules (ex: kills M. tuberculosis)
Th1 cell function vs CTL cell function
Th1 cells activate macrophages, NK cells, and CTLs, enabling them to destroy pathogens efficiently
CTLS kill infected target cells
clinical applications of CMI effector cellls
delayed type hypersensitivity (DTH) is a response mediated by sensitized helper T cells, which release various cytokines. DTH plays a part in macrophage clearance and killing of intracellular pathogens. DTH is the result of cytokine release from CMI effector cells
DTH responses
DTH responses in the skin have been used to test CMI in vivo
- if testing for TB, a positive test requires a subject’s exposure to the antigen to be at least 4-6 weeks prior to skin testing. positive tests are seen as erythema and induration 48-72 hours post injection
- a 10 mm induration is considered positive, and indicates immune sensitization only
effector cells in DTH
cytokines released from activated helper T cells elicits a DTH reaction
- cytokines modulate expression of cell adhesion molecules and promote monocyte to macrophage differentiation in the tissue
effects of activated CMI effector cells
- cytokines: Th1 effector cells from previous immunization recognizes the antigen and releases cytokines
- recruitment: recruitment and activation of more T cells, neutrophils, and macrophages to the site of antigen injection
- inflammation: recruited cells cause inflammation to the area
phagocytosis steps
attachment
ingestion
fusion
digestion
release
effects of macrophages in CMI
- production of reactive oxygen intermediates, nitric oxide, and increased lysosomal enzymes: kills microbes in phagolysosomes
- secretion of cytokines: TNF, IL-2: recruits leukocytes for inflammation. IL-12: Th1 differentiation
- increased expression of B7 costimulators, MHC molecules: increased T cell activation
effects of increased IL-12 secretion
IL-12 induces differentiation of naive CD4 Th0 cells into Th1 cells
NK cells
activated by interferons and macrophage derives cytokines to serve as an early defence against certain intracellular infections
NK cells target recognition recognition
killer inhibitory receptors (KIRs) are present on NK cells and recognize MHC class I
- all normal host cells/somatic cells express MHC class I molecules, therefore, when KIRs recognize these molecules, there is no killing of the normal cells
- virally infected cells have a downregulated expression of MHC class I molecules. In the absence of MHC class I molecules, KIRs are not engaged, ultimately resulting in signalling the NK cells to kill the cell
antibody dependent cell mediated cytotoxicity (ADCC)
a mechanism by which NK cells interact and kill target cells
- steps include antibody opsonization, NK cell CD16, activated NK cell
ADCC antibody opsonization
infected target cells express pathogen antigens on their cell surface
- these cells are recognized by specific antibodies produced by antibody secreting plasma cells
- this process is called opsonization
ADCC: NK Cell-FcyRIII (CD16)
NK cells express CD16 receptors, which recognize and bind the Fc portion of antibodies
- once CD16 of an NK cell binds to an antibody attached to a target cell, NK cell is activated
ADCC: activated NK cell
activated NK cells bound to a target cell release the content of their cytolytic granules by directly killing the target cell or by inducing Fas L/Fas-mediated apoptosis
cytotoxic T lymphocytes (CTLs)
the process of antiviral T cell activation is delayed in the generation of antigen specific CTLs
- due to their specificity, CTLs mount a more effective and lasting response against the virally infected cell than the initial non-specific immune response by NK cells
CTL activation
once immunocompetent naive CTLs are circulating in the organism, they require 3 signals to kill a target cell:
- adhesion
- antigen presentation
- activation
CTL activation: adhesion
interactions of T-cells with their targets initially involves non-specific adhesion molecules
- the initial interaction is formed by integrin LFA-1 on the T cell, and ICAM-1 on their target cell
CTL: antigen presentation
the interaction between adhesion molecules ensures that the T cell remains in contact with the target cell
- TCR peptide: MHC complex, in association with CD28 and B7, provides a signal to activate the CTL
CTL: activation
poised or partially activated CTLs express high levels of the IL-2R beta chain and low levels of the IL-2R alpha chain
poised CTLs secrete small amounts of IL-2, but not enough for full activation of the CTL. Poised CTL requires additional IL-2 from CD4+ Th1 cells
clonal expansion of CTLs
secretion of IL-2 from CD4+ helper Th1 cells increases p55-alpha expression for IL-2R, which permits the formation of high affinity IL-2R
- increased expression of IL-2R binds more IL-2 secreted from Th1 cells, allowing for clonal expansion of activated CTLs
clonal expansion
responsible for the strength and specificity of the cell mediated immune response
- it results in an increased number of cytotoxic T cells that are specific for an processed antigen expressed on MHC class I that can travel throughout the body to fight infection
MHC restriction of CTLs
during clonal expansion, activated CTLs develop:
- cytoplasmic granules that contain perforins, serine esterases, granzymes, toxins, etc.
- cytotoxic cytokines that induce IFN-y, TNF-b, which can inhibit viral replication and are involved in the activation and recruitment of macrophages
MHC Class I restricted CTL
two responses can be elicited when CTLs interact with processed antigenic peptide presented by MHC on a target cell
- either no recognition, or peptide/MHC specific recognition
no recognition
no recognition = no killing
if the TCR of a CTL does not recognize both the peptide and the MHC complex presented on a target cell, then the activated CTL will not recognize the antigen presenting target cell
- the CTL will separate from the target cell and not result in killing of the target cell
peptide/MHC specific recognition
recognition = killing
when the peptide:MHC complex is recognized by TCR, a series of events will activate the CTL effector functions
- the effector CTL will kill the target cell
polarization of T cells during killing steps
- TCR of the CTL recognizes peptide:MHC complex. The LFA-1 of the T cell adheres to ICAM-1 of the target cell
- TCR:MHC binding and adhesion molecules signals for the effector molecules in T cell to re-orient towards the target cell. effector molecules include MTOC, Golgi, and cytotoxic granules 3. re-orientation results in a lethal hit of cytotoxic granules to target cell at the point of contact (first punch) 4. cytotoxic granules release perforin in a directed fashion to target cell. at the site of cell-cell contact, perforins undergo Ca2+-dependent polymerization which forms pores in target cell membrane
- pores formed allow entry of granules (ex: granzymes) which initiates cell death by apoptosis
membrane damage to target cells
results in a visible cavity, influx of water, and swelling of the target cell ultimately, killing the target cell
T cell polarization
once the CTL is activated, it becomes polarized
- polarization of T cells results in conformational changes of the components of the T cell that promote pore formation in the target cell
- polarization of T cells leads to pore formation in the target cell
what would happen with a perforin deficiency
FHL is a disease caused by perforin deficiency, meaning CTLs and NKs cannot create perforin
- as a result, the CTL stays intact and its ability to kill target cells (its effector function) is reduced
- Fas/FasL mediated apoptosis will take over to compensate for perforin deficiency
apoptosis via Fas/FasL
Fas mechanism can be used to kill the target cell
- FasL is a membrane protein expressed by activated CD8+ T cells
- it binds to its target protein Fas, which is expressed on somatic cells
- interaction results in activation of capsase enzymes resulting in apoptosis of target cell
- target cell debris is removed by macrophages to prevent an inflammatory response
CTL killing process review
- adhesion: LFA-1 : ICAM-1
- recognition: TCR: peptide/MHC molecule
- movement of granules and reorientation
- killing of the target cell: “lethal hit”, pore formation, and cell apoptosis
CTL killing cancer cells
CTLs can kill cancerous cells in a similar manner
- cancer cells express self-antigens on MHC, allowing CTLs to recognize them
