Distinct migratory pathways and trafficking of dendritic cells to the central nervous
system (CNS): The immune system is a host defense mechanism protecting against invaders,
such as bacteria and viruses, while maintaining tolerance to self. Nonetheless, a
few sites throughout the body are believed to be immunologically inert, such as the
testes, the eye and the brain. Indeed, experiments in the mid-20th century gave rise
to the concept of the brain as a site of immune privilege. Originally, the immune
privilege of the brain was thought to be absolute, attributed by a physical blood-brain
barrier (BBB) protecting the CNS from the entry of pathogens and circulating immune
cells. These views have changed and currently, the CNS is seen as an immune-specialized
site regulated by immunological components into and within the CNS. However, in neuroinflammatory
disorders, such as multiple sclerosis (MS), the resident and infiltrating immune cells
damage components of the CNS resulting in neurodegeneration. Among the various immune
cells that infiltrate the CNS are dendritic cells (DCs), professional antigen-presenting
cells capable to initiate both immunity and tolerance. DCs are known to transmigrate
into the CNS during neuro-inflammation via different routes, one of them is through
the activation and breakdown of the BBB. The infiltration of peripheral DCs in the
CNS follow a classical multistep model, which are arbitrated by the expression of
chemokine receptors and adhesion molecules on the surface of DCs (
Figure 1
). Previous findings from our group have demonstrated aberrant expression of migration
markers and increased chemotaxis, besides aberrant expression of maturation markers,
by circulating DCs of MS patients as compared to DCs from healthy controls (Thewissen
et al., 2014). A better understanding of immune cell infiltration, explicitly DC transmigration
into the CNS, can provide a better comprehension of the underlying processes driving
neuroinflammation, such as in MS, ultimately moving forward the field by identifying
new treatment targets. Indeed, although currently available therapeutics can modulate
immune cell migration in general, selective hampering of pathogenic DC recruitment
into the CNS in particular, might form the basis for the design of new therapeutic
strategies for MS.
Figure 1
Different routes of entry of DCs to the CNS parenchyma following neuroinflammation.
(A) Blood-brain barrier: DC undergo the migration process through the BBB in different
steps. In the steady state DCs are normally circulating in the bloodstream and crosstalk
with the brain endothelium layer via several factors. DC interaction with the endothelial
cell of the brain proceeds in a step-wise manner both in steady state and during inflammation.
These cells interact with the ICAM-2/3 expressed on the EC which binds to the DC-SIGN
expressed on the DCs. Additionally the chemokine receptors binding to their respective
ligand leads to the integrin activation resulting in the rolling of DCs on the endothelium.
DCs also express PSGL-1 which interacts with P/E-selectins on the endothelial cell
layer. Further DC interact with ICAM-1 on the EC via LFA-1 expressed on the DC surface
leading to firm adhesion to the EC layer. In normal conditions, very low number of
DCs are observed in the perivascular region with almost none in the CNS. While in
the inflamed state the EC layer is highly activated with a highly increased expression
of the adhesion molecules including ICAM-1 and VCAM-1. This results in a higher DC
interaction and adhesion to the EC and hence a greater migration to the MS lesion
sites. Along with DC different subsets of T cells (CD4+ and CD8+) also infiltrate
the CNS and are present in the inflammation sites. (B) Choroid plexus: In a healthy
state, low number of DC migrate through the stromal space via the CP epithelium but
no DC invade the CNS parenchyma. Whereas, in the inflammatory conditions the different
layers of the choroid plexus are again activated with an increased number of selectins
and activation molecule expression leading to a much higher invasion of DC towards
the lesion sites in MS. (C) Meningeal vessels: Similar to the CP, under normal conditions
DC remain circulating in the subarachnoid spaces although a drastically high number
is observed during inflammation where DC interact with the highly expressed adhesion
molecules and proceed to move towards the CNS along with T cells. Despite several
ligands involved in the process of attachment and transmigration of DC to the epithelium
in choroid plexus and meninges, their involvement and salient role in the different
steps of DC migration still remains to be evaluated. Adapted from De Laere et al.
(2018). BBB: Blood-brain barrier; CNS: central nervous system; EC: endothelial cells;
DC-SIGN: dendritic cell-specific ICAM-grabbing nonintegrin; ICAM-1: intercellular
adhesion molecule-1; CCL: chemokine ligand; CCR: chemokine receptor; LFA-1: lymphocyte
function-associated antigen-1; VLA-4: very late antigen-4; VCAM: vascular cell adhesion
molecule; CP: choroid plexus; CSF: cerebrospinal fluid; DC: dendritic cell; ICAM-2:
intercellular adhesion molecule-2; MS: multiple sclerosis; PECAM-1: platelet and endothelial
cell adhesion molecule-1; PSGL: P-selectin glycoprotein ligand.
DC traffic via different migratory routes into the CNS during steady state and inflammation:
DCs, professional antigen-presenting cells, serve as the sentinels of the immune system
continuously surveying their local environment. Also in the brain, they play a role
in the regulation of immune surveillance as well as in the development of neuroinflammation.
For instance, selective disruption of the antigen-presenting capacity of DC renders
mice completely resistant to neuroinflammation (Greter et al., 2005). In contrast,
others found that depletion of DC results in aggravated disease in experimental autoimmune
encephalomyelitis (EAE), the mouse model for MS (Yogev et al., 2012). DC populate
the CNS in low numbers in the steady state and reside mostly in the perivascular regions
suggestive of their peripheral origin. The majority of DCs in the CNS is thought to
be derived from circulating bone marrow-derived precursor cells that have passed the
barriers protecting the CNS. During neuroinflammation, DCs annihilate these barriers
to different levels and overpopulate the CNS, resulting in a highly increased accumulation
of DCs in cerebrospinal fluid (CSF) including the perivascular lesions. This augmented
trafficking of DCs to the CNS is thought to be a result of increased activation of
the barriers normally protecting the brain although their derivation and function
during neuroinflammation still remains unclear.
Immune cells can invade the CNS via different routes (De Laere et al., 2018), the
choroid plexus, the meninges and the BBB. The choroid plexus is considered to play
an important role in leukocyte trafficking in the initial phases of neuroinflammatory
disease. It consists of an epithelial layer, which constitutes the actual blood-CSF
barrier, and expresses several tight junction proteins. Conventional DCs (cDCs) found
in the CSF of MS patients display a higher expression of CD80, CD86 and CD40 suggesting
a more mature phenotype than their blood counterparts (Pashenkov et al., 2001). Although
several findings underscore the role of the choroid plexus in DC trafficking, the
precise mechanisms involved in the process remain elusive.
Other than the choroid plexus, the meninges are also known to be suitable for entry
of DCs into the CNS due to its constitutive high expression levels of adhesion molecules,
in particular intercellular adhesion molecule 1 (ICAM-1). DCs are known to be present
at the meningeal sites in healthy human CNS. Also, in EAE, DCs were found in the meninges
before they further accumulate in the spinal cord parenchyma. Nonetheless, the specific
homeostatic recruitment of DCs to the meninges still remains indefinable and requires
further studies focusing on the mechanisms involved.
Among these routes of entry, the BBB is considered to be the most ideal and the best-studied
site of immune cell trafficking to the CNS. The BBB plays both a role in immune cell
recruitment to the CNS under normal conditions but is activated and broken down during
neuroinflammation, being even more permissive for immune cell migration into the CNS.
The BBB has a specialized structure which typically constitutes of tight junctions
and low basal expression of adhesion molecules including ICAM-1, vascular cell adhesion
protein-1 (VCAM-1) and P- and E-selectin, whereas ICAM-2 and platelet endothelial
cell adhesion molecule are expressed at high levels in cerebral microvessels under
non-inflammatory conditions. The expression of adhesion molecules ICAM-1 and VCAM-1
is upregulated following inflammation while ICAM-2 expression seems to be significantly
downregulated upon pro-inflammatory inflammation as previously presented by our group
(De Laere et al., 2017). Also, the expression of P- and E-selectin is upregulated
in MS patients. The occurrence of an increase in immune cell infiltration across the
BBB, uncontrolled activation and antigen presentation in MS is indicative of a compromised
BBB during neuroinflammation. In a healthy CNS, both the mature and immature monocyte-derived
DCs adhere to the human brain endothelial cells, albeit that immature DCs adhere with
a greater efficacy when compared with the mature population as demonstrated by intravital
fluorescence video microscopy (Jain et al., 2010). The adhesion molecules ICAM-1,
ICAM-2, VCAM-1 and platelet endothelial cell adhesion molecule on the endothelial
cell layer of the BBB facilitate this adherence by immature DCs, whereas only ICAM-1
plays a role in the adherence of mature DCs. In relapsing-remitting (RR) MS and chronic
progressive MS patients, higher numbers of CCR5-expressing cDCs and pDCs are seen
when compared to their healthy counterparts, in addition to upregulated expression
of CCR7 on pDC of RRMS patients. Similarly, also monocyte-derived DCs from MS patients
expressed significantly higher levels of CCR7 when compared with healthy controls
(Nuyts et al., 2014).
The development of realistic in vitro BBB models that imitate the in vivo expression
of enzymes, transporters, receptors, and structural proteins on the BBB will demonstrate
to be an invaluable tool for understanding the pathological factors involved in the
development of various CNS disorders. For instance, using a static in vitro BBB model,
we previously showed that the inflammatory chemokine CCL3 alone was unable to drive
the transmigration of the immune cell via the BBB under inflammatory conditions in
vitro (De Laere et al., 2017).
Current therapeutic strategies for MS amend the migration of DCs into the CNS: Although
a cure for MS is still lacking, the medical arsenal for the treatment of MS expands
continuously. These disease-modifying therapies have been shown to slow disease progression
and prevent disability symptoms, thereby improving the course of MS. Whereas most
of the currently available treatments function in an anti-inflammatory fashion, pharmacological
targeting of immune cell trafficking to the CNS during pathogenesis also presents
an attractive treatment strategy for MS. One of the first biological treatments specifically
developed to intervene with migration of immune cells was natalizumab. It is a humanized
monoclonal antibody that selectively binds to the α4-chain of α4β1- and α4β7-integrins
expressed on the surface of human leukocytes. In doing so, natalizumab directly interferes
with the adhesion of immune cells to endothelial cell layers including the BBB. It
was observed that treatment of MS patients with natalizumab reduces the proportion
of α4β1-expressing circulating pDCs and cDCs after 48 hours of initiating therapy
and consequently the coagulation of DCs in the perivascular space of RRMS patients
(Andrés et al., 2012). Fingolimod, a sphingosine-1-phosphate receptor, has also been
demonstrated to affect the migratory capacity of immune cells. In particular, fingolimod
traps the lymphocytes in the lymph nodes thereby preventing their migration to the
inflamed CNS. DCs, among other immune cells, express the sphingosine-1-phosphate receptor
isoforms 1–4 that are mainly targeted by fingolimod. In EAE, treatment with fingolimod
resulted in a significant decrease in CCR7 expression by circulating DC and in vitro
generated bone marrow-derived DC. This observation was further reflected by a reduced
in vitro migratory capacity by DCs towards the CCL19 chemokine (Lan et al., 2005).
Moreover, in vitro treatment of monocyte-derived DCs with therapeutic doses of fingolimod
resulted in a dose-dependent reduction in chemotaxis, albeit without altering the
chemokine receptor expression by DCs. Also, interferon-β treatment is shown to ameliorate
the symptoms in patients with MS via modulation of the expression of migration-associated
molecules by various immune cells, besides the general immune-modulatory function
of interferon-β. In doing so, interferon-β reduces the amassing of inflammatory cells
in the BBB. For instance, IFN-β treatment in MS patients resulted in downregulation
of the expression of CCR7 in MS-derived pDCs upon TLR9 stimulation similar to the
expression level on pDCs from healthy controls (Aung et al., 2010).
Despite the increased efficacy of these drugs in the treatment of MS, they also pose
some downsides and sometimes life-threatening adverse events. For instance, natalizumab
therapy was seen to be associated with an increased risk of progressive multifocal
leukoencephalopathy, caused by the JC virus (Sadiq et al., 2010). This is thought
to be a consequence of broad-spectrum targeting of immune cell trafficking across
the blood-brain barrier to the CNS thereby impairing regular immune surveillance of
the brain. Hence, selective interference with the recruitment of inflammatory DCs
towards the CNS and consequently of their accumulation at strategic locations within
the CNS will aid in the design of novel therapeutic strategies to tackle the neuro-inflammation
in MS. In this context, a variety of molecules, known to be involved in DC migration,
could be key including adhesion molecules, matrix metalloproteinase, chemokine receptors
and/or the signaling molecules including NF-κB and extracellular signal-regulated
kinases. These specialized molecules can be specifically exploited to indirectly or
directly interfere with DC recruitment to the CNS. For instance, neutralizing antibodies
for CCR1 and CCR2 have been tested in several clinical trials in MS but with rather
disappointing results where these agents failed to meet the therapeutic expectations
showing almost no efficacy. Antagonists against CCR5 are also being tested in clinical
trials for the treatment of autoimmune disorders other than MS and show positive results
on the blocking of DC trafficking in the human immunodeficiency virus infection. Moreover,
G protein-coupled receptors, which are also known to play a vital role in the control
of DC migration, are being explored for their therapeutic potential as antibody target.
Additionally, MMP inhibitors are being clinically explored and tested in an early
phase of development as therapeutic compounds in inflammatory diseases such as MS,
and are shown to display a higher specificity and efficacy (De Laere et al., 2018).
Nonetheless, caution needs to be paid while targeting these specialized migratory-associated
molecules as their expression is likely not to be restricted to disease-causing DCs,
but also on tolerance-inducing DC subsets and other lymphocytes resulting in aggravation
of the disease instead of the anticipated amelioration. Hence, there is still an increased
need for additional studies evaluating the effectiveness and the possibility to use
DC-specific approaches as a treatment for autoimmune diseases. Addressing DC-targeting
strategies, alone or in combination with established therapies, will improve our ability
to limit disease activity and progression in patients with MS.
Conclusion and future perspective: Migration of immune cells, among other immune effector
cell functions, serves as a vital therapeutic target of several of the approved drugs
for MS. Future studies explicating the mechanisms and forces specifically driving
DC accumulation in the meninges, choroid plexus and in the CSF, will provide a better
understanding of the underlying disease processes. In this context, accurate three-dimensional
models of the BBB will aid in moving the study forward towards the direction of developing
new therapies for various neuroinflammatory disorders.