C H A P T E R 108
`GRAFT-VERSUS-HOST DISEASE AND
`GRAFT-VERSUS-LEUKEMIA RESPONSES
`Pavan Reddy and James L.M. Ferrara
`
`The ability of allogeneic hematopoietic cell transplantation (HCT)
`to cure certain hematologic malignancies is widely recognized. An
`important therapeutic aspect of HCT in eradicating malignant cells
`is the graft-versus-leukemia (GVL) effect. The importance of the
`GVL effect in allogeneic HCT has been recognized since the earliest
`experiments in stem cell transplantation. Forty years ago, Barnes and
`colleagues noted that leukemic mice treated with a subtherapeutic
`dose of radiation and a syngeneic (identical twin) graft transplant
`were more likely to relapse than mice given an allogeneic stem cell
`transplant.1,2 They hypothesized that the allogeneic graft contained
`cells with immune reactivity necessary for eradicating residual leuke-
`mia cells. They also noted that recipients of allogeneic grafts, though
`less likely to relapse, died of a “wasting syndrome” now recognized
`as graft-versus-host disease (GVHD). Thus in addition to describing
`GVL, these experiments highlighted for the first time the intricate
`relationship between GVL and GVHD. Since these early experi-
`ments, both GVHD and the GVL effect have been studied exten-
`sively.3 This chapter reviews the pathophysiology, clinical features,
`and treatment of GVHD and summarizes current understanding of
`the relationships between GVHD and the GVL effect.
`
`GRAFT-VERSUS-HOST DISEASE: CLINICAL AND
`PATHOLOGIC ASPECTS
`
`Ten years after the work of Barnes and Loutit, Billingham formulated
`the requirements for the development of GVHD: the graft must
`contain immunologically competent cells, the recipient must express
`tissue antigens that are not present in the transplant donor, and the
`recipient must be incapable of mounting an effective response to
`destroy the transplanted cells.4 According to these criteria, GVHD
`can develop in various clinical settings when tissues containing
`immunocompetent cells (blood products, bone marrow, and some
`solid organs) are transferred between persons. The most common
`setting for the development of GVHD is following allogeneic HCT;
`without prophylactic immunosuppression, most allogeneic HCTs will
`be complicated by GVHD. GVHD is induced by mismatches between
`histocompatibility antigens between the donor and recipient. Match-
`ing of the major histocompatibility complex (MHC) antigens hastens
`engraftment and reduces the severity of GVHD.5 In humans, the
`MHC region lies on the short arm of chromosome 6 and is called the
`HLA (human leukocyte antigen) region.6 The HLA region is divided
`into two classes, class I and class II, each containing numerous gene
`loci that encode a large number of polymorphic alleles. MHC class I
`molecules are involved in the presentation of peptides to CD8+ T cells,
`and class II molecules present peptides to CD4+ T cells.6,7 The deter-
`mination of HLA types has become much more accurate with
`molecular techniques that replace earlier serologic or cellular methods.
`In patients whose ancestry involves extensive interracial mixing, the
`chances of identifying an HLA identical donor are diminished.8
`Despite HLA identity between a patient and donor, substantial
`numbers of patients still develop GVHD because of differences in
`minor histocompatibility antigens (MiHAs) that lie outside the HLA
`loci. Most minor antigens are expressed on the cell surface as degraded
`peptides bound to specific HLA molecules, but the precise elucida-
`tion of many human minor antigens is yet to be accomplished.9 In
`
`1650
`
`the United States, the average patient has a 25% chance of having an
`HLA match within his or her immediate family.8 Patients who lack
`an HLA-identical family member donor must seek unrelated donor
`volunteers or cord blood donations.
`
`Acute Graft-Versus-Host Disease
`
`Acute GVHD can occur within days (in recipients who are not
`HLA-matched with the donor or in patients not given any prophy-
`laxis) or as late as 6 months after transplantation. The incidence ranges
`from less than 10% to more than 80%, depending on the degree of
`histoincompatibility between donor and recipient, the number of T
`cells in the graft, the patient’s age, and the GVHD prophylactic
`regimen.10 The principal target organs include the immune system,
`skin, liver, and intestine. GVHD occurs first and most commonly in
`the skin as a pruritic maculopapular rash, often involving the palms,
`soles, and ears; it can progress to total-body erythroderma, with bullae
`formation, rupture along the epidermal-dermal border, and desqua-
`mation in severe cases.10 Gastrointestinal (GI) and liver manifestations
`often appear later and rarely represent the first and only findings.
`Intestinal symptoms include anorexia, nausea, diarrhea (sometimes
`bloody), abdominal pain, and paralytic ileus.10 Liver dysfunction
`includes hyperbilirubinemia and increased serum alkaline phosphatase
`and aminotransferase values. Coagulation studies may become abnor-
`mal, and hepatic failure with ascites and encephalopathy may develop
`in severe cases.10–12 Hepatic GVHD can be distinguished from hepatic
`venoocclusive disease by weight gain or pain in the right upper
`quadrant in the latter.12 Acute GVHD also results in the delayed
`recovery of immunocompetence.10 The clinical result is profound
`immunodeficiency and susceptibility to infections, often further
`accentuated by the immunosuppressive agents used to treat GVHD.10
`Pathologically, the sine qua non of acute GVHD is selective epi-
`thelial damage of target organs.13,14 The epidermis and hair follicles
`are damaged and sometimes destroyed. Small bile ducts are pro-
`foundly affected, with segmental disruption. The destruction of
`intestinal crypts results in mucosal ulcerations that may be either
`patchy or diffuse. Other epithelial surfaces, such as the conjunctivae,
`vagina, and esophagus, are less commonly involved. A peculiarity of
`GVHD histology is the early paucity of mononuclear cell infiltrates;
`however, as the disease progresses, the inflammatory component may
`be substantial. Studies that identified inflammatory cytokines as
`soluble mediators of GVHD have suggested that direct contact
`between target cells and lymphocytes is not always required (see
`following sections). GVHD lesions are not evenly distributed: in the
`skin, damage is prominent at the tip of rete ridges; in the intestine,
`at the base of the crypts; and in the liver, in the periductular epithe-
`lium. These areas all contain a high proportion of stem cells, giving
`rise to the idea that GVHD targets may be undifferentiated epithelial
`cells with primitive surface antigens.15
`The histologic severity of GVHD is at best semiquantitative, and
`consequently pathologic scores are not used to grade GVHD. Because
`it is often difficult to obtain an adequate tissue biopsy, and because
`it can be very difficult to distinguish GVHD from other post-HCT
`complications such as drug eruptions or infectious complications, the
`physician is left to use clinical judgment.
`
`

`

`Chapter 108 Graft-Versus-Host Disease and Graft-Versus-Leukemia Responses
`
`1651
`
`An independent committee of a multicenter phase III trial that
`assessed the presence and severity of GVHD was unable to confirm
`a high incidence of GVHD16,17 Standard grading systems generally
`include clinical changes in the skin, GI tract, liver, and performance
`status (Table 108.1).18 Although the severity of GVHD is often dif-
`ficult to quantify, the overall maximal grade correlates with disease
`outcome: mild GVHD (grade I or II) is associated with little mortal-
`ity, whereas higher grades are associated with significantly decreased
`survival.18,19 Recent advances in the use of biomarkers at the onset of
`disease may soon be sufficiently accurate to guide therapy.19
`
`Clinical Features of Acute
`Graft-Versus-Host Disease
`
`The clinical features, staging, and grading of acute GVHD are sum-
`marized in Tables 108.1 and 108.2. In a comprehensive review of
`patients receiving therapy for acute GVHD, Martin and colleagues20
`found that 81% had skin involvement, 54% had GI involvement,
`and 50% had liver involvement at the initiation of therapy. After
`high-intensity (myeloablative) conditioning, acute GVHD generally
`occurs within 14–35 days of stem cell infusion. The time of onset
`may depend on the degree of histocompatibility, the number of
`donor T cells infused, and the prophylactic regimen for GVHD. A
`
`rapid and severe form of GVHD may occur in patients with severe
`HLA mismatches and in patients who receive T-cell replete trans-
`plants without or with inadequate in vivo GVHD prophylaxis.21
`Although such GVHD is sometimes called “hyperacute,” this term
`is misleading because it is pathophysiologically distinct from hyper-
`acute rejection after solid organ allografting, which is caused by
`preformed antibodies. This form of GVHD, which is manifested by
`fever, generalized erythroderma and desquamation, and often edema,
`typically occurs about 1 week after stem cell infusion and may be
`rapidly fatal. In patients receiving standard (in vivo) GVHD prophy-
`laxis such as a combination of cyclosporine and methotrexate, the
`median onset of GVHD is typically 21–25 days after transplantation;
`however, after in vitro T-cell depletion of the graft, the onset of
`GVHD symptoms may be much later.21 Thus the findings of rash
`and diarrhea by 1 week after transplantation would very likely be
`because of ineffective prophylaxis and would be very unlikely with
`the use of calcineurin inhibitors or in vitro T-cell depletion of the
`stem cell inoculum. A less ominous syndrome of fever, rash, and fluid
`retention occurring in the first 1–2 weeks after stem cell infusion is
`the “engraftment syndrome.” These manifestations may be seen with
`either allogeneic or autologous transplantation. Although this syn-
`drome’s pathophysiology is poorly understood, it is thought to be
`caused by a wave of cytokine production as the graft starts to recover.
`These symptoms are related to, but distinct from, the “cytokine
`storm”22 of acute GVHD because there is no concomitant T-cell–
`mediated attack. This syndrome responds immediately to steroids in
`most patients, and it typically presents earlier than acute GVHD.15
`Skin is the most commonly affected organ (Fig. 108.1). In patients
`receiving transplants after myeloablative conditioning, the skin is
`usually the first organ involved, and GVHD often coincides with
`engraftment. However, the presentation of GVHD is more varied
`following nonmyeloablative transplants or donor lymphocyte infu-
`sions.23 The characteristic maculopapular rash can spread throughout
`
`Clinical Manifestations and Staging of Acute
`Graft-Versus-Host Disease
`Clinical Manifestations
`
`Staging
`Stage 1: <25% rash
`Stage 2: 25%–50% rash
`Stage 3: generalized
`erythroderma
`Stage 4: bullae
`
`TABLE
`108.1
`Organ
`
`Skin
`
`Liver
`
`Gastrointestinal
`tract
`
`Erythematous,
`maculopapular rash
`involving palms and
`soles; may become
`confluent
`Severe disease: bullae
`Painless jaundice with
`conjugated
`hyperbilirubinemia
`and increased
`alkaline phosphatase
`Upper: nausea,
`vomiting, anorexia
`Lower: diarrhea,
`abdominal cramps,
`distension, ileus,
`bleeding
`
`Stage 1: bili 2–3 mg/dL
`Stage 2: bili 3.1–6 mg/dL
`Stage 3: bili 6.1–15 mg/dL
`Stage 4: bili >15 mg/dL
`
`Stage 1: diarrhea >500 mL/
`day
`Stage 2: diarrhea >1000 mL/
`day
`Stage 3: diarrhea >1500 mL/
`day
`Stage 4: ileus, bleeding
`
`TABLE
`108.2
`Overall Grade
`
`Glucksberg Criteria for Staging of Acute
`Graft-Versus-Host Diseasea
`Skin
`Liver
`
`Gut
`
`0
`0
`1–2
`I
`1
`and/or
`1
`1–3
`II
`2–3
`and/or
`2–4
`2–3
`III
`2–4
`and/or
`2–4
`2–4
`IV
`aSee Table 108.1 for individual organ staging. Traditionally, individual organs
`are staged without regard to attribution. The overall grade of graft-versus-host
`disease, however, reflects the actual extent of graft-versus-host disease. To
`achieve each overall grade, skin disease, liver and/or gut involvement are
`required.
`
`A
`
`B
`
`C
`
`Fig. 108.1 GRAFT-VERSUS-HOST DISEASE, SKIN BIOPSY. This 40-year-old man with a history of
`relapsed Hodgkin lymphoma was status-postallogeneic stem cell transplant with donor lymphocyte infusion.
`He developed painful oral ulcers and a macular-papular rash on the arms, hand, and chest. The skin biopsy
`is from the palmar surface of the hand (A). It shows a scant lymphoid infiltrate in the dermis with a developing
`subepithelial blister (right). There is basal vacuolar change with single lymphocytes in the epithelium, as well
`as apoptotic keratinocytes accompanied by lymphocytes (B, and detail, C). (Courtesy Vesna Petronic-Rosic and
`Mark Racz, University of Chicago.)
`
`

`

`1652
`
`Part X Transplantation
`
`the rest of the body but usually spares the scalp; it is often described
`as feeling like a sunburn, tight or pruritic. In severe cases the skin
`may blister and ulcerate.24 Histologic confirmation is critical to rule
`out drug reactions, viral infections, etc. Apoptosis at the base of
`dermal crypts is characteristic. Other features include dyskeratosis,
`exocytosis of lymphocytes, satellite lymphocytes adjacent to dyskera-
`totic epidermal keratinocytes, and dermal perivascular lymphocytic
`infiltration.25
`GI tract involvement of GVHD may present as nausea, vomiting,
`anorexia, diarrhea, and/or abdominal pain.26 It is a panintestinal
`process, often with differences in severity between the upper and lower
`GI tracts. Gastric involvement gives rise to postprandial vomiting that
`is not always preceded by nausea. Although gastroparesis is seen after
`bone marrow transplant, it is usually not associated with GVHD. The
`diarrhea of GVHD is secretory; significant GI blood loss may occur
`as a result of mucosal ulceration and is associated with a poor prog-
`nosis.27 In advanced disease, diffuse, severe abdominal pain, and dis-
`tension is accompanied by voluminous diarrhea (>2 liters/day).19,28
`Radiologic findings of the GI tract include luminal dilatation with
`thickening of the wall of the small bowel and air/fluid levels sugges-
`tive of an ileus on abdominal flat plates or small bowel series.
`Abdominal computed tomography may show the “ribbon” sign of
`diffuse thickening of the small bowel wall.24 Little correlation exists
`between the extent of disease and the appearance of mucosa on
`endoscopy, but mucosal sloughing is pathognomonic for severe
`disease.29 Nevertheless, some studies have shown that antral biopsies
`correlate well with the severity of GVHD in the duodenum and in
`the colon even when the presenting symptom is diarrhea.29 Histologic
`analysis of tissue is imperative to establish the diagnosis. The histo-
`logic features of GI GVHD are the presence of apoptotic bodies in
`the base of crypts, crypt abscesses, crypt loss, loss of Paneth cells, and
`flattening of the surface epithelium.28,30,31
`Liver function test abnormalities are common after bone marrow
`transplant and occur secondary to venoocclusive disease, drug toxic-
`ity, viral infection, sepsis, iron overload, and other causes of extrahe-
`patic biliary obstruction.12 The exact incidence of hepatic GVHD is
`unknown because many patients do not undergo liver biopsies. The
`development of jaundice or an increase in the alkaline phosphatase
`and bilirubin may be the initial features of acute GVHD of the liver.
`The histologic features of hepatic GVHD are endothelialitis, lym-
`phocytic infiltration of the portal areas, pericholangitis, and bile duct
`destruction and loss.19,32
`
`Other Organs
`Whether GVHD affects organs other than the classic triad of skin,
`liver, and gut has remained a matter of debate, although numerous
`reports suggest additional organ manifestations. The most likely
`candidate is the lung. Lung toxicity, including interstitial pneumonitis
`and diffuse alveolar hemorrhage, may occur in 20% to 60% of
`allogeneic transplant recipients but in fewer autologous transplant
`recipients. Causes of pulmonary damage other than GVHD include
`engraftment syndrome (see earlier), infection, radiation pneumonitis,
`and chemotherapy-related toxicity (e.g., methotrexate, busulfan).21,33
`One retrospective analysis failed to link severe pulmonary complica-
`tions to clinical acute GVHD per se.34 The mortality caused by
`pneumonia increases with the severity of GVHD, but this association
`may be related to increased immunosuppressive therapy.21 A histo-
`pathologic signature of lymphocytic bronchitis has been associated
`with GVHD,33 although not always.
`Despite the ability of kidneys and hearts to serve as targets of
`transplant rejection, there is no convincing evidence for direct renal
`or cardiac damage from acute GVHD that is not secondary to drugs
`or infection. Similarly, neurologic complications are also common
`after transplantation but most can be attributed to drug toxicity,
`infection, or vascular insults.
`
`Differential Diagnosis
`Acute GVHD ought to be distinguished from any process that causes
`a constellation of fever, erythematous skin rash, and pulmonary
`edema that may occur during neutrophil recovery and has been
`
`termed engraftment or capillary leak syndrome.35,36 In allogeneic
`transplant recipients distinction from acute GVHD is difficult.
`Engraftment syndrome is thought to reflect cellular and cytokine
`activities during early recovery of (donor-derived) blood cell counts
`and/or homeostatic proliferation of lymphocytes, but a precise
`delineation of the activated cells and mechanisms has not been
`demonstrated. Engraftment syndrome may be associated with
`increased mortality, primarily but not exclusively from pulmonary
`failure. Corticosteroid therapy may be effective particularly for the
`treatment of pulmonary manifestations.37 Skin rashes may reflect
`delayed reactions to the conditioning regimen, antibiotics, or infec-
`tions; furthermore, histopathologic skin changes consistent with
`acute GVHD can be mimicked by chemoradiotherapy and drug
`reactions.21,38 Diarrhea can be a consequence of total-body irradiation
`(TBI), viral infection (especially with cytomegalovirus and other
`herpes viruses), parasitic infection, Clostridium difficile infection,
`nonspecific gastritis, narcotic withdrawal, and drug reactions: all of
`which mimic GVHD of the gut. Liver dysfunction can be caused by
`parenteral nutrition, venoocclusive disease, and viral or drug-induced
`hepatitis.
`
`Genetic Basis of Graft-Versus-Host Disease
`
`The graft-versus-host (GVH) reaction was first noted when irradiated
`mice were infused with allogeneic marrow and spleen cells.39 Although
`mice recovered from radiation-induced injury and marrow aplasia,
`they subsequently died with “secondary disease,”39 a phenomenon
`subsequently recognized as acute GVHD. Three requirements for the
`development of GVHD were formulated by Billingham.4 First, the
`graft must contain immunologically competent cells, now recognized
`as mature T cells. In both experimental and clinical allogeneic HCT,
`the severity of GVHD correlates with the number of donor T cells
`transfused.40,41 The precise nature of these cells and the mechanisms
`they use are now understood in greater detail (see later). Second, the
`recipient must be incapable of rejecting the transplanted cells (i.e.,
`immunocompromised). After allogeneic HCT, the recipient is typi-
`cally immunosuppressed by chemotherapy and/or radiotherapy
`before the hematopoietic cell infusion.42 Third, the recipient must
`express tissue antigens that are not present in the transplant donor.
`Thus Billingham’s third postulate stipulates that the GVH reaction
`occurs when donor immune cells recognize disparate host antigens.4
`These differences are governed by the genetic polymorphisms.42
`
`HLA Matching
`Recognition of alloantigens depends on the match with the presenting
`major histocompatibility molecule.43–45 In humans, the MHC is
`governed by the HLA antigens that are encoded by the MHC gene
`complex on the short arm of chromosome 6 and can be categorized
`as class I, II, and III. Class I antigens (HLA-A, HLA-B, and HLA-C)
`are expressed on almost all cells of the body.46 Class II antigens (DR,
`DQ, and DP) are primarily expressed on hematopoietic cells, although
`their expression can also be induced on other cell types following
`inflammation.46 The incidence of acute GVHD is directly related to
`the degree of MHC mismatch.42 The role of HLA mismatching of
`cord blood (CB) donors is more difficult to analyze compared with
`unrelated donor HCT, because allele typing of CB units for HLA-A,
`HLA-B, HLA-C, DRB1, and DQB1 is not routinely performed.47
`Nonetheless, the total number of HLA disparities between the recipi-
`ent and the CB unit has been shown to correlate with risk for acute
`GVHD as the frequency of severe acute GVHD is lower in patients
`transplanted with HLA-matched (6/6) CB units.47–49
`
`Minor Histocompatibility Antigens
`In most clinical allogeneic transplants where MHC of donor and
`recipient are matched, donor T cells recognize MHC-bound pep-
`tides derived from the protein products of polymorphic genes
`(MiHAs) that are present in the host but not in the donor.9,50–55
`Substantial numbers (50%) of patients will develop acute GVHD
`despite
`receiving HLA-identical grafts as well as optimal
`
`

`

`Chapter 108 Graft-Versus-Host Disease and Graft-Versus-Leukemia Responses
`
`1653
`
`postgrafting immune suppression.9,42,56 MiHAs are widely but vari-
`ably expressed in different tissue,51,56 which is one possible explana-
`tion for the unique target organ distribution in GVHD. Many
`MiHAs such as HA-1 and HA-2 are expressed on hematopoietic
`cells, which may be one reason for the host immune system to be a
`primary target for the GVH response, and helps explain the critical
`role of direct presentation by professional recipient antigen-
`presenting cells (APCs) in the GVH response.57 By contrast, other
`MiHAs such as H-Y and HA-3 are expressed ubiquitously.56 MiHAs
`do not all equally induce lethal GVHD but show hierarchic immu-
`nodominance.58,59 Furthermore, the difference in a single immuno-
`dominant MiHA is insufficient to elicit GVHD in murine models,
`even though a single MiHA can elicit T-cell–mediated damage in a
`skin explant model.60,61 However, the role of specific MiHAs that
`are able to induce clinical GVHD has not been systematically evalu-
`ated in large groups of patients.62
`
`Other Non-HLA Genes
`Genetic polymorphisms in several non-HLA genes such as in killer-cell
`immunoglobulin-like receptors (KIRs), cytokines, and nucleotide-
`binding oligomerization domain containing 2 (NOD2) genes have
`recently been shown to modulate the severity and incidence of GVHD.
`KIRs on natural killer (NK) cells that bind to the HLA class I
`gene products are encoded on chromosome 19. Polymorphisms in
`the transmembrane and cytoplasmic domains of KIRs govern whether
`the receptor has inhibitory (such as KIR2DL1, 2DL2, 2DL3, and
`3DL1) or activating potential. Two competing models have been
`proposed for HLA-KIR allorecognition by donor NK cells following
`allogeneic HCT: the “mismatched ligand” and the “missing ligand”
`models.5,63–66 Both models are supported by several clinical observa-
`tions, albeit in patients receiving very different transplant and
`immunosuppressive regimens (see Chapters 20 and 102).64,67–69
`Proinflammatory cytokines involved in the classic cytokine storm
`of GVHD cause pathologic damage to target organs, such as the skin,
`liver, and GI tract (see later).22 Several cytokine gene polymorphisms
`in both recipients and donors have been implicated. Specifically,
`tumor necrosis factor (TNF) polymorphisms (TNFd3/d3 in the
`recipient, TNF863 and TNF857 in donors and/or recipients and
`TNFd4, TNF-α-1031C, and tumor necrosis factor receptor (TNFR)
`II-196R in the donors) have been associated with an increased risk
`for acute GVHD and transplant-related mortality (TRM).70,71 The
`three common haplotypes of the interleukin (IL)-10 gene promoter
`region in recipients, representing high, intermediate, and low produc-
`tion of IL-10, have been associated with severity of acute GVHD
`following HLA-matched sibling donor allogeneic HCT.72 By contrast,
`smaller studies have found neither IL-10 nor TNF-α polymorphisms
`to be associated with GVHD following HLA-mismatched cord blood
`transplantation.71,73 Interferon-gamma (IFN-γ) polymorphisms of
`the 2/2 genotype (high IFN-γ production) and 3/3 genotype (low
`IFN-γ production) have been associated with decreased or increased
`acute GVHD, respectively.71,74
`NOD2/caspase-activating recruitment domain 15 (CARD15)
`gene polymorphisms in both the donors and recipients were recently
`shown to have a striking association between GI GVHD and overall
`mortality following related and unrelated donor allogeneic HCT.75
`Several of the associations with non-HLA polymorphisms will need
`to be confirmed in larger and more diverse populations. Furthermore,
`it is likely that the importance of non-HLA gene polymorphisms in
`GVHD will differ depending on the donor source (related versus
`unrelated), HLA disparity (matched versus mismatched), graft source
`(CB versus bone marrow [BM] versus peripheral blood stem cells),
`and the intensity of the conditioning.
`
`PATHOPHYSIOLOGY OF ACUTE
`GRAFT-VERSUS-HOST DISEASE
`
`It is helpful to remember two important principles when considering
`the pathophysiology of acute GVHD. First, acute GVHD represents
`exaggerated but normal inflammatory responses against foreign
`
`antigens (alloantigens) that are ubiquitously expressed in a setting
`where they are undesirable. The donor lymphocytes that have been
`infused into the recipient function appropriately, given the foreign
`environment they encounter. Second, donor lymphocytes encounter
`tissues in the recipient that have been often profoundly damaged.
`The effects of the underlying disease, prior infections, and the
`intensity of conditioning regimen all result in substantial changes not
`only in the immune cells but also in the endothelial and epithelial
`cells. Thus the allogeneic donor cells rapidly encounter not simply a
`foreign environment, but one that has been altered to promote the
`activation and proliferation of inflammatory cells. Therefore the
`pathophysiology of acute GVHD may be considered a distortion of
`the normal inflammatory cellular responses that, in addition to the
`absolute requirement of donor T cells, involves multiple other innate
`and adaptive cells and mediators.76 The development and evolution
`of acute GVHD can be conceptualized in three sequential phases
`(Fig. 108.2) to provide a unified perspective on the complex cellular
`interactions and inflammatory cascades that lead to acute GVHD:
`(1) activation of the APCs; (2) donor T-cell activation, differentia-
`tion, and migration; and (3) effector phase.76 It is important to note
`that this three-phase description permits a unified perspective on
`GVHD biology but it is not meant to suggest that all three phases
`are of equal importance or that GVHD occurs in a stepwise and
`sequential manner. The spatiotemporal relationships among these
`biologic processes, depending on the context, are likely to vary and
`their relevance to the induction, severity, and maintenance of GVHD
`may depend on the factors cited earlier.
`
`Phase 1: Activation of Antigen-Presenting Cells
`
`The earliest phase of acute GVHD is initiated by the profound
`damage caused by the underlying disease and infections and further
`exacerbated by bone marrow transplantation (BMT) conditioning
`regimens (which include TBI and chemotherapy) that are adminis-
`tered even before the infusion of donor cells.77–81 This first step results
`in activation of the APCs.7 Specifically, damaged host tissues respond
`with multiple changes, including the secretion of proinflammatory
`cytokines, such as TNF-α, IL-1 and IL-6 described as the cytokine
`storm.79,80,82,83
`Such changes increase expression of adhesion molecules, costimu-
`latory molecules, MHC antigens, and chemokine gradients that alert
`the residual host and the infused donor immune cells.80 These “danger
`signals” activate host APCs.84,85 Damage to the GI tract from the
`conditioning is particularly important in this process because it allows
`for systemic translocation of immunostimulatory microbial products
`such as lipopolysaccharide (LPS) that further enhance the activation
`of host APCs, and the secondary lymphoid tissue in the GI tract is
`likely the initial site of interaction between activated APCs and donor
`T cells.80,86,87 This scenario accords with the observation that an
`increased risk for GVHD is associated with intensive conditioning
`regimens that cause extensive injury to epithelial and endothelial
`surfaces with a subsequent release of inflammatory cytokines and
`increases in expression of cell surface adhesion molecules.80,81 The
`relationship among conditioning intensity, inflammatory cytokine,
`and GVHD severity has been supported by elegant murine studies.82
`Furthermore, the observations from these experimental studies have
`led to two recent clinical innovations to reduce clinical acute GVHD:
`(1) reduced intensity conditioning to decrease the damage to host
`tissues and thus limit activation of host APC and (2) KIR mismatches
`between donor and recipients to eliminate the host APCs by the
`alloreactive NK cells.65,88
`Host-type APCs that are present and have been primed by con-
`ditioning are critical for the induction of this phase; recent evidence
`suggests that donor-type APCs exacerbate GVHD, but in certain
`experimental models, donor-type APC chimeras also
`induce
`GVHD.85,89–91 In clinical situations, if donor-type APCs are present
`in sufficient quantity and have been appropriately primed, they too
`might play a role in the initiation and exacerbation of GVHD.92–94
`Among the cells with antigen-presenting capability, dendritic cells
`
`

`

`1654
`
`Part X Transplantation
`
`(II) Conditioning
`
`Tissue damage
`
`Host
`tissues
`
`TNF-α
`IL-1
`LPS
`
`Small
`intestine
`
`LPS
`
`Mφ
`
`TNF-α
`IL-1
`
`IFN-γ
`
`Th
`
`CD4
`CTL
`
`TNF-α
`IL-1
`
`Target cell
`apoptosis
`
`CD8
`CTL
`
`(III)
`Cellular and
`inflammatory
`effectors
`
`Host
`APC
`
`Donor
`T cell
`
`(II)
`Donor T-cell
`activation
`
`Fig. 108.2 PATHOPHYSIOLOGY OF GRAFT-VERSUS-HOST DISEASE. During step 1, irradiation and
`chemotherapy both damage and activate host tissues, including intestinal mucosa, liver, and the skin. Activated
`cell hosts then secrete inflammatory cytokines (e.g., TNF-α and IL-1), which can be measured in the systemic
`circulation. The cytokine release has important effects on APCs of the host, including increased expression of
`adhesion molecules (e.g., ICAM-1, VCAM-1) and of MHC class II antigens. These changes in the APCs
`enhance the recognition of host MHC and/or minor H antigens by mature donor T cells. During step 2,
`donor T-cell activation is characterized by proliferation of GVHD T cells and secretion of the Th1 cytokines
`IL-2 and IFN-γ. Both of these cytokines play central roles in clonal T-cell expansion, induction of CTL and
`NK cell responses, and the priming of mononuclear phagocytes. In step 3, mononuclear phagocytes primed
`by IFN-γ are triggered by a second signal such as endotoxin LPS to secrete cytopathic amounts of IL-I and
`TNF-α. LPS can leak through the intestinal mucosa damaged by the conditioning regimen to stimulate
`gut-associated lymphoid tissue or Kupffer cells in the liver; LPS that penetrates the epidermis may stimulate
`keratinocytes, dermal fibroblasts, and macrophages to produce similar cytokines in the skin. This mechanism
`results in the amplification of local tissue injury and further production of inflammatory effectors such as
`nitric oxide, which, together with CTL and NK effectors, leads to the observed target tissue destruction in
`the stem cell transplant host. CTL effectors use Fas/FasL, perforin/granzyme B, and membrane-bound cyto-
`kines to lyse target cells. APC, Antigen-presenting cell; CTL, cytotoxic T lymphocyte; GVHD, graft-versus-host
`disease; ICAM, intercellular adhesion molecule; IFN, interferon; IL, interleukin; LPS, lipopolysaccharide;
`MHC, major histocompatibility complex; NK, natural killer; TNF, tumor necrosis factor; VCAM, vascular cell
`adhesion molecule.
`
`(DCs) are the most potent and play an important role in the induc-
`tion of GVHD.95 Experimental data suggest that GVHD can be
`regulated by qualitatively or quantitatively modulating distinct DC
`subsets.96–101 Langerhans cells were also shown to be sufficient for the
`induction of GVHD when all other APCs were unable to prime
`donor T cells, although the role for Langerhans cells when all APCs
`are intact is dispensable.102,103 Studies have yet to define roles for other
`DC subsets. In one clinical study persistence of host DC after day
`100 correlat