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Virus interactions with endocytic pathways in
macrophages and dendritic cells
Jason Mercer and Urs F. Greber
ETH Zürich
Institute of Biochemistry
Schafmattstr. 18
CH-8093, Zürich, Switzerland
[email protected]
University of Zurich,
Institute of Molecular Life Sciences
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
[email protected]
Keywords:
Endocytosis,
macropinocytosis,
receptors,
trans-infection,
macrophage, dendritic cells
1
Abstract
Macrophages and dendritic cells are at the front line of defence against fungi,
bacteria and viruses. Together with physical barriers such as mucus, and a
range of antimicrobial compounds they constitute a major part of the intrinsic and
innate immune systems.
They have elaborate features, including pattern
recognition receptors and specialized endocytic mechanisms, cytokines and
chemokines, and the ability to call on reserves. As masters of manipulation and
counter-attack viruses shunt intrinsic and innate recognition, enter immune cells,
and spread from these cells throughout an organism. Here, we review recently
described mechanisms by which viruses subvert endocytic and pathogensensing functions of macrophages and dendritic cells, while highlighting possible
strategic advantages of infecting cells normally tuned into pathogen destruction.
2
The cell biology of virus infection
The use of cell biological and molecular techniques has enabled virologists to
understand the interplay between viruses and cells at the heart of the infection
process. This is also shown in a recent surge of system-wide approaches, such
as high-throughput RNAi screening, functional genomics or large-scale
proteomics [for reviews, see 1, 2-4]. While these approaches are informing us
about interactions between viruses and their host cells, for technical reasons,
they have been performed in cell lines that may not be the major targets during
natural infections. In the past few years there has been a push towards novel
approaches and the use of primary cell lines or tissue culture systems that more
accurately reflect the situation viruses encounter in an organism.
This must
involve the consideration of phagocytic cells, such as macrophages and dendritic
cells (DCs). A recent example for a non-autonomous action of macrophages in
viral entry has been the finding that apical entry of human adenovirus (HAdV)
into polarized epithelial cells is facilitated by macrophage-derived chemokines,
such as CXCL-8 [5]. These cytokines are released from macrophages upon
contacting the virus. On highly polarized epithelial cells the cytokines trigger a
redistribution of the viral entry receptors CAR and integrins from the basolateral
to the apical membrane, thereby enabling infection from the luminal side of
airways.
The professionals: macrophages and DCs
Why do macrophages and DCs take center stage for viral infections? These
cells are specialized immune cells located throughout the body. By virtue of their
fast chemotactic movements to sites of infection, they are often the first to
encounter, identify, and engulf invading pathogens [6-10].
They decode the
nature of the pathogen, and distinguish between soluble components of
pathogens and the agents themselves, hence tuning inflammatory responses
3
[11]. Professional phagocytic cells generate reactive oxygen species highly toxic
for pathogens, or they perforate membranes or digest invading pathogens by
hydrolytic enzymes.
The immune surveillance function of macrophages and DCs crucially depends on
highly active endocytic processes [for reviews, see for example 7, 12]. This
allows them to not only engulf and digest invading pathogens, but also present
the products on their cell surface to T lymphocytes using major histocompatibility
complex (MHC) molecules [reviewed in 11, 13]. T cells themselves produce
soluble mediators, such as interferon gamma, which activate and enhance
macrophage killing ability [14]. Macrophages and DCs fulfil two major functions
in immunity, an innate and an adaptive function. Through efficient ingestion and
destruction, pathogens are eliminated from circulation within the body, and
second, the displayed antigens act in the initiation of an adaptive immune
response.
These events involve the production of cytokines, chemokines or
reactive oxides, recruitment of other immune cells, and regulation of T cell
activity and inflammation in response to the detected pathogen [11, 15, 16]. In
the case of DCs these functions often overlap between subsets, with classical
DCs being more efficient professional APCs and plasmacytoid DCs (pDCS),
playing the role of the primary producer of type I interferon and pro-inflammatory
cytokines in response to viral infection [17].
Endocytic mechanisms in macrophages and DCs
Macrophages and classical DCs, referred to as DCs throughout, are professional
phagocytes with exceptionally high endocytic activity.
Besides phagocytosis,
they carry out pinocytosis, such as clathrin- and non-clathrin-mediated
endocytosis and macropinocytosis (Figure 1). These endocytic machineries are
elaborate and deeply interconnected with signalling and the physiology of the
particular cell type [for excellent reviews, see 18, 19-22]. The best understood
4
endocytic process is clathrin-mediated endocytosis (CME). It is strictly receptordependent and requires the coat protein clathrin, and the large GTPase dynamin
for fission and release of clathrin-coated vesicles from the plasma membrane [23,
24].
Clathrin-independent mechanisms in turn give rise to distinct endocytic
pathways generally independent of dynamin, as schematized in Figure 1 [for
reviews, see 25, 26]. Some of these pathways have been implicated in repair
processes, for example upon membrane insults [27].
Among the clathrin-independent pathways is macropinocytosis, a signal-induced
transient actin-dependent process [18, 28]. In macrophages and DCs, however,
macropinocytosis is wired as an on-going constitutive process [29, 30].
It is
coupled to the migratory phenotype of these cells and essential for their immune
function. Macropinocytosis is strictly dependent on actin and Na+/H+ exchangers.
Na+/H+ exchangers rectify the cytosolic acidic pH proximal to activated tyrosine
kinase receptors near the plasma membrane, and thereby relieve inhibition of
guanine nucleotide exchange factors of Rho family GTPase [31].
Phagocytosis is mechanistically similar to macropinocytosis, albeit, unlike
macropinocytosis, dependent on dynamin and a close apposition of a multivalent
ligand with plasma membrane receptors [28, 32, 33]. It is key for maintaining
tissue homeostasis and balancing innate and adaptive immune response to
bacterial and fungal infections (Box 1). Given that phagocytosis is specified for
particles typically larger than 0.5-1 µm [34, 35], viruses smaller than 0.5 µm are
taken up by endocytosis rather than phagocytosis.
Notable exceptions are
reports for herpes simplex virus 1 (HSV-1) and Mimivirus [36, 37], or HAdV-C2
aggregated by soluble receptor fragments and targeted to the high affinity Fc
receptor CD64 of human monocytes [38]. The vacuolar compartments formed
during pinocytosis, macropinocytosis and phagocytosis are classical endosomes,
macropinosomes and phagosomes, respectively. They follow similar maturation
programs controlled by Rab GTPases and signalling phosphoinositide lipids [see
5
Figure 3, and 18, 33, 39]. Despite the similarities between these processes, we
should keep in mind that pinocytosis, macropinocytosis and phagocytosis are
tuned by specific machineries emerging from divergent receptor engagements
and responses of subcellular networks [11, 40]. This leads to different forms of
information processing and physiological reactions.
Virus entry into macrophages and DCs
Virus entry requires distinct cellular processes, typically attachment to receptors,
signalling and movements of viruses on the cell surface, endocytic uptake and
trafficking, and finally penetration and genome uncoating [for illustration, see
Figure 2, and reviews by 41, 42, 43]. Despite their armaments, macrophages
and DCs are not immune against viruses, and both DNA and RNA viruses can
infect macrophages and/or DCs (Table 1). This is not surprising, since viruses
have co-evolved with their hosts [44]. Mammals, for example, do not exclusively
rely on macrophages and DCs for defence, but possess other potent pathogen
killers, such as neutrophils and natural killer cells, which mount rapid responses
against infected cells [34, 45].
Virus Receptors
Virus attachment and receptor engagement on target cells have an important role
in determining cell-type specificity and host range [reviewed in 46, 47]. A special
feature of macrophages and DCs is the expression of pattern recognition
receptors (PRRs), which function in pathogen detection and destruction [48].
PRRs bind to pathogen-associated molecular patterns (PAMPs) and thereby
distinguish between ‘self’ and ‘non-self’. PRRs present in macrophages and DCs
include RIG-I-like receptors (RLRs) and Toll-like receptors (TLRs) [49, 50]. The
cytoplasmic RLRs (RIG-I and MDA5), and endosome-associated TLRs (TLR3, 7,
6
and 9) serve to decode viral features, such as capsid structures or nucleic acids,
and initiate a fine-tuned production of antiviral pro-inflammatory cytokines and
type I interferon [51]. The TLRs can act together with another class of PRRs, the
scavenger receptors, which mediate non-opsonic phagocytosis of pathogens,
and modulate the inflammatory response to TLR agonists [52].
Another set of PRRs comprises the C-type lectin receptors (CLRs) including
mannose receptor, LSECtin (Liver and lymph node sinusoidal endothelial cell Ctype lectin), Siglec1, DC-SIGN (dendritic cell-specific intercellular adhesion
molecule-3 grabbing non-integrin), and its homologue DC-SIGNR.
These
receptors, present on both macrophages and DCs, are responsible for the
recognition of viruses and other pathogens displaying the surface PAMP highmannose glycoproteins [53, 54]. Binding to these receptors leads to an innate
immune response against these viruses [53, 55]. Interestingly, HIV, hepatitis C
virus (HCV) or dengue virus can take advantage of DC-SIGN for attachment to
cells (Table 1). Using a collection of phleboviruses, including Rift Valley fever
and Uukuniemi viruses (UUKV), a recent study found that virus engagement of
DC-SIGN by UUKV initiates a signalling cascade resulting in endocytosis and
trafficking of the virus-receptor complex and infection [56]. Phleboviruses from
the family of bunyaviridae are arboviruses transmitted during a blood meal of
arthropods, such as insects.
Dermal DCs are likely to be the first cells to
encounter these viruses after delivery into the mammalian host.
DC-SIGN
mediated the delivery of UUKV to early endosomes. There the virus dissociated
from the receptor and trafficked to late endosomes, where it penetrated to the
cytosol, as shown by live cell fluorescence imaging experimentations. Whether
DC-SIGN serves as an authentic receptor for phleboviruses other than UUKV, if
DC-SIGN-mediated phlebovirus endocytosis requires a co-receptor, and what
kind of endocytic mechanism is activated for DC-SIGN-mediated virus
internalization will require further investigation.
7
Avoiding destruction in lysosomes
The endo-lysosomal system of macrophages and DCs is designed for pathogen
destruction and antigen presentation [11, 34]. For a pathogen, the lysosome of
an antigen-presenting cell (APC) is a most hostile intracellular environment. It
has extremely high proton concentrations (~pH 4.0), a combination of over 50
different proteases, lipases, DNAses, and other hydrolases, and reactive oxygen-,
nitrogen-, and chloride-species.
So why would a virus take such a risk? A likely answer is that endocytosis offers
an evolutionary, immune-evasion advantage over direct fusion at the plasma
membrane. Most importantly, this includes the presentation of a range of preexisting or induced entry portals along a dynamically changing endocytic
pathway [57]. Maturation of vesicular compartments provides a cargo, such as a
virus with a plethora of cues, such as decreasing pH, proteases or inorganic ions
[reviewed in 58, 59]. Such cues can trigger the uncoating of virus genomes [43],
or enable virus release from hostile compartments [60]. It is thus not surprising
that even viruses that can fuse their envelope with the host membrane at neutral
pH, such as HIV-1, HSV, human cytomegalovirus (HCMV) or respiratory
syncytial virus (RSV) use endocytosis to gain entry into appropriate host cells
[61-65].
A pathway of emerging interest for virus entry into immune cells is
macropinocytosis [28, 39]. This has been shown for HAdV of the species B [66,
67], and vaccinia virus (VACV) mature- and extracellular- virions (MVs and EVs)
in many different cell types [reviewed in 68]. A recent small compound inhibitor
and localization study revealed that entry of both forms of VACV into monocytederived DCs (MDDCs) occurs by macropinocytosis, independent of the C-type
lectin receptors [69].
Consistent with macropinocytic uptake, entry of virus
particles was dependent on actin, Na+/H+ exchangers, phosphoinositide 3-kinase,
intracellular calcium, and cholesterol. Just as important, entry was independent
8
of Fc- and complement receptors, clathrin, caveolin-1 and dynamin-2.
Interestingly, after internalization virus did not co-localize with classical endocytic
markers including EEA1 and Lamp2, or require low pH for fusion. Since this
pathway is constitutive in non-activated macrophages and DCs, it would not
require the use of a particular receptor for cell activation but rather be provided a
‘free ride’ into the cell.
Although the ride is free for a virus, macropinocytic cargo can in fact be
processed and presented on MHC I molecules [70], albeit not on MHC II [71].
Hence, viral escape from macropinosomes prior to lysosome fusion may be
important for the avoidance of processing and reduced MHC I/II presentation. In
line with this, delayed degradation of HIV-1 has been observed to occur upon
macropinocytic uptake into macrophages [72]. In addition, as macropinosomes
mature in parallel rather than merge into existing trafficking pathways, they may
not expose the virus to the full repertoire of detection and destruction
mechanisms present in classical endosomes and phagosomes. In fact, there is
currently no evidence that endosome and phagosome associated TLRs are
present in macropinosomes.
In summary, a number of important human pathogens infecting macrophages
and DCs (Table 1) utilize macropinocytosis for entry into other cells types, for
example HIV, influenza A virus, Ebola, Kaposi’s sarcoma-associated virus
(KSHV) and HAdV [66, 67, 73-79]. It will be of importance to determine if these
and other viruses are using macropinocytosis to enter APCs, and if so, what
consequences for infection and pathogen detection this may provide.
Autophagy – anti-viral and pro-viral
Macrophages and DCs respond to invading viruses by inducing autophagy, a
process operating in close association with endosomal functions.
The best
studied autophagic process in immune cells is macroautophagy. This process
9
involves the formation of a cytosolic double-membrane which engulfs
cytoplasmic material resulting in a new vesicle, termed an autophagosome [80].
Engulfed material can be degraded upon autophagosome fusion with late
endosomes
or
lysosomes.
It
is
becoming
increasingly
clear
that
macroautophagy and innate immune recognition modulate each other, to
facilitate pathogen elimination in both professional and non-professional immune
cells [81]. For example, the detection of extracellular pathogens by TLRs upregulates macroautophagy in mouse macrophage cell lines [82].
More
specifically, pathogen elimination can occur by enhanced degradation of viral
proteins or nucleic acids [83], by tinkering with the balance between cell death
and survival of infected and bystander cells [84], through channelling cytosolic
viral components to endosomal TLRs in plasmacytoid dendritic cells [85], or by
feeding antigens to MHC class II compartments for activation of adaptive
immunity [86].
On the other hand, viruses using the cytosol for transit or
replication have adapted a multitude of strategies to subjugate macroautophagy.
For example, to supply membrane for viral replication, to enhance cell survival or
suppress cell death [reviewed in 87].
Further research is clearly needed to
untangle the benefits and liabilities of macroautophagy for both viral pathogens
and hosts, and to place these results into the context of viral endocytosis.
Virus infiltration, hiding, and killing
As discussed above, virus entry into macrophages and DCs may be a risky
venture for a virus. Yet, viruses have come up with divergent ways to turn the
tables on these sentinel cells [88].
For example, they modulate cytokine
secretion or reduce antigen presentation, alter cellular trafficking, undergo
latency or productive infection, and even interfere with cell death. Rather than
discussing the underlying mechanisms, we highlight three general strategies for
infiltration, hiding and killing APCs (Figure 4).
10
Infiltration
In some cases, virus infection of DCs causes their activation and lowers their
endocytic capacity thus shunting surveillance. Once activated by infection DCs
migrate to the lymph nodes. Interestingly, several viruses that productively infect
these cells take advantage of this process to facilitate virus spread. Both Ebola
and HIV-1 infections result in partial maturation of DCs while promoting their
migration [89, 90].
It is thought that HIV-1 uses DCs as a ‘Trojan horse’ for transport to lymph nodes
where it infects CD4 T cells. Unlike DCs, CD4 T cells are major targets of HIV-1
for progeny production. T cell infection through DCs can occur by budding of
virions from DCs and cell-free transmission. T cells can also be infected through
DC-mediated cell-to-cell transmission via an immune-protective viral synapse, a
process called trans-infection [91].
Trans-infection from immature DCs was
suggested to require DC-SIGN [92] or other C-type lectins [93], although this was
later found to be highly inefficient due to rapid degradation of virions after uptake
[94, 95]. Conversely, trans-infection from mature DCs that relies on the CLR
Siglec-1, rather than DC-SIGN, occurs efficiently [96]. In both cases these factors
serve to capture but not promote virion fusion. In the simplest scenario, the
captured HIV-1 is transferred directly to CD4 T-cells [97].
However, trans-
infection has also been reported to involve macropinocytic internalization and
subsequent regurgitation of virions by DCs [91, 98, 99]. Although up for debate
[100], the internalization of virions may serve to effectively hide virions prior to
trans-infection. Perhaps targeted exocytosis provides specific release of a large
number of virions to the viral synapse only after formation of this immuneprotective environment has occurred. However there are still many questions
regarding the mechanism of trans-infection, above all no in vivo evidence of this
process has been documented.
11
Hiding
Perhaps the best strategy for long-term viral survival is to simply hide from, rather
than attack, host defences as evidenced by several chronic human viruses such
as HIV-1, HCV or KSHV. These viruses can be latent, interfere with antigen
processing and presentation, prevent APC maturation, or alter cytokine secretion
[for a review, see 88].
Latency is most effective. It minimizes viral gene
expression and under appropriate conditions, such as the encounter of a DC, can
lead to viral gene expression from the latently integrated provirus in activated T
cells [101]. For non-latent viruses the use of virus encoded proteins to prevent
APC maturation and cytokine secretion is a common strategy [reviewed in 102].
The VP35 protein of Ebola virus, for example, has been demonstrated to be a
multifunctional protein with immunosuppressive functions including inhibition of
DC maturation and prevention of T cell activation [102, 103]. It effectively blocks
the link between the innate and adaptive immune response during infection.
Consistent with these reports, infection with some Ebola virus strains, such as
Zaire, can be associated with a mortality rate approaching 90% [104], while
Ebola virus lacking VP35 does not cause lethality [105].
These studies
emphasize the importance of virus-encoded immune modulators to the outcome
of infection.
Killing
Given their important immunological role macrophages and DCs would be
expected to be prime targets for elimination by invading pathogens. However,
only a few viruses, such as VACV or HSV-1 do so. Infection by these viruses
triggers apoptosis of DCs [106, 107]. In these cases infection is abortive, but
results in inhibition of DC maturation, antigen presentation, and subsequent T
cell activation. The viral interference with DC function is crucial, since uninfected
DCs can effectively activate T cells using antigens obtained by clearance of
12
infected apoptotic cells, for example in the case of HSV-1 or influenza A virus
(IAV) [107, 108]. By killing of DCs, VACV and HSV-1 effectively reduce the
number of APCs that can respond to the infection. That these cells no longer
mature or secrete cytokines is likely to shunt the recruitment of additional
immune cells to the site of infection. Additionally, as these cells are unable to
activate T cells or migrate to secondary lymphoid tissue the activation of the
adaptive immune response is likely slowed.
Concluding remarks
Multiple layers of cell defense control viral infections in an individual.
Professional phagocytic DCs and macrophages are at the front line of this
defense. Their phagocytic activities are key to activate natural killer cells, and
limit the initial viremia prior to initiation of B and T cell mediated adaptive
immunity. Further, DCs can cross-present viral antigens from infected B cells for
T cell stimulation, and this may be key for virus-specific immune control.
Neutralizing antibodies in turn control the plasma levels of viruses particularly
upon lytic infections. To date in depth knowledge of virus entry and infection in
macrophages and DCs is limited to only a subset of viral pathogens. It will be
important to elucidate the major endocytic mechanisms used by viruses to enter
macrophages and DCs, and how these pathways are linked to autophagy, an
immediate antiviral activity within pathogen-sensing cells. It will be interesting to
consider the different types of dendritic cells and macrophages within tissues
exposed to pathogens, such as the lung. Focus will need to be placed on
elucidating their virus interaction biology and innate responses in isolation or
tissue context [109]. All of this can serve as a basis to determine how these
pathways are linked to innate immunity and eventually immune evasion. Future
work will also have to be directed at better understanding the in vivo implications
of these findings.
13
Acknowledgments
We would like to thank Dr. Maarit Suomalainen for critical reading and
suggestions. Apologies to those whose work has not been referenced due to
space limitations. JM is funded by an Ambizione grant from the Swiss National
Science
Foundation
(SNF),
and
UFG
by
an
SNF
project
grant
(31003A_141222/1).
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Figure legends
Figure 1. Endocytic mechanisms.
The endocytic activities executed by cells are classified into pinocytic,
macropinocytic, or phagocytic mechanisms. The various pinocytic mechanisms
are classified based on their cellular requirements, that is, dynamin or actin. For
each mechanism the major players are listed (in bold). While a major fraction of
studies have been conducted to understand clathrin-mediated, macropinocytic
and phagocytic mechanisms in non-professional phagocytes, professional
phagocytes may use the full range of endocytic mechanisms. Although not fully
understood, it is likely that viruses have learned to subjugate the majority of
these pathways for infection of both professional and non-professional
phagocytes.
Figure 2. A classical virus entry program.
The entry of a virus into a cell is a complex multi-step program involving a
multitude of cellular and viral factors. After initial binding viruses often transit the
cell surface in search of their preferred receptor. For direct penetration, receptor
binding may result in a series of changes in the virus fusion machinery that
facilitate its activation to promote fusion with the plasma membrane and delivery
of the virus capsid into the host cytosol. In the case of endocytosis, receptor
engagement causes the host cell to respond through the induction of a signaling
cascade that initiates endocytic uptake of the virus itself. The passage through
endosomes then provides cues for virus penetration.
In both scenarios, free
virus capsids are transported to the site of viral genome uncoating, for example
in a microtubule-dependent fashion.
The final stage of virus entry is the
complete uncoating of the viral genome, which can occur either in the cytoplasm
or the nucleus.
20
Figure 3. Endosomal trafficking and antigen presentation.
Presented are the organelles associated with the main endocytic mechanisms at
work in macrophages and DCs. The classic endocytic pathway is responsible for
trafficking of cargoes derived from clathrin- and non-clathrin- mediated
endocytosis. The organelles associated with this pathway are early endosomes
(EEs), late endosomes (LEs), and lysosomes (LYs). The maturation program of
these organelles, consisting of changes in Rab (Ras related in brain) proteins
and lipid-composition profiles, as well as a gradual lowering of pH, and their
centripetal trafficking are relatively well defined.
Both macropinosomes and
phagosomes mature in a fashion parallel to that of classical endosomes, going
through distinct early macropinosomes (EM), early phagosomes (EP) and late
macropinosomes (LM), late phagosomes (LP) stages.
All of the pathways
converge at the level of lysosome fusion where upon incoming pathogens are
destroyed and ultimately presented as antigenic non-self peptides at the cell
surface in the context of MHC class I and II. Whether phagolysosomal and
macrolysosomal organelles are identical or represent distinct entities remains
unknown.
Figure 4. Virus exploitation of DC infection.
Often virus infection of immature DCs stimulates their activation. If everything
were to go to plan this would result in a maturation program marked by the
expression of cell surface molecules which signal that a possible invader has
been detected. This includes the cell surface expression of maturation markers
(CD80/CD40), induction of DC migration to the lymph through up-regulation of
CCR7, the release of cytokines to attract and stimulate the activity of additional
immune cells, and the processing and presentation of antigen which in turn
drives T cell activation. However, viruses have devised multiple strategies to
prevent the full assault from DCs and slow the activation of the adaptive immune
response. These include infiltrating, hiding, and in some cases killing of infected
21
DCs. Infiltration is used by some viruses to rapidly spread infection to additional
target cells, such as T cells. In this scenario, maturation, antigen presentation,
and cytokine secretion is shunted, while migration of the infected DCs is
unabated. Depending on the virus, infection can be productive, or virions can be
captured on the cell surface and passed along. Another strategy used by viruses
to subvert DCs is hiding.
Given the number of successful chronic human
infections that practice hiding, it is probably the most successful mechanism. In
most cases the viral infection results in latency or only low-level productive
infection. As seen during infiltration DC maturation, antigen presentation, and
cytokine secretion are blocked by specific viral effectors. Although only practiced
by a few viruses, the most straight-forward strategy used by viruses to deal with
DCs is killing. In these cases, infection tends to be abortive and all facets of DC
activation and maturation are attenuated.
Box1. Phagocytosis
Phagocytosis is one of the major endocytic mechanism employed by
macrophages and dendritic cells for the uptake of large particulate matter. It is a
highly conserved, specialized endocytic mechanism involved in several essential
functions including the anti-inflammatory clearance of apoptotic debris. While not
typically involved in virus uptake, phagocytosis is essential for ingestion,
destruction, and antigenic presentation of foreign pathogens, such as bacteria
and fungi, underscoring its importance in innate immunity (for details, see main
text).
Particular features of phagocytosis include:






Exclusive to professional phagocytes (i.e. neutrophils, macrophages and
dendritic cells).
Reserved for particles larger than 0.5-1 µm.
The particle being internalized drives phagosome size and shape.
Activation and uptake are site-specific.
Typically requires opsonins (i.e. immunoglobulin G and complement).
Can mediate pro- or anti-inflammatory responses depending on antigen.
22
Figure 1
Pinocytosis Macropinocytosis Phagocytosis Ac>n RhoA Caveolae dependent IL-­‐2 pathway CLIC/ GEEC Arf6 dependent Flo>llin dependent Clathrin mediated Caveolin-­‐1 Cavins RhoA Rac1 GRAF1 CDC42 Arf6 PIP(5)K Flo>llin Fyn Clathrin AP-­‐2 Na+H+ Ac>n Ac>n Ab/FcR Dynamin-­‐II Clathrin Ab-­‐coated par>cle Figure 2
Binding & Receptor Engagement Direct Penetra;on Signalling & Endocytosis Trafficking Microtubule Transport Penetra;on Genome Uncoa;ng Nucleus Cytoplasm Figure 3
Macropinocytosis Pinocytosis CME Phagocytosis Non-­‐CME EE EM LM LE EP Rab5/EEA1 Rab7/Lamp1 LP Lamp1 LY ? Dynamin-­‐II 6.5 pH Clathrin Ac=n Ab-­‐coated par=cle Ab/FcR MHC I MHC II 4.0 An=genic pep=des Figure 4
Immature Dendri-c Cell Ac-vated/Mature Dendri-c Cell An-gen presenta-on/ T cell ac-va-on Matura-on (CD80/CD40) Immature CD4+ or CD8+ T cell Cytokine secre-on Migra-on (CCR7) Immature T cell Infiltra-on Produc-ve Infec-on Hiding Killing Latent/Produc-ve Infec-on (low) Abor-ve Infec-on Matura-on Matura-on Matura-on An-gen presenta-on An-gen presenta-on An-gen presenta-on Cytokine secre-on Cytokine secre-on Cytokine secre-on Migra-on Migra-on Migra-on Viral genome Viral effector proteins MHC I MHC II An-genic pep-des Box1 Figure
Click here to download high resolution image
Table I
Table 1. Viruses that infect DCs and macrophages
a
Virus
Genome/envelope (+/-)
Infections of DCs
HIV-1
ssRNA+ (+)
SARS
ssRNA+ (+)
HCV
ssRNA+ (+)
HRV6
ssRNA+ (-)
Ebola
ssRNA- (+)
Measles
ssRNA- (+)
Dengue
ssRNA+ (+)
RSV
ssRNA- (+)
Influenza A
Segmented ssRNA- (+)
Uukuniemi
Segmented ssRNA- (+)
Rift Valley fever Segmented ssRNA- (+)
LCMV
Segmented ssRNA- (+)
HBV
ss/dsDNA (+)
Adenovirus
dsDNA (-)
HSV-1
dsDNA (+)
HHV8/KSHV
dsDNA (+)
HCMV
dsDNA (+)
VZV
dsDNA (+)
EBV
dsDNA (+)
Vaccinia
dsDNA (+)
Infections of macrophages
HIV-1
ssRNA+ (+)
Ebola
ssRNA- (+)
RSV
ssRNA- (+)
Influenza A
Segmented ssRNA- (+)
a
b
Productive
c
Receptor(s)
d
+/+
+
+
+
+
+/+
+
+
+
+
+
-
DC-SIGN
DC-SIGN
CD81/CLDN1
N.D.
NPC1
SLAM
DC-SIGN
N.D.
DC-SIGN
DC-SIGN
DC-SIGN
α-dystroglycan
NTCP
DC-SIGN/Lf
DC-SIGN
DC-SIGN
DC-SIGN
IDE
N.D.
N.D.
+/+/-
DC-SIGN/ICAM3
NPC1
N.D.
DC-SIGN
Virus abbreviations: HIV-1, human immunodeficiency virus-1; SARS, sudden acute respiratory
syndrome; HCV, hepatitis C virus; HRV6, human rhino virus 6; RSV, respiratory syncytial virus;
LCMV, lymphocytic choriomeningitis virus; HBV, hepatitis B virus; HSV-1, herpes simplex virus-1;
HHV8/KSHV, human herpes virus 8/Kaposi’s sarcoma herpes virus; HCMV, human
cytomegalovirus; VZV, varicella-zoster virus; EBV, Epstein-Barr virus.
b
Genome structure abbreviations: ss, single-stranded; ds, double stranded. For RNA genomes +/indicates the sense of the RNA. If the virus is enveloped (+) or non-enveloped (-) is indicated
c
Whether virus infection is productive (+), non-productive (-), or varies depending on the literature
(+/-) is indicated.
d
Receptor abbreviations: DC-SIGN, dendritic cell-specific intercellular adhesion molecule-3grabbing non-integrin); CLDN1, claudin-1; NPC1, Niemann–Pick C1; SLAM, signalling
lymphocyte-activation molecule; NTCP, sodium taurocholate cotransporting polypeptide; Lf,
lactoferrin; IDE, insulin-degrading enzyme; ICAM3, intercellular adhesion molecule 3; N.D., not
determined.