Download Rapid innate control of antigen abrogates adaptive immunity

IMMUNOLOGY
REVIEW ARTICLE
Rapid innate control of antigen abrogates adaptive immunity
Thomas P. I. Pembroke, Awen M.
Gallimore and Andrew Godkin
Institute of Infection and Immunity, School of
Medicine, Cardiff University, Cardiff, UK
doi:10.1111/imm.12048
Received 07 September 2012; revised 22
November 2012; accepted 26 November
2012.
Correspondence: Thomas P. I. Pembroke,
Institute of Infection and Immunity, School
of Medicine, Cardiff University, Henry
Wellcome Building, Cardiff CF14 4XN, UK.
Email: [email protected]
Senior author: Dr Andrew Godkin,
email: [email protected]
Summary
Natural killer (NK) cells provide an immediate first line of defence against
viral infections. Memory responses, maintained by CD4+ T cells, require
exposure to viral antigen and provide long-term protection against future
infections. It is known that NK cells can promote the development of
the adaptive response through cytokine production and cross-talk with
antigen-presenting cells. In this paper however, we summarize a series of
recent publications, in mouse models and for the first time in man, with
the unifying message that rapid viral antigen control by the innate
immune system limits antigen exposure to CD4+ cells thereby abrogating
the development of a memory response. We discuss the significant implication of these studies on viral treatment strategies and immunization
models.
Keywords: CD4+ T cells; natural killer cells; viral immunity.
Introduction
Natural killer (NK) cells form an important part of the
innate immune response against viral infections. They
were first described as lymphocytes, which possess cytotoxic functions without the need for previous antigen
exposure.1 The NK responses are not only directly cytotoxic against virus-infected cells but also serve as a bridge
between the innate and adaptive branches of the immune
system, as they can stimulate CD4+ and cytotoxic T lymphocytes by cytokine production (Box 1 and reviewed in
ref. 2). These adaptive immune responses, which confer
antigen specificity, can also provide long-term protection
against further infections. To develop an antigen-specific
response, cognate CD4+ T cells must recognize peptide
epitopes presented by MHC class II heterodimeric glycoproteins, on antigen-presenting cells (APC) such as dendritic cells (DC) and B cells. If antigen can access the
peripheral lymphoid compartment, and if appropriate
timely co-stimulation is given to the T cells, then this
process can start within hours of infection.3
apparent that they are able to release cytokines and even have a
role in uterine vascular homeostasis during pregnancy. CD4+ T
lymphocytes include T helper cells and are activated when peptide antigens are presented to them by MHC class II glycoproteins. Once activated CD4+ cells divide and release cytokines to
stimulate and regulate the immune response. CD8+ T cells are
also known as cytotoxic T cells and kill target cells by binding
to specific antigens associated with MHC class I glycoproteins.
Professional antigen-presenting cells (APC) display foreign antigen particles (usually bound by MHC glycoproteins) on their
cell surface to T cells. Professional APC include macrophages,
dendritic cells (DC) and B cells; some epithelial cell types can
up-regulate MHC molecules after cytokine stimulation and act
as APC.
It is established that NK cells can influence CD4+ T cells in the
following ways:
•
•
Direct – NK cells release interferon-c (IFN-c) and pro-inflammatory cytokines promoting antigen processing and presentation to T cells and polarizing T helper type 1 responses.
Indirect – stimulation of DC; lysis of excess immature DC,
chemokine expression (IFN-c-induced protein-10 CCL-21) to
promote DC trafficking to lymph nodes.
Box 1 Natural killer and adaptive immune cells and their
interactions
Recent studies described in this paper report the following
mechanisms:
Natural killer (NK) cells are large granular lymphocytes that are
characteristically CD56+ CD3 . Although they were first given
their name natural killer cells based on their ability to kill ‘nonself’ tumour cells deficient in MHC class I, it has become
•
•
•
ª 2012 Blackwell Publishing Ltd, Immunology, 138, 293–297
Clearance of antigen before a CD4+ T-cell memory response
is established.
Perforin-mediated NK-cell reduction of antigen-bearing APC
Perforin-mediated NK-cell reduction of activated T cells.
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T. P. I. Pembroke et al.
Stimulation by DC-produced cytokines (interleukin-12
or interleukin-18) can induce interferon-c (IFN-c) production by NK cells, which promotes T helper type 1
polarization.4 The importance of IFN-c production by
NK cells is also highlighted in a recent publication examining the role of NK cells in directing influenza-specific
CD8+ T-cell responses. In this study, NK-cell depletion
attenuated T-cell responses by reducing DC antigen presentation and limiting the recruitment of both DC and T
cells into the draining lymph nodes. This effect of NK
cells was IFN-c-dependent.5 The bi-directional stimulation between NK cells and DC to promote CD4+ T cells
described could be considered a potential target of viral
immune evasion. Indeed, Mandaric et al.6 have demonstrated that mouse cytomegalovirus (MCMV) -induced
interleukin-10 limits NK–DC cross-talk and suppresses
CD4+ T-cell priming through reduced interleukin-12
expression.
However, several recent studies using mouse models
have thrown a different light on the relationship between
NK-cell responses to infection and subsequent adaptive
immune responses.7–12 The NK cells appear to be capable
of shaping adaptive immune responses by rapid elimination of antigen actually preventing formation of CD4+
T-cell responses or even by direct lysis of APC and CD4+ T
cells. Hence, NK cells might provide a previously unrecognized challenge for the treatment of viral infections if
life-long immunity is desired.
NK-cell depletion increases anti-viral CD4+ T-cell
responses
To study the role of NK cells in the development of
adaptive T-cell responses Andrews et al.7 measured the
effects of NK-cell depletion on CD4+ and CD8+ T-cell
responses following MCMV infection. Depletion of NK
cells before infection actually increased virus-specific
CD4+ T cells, measured ex vivo by IFN-c production. In
vivo NK cells reduced the number and longevity of DC
infected with MCMV, through a perforin-dependent
mechanism, so reducing the quantity of viral antigen
presented to CD4+ cells. Effective CD4+ responses were
required for control of MCMV replication in the salivary glands, which are the major source of transmitted
virus.
The importance of temporary viral antigen persistence
in the development of a CD4+ response is further demonstrated in a mouse model using vesicular stomatitis virus
(VSV) infection. VSV is effectively cleared from most tissues by type I IFN produced by an early rigorous innate
response. However, a subpopulation of splenic marginal
zone CD169+ macrophages is permissive to ongoing viral
replication because of inherent cellular resistance to type
I IFN, and hence enables persistent antigen presentation.
This allows an adaptive response to develop. Depletion of
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these CD169+ macrophages deprives the adaptive arm of
adequate exposure to antigen, and hence, although VSV
is controlled and removed by the innate response, no specific adaptive memory response ensues.8
There may be a host advantage to innate immunity
preventing adaptive responses during a disseminated
infection with high viral load whereby a robust CD4+mediated T-cell response could induce widespread
immunopathology (Fig. 1). Using lymphocytic choriomeningitis virus (LCMV) infection, Waggoner et al.9
demonstrated the effects of increasing the infecting dose
of virus on interplay between NK cells and the adaptive
immune response. Unlike MCMV infection, LCMVinfected cells appear to be resistant to direct NK-cell
lysis. During high-dose LCMV infection, NK cells targeted and depleted virus-specific CD4+ T cells, facilitating CD8+ T-cell exhaustion, resulting in subclinical viral
persistence but avoiding lethal immunopathology. Hence
in this situation depletion of NK cells has lethal consequences.
Evidence points to NK cells controlling adaptive
responses by control of antigen availability, or by directly
impinging on effector lymphocytes. The function of NK
cells seems to vary depending on the pathogen, infecting
dose etc. This role of NK cells was explored further comparing control of MCMV and recombinant Listeria monocytogenes expressing ovalbumin.10 Narni-Mancinelli et al.10
demonstrated that mice bred with hyper-responsive NK
cells exhibited reduced CD4+ and CD8+ T-cell responses
on repeated challenge compared with wild-type mice. The
NK responsiveness was enhanced in these mice because of
failure to express the activating receptor NKp46. These
results could be replicated by treating mice, after NK-cell
depletion, with an NKp46-blocking antibody. So again
the theme, which emerges, is that rapid innate control of
pathogens and hence antigen supply comes at a cost to
adaptive responses.
NK-cell activity limits CD8+ T-cell immunity
The effects of NK-cell activation on CD8+ T-cell antiviral activity are demonstrated by Lang et al.11 in a
mouse model of LCMV infection. LCMV infection
induces increased perforin-mediated NK cytotoxicity
and alters the phenotype of NK cells with increased
activating receptor NKG2D expression and reduced
inhibitory 2B4 expression. However, NK depletion,
NKG2D blockade and perforin-deficient strains of mice
exhibit increased CD8+ T-cell immunity. Importantly,
depletion of NK cells prevented viral persistence and
chronic viral infection by enhancing CD8+ T-cell-mediated clearance of LCMV. Furthermore, there was
reduced LCMV hepatitis with a greatly reduced number
of infected cells in the liver and lower levels of liver
cell damage as measured by alanine aminotransferase
ª 2012 Blackwell Publishing Ltd, Immunology, 138, 293–297
NK control of adaptive immunity
(a)
1. Moderate NK cytotoxic
anti-viral response
NK
Relative viral load &
effector function
Immunity
Infection
3
1
2. APC
prime CD4+
CD8+
CD8
4
2
CD4+
+
4. Robust viral clearance
Risk of fatal
CD8+
immunopathology
CD4+
CD8+
Time
Viral load
NK activity
APC priming
CD4+ response
Immunopathology
+
CD4+
3. CD4 expansion
& CD8+ recruitment
CD4+
CD8+
(b)
NK
1. Excessive NK cytotoxic
anti-viral response rapid viral clearance
Relative viral load &
effector function
Infection
Figure 1. Models of immune response to viral
pathogens and development of adaptive immunity. (a) A moderate natural killer (NK) cell
response to viral infection allows antigen-presenting cell (APC) priming of CD4+ T cells.
The adaptive immune response may result in
fatal immunopathology (Waggoner et al.9).
(b) An excessive NK cytotoxic response may
clear viral antigen without appropriate APC
priming of CD4+ T cells, leaving the host at
risk of future infection (Andrews et al. 7 and
Narni-Mancinelli et al.10). (c) CD169+ macrophages, interferon-a (IFN-a) resistant and permissive to viral replication, enable on-going
priming of adaptive immune responses during
rapid innate viral control (Honke et al.8).
APC, antigen-presenting cell; IFN, interferon;
NK, natural killer
Secondary
infection
3. Failure to mount effective
memory response – loss of
potential immunopathology
at cost of immunity
(c)
NK
1
2. APC
failure to
prime CD4+
2
Time
CD4+
1. Excessive NK cytotoxic
anti-viral response –
failure of conventional
APC priming
Infection
Relative viral load &
effector function
1
Immunity
2
3
Time
CD8+
3. CD4+ priming &
clonal expansion immunity
(ALT) in NK-cell-deficient mice as CD8+ T cells cleared
the virus.11 In a mouse tumour model NK-cell killing
of tumour antigen-specific CD8+ T cells has been found
to be perforin-dependent and NK activation receptor
NKG2D-dependent. In this mouse tumour model,
depletion of NK cells has also been shown to enhance
a greater number of antigen-specific CD8+ T cells,12
suggesting that this effect is not restricted to viral
pathogens.
In humans the study of innate and adaptive immune
responses to viral infections is largely limited to sampling
peripheral blood and using serum viral loads as substitutes for true antigen availability. We have previously
demonstrated successful clearance of hepatitis C virus
(HCV) in a subject lacking T-cell and B-cell responses,
suggesting an important role for innate mechanisms.13
We extended this study to a cohort of 33 patients being
treated for HCV with type I IFN. For the first time in a
ª 2012 Blackwell Publishing Ltd, Immunology, 138, 293–297
+
CD4+
CD4+
CD8+
CD4+
CD4+
2. CD169 macrophage
permissive to viral
replication & resistant to
type I IFN – primes CD4+
human ‘model’ we have demonstrated that during the
IFN-a-based treatment, the most rapid reduction in viral
load (a strong predictor of treatment success) was associated with a striking absence or paucity of CD4+ T-cell
responses. The group of patients who were successfully
treated but who had a relatively slower rate of viral clearance had robust anti-viral CD4+ responses measured in
vitro.14 Tellingly, the rapid viral clearance was associated
with an increased potential of NK cells to be activated by
strong stimuli (high-dose IFN-a/K562 cells) compared
with weaker stimuli (low-dose IFN-a/Huh7.5 cells). It is
intriguing that a lower NK-cell expression of NKp46 pretreatment was associated with this successful rapid viral
clearance, reflecting the findings of Narni-Mancinelli
et al. on the importance of this NK-cell activating marker
(Fig 2). This study suggests that similar principles may
apply to the human system reflecting the results emerging
from mouse models.7–12
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T. P. I. Pembroke et al.
IFN-α
IFN-α
1
2
NK
NK
1.Patients with low NKp46
expression have greatest
relative increase in cytotoxic
NK response to IFN-α and
rapidly clear HCV without
CD4+ memory responses
2.High NKp46 expression has
lower relative increase in
cytotoxic response to treatment
slowly clear the virus and are
dependent on CD4+ responses
for treatment success
Slow viral clearance
ψ = NKp46
Rapid viral clearance
CD4+
Weak
response
t
us e
ob nc
R po
s
re
Treatment successpoor/absent CD4+
T cell response
Risk of re-infection
Treatment
failure
Box 2 Implications for immunotherapies and vaccination strategies
A major aim of vaccination is to provide individuals with a
long-lasting pool of memory cells that can quickly respond to
secondary exposure to a pathogen or tumour antigen and elicit a
powerful immune effector response. The ability of the innate
immune system to remove antigen without priming an adaptive
immune response poses a challenge to this goal. Possible strategies to avoid this outcome include:
•
•
•
•
Use of vaccine adjuvants to induce antigen-presenting cell
(APC) uptake (and CD4+ T-cell priming) without natural
killer (NK) -cell activation
Maintenance of vaccine antigen to allow optimal CD4+ T-cell
exposure
Depletion of NK cells during vaccination
Vaccination of individuals who have been exposed to and
have cleared viruses without successfully mounting an adaptive immune response
Conclusions
Taken together these studies demonstrate that in both
mouse and human models robust innate activity, including NK-cell antiviral responses, may clear viral infections
without the immediate need for CD4+ T-cell responses.
However, increased exposure to antigen seems to be
required for optimal adaptive responses, and these studies above suggest that NK cells influence adaptive
responses by the control of antigen. In evolutionary
terms, a balance may have been sought to ensure that
296
Treatment
success
Figure 2. Innate and adaptive response during
the treatment of chronic hepatitis C virus
infection. APC, antigen-presenting cell; HCV,
hepatitis C virus; IFN, interferon; NK, natural
killer.
NK cells and/or the anti-viral effects of IFN-a are moderated, to allow such long-term protective responses to
develop. Hyper-responsive innate immune responses
may provide a benefit to the host (i.e. rapid pathogen
removal with reduced duration of illness) but at the cost
of no protective herd immunity and an easy path to
pathogen re-infection. There is also the possibility that
NK cells have evolved a function to prevent T-cell
immunopathology by direct killing of APC and activated
T cells, at the cost of subclinical persistent viral infections; this has not been demonstrated convincingly in
humans.
The above results also offer potential for optimizing
therapeutic strategies (Box 2). To induce enhanced adaptive memory responses after vaccination, transient manipulation of NK cells may be required in a similar manner
to depletion of regulatory T cells.15–17 It is also important
to consider during treatment of chronic viral infections
such as HCV that successful treatment does not necessarily confer an adaptive protective memory response and
post-treatment vaccination, when available, should be
considered as part of the treatment protocol to subjects
at risk of re-exposure.
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