Aplastic Anemia

Hematopoetic stem cells in the bone marrow are continuously being stimulated to produce differentiated granulocytes, erythrocytes, and platelets. An adequate pool of these cells is required to deliver oxygen to tissues (erythrocytes), prevent infection (granulocytes), and maintain blood clotting ability (platelets), as well as maintaining hematopoesis. The high production rates and relatively short life spans, which range from 2 to 120 days, of the differentiated end cells are key in producing rapidly serious clinical consequences following compromised bone marrow function.

Aplastic anemia, first described by Paul Ehrlich in 1888, is characterized by the development of peripheral blood pancytopenia and a hypocellular bone marrow. The normal hematopoetic tissue is replaced to a greater or lesser extent by fatty marrow. The absent or decreased bone marrow function results in inadequate production of granulocytes, erythrocytes, and platelets. Table 1 is a breakdown of the pathogenesis, etiology, and characteristics of the varied forms of aplastic anemia.

The disease can be congenital or acquired, with the latter constituting the majority of cases. Typically, the condition arises in an individual who was previously healthy with no evidence of malignant disease, or exposure to cytotoxic drugs or radiation. Although 50 to 70% of cases are idiopathic, drugs, chemicals, toxins, radiation, and infection may cause aplastic anemia. Other rare causes of acquired aplastic anemia include pregnancy and thymoma. It is possible that several inhibitory mechanisms operate at the same time to prevent hematopoesis, and that in any individual different mechanisms may be more or less important.

Pathology can vary from mild to fatal, and can affect any or all of the marrow-derived cell lines. Severe aplasia is defined by a leukocyte count less than 0.5 X 10⁹ /L, platelet and reticulocyte counts less than 20 X 10⁹ /L, and a hypocellular bone marrow. Patients diagnosed with severe disease have a grave prognosis with median survival of under 6 months, and 80% mortality within 1 to 2 years.

Data suggest an incidence of 2 to 5 cases per million per year in the United States, which translates into approximately 500 to 2500 cases annually. Men and women are affected non-preferentially. A biphasic distribution has been proposed where incidence rises in patients under 20, and peaks again in those over 60.

Drugs

As a well-known cause of aplastic anemia, drugs may produce predictable marrow failure, such as a myelosuppression following cancer chemotherapy. In Aplastic anemia, drug-induced bone marrow failure is usually an undesirable side effect that may or may not be reversible. Reversible effects are usually dose-dependent and may disappear following withdrawal of the drug. Chloramphenicol, thyroid medications, phenytoin, and chlorpromazine are all examples that may produce such reversible failure. Drug-induced toxicity that is not dose-dependent, and develops unpredictably weeks or months post-treatment is unaffected by withdrawal of the drug and may reflect a genetic predisposition.

Although the mechanism for such drug-induced damage is yet elusive, some drugs and their metabolites appear to be directly toxic to hematopoetic stem cells. Others are implicated in immune mechanisms such as antibody-mediated suppression of marrow function.

Chemicals and Toxins

In 1988, approximately 2,000 cases of aplastic anemia were diagnosed. According to the Aplastic Anemia Foundation in Baltimore, the rate of incidence has risen to about 5,000 cases annually as of 1997. It remains ambiguous as to why exposure to certain chemicals may produce symptoms in some people and not others. The Aplastic Anemia Foundation has identified a list of suspect chemicals that include some medicines, insecticides, household cleaners, mothballs, dry-cleaning liquids, glues, hair dyes, paint removers, varnishes and other petrochemical products. Benzene and insecticides are most commonly implicated in the pathogenesis of aplastic anemia. Their effect is dose-related and therefore potentially reversible. Length and amount of exposure to such potential mutagens may also affect presentation of symptoms. Genetic studies from Midwest farmers indicated that the rate of chromosomal mutation in the farmer’s blood rose sharply in the spring as compared to the fall. Presumably, pesticide and herbicide use were highest in the spring, as compared to the fall and winter when chemical use was absent or minimal. Further epidemiological studies may elucidate the role of such chemicals and toxins in causing aplastic anemia.

Radiation

Much evidence exists that radiation has dose-related suppressive effects on marrow function. Individuals with prolonged exposure to radiation, such as patients receiving chemotherapy , radiologist, and those exposed to nuclear weapons or nuclear reactors are appear to be at greatest risk for developing aplastic anemia. Marrow function usually recovers following withdrawal of the agent.

Infection

Association of viral infection with aplastic anemia is well documented. Parvovirus B19 has been shown to cause aplastic crises. Dengue viruses often cause transient marrow suppression. Human immunodeficiency virus can produce neutropenia or pancytopenia with hypocellular marrow. Epstein-Barr virus (EBV), which causes infectious mononucleosis, is present in the bone marrow of some aplastic patients. Although the pathogenesis is unclear, EBV is thought to induce suppression of hematopoesis via T cell mediated mechanisms. Immunologic induction of abnormal T suppressor-T helper cell ratios, abnormal T cell activation and T suppressor proliferation, and enhanced lymphokine production are possible contributors.

Most cases with a clear viral-association are in hepatitis patients, usually adolescent boys within 6 months of infection. Aplastic anemia has been reported to occur either during the acute or recovery phase of hepatitis infection. Although types A and B may also be involved, type C hepatitis (HCV) is considered to be the main culprit in hepatitis-associated aplastic anemia. The severity of the viral infection does not appear to correlate with the subsequent development of aplastic anemia. Interestingly, unlike EBV, HCV is absent in both marrow and serum. Anti-HCV antibodies are also absent despite progression towards a chronic phase of viral infection. These data suggest that another non-A, non-B, non-C agent may produce both the hepatitis and subsequent aplasia. Unfortunately, the prognosis for such patients is particularly grim, with median survival less than 6 months and 90% mortality at 1 year.

Pregnancy

Pregnancy-associated aplastic anemia is extremely rare since marrow cellularity and erythropoesis normally increase during pregnancy. The origin is attributed to the presence of an inhibitor or an absence of a stimulator of hematopoesis. A placenta-crossing inhibitor has been implicated in thrombocytopenia in infants born to mothers diagnosed in pregnancy. Since some of these mothers were transfusion recipients, anti-platelet or anti-erythrocyte alloantibodies may be responsible for the infant cytopenia, which generally undergoes spontaneous recovery a few weeks after birth. Patients with pregnancy-associated aplastic anemia are generally treated with supportive therapy, including platelet and erythrocyte transfusions before delivery. A third of the cases recover following termination of the pregnancy, either following therapeutic abortion or delivery. Since symptoms may return upon subsequent pregnancies, the condition itself may induce pathogenesis. Although implicated in some species, estrogens appear not be responsible for the marrow failure in humans.

Thymoma

Although the association between thymoma and pure red cell aplasia has been well documented, aplastic anemia is a rare complication. Recovery following thymectomy has been reported in some cases.

Fanconi’s anemia

Fanconi’s anemia, frequently associated with dysplasia of the radius and other bones, is an autosomal recessive disorder that is slowly progressive throughout childhood. Although idiopathic, an increased frequency of chromosome breaks has led to the hypothesis that the condition results from faulty DNA repair mechanisms. Development of leukemia is a substantial risk.

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Rare Constitutional Forms

One third of patients with Shwachman Diamond Syndrome (a condition characterized by pancreatic insufficiency, malabsorption, and decreased numbers of granulocytes) develop aplastic anemia. Dyskeratosis congenita, another rare X-linked dermatologic syndrome, produces aplastic anemia in approximately half the cases. In addition, some cases of amegakaryocytic thrombocytopenia have been reported to progress to aplastic anemia.

The evidence for genetic predisposition is weak. Cases of blood dyscrasia in twins have been reported, but the nature of their condition appears to be atypical of idiosyncratic acquired aplastic anemia. Association with HLA type and drug metabolism constitutes another possible genetic predisposition, but interpretation is difficult without adequate knowledge of the pathogenesis of aplastic anemia.

Defects In or Absence of Stem Cells

Animal models provide evidence that loss of marrow function results from defects in or absence of stem cells. Boggs and Boggs suggested that marrow failure might result from an inappropriate balance between stem cell replication and differentiation following observations in irradiated mice. Depletion of hematopoetic stem cells via irradiation in mice produces compensatory increases in stem cell replication and a decrease in stem cell differentiation. Other studies in mice with drug-induced "latent" marrow failure indicate a large deficit in stem cells, while having normal blood counts and marrow cellularity.

Some aplastic anemia cases in humans show data consistent with a quantitative abnormality in stem cells (see Figure 1). In vitro studies of bone marrow cells from patients indicate a decrease in hematopoetic progenitor cells. Restoration of normal hematopoesis following bone marrow transplantation from a genetically identical twin or human leukocyte antigen (HLA)-identical sibling in most cases provides further support for this hypothesis. Providing adequate numbers of normal stem cells is the presumed cause for this recovery.

One way to determine if the intrinsic abnormality in hematopoetic stem cells is the pathogenesis of aplastic anemia is to study the effects of bone marrow transplantation in twins. If absence of or defects in stem cells are important, then infusion of genetically identical marrow cells from the healthy twin should restore normal marrow function. In one study patients failed to recover when transplantation was not preceded by immunosuppression. When given a second transplant, preceded by immunosuppression, all patients recovered normal marrow function, suggesting an immune mechanism operative in these cases. In vitro analysis, however, failed to detect antibody- or cell-mediated suppression of hematopoesis.

In other studies one half of the cases responded to the transplant as if they had a stem cell defect while others recovered normal function only when the transplant was preceded by immunosuppression. Such data indicate that the presence of mechanisms other than stem cell defect that may suppress hematopoesis.

Microenvironment Defects

The S1/S1^d mouse is a model of microenvironment defects causing marrow failure. Bone marrow cells from S1/S1^d mice can restore hematopoesis in lethally irradiated mice, but infusion of normal marrow cells into these mice does not correct their anemia. The results that a microenvironmental defect, and not stem cell function may be operative in S1/S1^d mice. A similar abnormality was noted in one patient with congenital hypoplastic anemia, but further investigation is warranted.

Abnormal hormonal regulation does not appear to be a frequent contributor to aplastic anemia pathogenesis because levels of erythropoetin and other granulopoesis stimulants are normal in most cases.

Immune Suppression of Hematopoesis

Antibodies to normal myeloid progenitor cells have been found in approximately 25% of aplastic anemia patients, as well as multiply transfused patients without marrow failure. Immune mechanisms of bone marrow failure that include transfusion related antibodies may only rarely cause aplastic anemia.

Other studies have suggested that when abnormal, cellular interactions (which are necessary for regulation of normal hematopoesis) may be important in pathogenesis. T lymphocytes have been reported to promote growth of human erythroid stem cells. Animal models of congenital anemia indicate immune defects involving regulatory T cells may be partly responsible for development of the disease. T lymphocytes from some patients have been shown to inhibit growth of bone marrow cells from normal donors.

Kallenberg and coworkers have shown that a defective stimulating capacity of leukocytes, caused by an increased number of suppressor T cells, may contribute to aplastic anemia in some constitutional forms. Before transplantation the patient’s mononuclear leukocytes displayed defective stimulating capacity and hyporesponsiveness in mixed allogenic leukocyte culture. After removal of the T cells, the stimulating capacity of leukocytes was completely restored in culture. Studies suggest that the generation of suppressor T cells may be a consequence of previous blood transfusions, but the inconsistency of this result between patients makes interpretation difficult.

Regulation of hematopoesis by T lymphocytes may be both stimulatory and suppressive. Decreased helper-inducer T cells, increased suppressor-cytotoxic T cells, or the state of activation of T cells are sometimes associated with hematopoesis inhibition, and may produce pathogenesis of aplastic anemia. Lymphokine abnormalities, such as increased interferon (IFN-g ) by peripheral blood mononuclear cells, are often present. IFN-g has an inhibitory effect on hematopoesis. Tumor Necrosis Factor- a (TNF-a ), secreted by activated monocytes and macrophages, and by lymphocytes, has been shown to have similar inhibitory effects. Shinohara et al have observed increased TNF-a production by peripheral blood mononuclear cells in aplastic anemia patients. In addition, the elevated activities of TNF-a and platelet count were inversely correlated, suggesting a role for TNF-a in the suppression of hematopoesis.

Immune Function

Interestingly, although most patients have profound abnormalities in hematopoesis, immune function is generally normal. The disparity may be related to the long life span of lymphocytes, which range from months to years, as compared to that of erythrocytes, platelets, and granulocytes. Therefore, abnormalities in marrow function are manifest in circulating hematopoetic cells long before they are noted in circulating lymphocytes.

Minor abnormalities have been noted in some patients, including increased T- or B- lymphocytes, decreased skin test reactivity, and decreased immunoglobulins. Patient heterogeneity and effects of treatment, however, often complicate critical analyses of such observations.

In a study of 94 patients with severe aplastic anemia 13 cases were hepatitis-associated aplasia and the rest idiopathic. Gale, Mitsuyasu, and Yale reported that the 13 hepatitis-associated patients had the most notable immune abnormalities. Tests revealed reduced levels of T and B cells and monocytes, functional abnormalities in immunoglobulin, decreased alloantigen and mitogen responsiveness in mixed lymphocyte culture and mitogen induced proliferation to phytohemagglutinin and concanavilin A, and impaired skin test reactivity. Similar results have not been reported in hepatitis patients without aplastic anemia, and whether aplastic anemia is the cause or the result of these abnormalities remains to be determined.

There is evidence that patients with a history of transfusions exhibit alloimmunity to lymphocytes from HLA-identical donors. Warren and coworkers noted that some patients exhibited autoimmune (rarely seen in normal individuals) as well as alloimmune phenomena in patients with no history of transfusion. This indicates an alteration in immunity in these aplastic anemia patients that is only rarely seen in normal individuals. Further, the autologous and allogeneic reactivities appear to share a common pathway for cytotoxicity in most instances. While transfusion-induced sensitization to non-HLA antigens may be responsible for some of the allogeneic reactivity, the alloimmunity noted in patients with no history of transfusion indicates another mechanism at work.

Warren et al have suggested two possible explanations which argue that pathogenesis of aplastic anemia may be mediated by immunologic mechanisms in some instances. One explanation is that the auto- and alloimmune phenomena may be secondary to an intrinsic stem cell defect producing the loss of a suppressor or regulator cell for cytotoxic cells. Another is that these phenomena may be a result of a primary event in aplastic anemia, such as the disappearance of suppressor cells, or infection-induced autocytotoxicity.

Withdrawal of Etiologic Agents

Removing the causative factor is of course the most direct approach to treatment of aplastic anemia, and is practical when induced by certain cytotoxic drugs and chemicals. In addition, pregnancy related aplastic anemia may be reversed following termination of the pregnancy either due to delivery or therapeutic abortion. Patients with thymoma-associated aplasia have also been noted to improve following thymectomy. However, since 90% of cases are idiopathic, indirect approaches are usually employed to restore normal marrow function.

Supportive care

Erythrocyte Transfusions

Transfusions are the most common method used to correct the deficiencies in erythrocytes, platelets, and granulocytes. The major complications of erythrocyte transfusions include transfusion reactions, hepatitis, and iron overload.

Platelet Transfusions

Most patients respond to platelet transfusions from random, unselected donors, but the majority eventually develop antiplatelet antibodies. Those who fail to respond to random donors usually respond to HLA-matched donors. However, the problem of sensitization to these platelet antigens persists. Therefore platelet transfusions generally should be reserved until platelet counts dip below 5 X 10⁹ /L or there is evidence of hemorrhaging.

Granulocyte Transfusions

Granulocyte transfusions remain controversial as potential therapy for granulocytopenia. Patients with granulocyte counts below 0.5 X 10⁹ /L and documented infections have been reported to benefit from transfusions, especially in cases where broad spectrum antibiotics fail. However, granulocyte transfusion can sensitize patients to HLA and non-HLA antigens, thereby compromising responsiveness to subsequent transfusions of both platelets and granulocytes. In addition, patients commonly suffer reactions such as febrile transfusion reactions, chills, and even hepatitis, cytomegalovirus, and other viral infections. Taken in consideration with logistic difficulties and expense, these factors generally outweigh the potential benefits of granulocyte transfusions in aplastic anemia patients.

Prevention and treatment of infections is paramount in treatment of patients with granulocytopenia. Endogenous microbial flora of the skin and gastrointestinal (GI) tract are the most common sources of infection. Total or selective GI tract decontamination with oral, nonabsorbable antibiotics, reverse isolation are commonly employed measures of prevention.

Granulocyte/Macrophage Colony-Stimulating Factor (GM-CSF)

In aplastic anemia GM-CSF, given either intravenously or subcutaneously, increase granulocyte and monocyte counts in patients with a granulopoetic reserve, but there is no increase in granulocyte-macrophage colony-forming units. The increase in granulocyte count persists as long as GM-CSF is administered, after which the count returns to baseline. Again, severe cases benefit moderately, if at all. GM-CSF also activates circulating and tissue granulocytes and macrophages, and inhibits their spontaneous migration. Whether the increase in circulating granulocytes is beneficial to managing infection is still unclear. Positive results from GM-CSF treatment are encouraging for further investigation with other hematopoetic growth factors such as granulocyte colony-stimulating factor (G-CSF), and interleukin 3 (IL-3).

Restoration of Hematopoesis

The two most widely used treatments, which are commonly coupled, are anabolic steroids and derivatives to stimulate hematopoesis and antilymphocyte globulin (ALG) for immunosuppression.

Androgens, usually in the form of anabolic steroids, are active in stimulating hematopoesis, but their effects are limited to patients with mild or moderate aplastic anemia. In addition, a 1- to 3-month delay is required before a response can be detected. Controlled trials indicate that severe cases are unlikely to benefit from androgen treatment, and definitive treatment, such as bone marrow transplantation, should not be delayed for such trials.

Evaluations of glucocorticoid treatment reveal responses that are typically rare and brief. In addition, glucocorticoid administration may predispose patients to infections. Lithium administration has also produced improvement in some patients, but severe cases remain generally unaffected.

Data implicating abnormal immunity in pathogenesis of aplastic anemia have led to investigations of immunosuppressive treatment to restore hematopoesis. Antilymphocyte globulin was first introduced as a treatment for aplastic anemia in Paris in 1970 by Mathé and colleagues. ALG was a preparation of crude horse serum from a horse immunized with peripheral blood lymphocytes from humans. The reported survival figures of ALG treatment in humans vary from 35% to 76%, but randomized trials have confirmed that ALG treatment is more effective than supportive care alone. ALG, haplo-identical bone marrow, and androgens given together have also proved to be more effective than androgen therapy alone. Age, infection, and disease severity both negatively affect ALG effectiveness.

ALG is now a commercially prepared biologic product. The immunogen may consist of thoracic duct lymphocytes, or lymphoblastoid cell lines. Antibodies are raised in rabbits or horses, and are then purified and absorbed with a number of human tissues including erythrocytes and human placenta. The mode of action of ALG is also unclear. As an immunosuppressant leading to a decrease in lymphocyte count, it may also have a possible immunostimulatory effect. ALG administration may activate lymphokine production, as well as acting on surface receptors of hematopoetic progenitor cells, directly stimulating them, or increasing their responsiveness to hematopoetic growth factors.

Response to ALG therapy is slow, taking 1.5 to 3 months after treatment. There are no definitive tests that indicate which patients will recover, but an early sign of improvement appears to be an increase in mean red cell volume. Patients who fail to respond to the first course, may be subjected to a second trial after 4 months. Response to the second course is seen in approximately one third of cases.

High-dose Methylprednisolone has been tried as an alternative immunosuppressant. Although response rates are comparable to ALG treatment, toxicity is considerably increased especially in the presence of infection, diabetes, and hypertension. The effects of combined ALG-high-dose Methylprednisolone treatment are not additive. In addition, lower doses loose effectiveness.

Cyclosporine has also been used as an alternative source for immunosuppression in aplastic anemia patients, especially on those who failed to respond to one or more courses of ALG therapy. A randomized study of ALG versus cyclosporine as first line treatment suggests that cyclosporine may produce comparable effects to ALG therapy. Prolonged cyclosporine use however, produces renal impairment, and a syndrome resembling thrombotic thrombocytopenic purpura.

Transplantation of Hematopoetic Stem Cells

Bone Marrow Transplantation from HLA-identical Donors

The major histocompatibility complex in humans is human leukocyte antigen (HLA). The HLA complex consists of a series of closely linked loci on chromosome 6 known as HLA-A, B, C, and D, and are inherited as mendelian codominant genes. HLA-A, B, and C antigens are defined by highly specific anti-HLA sera, while HLA-D antigens are defined by mixed lymphocyte culture test. Because HLA-A, B, and C are closely linked to HLA-D, HLA-A, B, and C identical siblings have over a 99% probability of being HLA-D identical as well. In families with two or more children, there is a 25% to 45% of finding an HLA-identical sibling donor. Parents of a patient, unfortunately, are unlikely to be HLA-identical to the patient unless they share the antigens.

Therefore, most patients receive bone marrow transplants from HLA-identical siblings. The transplantation itself involves selection of a histocompatible donor, bone marrow procurement and transplantation, and pre- and post-transplant immunosuppression to prevent graft rejection and graft-versus-host disease.

Course of a Typical Bone Marrow Transplant

Patients with aplastic anemia are immunocompetent and can reject grafts. Pretransplant immunosuppression is required to achieve engraftment and prevent graft rejection, as noted in studies with HLA-identical twins. Immunosuppression is especially important when donors are HLA-identical siblings that differ in non-HLA antigens that may be targeted for rejection.

Low-dose total-body radiation and high-dose cyclophosphamide (200 mg/kg body weight over 4 days) is a well-tolerated regimen in most patients, and reduces the risk of rejection to less than 5%. However, the regimen is associated with an increased risk of graft-versus-host disease and interstitial pneumonia. An alternative approach is to give the recipient a transfusion of viable peripheral blood leukocytes. Studies have reported contradictory results, however, with this approach.

After completion of the conditioning regimen, the patient is intravenously infused with 1 to 5 10⁸ bone marrow cells per kilogram. Engraftment usually occurs during subsequent 2 to 4 weeks, during which time blood count and marrow cellularity increase. In a successful transplant only donor type hematopoesis can be detected after the graft. Granulocytes, erythrocytes, and platelets are all consistent with donor type. Monocytes, bone-marrow-derived macrophages, pulmonary alveolar macrophages, hepatic macrophages, osteoclasts, T- and B-lymphocytes, as well as immunoglobulin isotypes all switch to the donor type, indicating successful engraftment.

Graft Rejection

Patients at high risk for graft rejection include those with a history of transfusions, and those without adequate pretransplantation immunosuppression. In vitro tests of recipient antidonor immunity to predict autoimmunity include relative response in mixed lymphocyte culture, antibody-dependent cellular cytotoxicity, and lymphocytotoxins. The technical demands and lack of reproducibility of theses tests has prevented their wide use.

In addition infusions of viable donor buffy coat cells in addition to marrow cells has decreased the rate of graft rejection in multiply transfused patients, and increased their survival rate.

Deeg and coworkers identified five major risk factors for graft rejection:

Graft-versus-Host Disease

Recent studies have shown that 80% of untransfused and 70% of transfused patients with aplastic anemia, treated with immunosuppression and marrow transplantation from HLA-identical family members become long term survivors without recurrence of the disease. Less than half of these patients suffer from mild to severe chronic graft-versus-host disease.

Graft-versus-host disease is believed to be caused by donor lymphocytes that react against recipient tissues, and is probably mediated by T-lymphocytes. The degree of genetic disparity between donor and recipient, the intensity of the conditioning regimen, and the amount of marrow cells transferred have been identified as major factors in determining prevalence and severity of the disease. Chronic and acute forms have been noted in humans.

The acute form usually develops within the first 2 months post-transplantation. The primary target organs are the lymphoid system, skin, liver, and GI tract. Acute graft-versus-host disease develops in many recipients of allogeneic grafts. Moderate to severe grades occur in about 25% of patients, and is fatal in approximately half of these. Methotrexate, high-dose corticosteroids, antithymocyte globulin, or cyclosporin A can all favorably influence the clinical features, but may not increase survival rates. Removal of T-lymphocytes from the marrow before it is transplanted, has been tried as a preventative measure in animals, but further research is required in humans.

The chronic form develops 6 months to a year after transplantation, either as an extension of the acute form or after a period of well-being. As a disease of disordered immunity, patients generally present with symptoms similar to autoimmune disorders with profound immunodeficiency, autoantibody production, and imbalances in T cell subsets. Storb et al identified three predictive factors in chronic graft-versus-host disease in patients treated by HLA-identical marrow transplants. Increasing patient age, minimal use of viable donor buffy coat cells in addition to the marrow to prevent graft rejection, and increased grade of acute graft-versus-host disease were all predictors for development of chronic disease. Combinations of prednizone, azothioprine, or cyclophosphamide in patients with early chronic disease have shown promising results, with regression seen in some patients.

Immune Deficiency

Late-occuring infection are generally due to the profound immunodeficiency for 3 to 6 months that follows marrow transplantation, after which patients slowly recover for 1 to 2 years. Factors included in determining the severity of the deficiency included the degree of donor-recipient genetic disparity, effects of pretransplant radiation and chemotherapy, status of the recipient's thymus, nature of the transplanted lymphocytes, and the use of posttransplantation immunosuppression. Possible mechanisms of immunodeficiency include intrinsic and extrinsic inhibitors of lymphocyte maturation, presence of suppressor cells or factors, and abnormal cell-cell interaction between donor and recipient lymphocytes. T- and B-lymphocytes, as well as granulocyte, monocyte, and macrophage function may be abnormal. The presence of acute graft-versus-host disease also retards recovery in immunodeficient patients as they have decreased T cell mitogens and impaired antibody responses. Similarly, patients with chronic graft-versus-host disease may be profoundly immunodeficient for a prolonged period.

Interventions that may accelerate immune reconstitution include thymus transplants and thymosin administration. However, attempting clicnical trials for these and other approaches have been hindered by the complexity of clinical bone marrow transplantation that makes it difficult to interpret therapeutic intervention. In addition, the possibility of elevated risk for graft-versus-host disease is also a string deterant from such interventions. At present, efforts are focuse on lowering the incidence of graft-versus-host disease, with hopes of concommitant immune reconstitution.

Alternative Sources of Hematopoetic Stem Cells

 Potential alternative sources for hematopoetic stem cells include bone marrow or peripheral blood cells from partially or fully HLA-matched related and unrelated donors, fetal liver cells, and hematopoetic stem cells from long term in vitro cultures of bone marrow. Unfortunately, the overall results have been disappointing because graft rejection and graft-versus-host disease have been major problems. Therefore, bone marrow transplantation from HLA-matched donors has evolved as the preferred therapy for patients with aplastic anemia.