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REVIEW ARTICLE
From the Northwestern University Medical Center,
Division of Immune Therapy and Autoimmune Disease, Chicago, IL;
Department of Bone Marrow Transplantation & Cancer Immunotherapy,
Hadassah University Hospital, Jerusalem, Israel; Bone Marrow
Transplantation, Medical College of Wisconsin, Milwaukee; Divisione di
Ematologia II, Centro Trapianti di Midollo Osseo, Azienda Ospedaliera
S. Martino, Genoa, Italy.
Hematopoietic stem cells (HSCs) are the earliest cells of the
immune system, giving rise to B and T lymphocytes, monocytes, tissue
macrophages, and dendritic cells. In animal models, adoptive transfer
of HSCs, depending on circumstances, may cause, prevent, or cure
autoimmune diseases. Clinical trials have reported early remission of
otherwise refractory autoimmune disorders after either autologous or
allogeneic hematopoietic stem cell transplantation (HSCT). By
percentage of transplantations performed, autoimmune diseases are the
most rapidly expanding indication for stem cell transplantation.
Although numerous editorials or commentaries have been previously
published, no prior review has focused on the immunology of
transplantation tolerance or development of phase 3 autoimmune HSCT
trials. Results from current trials suggest that mobilization of HSCs,
conditioning regimen, eligibility and exclusion criteria, toxicity,
outcome, source of stem cells, and posttransplantation follow-up need
to be disease specific. HSCT-induced remission of an autoimmune disease
allows for a prospective analysis of events involved in immune
tolerance not available in cross-sectional studies.
(Blood. 2002;99:768-784) Autoimmunity arises from the pathologic reaction of
B-cell-derived antibodies and/or T cells to self-epitopes. Proof of an autoimmune pathogenesis requires adoptive transfer of disease by either
immune cells or antibody.1,2 Transplacental or iatrogenic
transfer of autoreactive antibodies may cause disease. This condition
was first shown in Harrington's self experimentation using plasma from
a patient with idiopathic thrombocytopenic purpura (ITP).3
Mothers with ITP, myasthenia gravis, and/or systemic lupus
erythematosus (SLE) with SSA-Ro-SSB/La immunity may transfer antibodies
to their fetus, resulting in neonatal disease.4-7 Allogeneic stem cell transplantation from donors with autoimmune disease may also transfer the disease to
recipients.8-13
Clinical tolerance is failure of an organism to reject an antigen
or tissue without use of immune-suppressive medications but with intact
normal rejection of third-party or foreign antigens. The oldest theory
of tolerance, and now viewed as orthodoxy, is clonal selection of
lymphocyte repertoires.14 Self-reactive lymphocytes are
deleted and not allowed to mature. Clonal selection as an explanation
for tolerance was first proposed by Burnet15 in 1957 in
regards to antibody formation and self-recognition and
non-self-recognition. Subsequently, this concept was extended to
selection of T cells by deletion of autoreactive clones within the
thymus.16-21 T-cell precursors emigrate from the marrow to the thymus. In the thymus, if self-antigen of sufficient concentration and affinity for their specific T-cell receptor (TCR) repertoires is
present, the T cells undergo apoptosis (deletion) or anergy (functional
silencing).22-25 Because lymphocyte progenitors are continually generated from HSCs, clonal selection would have to be an
ongoing process occurring throughout life.
Thymic editing includes not only negative selection to delete
self-reactive clones but also positive selection to allow maturation of
self-reactive clones.17,26 If a particular TCR fails to engage a major histocompatibility complex (MHC) peptide/complex, or
binds it too tightly, it undergoes apoptosis. If it recognizes an
MHC/peptide complex with moderate avidity, it is positively selected
and undergoes further maturation. The avidity (concentration and
binding affinity) of an MHC/peptide complex appears to play a role in
positive versus negative selection of T lymphocytes.27,28 Intrathymic selection and anergy as a mechanism of maintaining tolerance of autoreactive repertoires was, therefore, amended by
theories concerning peripheral tolerance.29,30
Mechanisms of peripheral tolerance revolve, in part, around the
2-signal hypothesis of self-discrimination and non-self-discrimination introduced by Bretscher and Cohn31 in 1970. T cells,
positively selected within the thymus, remain anergic unless antigen is
presented with a second signal (ie, a costimulatory signal). Basically, antigen presentation to a T cell without costimulation maintains anergy, whereas TCR engagement of antigen combined with costimulation results in T-cell activation.32-35
The traditional costimulatory molecule for T-cell activation is CD28, a
ligand for B7-1 (CD80), and B7-2 (CD86) receptors on T
cells.36 CD28 binding increases transcription of
interleukin 2 (IL-2).35,37 A variety of other molecules,
including CD40L, inducible costimulator (ICOS), and various
adhesion molecules, also provide secondary or tertiary signals to
facilitate T-cell activation.38-43 Requirement of
costimulation for activation may place some constraints on peripheral
sites for cellular activation. Antigen-presenting cells (APCs)
that express costimulatory molecules are localized within secondary
lymphoid tissues (spleen and draining lymph nodes). Transfer of antigen
by immune cells to secondary lymphoid regions may be important to
induce T-cell activation.44 For example, allogeneic tissue
grafts are not rejected in mice that lack secondary lymphoid
tissue.45
Besides the requirement for costimulation, a variety of mechanisms
maintain peripheral tolerance. Some of these mechanisms are similar to
intrathymic tolerance but occur in the periphery, including peripheral
T-cell deletion and/or anergy induced by T-cell interaction with
parenchymal cells.46,47 Other checks to maintain
peripheral tolerance include activation-induced cell death,48 suppressor or regulatory cells,49-51
and peripheral antigen avidity (ie, antigen persistence, concentration,
and affinity).52,53 Theories on peripheral tolerance
explain how a T-cell repertoire selected intrathymically for reactivity
to self maintains peripheral tolerance. A further extension of
tolerance to what has been termed the "danger signal" explains the
context in which costimulation arises.54
The danger metaphor proposed by Matzinger54 involves the
use of the innate immune system (neutrophils, natural killer cells, and
macrophages) to break peripheral tolerance. T-cell-mediated immunity,
known as adaptive immunity, is an evolutionary development of
vertebrates.55 Adaptive immunity involves the
rearrangement of a limited number of germ line genes to produce a
highly diversified repertoire of approximately 1014 to
1018 somatically mutated T-cell (immunoglobulinlike)
receptors and B-cell immunoglobulin receptors. These T cells undergo
deletion and anergy within the thymus. However, the innate immune
system does not have pathogen-receptor repertoire
diversity.56 Response to infection is intrinsic to a
limited number of germ-line receptor genes that recognize
pathogen-specific molecular patterns. These patterns include receptors
for conserved pathogen structures like lipopolysaccharides, mannans,
bacterial DNA, and lipoteichoic acids. Receptor-mediated phagocytosis
of pathogens by macrophages leads to release of proinflammatory
cytokines and expression of costimulation molecules, along with MHC
presentation of pathogen-derived peptides, leading to T-cell
activation. Thus, pathogen stimulation of innate immunity can lead to
activation of the adaptive immune system.57-59
In animal models, active immunization with self-epitopes requires an
adjuvant (immune stimulant) to break tolerance. Adjuvant is often
nothing more than homogenized pathogens such as mycobacterium, which
provides the danger signal for activation of innate APCs such as
macrophages. Presentation of coinjected self-proteins by
adjuvant-activated APCs initiates antigen-specific autoreactive T
cells. Once activated to self by innate immunity, how is the adaptive
immune system prevented from causing autoimmune disease? This question
may be approached by viewing the immune system as dynamic and
constantly fluctuating.
In all prior theories of tolerance, lymphocytes are viewed as
responding or not responding, like a light switch that is on or off.
The perturbation theory postulated by Grossman and Singer60 and Grossman and Paul61,62 proposes that lymphocytes are
dynamically tuned much like a rheostat dims or brightens a room.
Lymphocytes selected intrathymically may maintain a steady tone by
repeated interaction with peripheral tissue. It is the sudden change in dynamic homeostasis that is perceived as a perturbation. By analogy, blood is always dynamically fluctuating between clotting and lysis. In
steady state, blood may be erroneously perceived as static. The immune
system may also be dynamically fluctuating between autoimmunity and
tolerance in a dynamic steady state not readily appreciated. A steady
state that may be controlled by clonal selection, activation, feedback
inhibition, and intracellular receptor and signal transduction tuning.
It is conceivable, but unproven, that immune ablation followed by
infusion of hematopoietic stem cells (HSCs) may "reset the immune rheostat."
All processes involving tolerance, even deletion, are ongoing
recurring events and may be broken. Both central and peripheral T-cell
tolerance may be broken by environmental exposure. Classic agents
capable of breaking tolerance include drugs and
infections.63-65
Drug-induced autoimmunity
Drug-induced disruption of central tolerance implies existence of a
functional thymus throughout adulthood. By using the membrane protein
CD45 to differentiate naive (CD45 RA) from memory (CD45RO) T cells,
thymic-dependent T-cell production appears to diminish markedly after
puberty, presumably because of thymic atrophy. If the thymus involutes,
new adult T cells would then be derived exclusively from peripheral
expansion of existing memory cells. However, with the advent of newer
DNA assays, the accuracy of differentiation between naive and memory T
cells by CD45 has been questioned.73-75
During TCR thymic development, rearrangement of TCR genes leads to
excision of circular DNA termed T-cell receptor rearrangement excision
circles (TRECs).73 TRECs are episomal, unique to T cells,
and do not duplicate during mitosis. Because TCR rearrangement occurs
during thymic development, TRECs may be used as a marker for recent
thymic emigrants. In the early post-hematopoietic stem cell
transplantation (HSCT) period, there is a substantial increase in
peripheral blood TREC-positive T cells.74 Although an
inverse correlation exists between age and TREC production after HSCT, TREC numbers increased in all age groups. Therefore, thymic-dependent generation of T cells occurs in all ages. A drug or
environmental-related disruption of thymic tolerance, which alters TCR
antigen avidity or TCR cytoplasmic or nuclear signaling events, may
allow escape of autoreactive lymphocytes. Once in the periphery,
long-lived autoreactive cells could cause a persistent autoimmune disease.
Infection-induced autoimmunity
An infection could precipitate an autoimmune disease by breaking self-tolerance through molecular mimicry,98,99 determinant or epitope spreading,100,101 or bystander activation.102 Molecular mimicry is the capacity of a lymphocyte activated to an infectious pathogen to cross-react with a similar host determinant. Because memory lymphocytes are long lived, the infectious agent that initiated molecular mimicry to self does not need to persist for autoimmunity to occur. This situation may be one reason for difficulty in proving an infectious etiology for autoimmune disorders. Bystander activation arises when activation of T cells specific for antigen X occurs during an immune response against a nonhomologous antigen Y. In contrast, molecular mimicry is targeted toward self-peptides homologous to the initiating determinant on a viral or other infectious agent. Immunization with adjuvant and peptide is an example of bystander activation to the coinjected nonhomologous peptide.103 Infection-related inflammation is associated with tissue destruction and presentation of self-epitopes, as well as up-regulation of APC costimulatory molecules that may also lead to bystander activation of T cells to self-determinants. Theiler murine encephalomyelitis virus (TMEV)-induced demyelination, an autoimmune demyelinating disease that mimics MS, is an example of viral-induced bystander activation.104 TMEV is a picornavirus (small RNA virus) that infects gray matter neurons but, through bystander activation of the immune system, leads to an autoimmune-demyelinating white matter disease.105 Superantigens may also cause bystander activation. Superantigens are
bacterial, mycoplasma, or viral proteins that activate polyclonal
groups of T cells.106-112 Polyclonal activation arises by
cross-linking the side of a MHC molecule to the V Once molecular mimicry, bystander activation, or superantigens initiate an autoimmune disease, the immune response spreads over time to epitopes that are distinct and non-cross-reactive to the inducing epitope, a phenomenon termed determinant or epitope spreading.113 Epitope spreading has been documented for both T- and B-cell immune responses. A hierarchical order of epitope spreading occurs according to immune dominance of the epitope. Determinant spreading may occur to different regions on the same protein (intramolecular epitope spread) or to a protein distinct from the protein containing the disease-initiating epitope (intermolecular epitope spreading). Temporal spreading of immune responses to other epitopes has been demonstrated in numerous animal autoimmune disorders, including experimental autoimmune encephalomyelitis (EAE),114 diabetes in nonobese diabetic (NOD) mice,115 and experimental autoimmune myasthenia gravis.116 Determinant spreading is suspected to be associated with several human autoimmune diseases, including MS,117 SLE,118 bullous skin diseases,119 myasthenia gravis,120 diabetes,121,122 and chronic rejection of organ allografts.123-125 The mechanism of epitope spreading may be related to costimulation, because in some models blocking CD28/B7 costimulation may prevent epitope spreading.100 Whatever the mechanism, epitope spreading makes it difficult to retrospectively determine the inducing epitope or antigen. Effectiveness of targeted immune interventions directed against one TCR or epitope may be limited by the phenomenon of epitope spreading.
MHC autoimmune-associated genes MHC antigens were initially referred to as tissue transplantation antigens. They were discovered, as the name implies (major histocompatability complex), to have a major role in rejection of transplanted organs. As later discovered by Zinkernagel and Doherty,126 the MHCs are peptide-presenting molecules resulting in MHC/peptide restriction for T-cell recognition.127 It is not, therefore, surprising that many autoimmune diseases are associated with particular MHC genotypes.Numerous suspected autoimmune disorders (such as MS, RA, spondyloarthropathies, diabetes, myasthenia gravis, Crohn disease, primary biliary cirrhosis, autoimmune hepatitis, SLE, vasculitis, pemphigus vulgaris, and Sjögren syndrome) are associated with MHC alleles.128 Because combined MHC/peptide presentation is essential for T-cell activation, a MHC association may be indirect evidence for an immune pathogenesis. RA-prone MHC alleles, their frequencies vary for different ethnic groups, share a similar amino acid epitope sequence (LLEQKRAA or LLEQRRAA) encoded by codons 67 to 74.129-131 The HLA sequence 67 to 74 is a HLA contact site for both peptide and TCR binding. This finding suggests HLA presentation of a common infectious or self-antigen to T cells is involved in the pathogenesis of RA. Spondyloarthropathies are linked with only some molecular subtypes of HLA-B27.132 Similar to RA, peptide-binding differences may explain differences in disease susceptibility. HLA-B27 may even present its own B27-derived peptides. In which case, the putative arthritogenic peptide may be a component of the HLA-B27 molecule. The autoimmune etiology for scleroderma is questionable because of poor response to immune suppressive medications. Similarly, scleroderma also has a relatively weak MHC association that may indicate only partial immune pathogenesis or weak linkage of scleroderma genes to MHC alleles or the absence of an autoimmune etiology.133 Although MHC genes correlate with autoimmune disease susceptibility, most patients with disease-associated MHC genes remain disease free throughout their lifespan. Environment and/or non-MHC genes must, therefore, contribute toward development of disease. Non-MHC autoimmune genes Multiple non-MHC genes that regulate cell proliferation (oncogenes), cell signaling (tyrosinases), immune response (costimulatory molecules, interleukins, and cytokines), and apoptosis (fas) may play a role in development of autoimmunity.134 Analysis of the diabetic-prone NOD mouse has revealed at least 18 insulin-dependent diabetes prone genes.135 SLE occurs in various strains of mice, including Murthy Roth lymphoproliferative (MRL/lpr) mice and New Zealand Black X New Zealand White F1 hybrid (NZB/NZW) mice.136 Various mating crosses of lupus-prone mice, as well as backcrosses to normal mice, have linked murine lupus to 38 different genomic loci.137 Some loci are associated with glomerulonephritis, others with vasculitis, some with anti-ds DNA, some with antichromatin antibody, some with lymphoproliferation, and others with splenomegaly. No single gene is sufficient to cause disease. Various combinations of SLE-prone genes among different patients may explain why patients with SLE can have highly variable organ involvement and clinical symptoms. Collagen-induced arthritis in rats is a model for RA and is induced by injection of collagen and adjuvant.138,139 At least 14 genomic intervals or collagen-induced arthritis (CIA) loci are associated with collagen-induced arthritis.140,141Although autoimmunity involves MHC and numerous non-MHC genes, environmental interactions with these genes are essential to manifest disease. Approximately two thirds of syngeneic twins with MS, RA, SLE, or type I diabetes are discordant for clinical disease.142 Although a concordance rate of 33% is much higher than the general population, it remains significantly below a predetermined dominant Mendelian penetrance of 100% and suggests that environmental factors continue to have a significant role in polygenic autoimmune diseases.
Animal models and anecdotal case reports Animal autoimmune diseases that are induced by immunization with adjuvant or self-peptide and adjuvant may be ameliorated by syngeneic or pseudo-autologous HSCT.143-155Immunization with adjuvant and either myelin basic protein or proteolipid protein peptides induces a T-cell-mediated demyelinating disease, EAE, that, depending on the animal model, may be monophasic, relapsing-remitting with secondary progression, or progressive from onset. EAE in Swiss Jackson Laboratory/Jackson (SJL/J) mice is a relapsing, remitting, and secondarily progressive disease. Several investigators have demonstrated cure, decreased relapse rates, or decreased disease severity in EAE animals undergoing syngeneic HSCT.146,149-151 Because of the expense of long-term animal housing, most experiments in EAE are performed before disease onset to abort disease initiation or shortly after disease onset to ameliorate its course. It is unlikely that such experiments are applicable to patients with a long duration of MS with accumulated disease burden and tissue damage. Syngeneic HSCT performed in mice with chronic EAE, unlike the results in acute EAE, failed to demonstrate neurologic improvement.146 Histologic analysis revealed chronic scarring with glial proliferation that is unaffected by HSCT.146 To be effective as therapy for EAE, HSCT needs to be performed early in the disease course during its inflammatory stage and before accumulation of disease burden. A principle that may also be important for MS. Murine bone marrow transplantations are performed by killing and removing the femur from the donor and using a syringe to flush out the marrow cells. It is technically difficult and inhumane to perform a murine autologous transplantation because the surviving recipient's legs would have to be amputated. However, marrow could be harvested from a syngeneic donor in the same active stage of EAE as the recipient, referred to as a pseudoautologous transplant. HSCT of EAE using pseudoautologous donors suggests that infused lymphocytes contaminating the graft may contribute to relapse.147 This suggestion indicates that lymphocyte depletion of grafts may be important in decreasing posttransplantation relapse after autologous HSCT. Besides immunization with myelin peptides, demyelinating central nervous system disease may be induced with viruses such as TMEV.156 Autologous HSCT of TMEV-induced demyelinating disease causes a high mortality from viral superinfection of the central nervous system during the postconditioning pancytopenic period.156 Autoimmune disease mediated by an infectious agent can be rapidly fatal after autologous HSCT but only if the infectious agent is still present at the time of transplantation. Several other environmentally induced animal autoimmune diseases are improved or cured by syngeneic HSCT. These diseases include experimental autoimmune myasthenia gravis,153 adjuvant arthritis 154,155 and collagen-induced arthritis.145 Encouraging results of syngeneic and pseudoautologous HSCT in animal-induced autoimmunity supported the design of autologous and syngeneic HSCT trials in patients with severe autoimmune disorders. Anecdotal case reports of patients with a coincidental autoimmune disease and a malignancy provided further support and rationale for trial design.157-166 Refractory autoimmune diseases entered remission sometimes for several years. Because the indication for transplantation was a malignancy, and the outcome was reported retrospectively, in most cases a detailed pretransplantation evaluation by a rheumatologist or neurologist is missing. The autografts were usually not purged of lymphocytes, and the transplantations were not tailored as therapy for an autoimmune disease. Duration of response appeared shorter for RA compared with SLE. Too few patients have been reported for other autoimmune diseases, and long-term results of response to treatment in those that relapse, as well as duration of remission in those who had not relapsed, remain unknown. Mobilization of HSCs Collection of stem cells from patients with autoimmune diseases is based on methods already established for patients with nonautoimmune disorders. The complications and risks of the procedure appear greater in patients with autoimmune disease and are specific for the autoimmune disease and involved organ system.167 The most common peripheral blood stem cell (PBSC) mobilization regimens are single-agent granulocyte colony-stimulating factor (G-CSF) or cyclophosphamide and G-CSF.Flares of MS and RA have occurred while patients were taking G-CSF for mobilization.167,168 MS flares have resulted in serious and irreversible neurologic deterioration. G-CSF-related flares of RA are relatively mild, being manifest as a transient increase in the number of swollen or tender joints that resolves with or without an increase in corticosteroid dose.167 The only complications of G-CSF PBSC mobilization in patients with scleroderma are transient telangiectasia that spontaneously resolves.167 In other diseases, such as SLE, there exists virtually no data on PBSC with G-CSF as a single agent. The simultaneous administration of G-CSF and steroids has been used in a limited number of patients without disease exacerbation.169 To prevent G-CSF-related disease flare, combined cyclophosphamide and G-CSF (Cy/G-CSF) may be used for mobilization. However, combined Cy/G-CSF PBSC mobilization has been complicated by neutropenic-related infection and disease-specific fatal visceral organ toxicity.167 Infections with opportunistic organisms may be more common in patients who have been on high-dose corticosteroids for prolonged intervals, such as patients with refractory SLE. Scleroderma patients with cardiac and/or pulmonary involvement undergoing PBSC with 4.0 g/m2 cyclophosphamide have succumbed to cardiac arrest and/or pulmonary alveolar hemorrhage.167 No significant regimen-related organ damage has been reported at doses of 2.0 g/m2 or for doses of 4.0 g/m2 in nonscleroderma patients. This finding emphasizes the importance of adjusting the mobilization regimen based on disease and organ involvement for the minimum mobilization-related morbidity. Although cyclophosphamide-based mobilization is generally associated with more toxicity from infection or organ damage, autoimmune diseases are generally ameliorated by the immune suppressive effects of cyclophosphamide.167 The duration of improvement from cyclophosphamide-based PBSC mobilization is unknown because most patients proceed within a relatively short time interval from mobilization to HSCT. As an exception, in at least one autoimmune disease (Evans syndrome), cyclophosphamide-based PBSC resulted in rapid and fatal acceleration of disease activity.170 This acceleration was attributed to a rapid cyclophosphamide-induced suppression of otherwise compensatory and accelerated hematopoiesis in the presence of persistent peripheral destruction from residual immunoglobulins against red blood cells and platelets. There is no single optimal mobilization regimen for PBSC in patients with autoimmune disease. The PBSC method should be individualized for the disease and organ system involved. Newer mobilizing agents such as stem cell factor, thrombopoietin, chemokines, and/or high-dose corticosteroids and G-CSF need to be evaluated to collect progenitor stem cells with minimum mobilization-related morbidity. After collection of progenitor cells, most but not all centers perform ex vivo lymphocyte depletion.167 Because the existence or identity of suppressor cells remains vague, graft depletion techniques are nonspecific without attempts at conserving regulatory cells. Positive enrichment for CD34+ cells has been performed by using either CEPRATE (CellPro, Bothel, WA), Isolex (Nexel, Irvine, CA), or CliniMACS (Miltenyi, Bergish Gladbach, Germany) cell separation systems. Negative selection was performed with T-cell antibodies by e-rosette or Nexel Isolex CD4/CD8 selection. Insufficient clinical data are currently available to compare an unmanipulated versus a T-cell-depleted graft in terms of disease response or relapse. Aggressive lymphocyte depletion may adversely affect immune reconstitution against pathogens, increasing the risk of serious posttransplantation opportunistic infections such as cytomegalovirus, fungemia, Pneumocystis carinii pneumonia, or Epstein-Barr virus posttransplantation lymphoproliferative disease (PTLD). Conditioning regimens and the role of immunosuppressive versus myeloablative conditioning for reinduction of self-tolerance The first convincing evidence that intense immunosuppression may cure life-threatening autoimmune diseases was obtained in a patient with mixed cryoglobulinemia in end-stage renal failure with a cryocrit level of 60%.171 In the early 1970s, a patient with monoclonal immunoglobulin (Ig)M and polyclonal IgG was treated with a combination of cyclophosphamide and azathioprine. Treatment was complicated by lymphocytopenia and sepsis because of neutropenia, but the patient recovered with no stem cell support. After recovery, renal function normalized in parallel with elimination of the cryoglobulinemia, and the patient is alive and disease free for more than 25 years.171 This case represents the longest observation of a patient with reinduced self-tolerance after elimination of self-reactive lymphocytes and reestablishment of immunity from uncommitted stem cells.Brodsky et al172 extended this early observation by treating a variety of autoimmune diseases with high-dose cyclophosphamide (200 mg/kg) without HSC infusion.172 For some autoimmune diseases such as SLE, early results from high-dose cyclophosphamide without stem cell support are encouraging. Although the response rate is high, depending on disease, relapse is common. With the exception of some diseases such as SLE, a more intense and myeloablative regimen with stem cell support may be required for durable responses. Infusion of mobilized HSCs shortens the duration of neutropenia by 5 to 7 days, theoretically decreasing the risk of serious infections. Ex vivo expansion of HSCs before infusion may completely eliminate neutropenic-related infections. For these reasons, a trial that randomized between cyclophosphamide with or without stem cell support is not currently being planned, and the rest of this review will be devoted to immune suppression with HSC support. Ideally, the conditioning regimen should be able to eliminate immune cells without neutropenia. Such a regimen does not exist. The more immune ablative a regimen becomes, the more likely it is to be myeloablative and require stem cell support for reconstituting hematopoiesis. The conditioning regimens being used in autoimmune transplantations were empirically developed for use in malignancies. Autoimmune conditioning regimens include cyclophosphamide (Cy)173-177; cyclophosphamide and polyclonal antilymphocyte antibodies such as antithymocyte globulin (ATG) or humanized monoclonal rat antihuman CD52 (Campath-1H) antibodies (Cy/ATG or Cy/Campath-1H, respectively)178-188; carmustine, etoposide, cytarabine, and melphalan (BEAM) 189-192; cyclophosphamide and total body irradiation (Cy/TBI)193; cyclophosphamide, TBI, and antithymocyte globulin (Cy/TBI/ATG)194,195; busulfan and cyclophosphamide (Bu/Cy)196,197; busulfan, cyclophosphamide, and ATG (Bu/Cy/ATG)198; cyclophosphamide and thiotepa (Cy/TT)199,200; and fludarabine-based regimens. Cy or Cy/ATG is the most common conditioning regimen used for HSCT of SLE.181-184,188 Pulse cyclophosphamide (500-1000 mg/m2) is a standard treatment for SLE. It is, therefore, reasonable to escalate cyclophosphamide to transplantation doses as the conditioning regimen for SLE. To avoid cardiac injury, transplantation doses of cyclophosphamide are limited to 200 mg/kg usually divided into 50 mg/kg per day. Cyclophosphamide is often used to mobilize stem cells before HSCT at doses of 2.0 to 4.0 g/m2. If cyclophosphamide is used in both the mobilizing and conditioning regimen, either the conditioning regimen dose may be decreased or the time interval between mobilization and HSCT may be delayed by several weeks to minimize the risk of cardiac toxicity from total cyclophosphamide dose. When the conditioning dose of cyclophosphamide is decreased, some centers add another agent such as thiotepa.199,200 Most patients with SLE eligible for HSCT are corticosteroid dependent and markedly cushingoid. There is a marked discrepancy between ideal and actual weight in terms of calculating cyclophosphamide dose. For safety reasons, in cushingoid patients, the dose is generally based on ideal or an adjusted ideal rather than actual weight. Cy and Cy/ATG are conditioning regimens for scleroderma176,187,188 and RA.173-175,180 High-dose cyclophosphamide may be associated with high cardiopulmonary mortality in patients with scleroderma.167 Volume shifts and infections that stress cardiovascular reserve are the likely culprit of HSCT-related cardiopulmonary collapse in scleroderma-associated pulmonary artery hypertension. In RA, organ function is generally normal, and cyclophosphamide-related toxicity is less problematic. The toxicity of a conditioning regimen, therefore, depends on the disease and disease-related organ dysfunction. Bu/Cy regimens have been used in a limited number of HSCTs for MS197 and RA.196 Busulfan is fat soluble and readily crosses the blood-brain barrier to the site of MS plaques. Busulfan is administered orally with variability in absorption and first-pass hepatic metabolism. Busulfex is an intravenous formulation that gives more uniform and less toxic serum levels. For RA, it may be equally important for efficacy that the conditioning regimen target not only lymphocytes but also synovial macrophages. Theoretically, HSCT results may be improved in RA by adding a more effective antimacrophage agent such as busulfan to a cyclophosphamide-based regimen.201 There are special concerns about the use of Bu/Cy in RA and MS. Patients with RA may have disease-related interstitial pneumonitis with little reserve for busulfan-related lung injury. The effects of alkylating agents on demyelinated neurons are unknown. In MS, the neurotoxicity of high-dose alkylating-based conditioning regimens remains unknown. BEAM and Cy/TBI are common lymphoma regimens being used to treat MS.189-191,193 TBI was selected because, unlike most agents, radiation readily crosses the blood-brain barrier. To avoid TBI-related pulmonary injury, radiation is generally given in the anteroposterior and posteroanterior position with 50% lung blocks with full dose to the mediastinal lymph nodes and spinal cord. A comparison of BEAM versus Cy/TBI regimen-related toxicity has not been performed. In general, TBI regimens are not used in RA because trials of nonmyeloablative total nodal irradiation in RA were associated with unexpected late complications such as myelodysplasia.202 Cy/TBI/ATG has been used as a conditioning regimen in the United States for scleroderma195 and MS,169 and in Europe for juvenile chronic arthritis (JCA).194 For patients with pulmonary scleroderma, TBI without lung shielding has been associated with lethal pulmonary deterioration.195 If attenuated with partial lung shields, TBI-related scleroderma lung injury appears less likely. Cy/TBI/ATG has been associated with lethal PTLD.358 The investigators attributed PTLD to use of high-dose rabbit ATG. Lower and less immune-suppressive doses of rabbit ATG or the use of horse ATG has not been reported to cause PTLD in autoimmune diseases. Independent of the conditioning regimen (Cy or Cy/TBI/ATG), when combined with aggressive T-cell depletion, HSCT in JCA has been complicated by lethal macrophage activation syndrome (MAS), manifesting as fever, lymphadenopathy, hepatosplenomegaly, and disseminated intravascular coagulation.186 MAS is a reactive hematophagocytic lymphohistiocytosis and has been associated with JCA independent of HSCT.203 The diagnosis is confirmed on bone marrow aspirate by macrophages (or histiocytes) actively phagocytosing hematopoietic cells and may arise from immune dysregulation perhaps in response to viral infections. To date, posttransplantation MAS appears to be a complication unique to JCA. No reports exist of late regimen-related organ toxicity from HSCT in autoimmune diseases. All patients need to be warned of infertility and of regimen-specific late toxicities such as cataracts from TBI. Late malignancies are also possible.204 Similar to mobilization regimens, conditioning regimens must be uniquely designed for the disease, organ impairment, disease-specific infection susceptibility, and extent of prior immune suppressive medication-related infectious risk to ensure minimum regimen-related mortality. Mortality Transplantation-related mortality (TRM) for all autoimmune diseases has been reported to be 8.6%.205 TRM is disease specific, in order of highest to lowest TRM: scleroderma, SLE, MS, and RA. This mortality is higher than expected because of phase 1 trials that selected patients with advanced end-organ dysfunction and/or active and refractory disease. Judicious selection of patients earlier in disease or in remission, but with a high risk of relapse or further progression, will diminish TRM. Variability in TRM based on the center performing the transplant, also known as the center effect,206 may be occurring for autoimmune diseases. Many factors affect TRM, including patient selection, supportive care, conditioning regimen, degree of lymphocyte depletion of the graft, use of disease-specific versus generic protocols, and so forth. A lower mortality in centers dedicated to autoimmune HSCTs may be obscured within the variability of multicenter registry data.Posttransplantation immunization After HSCT, a patient's titer from prior immunizations (eg, diphtheria, measles, tetanus, hepatitis B, etc) is often low or undetectable. As discussed in the "Breaking tolerance by environmental exposure" section, immunization could, theoretically, reinduce autoimmune disease. The risk of relapse may vary according to the type of immunization. For example, there was concern that onset and flare of MS may be associated with hepatitis B vaccination, although recent studies have shown no association.207 Although the risk of infection-related mortality or infection-induced autoimmunity in a nonimmunized individual probably outweighs any theoretical risk of immunization-induced disease relapse, guidelines on posttransplantation vaccination in autoimmunity have yet to be written.
MS, SLE, RA, and scleroderma will be discussed further because phase 3 autologous HSCT trials are being prepared in these diseases. In Europe, the European Bone Marrow Transplant/European League Against Rheumatism (EBMT/EULAR) autoimmune committee is designing these trials. In the United States, the trials are funded by the National Institutes of Health and are being designed by disease-specific working groups composed of transplant physicians, rheumatologists, and neurologists. Autologous HSCT for MS MS is a relatively common North American and European disease with a prevalence of approximately 1 in 1000 people.208 It is at onset an immune-mediated disease confined to the central nervous system. The disease is characterized by a variable course.209-211 Patterns are (1) relapsing-remitting MS defined as relapsing disease without progression between relapses with or without residual neurologic deficits from each relapse, (2) secondary progressive MS defined as continuous (often insidious and steady) neurologic deterioration with or without superimposed relapses after an initial relapsing-remitting course, and (3) primary progressive MS defined as steady continuous deterioration from onset. At onset, approximately 15% of the cases are primary progressive and 85% are relapsing-remitting.209-211 Within 10 years, 50% of relapsing-remitting cases become secondary progressive, and by 25 years, 90% have progressive disease. Relapse frequency in the first year of diagnosis influences time interval to disability.209-211 The median time to difficulty ambulating without unilateral assistance (an extended disability status score [EDSS] of 6.0) is 7 years for 5 or more relapses; 13 years for 2 to 4 relapses; and 18 years for 1 to 2 relapses.Accepted immune-modulating agents for MS are interferon beta (Avonex,
Betaseron)212-216 or Copaxone (copolymer 1 or glatiramar acetate)217,218 known as ABC therapy. Avonex and Betaseron
are different formulations of interferon- Natural history magnetic resonance imaging (MRI) studies have demonstrated that neurologic progression can continue despite lack of new demyelinating events on MRI.222,223 Although early relapse frequency within the first year of diagnosis appears to correlate with onset of late disability, Confavreux et al224 reported that relapse frequency in disease of longer duration and EDSS scores more than 4.0 do not correlate with disability. This finding indicates that treatment designed to prevent relapses (ie, immune-modulating therapy) used late in disease is probably not adequate to prevent progressive disability. Demyelination alone does not adequately explain the progressive disability that occurs in patients with progressive MS. Yet, the most important therapeutic goal is to prevent disability and maintain neurologic function. An evolving amount of literature on MS supports the concept that, although initially an inflammatory demyelinating disease, MS transitions into or is also an axonal degenerative disease.225-228 HSCT for MS was suggested in 1995.229 In general, initial
HSCTs were phase 1 studies and captured patients with progressive disease and high disability (EDSS) scores189-191,230-233
(Table 1). The Thessaloniki group
has reported a 3-year progression-free survival for primary progressive
MS (39%), which appears significantly lower than for secondary
progressive (92%).190 In an Italian study, Mancardi et
al231 reported 10 subjects undergoing HSCT followed with a
frequent MRI protocol who demonstrated lack of enhancing lesions and
accumulation of T2 burden of disease over an observation period of 4 to
30 months. A second study with a 5-year follow-up has noted a
discordant response between MRI and clinical results.230
Some patients had clinical progression of disability, defined as an
increase in the EDSS by one or more points but no new attacks or change
on MRI in terms of T2 disease burden. The patients whose EDSS increased
despite lack of MRI changes had significant pretransplantation
disabilities (EDSS of 7.0 to 8.5). Although longer follow-up is
necessary, it appears that HSCT slows or halts acute attacks and
further immune-mediated demyelination but not progressive disability,
especially in disease of increasing duration or higher disability
scores.
Two possible phase 3 MS trial designs are being proposed to run simultaneously. For secondary progressive MS, the trial would be aimed at suppressing relapses in patients with progressive disability. Patients with accumulated baseline deficits, but still inflammatory disease, could be considered candidates. This group could include ambulatory patients with an EDSS of 3.5 to 6.0 and continued relapses (or MRI evidence of active disease) randomized between a TBI and Cy regimen with or without low-dose ATG and CD34-selected HSCs versus mitoxantrone every 3 months for 2 years. However, suppression of relapses may be insufficient to halt progressive neurologic impairment, particularly as the duration of disease and the level of disability increase. For relapsing-remitting MS, the protocol would be aimed at suppressing relapses in patients at risk for progressive disability. Patients with relapsing-remitting disease who have failed interferon may be randomized between cyclophosphamide (200 mg/kg, with or without low-dose ATG) with CD34-selected HSC support versus best standard therapy (ie, continued interferon or interferon and adjuvant immunotherapy) (azathioprine, methotrexate, mitoxantrone, or cyclophosphamide). Because patients in this study would be earlier in the disease course, a safer conditioning regimen that does not include TBI would be indicated. Efficacy of earlier intervention in MS is supported by the Controlled High-risk Subjects Avenox Multiple Sclerosis Prevention Study (CHAMPS), in which over a 3-year interval treatment with interferon after the first clinical event significantly lowered the probability of developing clinically definite MS.234 If early intervention before onset of progressive disease is important in preventing late disability, a safe but intense immune suppressive regimen might be indicated in patients with relapsing-remitting MS who have failed interferon. Although the primary outcome of these trials would be progressive disability defined by the EDSS, other outcome measures would include clinical status by the Neurologic Rating Scale and Multiple Sclerosis Functional Composite, measurement of accumulated atrophy on MRI of the brain and cervical spinal cord, and potentially measures of whole brain N-acetyl-aspartate on magnetic resonance spectroscopy that reflects neuronal and axonal integrity. Autologous HSCT for SLE Although studies have suggested that SLE encompasses several genetic diseases with some clinical commonalties,235,236 the disease will be considered here as a single entity with protean clinical expressivity.237,238 SLE has an overall prevalence that has varied from 12 to 50.8 cases per 100 000 persons.239 Survival has improved dramatically, reaching a 90% 10-year survival and a 70% 20-year survival in the 1990s. Within the first 5 years, the main cause of death is active disease (neurologic, renal, systemic) or infection. Thereafter, causes of death tend to be infectious or cardiovascular events (strokes and/or myocardial infarction) related to hypertension and hyperglycemia/hypercholesterinemia because of chronic corticotherapy.Three consecutive but separable levels of etiology, ethiopathogenesis, and pathogenesis have been considered for SLE.240 It has been thought imperative to identify the specific molecular defects as the only way to design and use any novel and rational treatments.241 In practice, however, SLE is treated with a variety of drugs, mainly immunosuppressive, that have been discussed recently.242 Along with corticosteroids, intravenous pulse cyclophosphamide has been used in a National Institutes of Health-developed protocol specifically directed toward lupus nephropathy.243 At the pinnacle of the lupus iceberg, however, there are cases of refractory-relapsing ("intractable")244 disease. For such patients, following the considerable experimental evidence discussed formerly and also on the basis of serendipitous case reports of coincidental diseases, HSCT was proposed in 1993.245 Several cases of concomitant SLE and malignancy have been treated with HSCT and published. They include chronic myeloid leukemia/SLE,166 non-Hodgkin lymphoma (NHL)/SLE,157 and Hodgkin disease/SLE.160 The first patient eventually died of his leukemia without any evidence of active SLE. In another case, the NHL did not relapse, but ITP supervened in conjunction with an anticentromere antibody.163 The first patient with SLE received a transplantation of her own
T-cell-depleted marrow in 1996.199 The first report on
HSCT for SLE in the United States was published 1 year later in
1997.183 There are now several fully published case
reports of nonconcomitant SLE patients having undergone HSCT (Table
2).181-184,192,199,246 All
received transplantations of cyclophosphamide and G-CSF-mobilized CD34+ cells, and conditioning regimens varied from Cy/TT to
Cy/ATG (200 mg) to BEAM. All patients reached complete remission, but in several there was a serologic antinuclear antibody (ANA) relapse after 2 to 3 years from transplantation. In the patient with the longest posttransplantation follow-up, after 3 years of corticoid-free remission, there was a reappearance of ANA/DNA antibodies, and, after
another year, there was also a mild proteinuria, which is currently
being treated with a combination of corticosteroids and mycophenolate
mofetil.247
In the most extensive single-center clinical study published to date,1 |
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Abstract
Autoimmunity: definition
portion of a TCR.
Superantigen binding occurs outside the MHC peptide-binding groove and
outside the TCR CDR3 antigen-specific recognition site. Activation by
superantigen results in overexpansion and/or deletion of entire
V