Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation. Models in Discovery and Translation

521 Pages·2013·10.982 MB·English

by  Gerard Socie and B.R. Blazar (Eds.)

1
c h
Overview of the immune biology
of allogeneic hematopoietic
stem cell transplantation
Gérard Socié
Service d’Hématologie-Greffe de Moelle, Hôpital Saint-Louis, AP-HP, Paris, Université Paris VII Denis-Diderot, and Unité
INSERM U940, Paris, France
Bruce R Blazar
Cancer Center and Department of Pediatrics, Division of Blood and Marrow Transplantation, University of Minnesota,
Minneapolis, Minnesota, USA 1
Introduction
Much of our understanding of the biology of graft-versus-host disease
(GVHD) has developed from two preclinical animal models, the mouse
and the dog (reviewed in references [1–6]). Since there are significant spe-
cies differences between humans and mice, five points are important to
consider when drawing conclusions from studies with animal models and
before correlation to the clinical allogeneic hematopoietic stem cell trans-
plantation (HSCT) scenario (Box 1.1).
In this overview we introduce the main concepts and experimental results
concerning the major areas developed in the book including the GVHD, the
graft-versus-leukemia (GVL) effect, rejection and immune deficiency.
We will focus on recent advances and their translation into clinical knowl-
edge or therapies. In each section we summarize key experimental data and
then provide a perspective as to how these data succeeded or failed to be
translated to the bedside. The main differences between experimental sys-
tems and human beings, as well the tools used to study GVHD and GVL, are
illustrated in Figure 1.1.
Immune rejection
Preclinical studies show that allogeneic hematopoietic stem cell (HSC) graft
rejection can be mediated by host natural killer (NK) cells, NK T cells, γδ T
cells, and/or CD4+ and CD8+ T cells that recognize histocompatibility anti-
gens (MiHA) on the donor cells (reviewed in chapter 5, and in references [6]
and [7]). In clinical practice, graft rejection of related HLA-identical bone
marrow (BM) or mobilized peripheral blood (PB) after myeloablative con-
ditioning is rare. Graft rejection or graft failure occurs primarily following
Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation.
http://dx.doi.org/10.1016/B978-0-12-416004-0.00001-x
Copyright © 2013 Elsevier Inc. All rights reserved.
Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation
BOX 1.1 Caveats in directly translating results from animal
models into human studies
1. Conditioning regimen

• In murine studies, usually irradiation alone (without chemotherapy)
• Irradiation often is used with large fraction doses and high dose rates not com-
monly used in patients

2. Immunological disparity between donor and recipient

• Inbred strain combinations are used, resulting in a variety of MHC- and/or minor
histocompatibility antigen (MiHA)-disparate models
• These different strain combinations have different Th1/Th2/Th17 as well as Treg
content and can sway the dominance of CD4+ or CD8+ T-cell effectors in GVHD
• Thus, conclusions in one model may not translate into other immunologically
distinct models or into the clinic

3. Source of donor cells

• Typically spleen cells and/or lymph node T cells are added to the bone marrow
graft, in contrast to peripheral blood or bone marrow grafts (contaminated with
peripheral blood cells) in human studies

2
4. Microbial baseline

• Whereas in rodents, mice are housed under specific pathogen free conditions
since birth, humans are not. Therefore, extrapolation of murine data between
laboratories may be difficult and clinical translation of such findings into HSCT
recipients may be even more challenging, especially for those innate and adap-
tive immune responses most readily influenced by the microenvironment

5. Age of the donors and recipients

• The majority of murine HSCT studies use primarily young adult mice and only
infrequently older mice will be used. Older age in mice is known to alter antigen-
presenting cell (APC) capacity, thymopoiesis and peripheral T-cell recovery, and
sensitivity to radiation

6. GVHD prophylaxis

• Generally not included systematically in the experimental setting

7. GVL

• Experimental studies are limited by the use of a limited repertoire of cell lines and
only infrequently transformed or mutated primary hematopoietic cells given to
the host

transplant of cells from related HLA-mismatched or matched unrelated
donors (MUDs) and/or use of T-cell-depleted grafts. Rejection or graft failure
can be assessed by the extent of donor chimerism measuring the propor-
tion of the recipient cells. However, early elimination of myeloid precursors
and their progeny does not always correlate with long-term engraftment,
and researchers should take care to avoid over-interpretation of results. Early
studies to characterize murine NK cells suggested that host NK cells could
reject donor BM in a non-MHC-restricted manner, as evidenced by a phe-
nomenon called hybrid resistance in which parental BM cells are rejected by
F1 hybrid recipients. Investigators now recognize that NK cells bear inhibitory
and activating receptors directed to MHC and other cellular determinants
Overview of the immune biology of allogeneic hematopoietic stem cell transplantation
3
FIGURE 1.1
(a) Tools in experimental GVHD. (b) Tools and challenges in studying human GVHD. KO = knock out; pbmnc = peripheral blood mononuclear cells.
that are critical to target cell identification and subsequent NK cell-mediated
killing (reviewed in chapter 15). During graft rejection, the effector pathways
used by recipient T cells differ on the basis of prior sensitization of the host
to alloantigen. In naïve, un-sensitized recipient mice, perforin, granzyme B
and Fas/FasL can mediate rejection of MHC- and/or MiHA mismatched BM
by CD8+ T cells. CD4+ T cells mediate allogeneic BM destruction. However,
CD8+ T cells from sensitized recipients with alloantigen can reject BM by
an unknown mechanism that appears independent of the numerous path-
ways, as determined using gene knockout mice and neutralizing antibod-
ies. Prior exposure to histocompatibility antigens, which can occur by blood
Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation
product transfusions, pregnancies or immunization in experimental mod-
els, is attributed to cytotoxic T cells, which can be identified in alloantigen-
sensitized recipients. However, antibodies capable of recognizing MHC or
MiHA on donor cells can induce graft rejection or lineage-specific aplasia.
Immune deficiency
A major problem limiting the efficacy of allogeneic HSCT is the issue of
promoting immune reconstitution without increasing GVHD (reviewed in
chapter 6 and in reference [6]). Patients are profoundly immunosuppressed
following transplant as a result of the cytoreductive conditioning, immuno-
suppressive drugs to prevent GVHD, and the paucity of transplanted T cells
compared with the size of the T-cell compartment in an immune compe-
tent person. In addition, acute GVHD induces lymphoid hypoplasia, thus
tying GVHD to immune impairment. This leaves the patient susceptible to
a number of opportunistic infections. Infectious complications associated
with neutropenia early post-transplant are no longer as prominent in clini-
cal practice. However, cytomegalovirus, Epstein–Barr virus (EBV) and fun-
gal infections, predominantly Candida species and Aspergillus fumigatus
4 that arise after neutrophil recovery, are now major contributors to morbid-
ity and mortality following allogeneic HSCT. Unfortunately, there are few
preclinical models that have been developed to study these opportunistic
infections and the complicating effects of GVHD on their occurrence.
There are two sources for T cells in the recovering recipient: peripheral
expansion of mature T cells and de novo production of naïve T cells derived
from transplanted stem cells and produced in the recipient thymus. How-
ever, the thymus begins to involute at puberty, and the capacity for thymic-
derived T-cell production is greatly diminished in adulthood. In addition,
the cytoreductive conditioning can induce tissue damage to the epithelial
cells of the thymus and a decreased ability to produce IL-7. Thus, a reduced
ability to generate new T cells is a function both of increasing age and of
conditioning dose intensity. An older HSCT recipient is especially prone to
limited recovery of the CD4+ T-cell repertoire following allogeneic HSCT. A
slow recovery is associated with an increased risk of opportunistic infec-
tions and a decreased ability to generate a response to vaccination. The
benefit of de novo generation of T cells post-transplant is the production
of donor-derived T cells that are tolerant of both the graft and the recipient
and generation of a broad T-cell receptor (TCR) repertoire.
Enhancing immune reconstitution is an area of intensive research. An
increasing variety of approaches has been explored pre-clinically and clini-
cally: infusion of IL-7, keratinocyte growth factor, growth hormone, mature
cytotoxic lymphocytes with defined immunological properties against
pathogens or tumor antigens and blockade of sex hormones. New devel-
opments of allogeneic HSCT, e.g. umbilical cord blood or haploidentical
graft preparations leading to prolonged immunodeficiency, have further
increased the need to improve immune reconstitution. While slow T-cell
reconstitution is regarded as primarily responsible for susceptibility to
infections with viruses and fungi, GVHD and propensity for post-HSCT
relapse, the importance of innate immune cells for disease and infection
control is currently being re-evaluated. In the future, individualized therapy
Overview of the immune biology of allogeneic hematopoietic stem cell transplantation
partially based on genetic features of the underlying disease will likely come
of age (reviewed in reference [8]).
GVHD pathophysiology
GVHD is a complex disease resulting from donor T-cell recognition of a
genetically disparate recipient that is unable to reject donor cells follow-
ing allogeneic HSCT. The classical scheme of GVHD [2,6,9] development
includes five basic steps:

Step 1: Priming of the immune response. Cytoreductive conditioning
induces tissue damage and the release of a storm of proinflammatory
cytokines that promote the activation and maturation of antigen-pre-
senting cells (APCs) and the rapid amplification of donor T cells [10–12].
Step 2: T-cell activation and costimulation. Activation occurs as the re-
sult of the recognition and interaction of the TCR and costimulatory mol-
ecules with their cognate ligands expressed on the surface of the APC.
Step 3: Alloreactive T-cell expansion and differentiation.
Step 4: Activated T-cell trafficking. Activated T-cell migration to GVHD
target tissues (gut, liver, skin and lung) is followed by the recruitment of 5
other effector leukocytes [13].
Step 5: Destruction of the target tissues by effector T cells. Destruction
occurs via exposure to cell surface and release of soluble immune effec-
tor molecules. Tissue damage then leads to increased inflammatory sig-
nals, perpetuating and augmenting the disease process by contributing
to the cytokine storm that fuels GVHD.
Previous reviews [2,6,9,10,13–15] and chapters in this book have detailed
these phases of GVHD initiation and tissue destruction.
Priming of the immune response
The earliest phase of acute GVHD is set into motion by the damage caused
by the underlying disease and exacerbated by conditioning regimens
(reviewed in chapter 8). Damaged host tissues secrete proinflammatory
cytokines, such as TNF-α and IL-1, which contribute to the “cytokine
storm” increasing the expression of adhesion molecules, costimula-
tory molecules, MHC antigens and chemokine gradients that alert the
residual host and the infused donor immune cells. These “danger sig-
nals” activate host tissue cells including APCs. Damage to the GI tract
from the conditioning is particularly important in this process, because
it allows for systemic translocation of lipopolysaccharide that further
enhances host APC activation [9,16]. This scenario is in accord with the
increased GVHD risk associated with intensive conditioning regimens in
some human randomized trials [17–19]. However, preclinical studies in
dogs [1,3,20–22] and clinical studies have indicated that reduced inten-
sity conditioning is associated with less morbidity and less early acute
GVHD [23].
It is noticeable that IL-1 blockade [24] or protection of epithelial tissue dam-
age by infusion of keratinocyte growth factor, although partially efficacious
in some experimental GVHD models [25,26], thus far have proved ineffec-
tive in preventing acute GVHD in randomized human trials performed in
Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation
matched sibling donors [27] (including both randomized IL-1 and KGF tri-
als). Because the mechanisms associated with acute (late onset) GVHD after
reduced (eventually minimal) conditioning have not been well elucidated,
additional studies are warranted that go back to the bench to develop the
so-called “mini transplant” in the mouse setting that may complement the
aforementioned canine investigations.
T-cell activation and costimulation
The core of the graft-versus-host immune reaction lies within the second
step, in which donor T cells proliferate and differentiate in response to host
APCs [28,29] (reviewed in chapter 9). Recent advances have indicated the
presence of a subset of post-mitotic, self-renewing CD44 (lo)/CD62L (hi)/
CD8+ T cells that can generate and sustain all allogeneic T-cell subsets in
GVHD reactions, including central memory, effector memory and effector
CD8+ T cells [30]. The danger signals generated in the first phase augment
this activation, at least in part, by increasing expression of costimulatory
molecules. In mouse models, in which genetic differences between donor
and recipient strains can be tightly controlled, CD4+ T cells induce acute
GVHD to MHC class II differences and CD8+ T cells induce acute disease to
6
MHC class I differences [29,31–38]. Under typical bone marrow transplan-
tation (BMT) conditions, murine studies with MiHA-disparate models
have demonstrated that GVHD initiation requires donor T-cell recognition
of host antigen in the context of host APCs [29,31–38]. Donor-derived APCs
are then able to augment CD8+ T-cell-mediated GVHD by acquiring and
presenting host antigens [34].
In humans, the incidence of acute GVHD is directly related to the degree
of mismatch between HLA determinants [39], mapped by high-resolution
DNA typing of HLA genes with PCR-based techniques, largely replacing
earlier cellular methods (reviewed in chapter 2). However, recipients of
HLA-identical grafts can still develop systemic acute GVHD due to genetic
differences that lie outside the MHC loci and that encode proteins referred
to as MiHAs (reviewed in chapter 3). Thus, there is strong evidence for
MiHA-mismatch mediated GVHD in humans [40–42]. Although individ-
ual human MiHA antigens associated with GVHD have been identified,
the relative contribution of diverse MiHA and the existence (if any) of
single, dominant MiHAs in humans (such as B6dom and H60 that have
been well characterized in rodents [43,44]) is unknown. With respect to
the donor-versus-host origin of APCs initiating GVHD in humans, little
data are available. However, recent studies on the fate of human Lang-
erhans cells, dermal dendritic cell and macrophages in patients suggest
that host-derived APCs at least participate to the early stage of the disease
[45–47]. Donor and recipient polymorphisms of cytokine genes ascribed
to the cytokine storm in rodents and humans have also been implicated
as risk factors for the disorder. For example, TNF-α, IL-10 and INF-γ vari-
ants have correlated with GVHD in some, but not all, studies (reviewed in
reference [48] and in chapter 16). Genetic polymorphisms of proteins con-
nected with innate immunity, such as NOD2, have been associated with
acute GVHD in patients [49]. Lastly, in some experimental models, poly-
morphisms in members of the Toll-like receptor family have been linked
to GVHD risk [50].
Overview of the immune biology of allogeneic hematopoietic stem cell transplantation
COSTIMULATORY MOLECULES PLAY PIVOTAL ROLES IN EXPERIMENTAL GVHD
A major role for GVHD initiation in rodent models has been ascribed to CD28/
CTLA-4 (CD152):B7 interactions which consists of both a positive (CD28/B7)
and an inhibitory (CTLA-4:B7) pathway (reviewed in chapter 10). Another B7
supergene family member, ICOS (inducible costimulator) (CD278), binds the
ligand B7h (CD275) expressed on host APCs and thereby promotes T-effector
responses. Blockade or absence of ICOS on donor T cells diminishes gut and
liver GVHD [51,52]. Other costimulatory molecules with potent implication
in GVHD include OX40 (CD134), CD40L (CD154), 4–1BB (CD137) and gluco-
corticoid-induced tumor necrosis factor receptor (GITR).
INHIBITORY PATHWAYS THAT DOWNREGULATE GVHD
In response to tissue injury and activated T cells, inhibitory pathways are
upregulated in an attempt to protect the host against injury. CTLA-4 and pro-
grammed death-1 (PD-1; CD279) are upregulated on donor T cells during
acute GVHD and serve to dampen the immune response. Both also are pri-
marily expressed in the cytoplasm of activated T cells and CD4+CD25+Treg cells
(reviewed in references [6,15]). In rodents, selective blockade of CTLA-4:B7
7
interactions accelerated acute GVHD lethality. Thus, an ideal reagent for inhib-
iting GVHD would be one that selectively blocks CD28/B7 blockade or absence
of PD-1 on donor cells accelerates GVHD and is associated with increased IFN-
γ production [53]. Conversely the future development of molecules that signal
via PD-1 or its downstream pathways may prove useful in inhibiting GVHD. In
addition to surface molecules, the intracellular tryptophan catabolic pathway,
indoleamine 2,3-dioxygenase, induced by IFN-γ in GVHD target organs espe-
cially in the GI tract, diminishes T effector cell destruction via local mecha-
nisms that result in both an increased donor T-cell apoptosis and decreased
proliferation [54–56]. Activation of indoleamine 2,3-dioxygenase or provision
of tryptophan catabolites has been shown in rodent models to reduce GVHD.
Translating data on costimulatory molecules for GVHD prevention into the
clinic turns out to be much more difficult. Data on a limited number of patients
suggest that costimulatory blockade accomplished by adding CTLA4-Ig to an
in vitro mixed lymphocyte reaction culture resulted in donor anti-host hypo-
responsive T cells that supported relatively rapid T-cell immune recovery and
a seemingly low propensity to cause acute GVHD when added to a haploi-
dentical stem cell graft [57,58]. More broadly directed in vitro methodologies
have been recently devised to depleted alloreactive T cells and such meth-
odologies have been applied to studies in a limited number of patients [59].
The new CTLA4-Ig derivative of Abatacept, Belatacept, which preferentially
blocks CD28/B7 interactions, is highly efficient in the treatment of rheuma-
toid arthritis and psoriasis and in preventing acute solid organ graft rejection,
but has not been tested to date for acute GVHD prophylaxis [60].
Acute GVHD and T-cell subpopulations
CONVENTIONAL T CELLS
Using new methods such as green fluorescent protein marking or biolumi-
nescence technology, it has been reported that T cells can undergo a mas-
sive and much earlier than previously thought expansion in lymph nodes
Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation
and Peyer patches (reviewed in chapter 11 and in reference [61]). In mice,
naive CD44(lo)CD62L(hi) CD8+ T cells generate and sustain allogeneic CD8+
T cells in GVHD reactions [62,63]. Murine memory T cells isolated from non-
allosensitized donors fail to induce GVHD in experimental models [62]. In
contrast, alloantigen-sensitized effector memory CD44(hi)CD62L(lo) as well
as naïve phenotype CD44(lo)CD62L(hi), but not central memory CD44(hi)
CD62L(hi) CD8+ T cells, cause GVHD following adoptive transfer into sec-
ondary recipients [30]. Both alloantigen-sensitized effector memory CD4+
and CD8+ T cells are involved in the transfer of GVHD under these conditions.
In the clinic, quantification of the degree and location of early T-cell expan-
sion is not readily possible given the limitations of current technology that
can be applied to HSCT recipients. Nonetheless, clinical studies currently
evaluate transferring enriched memory T cells rather than naïve T cells
to the recipient at the time of HSCT. Such studies will provide important
proof-of-concept as to whether the removal of naïve T cells from the donor
graft is sufficient to reduce or prevent acute GVHD.
REGULATORY T CELLS
8
CD4+CD25+Foxp3+ regulatory T cells (Tregs) have potent suppressor activity
both in vitro and in vivo (reviewed in chapter 12). Donor Treg cell infusion
blocks acute GVHD. Murine L-selectin (CD62L) expressing Treg cells prefer-
entially home to secondary lymphoid organs, and in particular lymph nodes,
resulting in GVHD prevention [64]. Conversely, depletion of CD25+ T cells
from the donor graft or in the recipient immediately following allogeneic
HSCT promotes acute and chronic GVHD in various mouse models while
still maintaining a graft-versus-hematopoietic cell malignancy response in
most but not all studies [65–69]. Because of the relatively low frequency of
Tregs in lymphoid organs, ex vivo expansion of Tregs has often been used to
increase the number and to activate Tregs prior to in vivo adoptive trans-
fer. Immunosuppressive drugs given to prevent or control GVHD also affect
Treg cell expansion and function. Calcineurin inhibitors such as cyclosporin
decrease IL-2 production, leading to a reduction in Treg proliferation and
function. In contrast, rapamycin preferentially spares Tregs as opposed to
effector T cells and induces or functionally increases murine and human
Tregs in ex vivo culture systems, albeit at the expense of overall cell yield [70].
Some challenges have arisen in the manipulation of human Tregs dur-
ing allogeneic HSCT. A combination of CD4, CD25 and CD127 (IL-7R) has
permitted the isolation of a highly purified Treg population that included
both CD4+CD25+ and CD4+CD25− T-cell subsets both of which were as sup-
pressive as the classic CD4+CD25(hi) Treg cell subset [71,72]. However, it
is unknown whether the expansion of this Treg subpopulation will permit
retention of as high a level of suppressor function as the CD4+25+ popula-
tion [73]. Furthermore experimental data both in mice and in vitro human
studies has demonstrated the extraordinary potential of T helper cell sub-
sets (Th1, Th2 and Th17) and of Tregs to exhibit plasticity, shifting from one
phenotype to another one (reviewed in references [74,75]). This aspect of
“plasticity” may also be of concern when administering Tregs to patients
with inflammatory diseases. However, early phase I–II clinical trials have
demonstrated the feasibility of using Treg in the clinical setting [76,77]
Overview of the immune biology of allogeneic hematopoietic stem cell transplantation
without significant GVHD or toxicities. New techniques of Treg expansion
[78] now allow the production of sufficient functional Treg for clinical use.
NKT CELLS
A second inhibitory population shown to inhibit acute GVHD lethality is the
NKT subset that co-expresses NK and T-cell surface determinants [79]. In
rodents, total lymphoid irradiation combined with anti-thymocyte globulin has
been shown to induce host NKT cells that also promote the generation of Tregs
and the production and release of anti-inflammatory cytokines [80]. In HSCT
human recipients, studies indicate that the reduced acute GVHD lethality seen
despite the infusion of high numbers of T cells contained in a G-CSF mobilized
PB stem cell graft is associated with increased donor NKT cells [81,82].
TH17 CELLS
Th17 cells (reviewed in chapter 13 and in reference [83]) have recently
emerged as a new player in GVHD. Although the role of this new T-cell sub-
set has been dissected in certain experimental models including inflamma-
tory bowel disease, lung and skin GVHD, experimental GVHD studies have
led to seemingly discordant GVHD lethality results that may be ascribed to 9
distinct differences in experimental GVHD conditions [84–86]. As yet the
role of Th17 cells in humans is uncertain [87].
T-cell trafficking
How T cells are recruited into tissues could be pivotal for understanding
the stereotypical involvement of skin, liver and bowel in GVHD. While the
migration of T cells into secondary lymphoid organs during GVHD and other
inflammatory responses has been well characterized, the migration of leu-
kocytes into parenchymal organs is less well understood. This process may
involve changes in vascular permeability and, in certain systems, has been
shown to require specific selectin–ligand, chemokine–receptor and integ-
rin–ligand interactions (reviewed in chapter 16 and in reference [13]). Dur-
ing a GVHD reaction, donor T cells initially migrate to spleen and peripheral
lymphoid tissues within hours [88]. Naïve donor T cells traffic to lymphoid
tissues, where the subset of alloreactive T cells receive activation signals by
APCs, and then subsequently migrate to specific GVHD target organ sites,
essential for the induction and pathogenesis of acute GVHD [89]. Almost all
tissues express transplantation antigens; however, acute GVHD pathology is
primarily limited to only a few locations rich in epithelial cells and express-
ing high levels of MHC antigens – gut, skin, liver, lung, secondary lymphoid
organs and thymus. The ability of alloreactive donor T cells to home to spe-
cific organs is regulated by a unique combination of signals that bind to cor-
responding receptors on host tissues and counter-receptors expressed on
donor T cells, including members of the chemokine family. MIP1a and other
chemokines (such as CCL2–CCL5, CXCL2, CXCL9, CXCL10, CXCL11, CCL17
and CCL27) are over-expressed during GVHD generation and can enhance
the homing of cellular effectors to GVHD target organs.
These results suggest that strategies that influence T-cell migration, particu-
larly to GVHD target organs, may offer promise for reducing GVHD target
organ specific injury, although the redundancy of chemokines and their
Immune Biology of Allogeneic Hematopoietic Stem Cell Transplantation
receptors may hinder clinical efficacy in the context of GVHD prevention
or therapy. As such, targeting lymphocyte/integrin interaction may be a
more promising way to explore this issue. Indeed the research of targeting
lymphocyte trafficking has been taken into the clinic in diseases related to
GVHD, such as rheumatoid arthritis and colitis.
Effector stage; T cells and others
After migration of alloreactive effector T cells to the target tissues of GVHD,
these cells can mediate tissue destruction through both direct cytotoxic
activity and the recruitment of other leukocytes. Targeting these effector
pathways has been studied as a strategy to prevent or reduce GVHD sever-
ity. Researchers have considered acute GVHD to be a Th1/T cytotoxic-type
(IL-12, IL-2 and IFN-γ) disease on the basis of the predominance of cyto-
toxic T-cell-mediated pathology and of increased production of Th1-type
cytokines. However, several recent studies have suggested that the influ-
ence of Th1 and Th2 cytokines in acute and chronic GVHD is not so simply
explained (reviewed in chapter 11).
The concentration and timing of cytokine release into the circulation and
10 relevant target organs appear to be critical for GVHD (reviewed in chapter
16). For example, IL-10 promotes Th2 and type 1 regulatory T-cell responses,
which can be important in the induction of tolerance to allografts (reviewed
in reference [6]). Higher production of IL-10, as demonstrated in human
recipients with an IL-10 polymorphism, is associated with reduced occur-
rence and severity of GVHD [90]. Paradoxically, high dose of IL-10 admin-
istration can accelerate GVHD in a murine model, and high-serum IL-10
levels in patients after HSCT are associated with a fatal outcome. However,
conversely, low-dose IL-10 administration can inhibit acute GVHD in mice
(reviewed in reference [6]). These findings highlight the pleiotropic, some-
times opposing, nature of cytokines during the different phases of GVHD
pathogenesis and on various effector and regulatory cell populations.
T cells mediate the final effector pathway in GVHD by multiple pathways
[30,91,92]. The expression of both Fas and FasL is increased on CD8+ and
CD4+ donor T cells during acute GVHD in patients and mice, and serum
levels of soluble FasL and Fas were found to correlate with GVHD severity
or the response to GVHD therapy. Several studies in experimental mouse
models have analyzed the role of the Fas–FasL and perforin–granzymes
pathways in the development of GVHD by using mice that are deficient for
FasL (gld mice), perforin or granzyme B as donors, or by the in vivo admin-
istration of neutralizing anti-FasL antibodies. Although these differences in
experimental design affect the opportunity to draw a uniform conclusion,
most studies have shown a role for the Fas–FasL pathway in GVHD mor-
tality. With respect to the perforin–granzyme pathway, approximately two-
thirds of studies demonstrated the importance of this pathway in GVHD
pathogenesis (reviewed in reference [5]).
In studies of transplant patients, polymorphisms in the TNF-α gene of
HSCT recipients are associated with higher levels of production of the cyto-
kine and are correlated with a higher incidence of severe acute GVHD [93]
(and reviewed in reference [48]), which suggests that, in humans, induc-
tion of TNF-α from recipient cells may make an important contribution