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VOL. 20 / N ÚM. 2 / A BRIL-JUNIO 2001
2001; PP 78-87
INMUNOLOGÍA,
Integrins: what are they doing
on leukocytes in vivo?
A. G. ARROYO
Servicio de Inmunología. Hospital de la Princesa. Madrid
INTEGRINAS: ¿QUÉ ESTÁN HACIENDO
EN LOS LEUCOCITOS IN VIVO?
RESUMEN
Las integrinas constituyen una familia importante de
receptores de adhesión que pueden reconocer tanto componentes de la matriz extracelular como ligandos celulares. Distintas aproximaciones experimentales in vitro han
mostrado que las integrinas pueden jugar un papel importante en diversas funciones leucocitarias. Recientemente,
la generación de ratones manipulados genéticamente ha
proporcionado una herramienta muy útil para diseccionar
las funciones que estos receptores de adhesión desempeñan in vivo en los leucocitos. De forma interesante, el análisis de ratones deficientes en alguna integrina ha revelado
funciones insospechadas para estos receptores como es el
caso del papel de las integrinas β1 en la migración de progenitores hematopoyéticos o del papel de la integrina
αMβ2 en la apoptosis de neutrófilos. Sin embargo, otros
hallazgos en estos ratones han confirmado las funciones
que se habían sugerido previamente por otros abordajes
como es el papel de algunas integrinas en el tráfico leucocitario o en la migración leucocitaria durante la inflamación. Finalmente, los ratones deficientes en integrinas pueden servir en algunos casos como modelos animales de
enfermedades humanas tales como la deficiencia de adhesión leucocitaria o el asma.
ABSTRACT
Integrins constitute a major family of adhesion receptors
that recognise extracellular matrix components as well as
cellular ligands. They have been shown to play roles in a
variety of leukocyte functions by different in vitro approaches. Recently, the generation of genetically manipulated mice
has provided a useful tool to dissect the in vivo functions that
these adhesion receptors play on leukocytes. Interestingly,
some of the revealed functions from the analysis of deficient
mice have been unsuspected such as the role of β1 integrins in
homing of hematopoietic progenitors or the role of αMβ2 in
neutrophil apoptosis. However, other findings in these mice
have confirmed the functions that had previously been suggested by other approaches including the role of distinct integrins
in lymphocyte homing or in leukocyte migration during
inflammation. Finally, integrin deficient mice can provide in
some cases animal models for human diseases such as the leukocyte adhesion deficiency or the asthma.
PALABRAS CLAVE: Integrina/ Adhesión/ Leucocito/
rratón/ Knockout.
KEY WORDS: Integrins/ Adhesion/ Leukocyte/ Mouse/
Knockout.
INTRODUCTION
poietic stroma had been shown to be important
for leukocyte development (1). Once lymphocytes mature, they recirculate constantly through
secondary lymphoid organs to exert their surveillance function. Moreover, both lymphocytes and
myeloid cells need to attach to the endothelium
and transmigrate during inflammation to reach
L
78
eukocyte development occurs in the foetal liver during the embryonic life and in
the bone marrow during postnatal life
except for T lymphocytes that develop in
the thymus. Interactions with the hemato-
INMUNOLOGÍA
the inflammatory foci (2). Leukocytes that normally circulate as non-adherent cells are able to
attach to the endothelium and stroma during all
these processes. To this purpose, they use adhesion receptors including integrins.
The integrin family is composed of at least 24
heterodimers formed by the association of 8β
subunits and 18α subunits (3). Integrins constitute an important family of adhesion receptors that
recognise extracellular matrix (ECM) proteins as
well as cellular ligands (4). Classically, subfamilies have been distinguished depending on the β
subunit present in the heterodimer. Thus, β1
subfamily contains members that mainly recognize ECM proteins such as fibronectin, collagen,
laminin, and others except α4β1 and α9β1 that
can also bind the cellular ligand VCAM-1. The β7
subfamily comprises the heterodimers α4β 7 and
αEβ7 that bind MAdCAM-1 and E-cadherin, respectively. The β2 subfamily constitutes the leukocyte integrins since they are expressed on this
cellular subtype. Other β chains (β3, β5, β6, and
β8) associate to αv and β4 to α6. Integrins are
important as adhesion receptors and also as linkers between the cell membrane and the cytoskeleton and as signal transducers (5).
As mentioned, β2 integrins, composed by four
heterodimers, are mostly expressed on different
leukocyte subtypes (T and B lymphocytes, and
myeloid cells) and they recognise different ligands
such as ICAM-1, -2, and –3, iC3b, and others (6).
The β1 subfamily is finely regulated in expression
and function on the hematopoietic lineage. Thus,
α4β1 and α5β1 are expressed early during hematopoiesis and later in lymphocytes and polymorphonuclear subsets. Other members are activated
upon T lymphoid activation (7).
Different in vitro approaches using monoclonal
antibodies (mAbs), ligands and other tools had
suggested important roles for integrins during leukocyte development, traffic and activation.
Genetic manipulation of mice has provided an useful tool to elucidate the in vivo roles that integrins
might be playing in leukocyte physiology (8). In
this review, we will try to summarise the major findings obtained from the analysis of mice deficient
and chimeric for integrins related to defects in any
aspect of leukocyte physiology including development, homing and migration, activation and
others. We will also comment specific knockout
mice that might be used as models for human
immune disease.
INTEGRINS DURING IN VIVO LEUKOCYTE
DEVELOPMENT
Leukocyte progenitors need to migrate to different organs and to interact with the hematopoietic stroma to successfully complete their develop-
A. G. ARROYO
ment. Adhesion receptors including integrins
have been suggested to play roles during this process (for review see 9). Thus, anti-β1 antibodies
decreased the formation of colony-forming units
and the lodging of progenitors in the spleen and
anti-α4 antibodies could inhibit B lymphocyte
development and delayed myeloid development
in vitro (10-12). Knockout and chimeric mice have
revealed interesting in vivo functions for integrins
during hematopoiesis including leukocyte development.
Homing failure of progenitors to hematopoietic
organs in the absence of 1 integrins
Homing of hematopoietic stem cells (HSCs) to
hematopoietic organs is required for the establishment of hematopoiesis during embryogenesis and
after bone marrow transplantation. Mice chimeric or deficient for the β1 subunit in an inducible
manner have shown that β1 integrins are critical
on foetal as well as adult HSCs for homing to appropriate environments including foetal liver, thymus, spleen, or bone marrow. However, the potential of β1 null cells to differentiate in vitro (in foetal
organ cultures or in adult spleen context) is not
affected. Thus, β1 integrins are crucial adhesion
molecules for the homing of HSCs (13,14) resulting in a complete blockade of hematopoiesis in
vivo. With regard to the β1 partner responsible for
these defects, no single integrin subunit knockout
mouse displays the same phenotype than β1 integrin deficient mice. It might be either a combination of a subunits that individually can be deleted
without effect, or an as yet unknown subunit, maybe developmentally regulated, that heterodimerizes with β1 integrin and mediates the functions
essential for the homing of HSCs to the hematopoietic organs.
4 integrins are essential to regulate
the proliferation/differentiation balance
of hematopoietic progenitors in vivo
α4 integrins had been suggested to play roles in
migration, attachment, transmigration, proliferation and differentiation of hematopoietic progenitors in vitro (9), but their role in vivo remained
undefined.
The first characterisation of α4 integrin chimeric mice showed that α4 integrins were essential
for B lymphocyte development in the bone marrow
during the postnatal life (15). T lymphocyte development appeared to be normal during embryonic
development and was only impaired thereafter
likely due to a defect at the progenitor level within
the bone marrow environment. Other lineages
such as monocytes and natural killer cells seemed
79
INTEGRINS: WHAT ARE THEY DOING ON LEUKOCYTES IN VIVO?
VOL.
to be less affected by the absence of α4 integrins.
A more detailed analysis of the hematopoiesis
during all stages of mouse development in α4 chimeric mice, showed that α4 integrins are essential
for development of all lineages in the foetal liver
and the dependence is greater at later stages in the
bone marrow (16). Homing of progenitors to foetal liver or bone marrow, however, seems to be normal. Interestingly, transmigration of the progenitors through the bone marrow or foetal liver
stroma is decreased in the absence of α4 integrins
resulting in a defect in proliferation of the hematopoietic progenitors within these environments.
These defects allow the differentiation of mature
blood cells but at much lower numbers in the
absence of these receptors. Thus, α4 integrins are
demonstrated to be essential for regulation of the
proliferation/differentiation balance of hematopoietic progenitors within the foetal liver and bone
marrow microenvironments.
In figure 1, a working model for the roles of α4
integrins in regulating hematopoiesis is proposed
(16). Hematopoietic progenitors normally seed
and attach to the stroma. It is possible to distinguish three compartments for self-renewal, expansion, and maturation of progenitors within the
stroma (17). In these stromal compartments, the-
20 NÚM . 2 / 2001
re might be a differential secretion of soluble factors such as cytokines and chemokines as well as
distinct stroma-progenitor cell interactions. The
balance among all these signals (18) and the competition for the niches among different progenitors would determine the fate of the hematopoietic progenitors. In the expansion stroma,
progenitors would transmigrate beneath in response to chemokines, and cytokines might also
facilitate this process by activating adhesion receptors such as integrins (9). In that niche, progenitors proliferate in contact with extracellular
matrix proteins such as fibronectin and growth
factors presented by the proteoglycans. After this
expansion stage, progenitors continue differentiation into mature cells under the influence of
other cytokines in the maturation compartment.
In this model, α4 integrins would act as a retention signal for progenitors to remain in the expansion compartment. In the absence of α4 integrins
two major steps would be impaired: first, the response to cytokines and chemokines and subsequent attachment and transmigration beneath the
stroma, and second, proliferation mediated by
contact with fibronectin and growth factors. As a
consequence, α4-deficient progenitors would
detach prematurely, skipping the expansion phase,
wild type
Progenitors
α4β1
Stroma
Chemokines
α4β1
Growth Factors
ECM
FN FN
α4 null
Mature cells
Progenitors
α4β1
Fetal liver
Stroma Spleen
Bone marrow
Chemokines
α4β1
Growth Factors
FN FN
ECM
Figure 1. α4 integrins are essential to control the normal balance between proliferation and differentiation of hema-
topoietic progenitors in vivo. A model for regulation of the proliferation of hematopoietic progenitors by α4 integrins in
vivo is proposed. α4 integrins would participate in the adhesion and subsequent transmigration of progenitors through
the stroma stimulated by chemokines. Moreover, α4 integrins would also play a role in recognising growth factors presented by the extracellular matrix beneath the stroma. In the absence of α4 integrins, these two steps would be impaired resulting in skipping of the expansion phase, leading the progenitors directly to differentiation and resulting in low
numbers of mature blood cells.
80
INMUNOLOGÍA
and shift towards differentiation. This might result
in low numbers of progenitors involved in hematopoiesis and lower yields of mature cells.
α4 integrin chain can associate with β1 or β7
subunits. Since β7 deficient mice do not display any
defect in leukocyte development (19), it seems that
α4β1 is the responsible for these critical interactions. α4β1 can bind fibronectin and VCAM-1. It is
not possible to know at this point which of the
ligands is essential since VCAM-1 null mice do not
have major defects in leukocyte development and
fibronectin null mice are lethal very early (20).
Moreover, conditional VCAM-1 deficient mice have
recently been generated and they mainly show
defects in migration of lymphocytes to the bone
marrow and in T-cell dependent humoral immune
response (21,22). These defects do not reproduce
those found in α4 chimeric mice. It is possible that
both ligands or as yet unknown one are involved since there is no compensation by other fibronectin
receptors such as α5β1 or αvβ1 nor by other VCAM1 receptors such as α9β1, in spite of its similarities
with α4β1 in structure and ligands (23), or αdβ2
that is expressed on a variety of leukocytes (24).
M 2 integrin is required for mast cell
development
αMβ2 is a member of the β2 subfamily expressed
in several leukocyte subsets. The analysis of αMβ2
null mice has surprisingly shown that it is required
for the expression of normal levels of mast cells in
the peritoneal cavity, peritoneal wall, and certain
areas of skin (25). αMβ2 is also shown to be important for maintaining adequate cell-dependent host
defence against bacterial infection since these mice
exhibit increased mortality in a model of acute septic peritonitis in which it is known that host resistance rely on mast cells and complement (25). The
mechanism for this regulation of resident mast cells
numbers by αMβ2 is unknown. Thus, αMβ2 might
be affecting migration of mast cell progenitors or
proliferation and survival of differentiated mast cells
rather than development itself.
In summary, so far only a few integrin receptors
(β1 and α4 integrins, and αMβ2) have been shown
to play roles in leukocyte development in vivo. β1
subunit as the partner for multiple α chains constituting the major adhesion receptor subfamily is
essential for the homing of progenitors to hematopoietic sites and its absence results in the earliest blockade in leukocyte development. α4 integrins participate at later stages when progenitors
are already within the hematopoietic organs and
regulate their transmigration and proliferation
with a decreased numbers of mature blood cells in
their absence. Finally, a more specific defect would
be caused by the deficiency of αMβ2 that seems to
only affect the number of mast cells.
A. G. ARROYO
INTEGRINS DURING IN VIVO LEUKOCYTE
HOMING AND MIGRATION
The most important functions of leukocytes are
the surveillance of exogenous pathogens and the
fight against them. In order to do so, lymphocytes
need to recirculate constantly through different
secondary lymphoid organs and leukocytes in
general need to adhere and go through anatomic
barriers including the endothelium and ECM to
reach the inflammatory foci (2). Thus, leukocytes,
that circulate normally as non-adherent cells in the
bloodstream, need to quickly switch to an adherent state to be able to reach different sites by using
adhesion receptors. Different approaches have suggested important roles for integrin receptors in leukocyte homing by binding to addressins and migration to inflammatory foci (26), but now the analysis
of genetically manipulated mice have demonstrated the role that these receptors play in vivo.
Integrins and homing
4, 7, and αE integrins are essential
for constitution of the gut-associated
lymphoid tissue
The gut-associated lymphoid tissue in the intestinal mucosa represents an integrated system of
secondary lymphoid organs (Peyer’s patches and
lymph mesenteric nodes) and mucosal effector
sites (lamina propria and the intraepithelial lymphocyte compartment located above the villus basement membrane). In vivo data from the analysis of
chimeric mice have shown that α4 integrins are
essential for migration of lymphocytes to Peyer’s
patches (15,27). The α4 integrin chain can associate with β1 or β7 subunits. The analysis of β7 deficient mice has shown a critical role for β7 receptors in migration of lymphocytes to Peyer’s patches
and an important role in trafficking to the intraepithelial intestinal compartment and lamina propria (19). The defect observed in migration to
Peyer’s patches in the absence of α4β7 fits with the
presence of its specific ligand, the addressin
MAdCAM-1 at this site. However, α4 integrins are
not critical for migration to intestinal epithelium
and lamina propria, suggesting that other β7 partners might be responsible for the defects. In this
regard, αE-null mice show decreased numbers of
intraepithelial and lamina propria lymphocytes
(28). In conclusion, α4β7 integrin is essential for
migration of lymphocytes to Peyer’s patches and
αEβ7 is critical for localisation of lymphocytes within the intestinal epithelium and lamina propria.
It is possible that β2 integrins might also participate in the establishment of the intestinal lymphocyte compartment in vivo as it has been shown in vitro
(29) but this point has not been investigated yet.
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INTEGRINS: WHAT ARE THEY DOING ON LEUKOCYTES IN VIVO?
L 2 (LFA-1) integrin participates in homing
of lymphocytes to peripheral lymph nodes
LFA-1 is expressed on lymphocytes, monocytes, granulocytes, and bone marrow cells. Its natural ligands include ICAM-1, -2, and –3 and it has
been suggested to play roles during the immune
response, lymphocyte recirculation, and inflammation. Its role in homing had been suggested by
in vitro and in vivo studies but the results were not
conclusive enough. Analysis of mice deficient in
the αL subunit showed a marked defect in the
migration of lymphocytes into peripheral lymph
nodes but preserved migration of lymphocytes
into mesenteric nodes, Peyer’s patches and the
spleen (30). Migration into these sites is abolished
in αL null mice by antibodies against α4 or the
relevant α4 integrin ligands, confirming a compensatory role for α4 integrins in mediating migration to these sites.
Interestingly, analysis of β2 integrin deficient
mice has also shown that β2-mediated adhesion is
necessary for the emigration of the precursors of
dendritic cells into the lung resulting in significantly lower numbers of lung dendritic cells in the
mutant mice. The defect is, however, partial, suggesting that other pathways, likely α4 integrins/
VCAM-1, might be involved (31).
6A integrin plays a role in migration
of lymphocytes on laminin and likely in homing
to peripheral and mesenteric nodes
Some integrin subunits display alternative splicing variants. This is the case of α6 integrin subunit whose cytoplasmic domain can be encoded
by two alternatively spliced exons. Interestingly,
inducible null mice for the variant α6A that can
only express α6B show an unsuspected role in
lymphocytes. Thus, lymphocytes from α6A null
mice demonstrate impaired migration on laminin,
and a decreased number of T cells in both peripheral and mesenteric lymph nodes, suggesting a
role for the α6A isoform in lymphocyte migration
(32). However, experiments injecting α6A null
lymphocytes into mouse recipients do not show
any defect in homing to these sites and so this point
needs to be clarified further.
Other integrin subunits such as α5 and αv have
been shown not to play a role in homing of leukocytes to immune organs (27).
Integrins and migration to inflammatory sites
2 integrins are required for migration
of leukocytes to inflammatory sites
During the inflammatory response, lymphocytes initiate contact with the activated endothe-
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VOL.
20 NÚM . 2 / 2001
lium, first by tethering and then by firm adhesion
and subsequent transmigration. During this process, the β2 integrin ligand ICAM-1 and the α4
integrin ligand VCAM-1 are up-regulated in the
endothelium (26).
αL null mice display modest delays in the influx
of polymorphonuclear leukocytes and monocytes
into the peritoneal cavity in response to an inflammatory stimulus (30). In contrast to expectations,
αM null mice do not have any defect in recruitment
of neutrophils to sites of inflammation likely due
to a compensation by LFA-1 (33,34). However, in
αM deficient mice a reduction in neutrophil infiltration is observed in a model of brain ischemia/
reperfusion injury, resulting in a decrease of the
damaged area (35).
β2 null mice show most of the features of the
leukocyte adhesion deficiency (LAD I) seen in
humans including leukocytosis, spontaneous
infections and impaired emigration of neutrophils
into inflamed or infected skin (36,37), and this
point will be discussed again later.
4 integrins also play a role in migration
of leukocytes to inflammatory sites
The role that α4 integrins might have during
inflammatory processes had been previously investigated using antibodies against α4 integrins or specific inhibitory peptides (38). These studies had
shown a role for α4 integrins during allergic asthma, delayed contact hypersensitivity, experimental
allergic encephalomyelitis, rheumatoid arthritis,
and chronic inflammatory bowel disease. The
analysis of chimeric mice for α4 integrins have
demonstrated that in fact α4 integrins participate
in vivo in the migration of T lymphocytes to the peritoneal cavity in a model of peritonitis (27). These
data reinforce the therapeutic approaches that are
under clinical trial using α4 inhibitors for the
treatment of different inflammatory diseases always
keeping in mind the possible side effects due to the
role of α4 integrins in hematopoiesis.
7 integrins are also important for migration
of lymphocytes during certain
infectious/inflammatory pathologies
A model of mice transgenic for an ovalbuminspecific major histocompatibility complex class-I
restricted T cell repertoire has been used to investigate the role of β7 integrins during a primary
antiviral CD8 T cell response in vivo(39). Analysis
of adoptive transfer experiments with β7 null cells
from these mice have shown that β7 integrins seem
to be critical for migration of activated CD8 T cells
to the mesenteric lymph node, Peyer’s patch, and
to the small and large intestinal mucosa including
INMUNOLOGÍA
A. G. ARROYO
the epithelium, and for entering of naive CD8 T
cells to the lamina propria and intestinal epithelium. However, one of its partners, αE integrin,
does not seem to play a role during this primary
immune response in vivo.
The role of β7 integrins has also been investigated in a model of intestinal inflammation
during pathogen-specific protective immunity to
enteric helminth infection. The absence of β7
integrins resulted in a delay and reduction of
intestinal eosinophilia and mastocytosis during
Trichinella spiralis infection of the small intestine, leading to an impaired host protection (40).
However, there was no defects in the recruitment
of leukocytes after Trichuris muris infection of
the large intestine in the β7 deficient mice. Thus,
β7 integrins are important for protective immunity against helminth infection of the small intestine.
Finally, it has been observed in a model of experimental autoimmune encephalomyelitis that
adoptive transfer of either wild-type encephalitogenic T cells to β7-deficient mice, or of β7 null
encephalitogenic T cells to wild-type recipients
resulted in attenuated forms of the disease indicating that β7 positive cells contribute to disease progression (41).
In figure 2, the main pathways of leukocyte
homing to lymphoid organs and inflammatory
sites demonstrated by the analysis of knockout
and chimeric mice are represented. Accordingly
to their ligand specificity, integrins (α4β1, α4β7,
αEβ7, and β2 integrins) that recognise cellular
ligands (VCAM-1, MAdCAM-1, E-cadherin and
ICAM-1,-2, and –3, respectively) play a role in
recognition of the first tissue barrier i.e. endothe-
lium to home to different tissues. On the contrary,
integrins such as α5β1 and αvβ1 recognising ECM
ligands beneath the endothelium do not seem to
play a role.
INTEGRINS DURING IN VIVO LEUKOCYTE
ACTIVATION
Signalling by integrins and other receptors
plays an important role in lymphocyte activation
(42). Thus, integrin function is regulated by lymphocyte activation. It has also been shown that several integrin ligands, including fibronectin and
VCAM-1, can act as co-activation signals for lymphocytes (43). All these data suggest that integrins
might participate in lymphocyte activation. This
point has been addressed in vivo by the analysis of
different mutant mice.
2 integrins act as costimulatory signals
for lymphocytes in vivo
αLβ2 (LFA-1) had been suggested to play
important roles in the induction of immune responses, lymphocyte proliferation, and CTL killing
of target cells but the results were controversial.
The analysis of mice deficient for either αL or β2
integrins have demonstrated the role of LFA-1 in
T cell co-activation by activation with soluble antiCD3, mixed lymphocyte reaction or SEA stimulation, confirming the importance of co-stimulatory signals from β2 integrins in vivo (30,37).
Moreover, natural killer cytotoxicity is markedly
Peripheral lymph nodes
α L, α6A?
Skin
β2
GUT-associated
lymphoid tissue:
Mesenteric nodes
α4, αL
α6A?
α4β7
Peyer´s patches
αEβ7
Peritoneum
β7
Gut (antiviral response, helminth infection)
αM, β7
CNS (ischemia/reperfusion, EAE))
Lamina propria
Intraepithelium
Figure 2. Integrins are important in vivo for establishing the normal pathways of lymphocyte homing and leukocyte
migration. In this scheme, we have summarised the integrins that have been demonstrated or suggested (?) to play a role
in vivo in the different lymphocyte homing pathways as well as in migration of leukocytes to the inflammatory foci
during diverse pathologies.
83
INTEGRINS: WHAT ARE THEY DOING ON LEUKOCYTES IN VIVO?
reduced, delayed-type hypersensitivity is absent,
and there is no rejection of tumour cells in αL deficient mice suggesting that recirculation of lymphocytes through the lymph node network is impaired. However, CTL activity and secondary antigen
restimulation against a systemic infection by virus
is normal. It could also be possible that LFA-1
might be differentially required for activation and
proliferation, depending on the type of antigen
presenting cell involved or the microenvironment
of antigen presentation. Altogether, these data
demonstrate the role of αLβ2 in TcR-dependent T
cell proliferation and in lymphocyte recirculation
in vivo.
4, 5, and v integrins are not essential
for lymphocyte costimulation in vivo
In vitro studies had shown that α4 and α5 integrins might act as co-stimulatory signals for lymphocytes (for review see 43). However, the analysis
of the proliferation and activation status of lymphocytes from mice chimeric for α5, αv or α4 integrins have not shown any defect (27). These data
suggest that α5, αv, or α4 integrins are not essential for activation of lymphocytes. However, synergism of these adhesion receptors in costimulating
lymphocytes under certain conditions in vivo
could still be possible. Moreover, because accessory pathways seem to be multiple, it is not possible to rule out replacement by other accessory
molecules. Inducible tissue-specific knock-out
mice will give further clues about the functions of
these receptors during the immune response in
vivo.
M 2 is critical for neutrophil effector
functions in vivo
Despite preserved migration of polymorphonuclear neutrophils during inflammation in the
absence of αMβ2, neutrophil effector functions
are impaired likely because of absent outside-in
signalling or deficient signal integration. Thus,
αMβ2 has been shown to be important in regulating immune complex-stimulated neutrophil
function in vivo resulting in an abrogation of sustained Fcγ receptor-dependent neutrophil adhesion and complement-dependent proteinuria
during acute glomerulonephritis in αM deficient
mice (44). Moreover, neutrophils deficient in
αMβ2 display impaired adherence to hemocyanin
coated-glass and fibrinogen coated-filters, iC3bmediated phagocytosis, degranulation, and
homotypic aggregation demonstrating the important in vivo role for this β2 integrin in mediating
neutrophil effector functions (34).
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20 NÚM. 2 / 2001
INTEGRINS AND LEUKOCYTE APOPTOSIS
M 2 regulates neutrophil apoptosis in vivo
αM null mice have revealed a completely unsuspected role for this β2 integrin subunit in regulating neutrophil physiology. In response to a chemoattractant stimulus in the peritoneal cavity,
these mice show a marked increase in neutrophil
accumulation likely due to enhanced neutrophil
survival (33). This effect appears to result from the
impaired degranulation and phagocytosis, events
that normally accelerate apoptosis in neutrophils
and that would be regulated by αMβ2 integrin as
it has been mentioned earlier (34).
2 integrins might participate
in T cell-mediated B cell apoptosis in vivo
It has been shown by mAb blocking assays, that
interaction of αLβ2 on Th1 T cells with ICAM-1, a
ligand for αLβ2, on B cells seems to be essential for
Fas-mediated B cell apoptosis. These studies have
also been confirmed using B lymphocytes from
ICAM-1 deficient mice but it would also be interesting to investigate this point in β2 mutant mice (45).
INTEGRIN-DEFICIENT MICE AS IN VIVO
MODELS FOR IMMUNE DISEASE
2 null mice as a model of human LAD I disease
Leukocyte adhesion deficiency type I (LAD I) is
a human disease that results from heterogeneous
mutations in the gene encoding the common integrin β2-subunit and it is characterised by delayed
separation of the umbilical cord, marked neutrophilia, and recurrent bacterial infections (for review
see 46). As it has already been mentioned, partial
and complete deficiency for β2 integrins and specific deletions of αLβ2, αMβ2, and αdβ2 (6) have been
generated. β2 null mice display most of the features
of human leukocyte adhesion deficiency type I
(36,37) including neutrophilia and a defect in accumulation of polymorphonuclear neutrophils in
inflamed skin. However, migration of neutrophils
to other sites such as lung or peritoneum in response to an inflammatory stimulus is preserved suggesting the existence of alternative adhesion mechanisms in mice in contrast to humans in which
migration to all inflammatory sites is impaired (46).
6 null mice as a model for asthma and lung
inflammation
αvβ6 integrin is only expressed in epithelial
cells and recognizes fibronectin, tenascin, and
INMUNOLOGÍA
A. G. ARROYO
85
INTEGRINS: WHAT ARE THEY DOING ON LEUKOCYTES IN VIVO?
v i t ronectin. This receptor is barely detected in
adult epithelia but it is upregulated upon epithelial injury. Intere s t i n g l y, β6 subunit null mice
manifest inflammatory baldness and lung inflammation (47). The skin and the lung lesions with
infiltration of macrophages and activated lymphocytes, re s p e c t i v e l y, appear to depend on an
e n v i ronmental stimulus. In fact, β6 deficient
mice demonstrated airway hyperresponsiveness
that resembled asthma. Whenβ6 null mice were
given bleomycin in a model of pulmonary fibrosis, they were completely protected from the
fibrotic effect (48). Both the exaggerated inflammation and the protection from lung fibro s i s
appear to be explained by the novel role recently
revealed for this integrin in binding to the latency
associated peptide of TGFβ1 and activating
extracellular latent TGFβ1 complexes (48). In
the absence of β6, there would be less active
T G Fβ1 available and it is known that this cytokine is critical for regulation of the immune response. In fact, the defects found in β6 deficient
mice partially correlates with the phenotype
observed in TGFβ mutant mice (49).
VOL.
human pathologies such as LAD I, asthma, and
lung inflammation and to try novel therapeutic
approaches.
CO RRESPONDENCE:
Alicia G. Arroyo
Servicio de Inmunología. Hospital de la Princesa.
C/ Diego de León 62
28006 Madrid
Tel.: 91 520 23 70
Fax : 91 520 23 74
E-mail : [email protected]
References
1.
2.
3.
CONCLUDING REMARKS
4.
The immune system function relies on a complex network of leukocyte interactions with each
other, with the endothelium or with other soluble
or ECM ligands. As outlined along this review, the
in vivo analysis of mice deficient for diverse members of the integrin superfamily of adhesion receptors has confirmed in many cases and revealed in
others the importance that ligand recognition by
integrins on the leukocyte surface has for all these
processes (Table I). Furthermore, in spite of
redundancy of integrin expression and, in some
cases, function there are unique and distinct roles
for some integrins in leukocyte physiology. Thus,
no leukocyte development is possible in the absence of all β1 integrins. Moreover, traffic of leukocytes to lymphoid organs and inflammatory foci
depends highly on the presence of integrins on
their surface, although other receptors such as
selectins can also play a role. During the immune
response, the establishment of the immune synapses between T lymphocytes and the APC is critical for activation of the T lymphocytes. The co-stimulatory signals mediated through β2 integrins
have been demonstrated to be important in vivo
for this proper signalling to occur. Also, αMβ2 is
absolutely required for proper effector functions
of neutrophils. Finally, other processes important
to maintain immune homeostasis such as apoptosis are regulated by αMβ2 on neutrophils and
likely by β2 integrins on B lymphocytes.
Interestingly, there are also integrin deficient animals that might be used to understand better some
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