front 1 Describe how NK cells circulate in the bloodstream and are able to
enter inflamed tissues to participate in the innate
immune response.
Describe the types of cytokines that activate and recruit NK
cells, and describe how NK cells can
influence the activity of
tissue macrophages through their release of IFN-γ | back 1  - NK cells circulate through the body in the bloodstream but can
be rapidly recruited to enter tissues. NK cells play an important
role in innate immunity and can contribute to inflammation. As
components of innate immunity, NK cells are rapid responders to
infection, and function in a more general manner, not in an
antigen-specific manner as do the B and T lymphocytes.
- Natural killer (NK) cells are a critical component of the innate
immune system, tasked with identifying and eliminating infected or
abnormal cells. These cells circulate throughout the bloodstream,
constantly surveying for signs of distress. The process by which NK
cells enter inflamed tissues involves a complex interplay of
adhesion molecules and chemokines.
- When tissues become
inflamed, various signals are released, including chemokines such as
CXCL8 (IL-8) and CXCL10 (IP-10), which act as chemoattractants. NK
cells express receptors that recognize these chemokines, allowing
them to follow the gradient towards the site of inflammation.
Additionally, adhesion molecules such as integrins and selectins are
upregulated on the surface of endothelial cells lining blood vessels
in inflamed tissues. NK cells express corresponding ligands for
these adhesion molecules, facilitating their extravasation from the
bloodstream into the tissue.
- Once inside inflamed tissues,
NK cells interact with other immune cells, including tissue
macrophages. NK cells are activated by cytokines such as
interleukin-12 (IL-12), interleukin-15 (IL-15), and type I
interferons (IFNs), which are typically produced by dendritic cells,
macrophages, and other immune cells in response to infection or
inflammation. Upon activation, NK cells release cytokines such as
interferon-gamma (IFN-γ), which plays a crucial role in shaping the
immune response.
- IFN-γ produced by NK cells can influence
the activity of tissue macrophages in several ways. Firstly, IFN-γ
enhances the antimicrobial activity of macrophages by stimulating
the production of reactive oxygen species (ROS) and nitric oxide
(NO), which are toxic to pathogens. Secondly, IFN-γ promotes the
upregulation of major histocompatibility complex (MHC) molecules on
the surface of infected cells, facilitating their recognition and
elimination by cytotoxic T cells. Lastly, IFN-γ can modulate the
polarization of macrophages towards a pro-inflammatory phenotype,
enhancing their ability to engulf and destroy
|
front 2 Describe the relationship between virus infection, interferon
production, and NK cell activation. | back 2 - Virus Infection: When a virus infects host cells, it triggers
various intracellular signaling pathways that are recognized by the
host's immune system as foreign. Infected cells may display viral
antigens on their surface, signaling danger to the immune
system.
- Interferon Production: In response to virus
infection, infected cells and nearby immune cells release signaling
molecules called interferons. These interferons, particularly type I
interferons (such as interferon-alpha and interferon-beta), serve as
alarm signals to neighboring cells, alerting them to the presence of
viral infection. This triggers an antiviral state in neighboring
cells, making them more resistant to viral replication and
spread.
- NK Cell Activation: Interferons, along with other
cytokines like interleukin-12 (IL-12) and interleukin-15 (IL-15),
play a crucial role in activating natural killer (NK) cells. NK
cells are a type of lymphocyte that forms a critical part of the
innate immune response. Upon activation, NK cells become cytotoxic
and start to target and kill virus-infected cells. They do so by
recognizing and engaging with infected cells that display abnormal
or stressed markers, such as reduced expression of MHC class I
molecules, which are often downregulated by viruses to evade
detection by cytotoxic T cells. The relationship between virus
infection, interferon production, and NK cell activation is
intricate and crucial for mounting an effective immune response
against viral pathogens.
- Effector Functions: Once
activated, NK cells exert their effector functions. They release
cytotoxic molecules such as perforin and granzymes, which induce
apoptosis (programmed cell death) in the infected cells. NK cells
also produce cytokines like interferon-gamma (IFN-γ), which further
amplify the immune response by activating macrophages, enhancing
antigen presentation, and promoting inflammation.
- Feedback
Loop: The production of type I interferons by infected cells not
only activates NK cells but also creates a positive feedback loop.
IFN-γ produced by activated NK cells can further stimulate infected
cells to produce more type I interferons, reinforcing the antiviral
state of neighboring cells and enhancing overall antiviral
immunity.
So, the relationship between virus infection, interferon
production, and NK cell activation is intertwined: virus infection
triggers the production of interferons, which in turn activate NK
cells to target and eliminate virus-infected cells, thus helping to
control the spread of the virus within the host.
In summary, virus infection leads to the production of
interferons, which not only establish an antiviral state in infected
and neighboring cells but also play a pivotal role in activating NK
cells. Activated NK cells then contribute to the immune response by
directly targeting and eliminating virus-infected cells, thus helping
to contain the spread of the infection and limit its severity. This
coordinated response between virus-infected cells, interferon
production, and NK cell activation forms an essential aspect of the
innate immune defense against viral pathogens. |
front 3 Describe the types of ligands that NK cell activating receptors bind
to (stress-induced proteins such as MIC).
Describe the types of
ligands that NK cell inhibitory receptors bind to (classic MHC-I or HLA-E). | back 3 - Stressed genes : they will express MIC on their surface
- Cells that are not stressed do a poor job of producing and
stimulating NK cells
NK cells express a variety of receptors that allow them to
interact with target cells, either activating their cytotoxic
functions or inhibiting them. These receptors recognize specific
ligands on the surface of target cells, influencing NK cell activity.
Here's a breakdown of the types of ligands recognized by NK cell
activating and inhibitory receptors:
- NK Cell Activating Receptors Ligands: a. Stress-Induced
Proteins: Activating receptors on NK cells, such as NKG2D (natural
killer group 2 member D), recognize stress-induced proteins that are
upregulated on the surface of target cells during cellular stress or
transformation. Examples of ligands for NKG2D include MICA (MHC
class I chain-related protein A) and MICB (MHC class I chain-related
protein B). These ligands are often expressed on virus-infected
cells, tumor cells, or cells undergoing cellular stress, marking
them for NK cell recognition and activation. b. Viral Proteins: Some
activating receptors on NK cells can directly recognize viral
proteins expressed on infected cells. For example, the NKp30
receptor can bind to viral hemagglutinins, while NKp44 and NKp46 can
recognize viral hemagglutinin-neuraminidase proteins. These
interactions trigger NK cell activation and cytotoxicity against
virus-infected cells.
- NK Cell Inhibitory Receptors Ligands:
a. Classical MHC-I Molecules: The most well-characterized inhibitory
receptors on NK cells are the killer cell immunoglobulin-like
receptors (KIRs) and the CD94/NKG2A heterodimer. These receptors
primarily recognize classical MHC class I molecules (HLA-A, HLA-B,
and HLA-C) on the surface of healthy cells. When these receptors
engage with MHC class I molecules, they deliver inhibitory signals
to the NK cell, preventing it from initiating cytotoxicity against
the target cell. This mechanism serves as a
"self-recognition" system, ensuring that NK cells spare
healthy cells expressing normal levels of MHC class I molecules. b.
HLA-E Molecule: Another ligand for the inhibitory receptor
CD94/NKG2A is the non-classical MHC class I molecule HLA-E. HLA-E
presents peptides derived from the leader sequences of other MHC
class.
In summary, NK cell activating receptors bind to stress-induced
ligands such as MICA, MICB, and ULBP, triggering NK cell activation
and cytotoxicity. In contrast, NK cell inhibitory receptors bind to
self-MHC class I molecules (classic MHC-I) or HLA-E, delivering
inhibitory signals that prevent NK cell activation and killing of
healthy cells. This balance between activating and inhibitory signals
helps regulate NK cell activity and ensure proper immune surveillance
without causing harm to healthy tissues. |
front 4 Explain the fundamental difference between direct and indirect
assessment of MHC-I expression on a target cell by
NK cells. How
do the mechanisms differ from one another? Understand the role of HLA-E. | back 4 Direct and indirect assessment of MHC class I (MHC-I) expression by
natural killer (NK) cells represent distinct mechanisms by which NK
cells evaluate the "self" status of target cells. These
mechanisms involve the recognition of MHC-I molecules on target cells,
which serve as ligands for inhibitory receptors on NK cells. The
fundamental difference lies in how MHC-I molecules are detected and
interpreted by NK cells:
- Direct Assessment: In direct assessment, NK cells directly
interact with classical MHC-I molecules (HLA-A, HLA-B, and HLA-C)
displayed on the surface of target cells. Classical MHC-I molecules
present peptides derived from endogenous proteins to T cells,
contributing to antigen presentation and immune surveillance. NK
cells possess inhibitory receptors, such as killer cell
immunoglobulin-like receptors (KIRs) and CD94/NKG2A, that recognize
specific epitopes on classical MHC-I molecules. When these
inhibitory receptors engage with MHC-I molecules on target cells,
they deliver inhibitory signals to the NK cell, preventing it from
initiating cytotoxicity. This mechanism ensures that healthy cells
expressing normal levels of MHC-I molecules are spared from NK
cell-mediated killing, thereby maintaining self-tolerance.
- Indirect Assessment: In indirect assessment, NK cells monitor
the levels of a non-classical MHC-I molecule called HLA-E on the
surface of target cells. HLA-E presents peptides derived from the
leader sequences of other MHC-I molecules to CD94/NKG2A heterodimers
on NK cells. These peptides are typically derived from nascent MHC-I
molecules in the endoplasmic reticulum and reflect the overall
intracellular MHC-I pool. Under normal conditions, abundant MHC-I
molecules are processed and loaded with peptides, leading to the
presentation of a diverse repertoire of peptides on HLA-E.
Engagement of CD94/NKG2A with peptide-presenting HLA-E delivers
inhibitory signals to NK cells, preventing them from attacking cells
with sufficient MHC-I expression. However, decreased MHC-I
expression results in reduced peptide presentation on HLA-E, leading
to decreased engagement of CD94/NKG2A and, consequently, reduced
inhibition of NK cell activity.
In summary, the fundamental difference between direct and indirect
assessment of MHC-I expression lies in the receptors and ligands
involved, as well as the mechanism by which MHC-I expression is.
The role of HLA-E in this context is crucial for immune
surveillance. By presenting peptides derived from the leader sequences
of other MHC-I molecules, HLA-E acts as a sensor of MHC-I synthesis
and trafficking within the cell. The recognition of HLA-E by the
CD94/NKG2A inhibitory receptor provides a mechanism for NK cells to
indirectly assess the overall integrity of the MHC-I antigen
presentation pathway in target cells. If HLA-E presents appropriate
peptides, indicating proper MHC-I expression and function, NK cell
cytotoxicity is inhibited, allowing healthy cells to be spared from
attack. However, if MHC-I expression is compromised or inhibited,
leading to reduced HLA-E presentation of MHC-I-derived peptides, NK
cell cytotoxicity may be unleashed against abnormal or infected cells
lacking MHC-I expression, thus contributing to immune surveillance and
elimination of target cells. |
front 5 Explain how the balance of signaling delivered through the two types
of NK cell receptors (activating/inhibitory) dictate
NK activity
(to kill or not to kill). | back 5 The balance of signaling through activating and inhibitory receptors
on natural killer (NK) cells plays a crucial role in determining NK
cell activity, specifically whether to initiate cytotoxicity against
target cells or spare them from destruction. This balance is often
referred to as the "missing self" or "induced
self" recognition model and is essential for maintaining immune
tolerance while enabling effective immune responses against infected
or abnormal cells. Here's how it works:
- Activating Receptors: Activating receptors on NK cells
recognize ligands expressed on the surface of target cells,
typically induced during stress, infection, or transformation. When
activating receptors engage their ligands, they trigger signaling
pathways within the NK cell that promote cytotoxicity and cytokine
production. These pathways can involve phosphorylation cascades,
calcium influx, and cytoskeletal rearrangements, leading to the
release of cytotoxic granules containing perforin and granzymes and
the secretion of pro-inflammatory cytokines such as interferon-gamma
(IFN-γ). Examples of activating receptors include NKG2D, NKp30,
NKp44, and NKp46. These receptors recognize stress-induced ligands,
viral proteins, or other markers of cellular distress.
- Inhibitory Receptors: Inhibitory receptors on NK cells recognize
self-antigens presented by MHC class I molecules on the surface of
healthy cells. When inhibitory receptors engage with their ligands,
which are typically classical MHC-I molecules (HLA-A, HLA-B, and
HLA-C), they deliver inhibitory signals that counteract the
activating signals from activating receptors. This inhibition
prevents NK cell activation and cytotoxicity against normal, healthy
cells expressing normal levels of MHC-I molecules. Examples of
inhibitory receptors include killer cell immunoglobulin-like
receptors (KIRs), CD94/NKG2A, and the Ly49 family of receptors.
-
Balance of Signaling: The balance of signaling
between activating and inhibitory receptors ultimately determines NK
cell activity. If activating signals dominate and outweigh
inhibitory signals, NK cells become activated and initiate
cytotoxicity against target cells expressing stress-induced or
infected markers. Conversely, if inhibitory signals predominate and
suppress activating signals, NK cell activity is inhibited, and
healthy cells expressing normal levels of MHC class I molecules are
spared from destruction. This delicate balance ensures that NK cells
selectively target and eliminate aberrant or infected cells while
sparing healthy cells, thus contributing to immune surveillance and
homeostasis.
In summary, the interplay between activating and inhibitory signals
received by NK cells dictates their activity, determining whether NK
cells will kill or spare target cells. This balance ensures effective
immune surveillance and helps maintain tissue integrity and function. |
front 6 Explain why virus-infected cells are particularly susceptible to
killing by NK cells (upregulation of stress molecules
and
decreased expression of MHC-I). | back 6 Virus-infected cells are particularly susceptible to killing by
natural killer (NK) cells due to two main factors: the upregulation of
stress molecules and the decreased expression of major
histocompatibility complex class I (MHC-I) molecules. These changes in
infected cells make them more visible to NK cells and reduce the
inhibitory signals that normally prevent NK cell activation. Here's
how these mechanisms contribute to the susceptibility of
virus-infected cells to NK cell killing:
- Upregulation of Stress Molecules: Virus infection triggers
intracellular stress responses within infected cells. As a result,
infected cells often upregulate stress-induced ligands on their
surface, which serve as targets for activating receptors on NK
cells. These stress molecules, such as MICA (MHC class I
chain-related protein A) and MICB (MHC class I chain-related protein
B), are recognized by activating receptors like NKG2D on NK cells.
The upregulation of stress molecules on virus-infected cells acts as
a danger signal, alerting NK cells to the presence of infection.
Engagement of activating receptors with stress molecules activates
NK cells and triggers their cytotoxic response against the infected
cells.
-
Decreased Expression of MHC-I Molecules: MHC-I
molecules play a crucial role in presenting viral antigens to
cytotoxic T cells, initiating adaptive immune responses against
viral infections. However, many viruses have evolved strategies to
evade immune detection by downregulating the expression of MHC-I
molecules on infected cells. Decreased expression of MHC-I molecules
reduces the ability of infected cells to present viral antigens to
cytotoxic T cells, allowing viruses to evade adaptive immune
surveillance. However, this downregulation of MHC-I expression also
renders virus-infected cells more susceptible to NK cell-mediated
killing. NK cells possess inhibitory receptors, such as KIRs and
CD94/NKG2A, that recognize MHC-I molecules on target cells. When
MHC-I expression is decreased or absent, inhibitory signaling
through these receptors is diminished, leading to NK cell activation
and cytotoxicity against the infected cells.
In summary, virus-infected cells are particularly susceptible to
killing by NK cells due to the upregulation of stress molecules, such
as MICA and MICB, which trigger activating signals on NK cells, and
the decreased expression of MHC-I molecules, which reduces inhibitory
signaling and enhances NK cell activation. This dual mechanism of
recognition allows NK cells to efficiently detect and eliminate
virus-infected cells, contributing to the early control of viral
infections and the maintenance of immune surveillance. |
front 7 Explain the role of antibody and Fc receptors in NK cell-mediated
ADCC (antibody-dependent cellular cytotoxicity). | back 7 Antibody-dependent cellular cytotoxicity (ADCC) is a vital mechanism
of the immune response mediated by natural killer (NK) cells in
conjunction with antibodies and Fc receptors. This process is crucial
for the elimination of antibody-coated target cells, such as
virus-infected cells or tumor cells, and plays a significant role in
both innate and adaptive immunity. Here's how the components interact
in ADCC:
- Antibody Production: ADCC begins with the production of
antibodies by B cells in response to a specific antigen, such as
viral proteins or tumor-associated antigens. These antibodies can be
of the IgG class, particularly IgG1 and IgG3 subclasses, which have
a high affinity for Fc receptors on NK cells. Upon encountering the
antigen, B cells differentiate into plasma cells, which secrete
antibodies into the bloodstream.
- Coating of Target Cells
with Antibodies: In ADCC, antibodies bind to specific antigens
expressed on the surface of target cells, such as viral-infected
cells or tumor cells. This binding results in the coating or
"opsonization" of target cells with antibodies, forming
immune complexes.
- Recognition of Antibody-Coated Cells by
NK Cells: NK cells express Fc receptors, primarily FcγRIIIA (CD16),
on their surface. These Fc receptors specifically recognize the Fc
portion of antibodies bound to target cells. When NK cells encounter
antibody-coated target cells, the Fc receptors on NK cells bind to
the Fc portion of the antibodies, forming a bridge between the NK
cell and the target cell.
- Activation of NK Cells:
Engagement of Fc receptors on NK cells with antibody-coated target
cells triggers intracellular signaling pathways within the NK cell,
leading to NK cell activation. This activation results in the
release of cytotoxic granules containing perforin and granzymes, as
well as the secretion of pro-inflammatory cytokines such as
interferon-gamma (IFN-γ).
- Cytotoxicity Against Target
Cells: Activated NK cells exert their cytotoxic functions against
antibody-coated target cells through several mechanisms. Perforin
forms pores in the target cell membrane, allowing entry of
granzymes, which induce apoptosis (programmed cell death) in the
target cell. Additionally, NK cells may also release cytokines such
as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α)
upon target cell recognition. These cytokines further enhance the
immune response by promoting inflammation, activating macrophages,
and modulating adaptive immune responses.
|
front 8 THIS IS WHERE TOP HAT BEGINS | back 8 THIS IS WHERE TOP HAT BEGINS |
front 9
True/False: The nude mouse (lacks a thymus) does not
produce NK cells. | back 9 False
*NK Cells do not go to the thymus |
front 10 SCID mice do not express a functional form of the VDJ recombinase
enzyme (RAG); therefore, SCID mice do not produce functional B or T
lymphocytes (fact). True/False: SCID mice do not
produce functional NK cells. | |
front 11
Which two (2) of the following statements about NK
cells are true?
a.NK cells are unable to kill extracellular bacteria.
b.NK cells function as components of adaptive immunity.
c.NK cells display a high degree of antigen specificity.
d.NK cells are unable present peptides to Tc cells.
e.NK cells are derived from hematopoietic stem cells that reside in
the bone marrow. | back 11 a.NK cells are unable to kill extracellular bacteria.
e.NK cells are derived from hematopoietic stem cells that
reside in the bone marrow. |
front 12 In a future lecture we will discuss how NK cells play an important
role in identifying and killing cancer cells that arise in the body.
Which two (2) of the following phenotypes displayed
by cancer cells do you think play a role in their recognition and
elimination by NK cells?
a.Their ability to divide in an uncontrolled manner.
b.Their ability to move into tissues (metastasis).
c.Their expression of MIC proteins on their surface.
d. Their ability to evade CTLs by decreasing expression of MHC-I on
their surface.
e. Their ability to mutate their DNA at a higher than normal rate. | back 12 c.Their expression of MIC proteins on their surface.
d. Their ability to evade CTLs by decreasing expression of MHC-I on
their surface. |
front 13
Which one of the following statements about NK
cells is false?
a.NK cells are lymphocytes and function as cellular components of
innate immunity.
b.NK cells are more likely to kill a "stressed" cell than
they are to kill a "non-stressed" cell.
c.NK cells develop within a primary lymphoid tissue.
d.NK cells possess antigen-specific surface receptors (different NK
cells bind to different non-self antigens).
e.NK cells are capable of killing virus-infected cells by
participating in the process of ADCC. | back 13 d.NK cells possess antigen-specific surface receptors (different NK
cells bind to different non-self antigens). |
front 14 Which complex (MHC-1 or MHC-2) is more likely to be bound by a NK
cell inhibitory receptor?
a.) MHC-1
b.) MHC-2 | |
front 15 NK cells and CTLs are both highly effective at identifying and
killing virus-infected cells (fact). True/False: NK
cells and CTLs use similar strategies/mechanisms for differentiating
between infected cells (which they kill) and uninfected cells (which
they do not kill). | |
front 16 Individual X has a genetic condition that results in the inability to
produce functional NK cells. Which one of the
following types of microbes do you think individual X is most
susceptible to due to this condition?
a.Worm infections
b.Virus infections
c.Bacteria infections
d.Fungal infections
e.Unicellular parasite infections. | |
front 17 One population of cells is infected with a virus called HCMV and a
second population of cells is infected with Zika virus. Infections are
conducted so that only 50% of the cells become
infected. Six hours after infection the cells are analyzed by
FLOW cytometry using a monoclonal antibody that bind to a virus
antigen (HCMV or ZIKA) which will bind to virus-infected cells, and a
monoclonal antibody that binds to the MIC A protein. Which
virus-infected cells (HCMV or Zika) appear to be more
susceptible to killing by NK cells?
a.)HCMV
b.)Zika | |
