Phase 2- Destruction of Target Cells
*Directional release of cytotoxic proteins (including perforin and granzyes) onto target cell.
*Interaction of the membrane-bound Fas ligand on CTLs with the Fas receptor on the surface of target cells.
Both mechanisms probably involve apoptosis.
Primary events in pathway involving directional release of cytotoxic proteins involves:
conjugate formation - CTL binds to target cell
membrane attack - Ca++ dependent infliction of membrane damage
CTL dissociation
target cell destruction
Conjugate formation -
CTL binds to target cell
TcR binds to Class I MHC + Peptide
LFA-1 on CTL binds to intercellular cell-adhesion molecules (ICAMs) on the target cell. High affinity following recognition of antigen but only for ~ 5-10 minutes.
CTLs contain storage granules filled with TNF-beta, perforins, granzymes [proteases].
Contents released by exocytosis - pores are created on target cell membrane. These pores then allow toxic substances to enter the target cell.
Granzymes trigger an apoptotic pathway which results in the fragmentation of the target cell DNA (occurs within 5 minutes of cell contact) [it has been shown that viral DNA is also fragmented in this process!]
Second pathway is due to the fact that CTLs possess Fas ligand.
This ligand binds to Fas on target cell and induces a signal in the target cell that activates the endogenous apoptotic pathway.
NK cell-mediated cytotoxicity
5-10% of the re-circulating lymphocyte population.
Lineage still uncertain.
They possess Thy1, CD2, the beta subunit of the IL-2 R , and CD16 (the receptor for Fc portion of IgG - also termed FcgRIII)
Monoclonal antibody against FcgRIII almost completely removes NK cell activity.
Activity stimulated by INF-alpha, INF-beta, and IL-12
NK cells are really our first line of defense against viral infection.
(It take almost a week in vivo to get good population of functional CTLs)
NK cells kill in a manner similar to CTL cells, but antigen-nonspecific and NON-MHC restricted. There are no TcRs or CD3 proteins. Also, no increase in cytotoxic activity is observed upon a secondary exposure to the antigen. NO MEMORY. Perforins and granzymes are present in granules in the cytoplasm constitutively. NK cells are also able to secrete TNF-alpha. In both cases, apoptosis is probably the major cause of death.
ADCC
antibody dependent cell mediated cytotoxicity
Cytotoxic potential by cells which bear receptors for the Fc portion of Ig.
NK cells, macrophages, monocytes, neutrophils, and eosinophils
DOES NOT involve complement, DOES NOT involve phagocytosis.
# of Different Mechanisms:
release of lytic components
release of TNF-alpha
release of perforins and granzymes by NK cells and eosinophils
DELAYED TYPE HYPERSENSITIVITY
Type IV Hypersensitivity
Common response to intracellular pathogens and to skin contact antigens (poison ivy and oak, metals, etc.)
Classic example is the immune response to Mycobacterium tuberculosis, first noted by Robert Koch who observed that individuals who had been exposed to M. tuberculosis would develop a raised red lesion if injected intradermally with M. tuberculosis antigen.
Response came to be known as the tuberculin reaction.
Response is characterized by large influx of nonspecific inflammatory cells. Initially neutrophils are observed but they soon wane and macrophages represent the predominant effector cells of the response.
Two Phases:
Sensitization Phase- Occurs within 1-2 weeks following the primary contact with the antigen or pathogen. This phase involves Th1 cell activation and clonal expansion. Both Langerhan cells (dendritic cells in skin) and Macrophages have been identified as important APCs in this response.
Effector Phase - Follows a secondary encounter with antigen. Involves additional proliferation of TH1 (Tdth) cells followed by intense cytokine secretion. This leads to the recruitment and activation of macrophages.
Occurs 24hours following secondary contact but activity peaks in 48-72 hours.
Macrophages are the principle effector cells - and they are activated by Tdth cytokines. Such macrophages show increased phagocytic activity, increased extravasation, increased respiratory burst and NO synthesis.
Protective Result: CLEARANCE OF PATHOGEN
An intense, prolonged DTH can have damaging results such as granuloma formation, blood vessel damage, and tissue necrosis [primarily due to the release of lytic enzymes by macrophages].
Cytokines involved:
IL-2 works in an autocrine fashion, amplifying the Tdth response.
IL-3 and GM-CSF - promote hematopoiesis of granulocyte- monocyte lineage
IFN g and TNF-b from Tdth cells
TNFa and IL-1 from macrophages or dendritic cells
Lead to increased expression of adhesion molecules on vascular endothelial cells, increased extravasation etc.
As monocytes extravasate into tissue they become macrophages and macrophages are chemotactically attracted to site of DTH response by MCAF - monocyte chemotactic and activating factor and by IFN-g. Once at site, MIF (migration inhibition factor) inhibits their further migration.
At site, macrophages become further activated by IFN g and membrane bound TNF-b from Tdth cells. In turn, the macrophages become even better APCs and can stimulate more Tdth cells.
AIDS patients have virtually no DTH reponse and succumb to intracellular pathogens which are even non-pathogenic in healthy individuals.
Previous
| Next
| 220 Syllabus
| Biology Home
| WKU Home
| Research Paper Topics
Lecture 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29