Acquired Disorders of Red Cell, White Cell, and Platelet Production

Etiology and Classification

Acquired forms of PRCA must be distinguished from congenital forms of PRCA, which usually manifest themselves early in life (see Chapter 30 ). Acquired PRCA occurring in childhood may be difficult to distinguish from DBA. As an acquired disease, PRCA may be a primary disorder or secondary to a variety of systemic diseases, including a number of hematologic malignancies ( Table 33.1 ).

Pathogenesis

The inciting events in the development of PRCA are not known. However, as with idiopathic aplastic anemia, viruses or exposure to chemicals can serve as potential triggers ( Fig. 33.1 ). Theoretically, a viral infection could lead to depletion of erythroid precursors. Studies of B19 parvovirus (discussed later) suggest such an etiology. Because hematopoietic stem cells are not affected by B19 parvovirus, myeloid cells and platelets are normally produced, and upon clearance of the virus, normal erythroid production can resume. Similarly, in immune-mediated PRCA, the mechanism of erythroid inhibition may vary and may include (1) antibodies to proteins specific to erythroblasts, (2) direct cytotoxic T lymphocyte (CTL)–mediated killing of erythroid precursors, and (3) production of soluble products by CTLs such as inhibitory or proapoptotic cytokines that directly affect the erythroid series.

Historically, initial studies concentrated on examining the effects of soluble serum inhibitors of erythropoiesis. These investigations revealed a decline in erythroid colony formation in the presence of patient serum or failure to induce erythroid colony formation in the presence of erythropoietin. Such serum inhibitors can be detected in 40% of patients with PRCA. In 60% of patients, erythroid colony formation can be induced in vitro with hematopoietic growth factors. The inhibitory activity is localized to the immunoglobulin (Ig)G fraction and disappears upon remission. The antigenic targets for autoantibodies have not been well characterized, but various stages of erythroid differentiation can be affected (also called PRCA type A), as seen in the reduction of burst-forming unit–erythroid or colony-forming unit–erythroid. In certain cases of antibody-mediated PRCA, the involvement of the complement system is a prerequisite to disease causation. Perhaps the exception and a model for antibody-induced red cell aplasia is the identification of PRCA associated with anti-erythropoietin antibodies (also called PRCA type B) in rare cases. Consistent with the specificity of the antibodies, myeloid colony formation is not impaired, making it unlikely that a more ubiquitous inhibitory cytokine mediates the specific erythroid inhibition.

Experimental and clinical observations have suggested that PRCA may also be mediated by CTLs, which specifically recognize and kill erythroid precursors similar to CTL-mediated killing of cells in aplastic anemia. Although such a T cell–mediated erythroid response is likely to be polyclonal, rare instances of T-cell large granular lymphocyte (T-LGL) leukemia associated with PRCA or erythroid inhibition may represent an extreme form of the clonal continuum of CTL responses (see T-Cell Large Granular Lymphocyte–Associated PRCA). In addition to CTLs expressing α/β T-cell receptors (TCRs), T cells with a γ/δ TCR can mediate PRCA. The antigens/antigenic peptides triggering such a response have not been well described. However, it has been shown that T-LGLs from PRCA patients are directly capable of suppressing erythropoiesis of CFU-E in vitro. Similarly, natural killer (NK) cells have also been implicated in mediating cytotoxicity directed against erythroid precursors. NK cells, like γ/γ T lymphocytes and unlike α/β CTLs, do not rely on major histocompatibility complex (MHC)–restricted cytotoxicity, but may use killer-cell immunoglobulin-like receptors (KIRs). KIRs inhibit cytolysis when they encounter a cell bearing human leukocyte antigen (HLA) class I molecules. A lack of appropriate KIRs may predispose cells to increased attack by NK cells or γ/δ CTLs. Physiologic downregulation of KIRs has been implicated in the pathogenesis of PRCA in a patient with concomitant γ/δ T-LGL clonal proliferation. An alternative NK-cell cytotoxic mechanism independent of KIR has been reported in healthy individuals.

Peripheral Th lymphocyte polarization has been implicated in the pathogenesis of PRCA. Polarization toward the Th2 functional subtype during disease relapse and normalization of Th1/Th2 ratio after effective treatment has been associated with both monoclonal gammopathy- and thymoma-associated PRCA.

Primary Pure Red Cell Aplasia

Primary PRCA occurs in the absence of any underlying disorder. It may be acute and self-limited or may be a chronic and refractory condition. Acute forms are uncommon. Most cases are protracted and chronic, unlike TEC, which is an acute and self-limited disorder. Most of the cases of classic primary PRCA are autoimmune in origin, but a significant proportion will remain idiopathic in origin in spite of an exhaustive workup.

Secondary Forms of Pure Red Cell Aplasia

Clinical Associations

B-Cell Chronic Lymphocytic Leukemia–Associated Pure Red Cell Aplasia

In B-cell chronic lymphocytic leukemia (CLL), PRCA can be observed in up to 6% of cases. A recent study found 0.5% of their 1750 CLL patients has PRCA. This diagnosis should be considered in CLL patients who demonstrate anemia and reticulocytopenia. Definite diagnosis requires bone marrow biopsy which will show absence of erythroid precursors and red cell aplasia. The underlying pathogenetic mechanisms are not clear, and the inhibition of the erythroid series does not appear to be mediated by a soluble factor. The distinction between whether the PRCA is a result of the primary B-cell CLL disease or its therapy becomes difficult in circumstances when PRCA presents as a late event. In most cases, PRCA cannot be attributed simply to infiltration of the marrow by lymphoma cells.

T-Cell Large Granular Lymphocyte–Associated Pure Red Cell Aplasia

Although neutropenia is a typical finding in T-LGL leukemia, PRCA with varying degrees of erythroblastopenia can also be observed in 10% to 68% of patients with T-LGL leukemia, with much higher rates in Asian populations. It is found to be commonly associated with signal transducer and activator of transcription (STAT)3 mutant T-LGL leukemia. In such a setting, PRCA is often accompanied by red cells with an increased mean corpuscular volume. It is possible that PRCA associated with T-LGL leukemia represents an extreme form of the T-cell–mediated disease that, if polyclonal, might be classified as idiopathic PRCA.

Thymoma-Associated Pure Red Cell Aplasia

Thymoma is associated with PRCA; thus a chest X-ray examination or computed tomographic (CT) scan should be included in the workup for PRCA. Antibodies with direct inhibitory effects against erythroid precursors may be present. T-cell–mediated inhibition of erythropoiesis has also been implicated in the pathogenesis of PRCA associated with either benign or malignant thymomas. A late onset immunodeficiency condition, Good syndrome, which is characterized by hypogammaglobulinemia and thymoma is associated with PRCA in 33% of the patients. Thymectomy is the usual initial treatment approach ; however, incomplete responders, nonresponders, and patients who relapse are common, necessitating additional therapies in the form of azathioprine, intravenous immunoglobulins (IVIg), and cyclosporine A (CsA).

Pregnancy-Associated Pure Red Cell Aplasia

Pregnancy-associated PRCA is a self-limited syndrome that may occur at any age of gestation. It has a high risk for relapse during subsequent pregnancies and can be safely managed with either blood transfusions or corticosteroids. Patients with other forms of PRCA may also be more prone to relapse during pregnancy.

Parvovirus B19 and Other Viral-Induced Pure Red Cell Aplasias

Parvovirus B19 is a single-stranded deoxyribonucleic acid (DNA) virus, which in normal individuals causes fifth disease (erythema infectiosum) in children and arthropathy in adults. It is of particular concern in patients with sickle cell disease. The cellular receptor for the virus is the P antigen, a blood group antigen also responsible for the agglutination reaction that occurs in the presence of the virus. Detection of parvovirus B19–specific IgM without antiparvovirus B19 IgG supports the diagnosis of acute infection, whereas the parvovirus B19–specific IgG suggests immunity. Addition of parvovirus B19 in vitro to cultures of erythroid progenitor cells completely abolishes erythroid colony formation. Primary infection causes lifelong immunity; however, it is possible that a latent virus may persist in a healthy individual for years.

A transient aplastic crisis is a typical complication of a primary parvovirus B19 infection in patients with increased red cell turnover (usually chronic hemolysis, e.g., hemoglobinopathies, and hereditary red blood cell [RBC] membrane disorders, e.g., hereditary spherocytosis). In typical cases, acute reticulocytopenia results in a sudden drop in hemoglobin (Hb)/hematocrit levels as RBC destruction is not supported by a suppressed marrow. Occasionally, characteristic giant pronormoblasts may be seen in marrow aspirates ( Fig. 33.2 ). Aplastic crisis is often self-limiting with the evolution of a protective IgG response. Viral titers in the serum of affected patients may be high.

A more chronic form, parvovirus B19–related PRCA, may develop in immunocompromised patients as, for example, in acquired immunodeficiency syndrome (AIDS) (see Chapter 152 ). In such cases, IVIg can produce remarkable responses. High doses of IVIg are required (>2 g/kg) because an insufficient dose may not produce the desired effect. DNA dot blot hybridization is the best diagnostic test for the detection of viremia. Parvovirus B19 can also be detected by polymerase chain reaction (PCR), a routinely available test, but this method may provide a high rate of false-positive results. However, if negative, it excludes B19 parvovirus–mediated disease. Improved tests have been developed that allow for the detection of neutralizing antibodies and infectivity of parvovirus B19.

Several other viral infections, including viral hepatitis (A and C ), Epstein-Barr virus (EBV), cytomegalovirus (CMV), human T-Lymphotropic virus (HTLV)-1, and human immunodeficiency virus (HIV), have been implicated as causative agents of PRCA. Little is known about the exact mechanisms underlying these disorders, but they likely involve T-cell–mediated suppression as observed during HTLV-1 infection and EBV or antibody-mediated destruction of RBC precursors, as in hepatitis C–induced PRCA. Frequently the presence of multiple comorbidities and medications makes it difficult to isolate the causative agent of PRCA.

Connective Tissue Disease–Associated Pure Red Cell Aplasia

The majority of connective tissue diseases associated with PRCA are autoimmune in nature. Several rheumatologic diseases have been associated with PRCA, including adult-onset Still disease, dermatomyositis, mixed connective tissue disease, polymyositis, rheumatoid arthritis, Sjögren syndrome, and systemic lupus erythematosus. The pathogenesis of the PRCA in this setting may vary and includes autoantibody-mediated erythroid inhibition, autoantibody directed against erythropoietin, and CTL-mediated killing of erythroid precursors.

Drug-Induced Pure Red Cell Aplasia

Various chemical agents and drugs have been associated with PRCA. The mechanisms responsible for erythroid inhibition may be diverse depending on the offending agent ( Table 33.2 ) but may include induction of antibodies targeting the drugs or drugs bound to cellular and plasma proteins. Another possible mechanism involves drug-mediated triggering of T-cell responses, or direct toxicity to the erythroid series as seen with diphenylhydantoin.

Table 33.2

Drugs and Chemicals Implicated in Pure Red Cell Aplasia

Alemtuzumab Allopurinol α-Methyldopa Aminopyrine Azathioprine Benzene Carbamazepine Cephalothin Chloramphenicol Chlorpropamide Cladribine Clopidogrel Cotrimoxazole d -Penicillamine Erythropoietin Estrogens Fludarabine FK506 Gold Halothane Interferon-α

Isoniazid Lamivudine Linezolid Maloprim Mycophenolate mofetil Penicillin Phenylbutazone Phenytoin Procainamide Ribavirin Rifampicin Sulfasalazine Sulfathiazole Sulindac Tacrolimus Thiamphenicol Ticlopidine Valproic acid Zidovudine

Erythropoietin Antibody–Associated Pure Red Cell Aplasia

Recombinant erythropoietin is used in the treatment of anemia of various origins, including anemia of chronic disease, renal disease, and a variety of bone marrow failure syndromes, particularly myelodysplastic syndromes (MDS). Cases of PRCA have developed as a consequence of antibody formation against endogenous erythropoietin or while receiving treatment with recombinant erythropoietin. The latter condition has been referred to as epoetin-induced PRCA or EPO-PRCA with initial cases related to exposure to epoetin alpha (Eprex; 92%) and epoetin beta (NeoRecormon; 8%). There are reports of human erythropoietin (HuEpo) neutralizing antibody PRCA related to the use of biosimilar recombinant HuEpo in Thailand. There are several risk factors associated with the development of EPO-PRCA, including subcutaneous (SC) route of administration, use of epoetin alpha stabilized in a human serum albumin (HSA)–free formulation, use of silicone oil as lubricant in prefilled Eprex syringes, and use in patients with chronic renal disease. This observation has led to modification in the storage, handling, and administration of Eprex favoring IV administration, especially with Eprex stabilized with HSA-free formulation, and avoidance of the SC non-HSA–stabilized Eprex. Diagnostic criteria have also been proposed incorporating major features (treatment with epoetin for at least 3 weeks, decrease in Hb by 0.1 g/dL/day without transfusions or transfusion requirement of about 1 unit/week to keep Hb levels stable, reticulocyte count less than 10 × 10 ⁹ /L, no major drop in other blood lineages), minor features (skin and systemic allergic reactions), and accessory investigations to exclude other causes. The most commonly used tests to detect antibodies are enzyme-linked immunosorbent assay, radioimmunoprecipitation assay, and surface plasmon resonance. Once antibodies are detected, their neutralizing ability is tested using an in vitro bioassay. Following establishment of diagnosis, management should include discontinuation of exogenous erythropoietin, administration of immunosuppressive agents, or, in cases of anemia secondary to renal insufficiency, renal transplantation.

Pure Red Cell Aplasia Following Allogeneic Stem Cell Transplantation

In contrast to HLA matching, ABO blood group incompatibility plays a minor role in the success of allogeneic hematopoietic stem cell transplantation (HSCT). However, PRCA may be associated with major ABO incompatibility between the donor and recipient, leading to inhibition of donor erythroid precursors by residual host isoagglutinins. This complication is more commonly observed following the use of nonmyeloablative conditioning regimens. PRCA may also be resistant to the withdrawal or decrease of immunosuppression, or donor lymphocyte infusions. Responses to rituximab, erythropoietin, plasma exchange, and azathioprine have been reported. A case responsive to a purified CD34 ⁺ cell infusion has also been reported. Multiple cases of successful treatment with daratumumab (IgG1k monoclonal antibody directed against CD38) have also been reported. Resolution of PRCA is generally associated with a decrease and subsequent disappearance of host isoagglutinins.

Pure Red Cell Aplasia Associated With Immune Dysregulation, Polyendocrinopathy, Enteropathy, X-Linked Syndrome

Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome is a rare X-linked recessive condition typically seen during infancy. Clinical features may include type 1 diabetes mellitus, eczema, and autoimmune hepatitis. Anemia can be severe at diagnosis because the fall in Hb occurs over a protracted period of time and patients often exhibit a good degree of adaptation. Arrest of erythropoiesis is obvious with a profound reticulocytopenia.

Laboratory Evaluation

A complete blood cell count with differential, peripheral smear review, reticulocyte count, and a bone marrow examination remain the cornerstone in the diagnosis of PRCA. The classic hematologic picture of PRCA includes a normocytic, normochromic anemia (anemia associated with T-LGL leukemia is often macrocytic) with a normal white blood cell and platelet count. The reticulocyte count is reduced to less than 1% when corrected for the degree of anemia (a reticulocyte level greater than 2% is not compatible with the diagnosis of PRCA) ( Fig. 33.3 ).

The bone marrow examination generally shows absence of cells belonging to the erythroid lineage and the normal appearance of granulocytic and monocytic precursors and megakaryocytes. Erythroid precursors, if present, are usually less than 1%, and only a few residual proerythroblasts or basophilic erythroblasts may be seen. Blast cell numbers and cellularity are within normal limits. There are no dysplastic changes, ringed sideroblasts, or reticulin fibrosis. Cytogenetic evaluation is normal. In some cases, neutropenia, mild thrombocytopenia, eosinophilia, thrombocytosis, leukocytosis, or relative lymphocytosis may be seen. Cytogenetic abnormalities, if present, may indicate concomitant MDS and is a poor prognostic marker for both response to treatment and propensity to leukemic transformation. During the course of PRCA patients, ineffective erythropoiesis characterized by a maturation arrest at the proerythroblast or basophilic erythroblast stage may be observed and signifies either partial response to treatment or initial recovery from treatment or a prelude to the development of full-blown PRCA.

It is important to exclude vitamin B ₁₂ and folate deficiencies, and depending on the etiology and associated disease, other blood and bone marrow findings may be seen. The presence of giant and vacuolated pronormoblasts in the bone marrow examination should raise the suspicion for parvovirus B19 infection. The presence of large granular lymphocytosis, neutropenia, and/or thrombocytopenia, expansion of CD3 ⁺ CD8 ⁺ CD57 ⁺ T cells, clonal cytotoxic T cells as documented by TCR gene rearrangement, expansion of specific TCR Vβ region family gene segment, and splenomegaly may point toward concomitant T-LGL leukemia. B-cell lymphocytosis, especially of the CD5 ⁺ /CD19 ⁺ /CD20 ⁺ /CD23 ⁺ /cyclin D ⁻ /SmIg ^−dim phenotype with concomitant lymphadenopathy, hepatosplenomegaly, hypogammaglobulinemia, and thrombocytopenia may be very suggestive of a B-cell CLL. Another laboratory finding that may help point to a secondary cause of PRCA includes the presence of monoclonal gammopathy. Parvovirus B19 DNA titers (DNA hybridization and amplification techniques) may show high levels of the virus at 10 genome copies per milliliter, but it is important to note that serologic (IgM and IgG) titers are usually absent. Erythropoietin antibodies, antinuclear antibodies, and/or complement consumption may point toward a specific disease mechanism. A radiographic workup may also be useful in the clinical workup because chest X-ray examination or CT scan may show evidence of a thymoma.

Medical