Immune pathophysiology of aplastic anemia

Elevated expression of CX3C chemokine receptor 1 mediates recruitment of T cells into bone marrow of patients with acquired aplastic anaemia

OBJECTIVE

Acquired aplastic anaemia (AA) is a T-cell-mediated, organ-specific autoimmune disease characterized by haematopoietic stem cell destruction in the bone marrow. The exact molecular mechanism of T-cell trafficking into the bone marrow is unclear in AA. Very late activation antigen-4 (VLA-4) and CX3C chemokine receptor 1 (CX3CR1) play active roles in many autoimmune diseases. Therefore, we investigated whether VLA-4 and CX3CR1 also contribute to T-cell migration into the bone marrow in acquired AA.

DESIGN, SETTING AND SUBJECTS

Expression levels of CX3CR1 and VLA-4 and their ligands [fractalkine (CX3CL1) and vascular cell adhesion molecule-1 (VCAM-1)] were examined in 63 patients with AA and 21 healthy control subjects. T-cell chemotaxis and adhesion were analysed in 17 patients with severe AA. We also prospectively evaluated the expression pattern of CX3CR1 during treatment with antithymocyte globulin plus cyclosporine in 11 patients with severe AA.

RESULTS

The proportion of peripheral and bone marrow CD4(+) and CD8(+) T cells expressing CX3CR1 and the level of CX3CL1 was increased in patients with AA. However, there was no significant difference in VLA-4 expression or VCAM-1 levels. Functional studies demonstrated that chemotaxis towards autologous bone marrow plasma or soluble CX3CL1 was significantly higher in T cells from AA patients and could be blocked by CX3CR1 inhibitors. CX3CR1-mediated T-cell adhesion was also upregulated in these patients. The expression of CX3CR1 was associated with the efficacy of immunosuppressive therapy.

CONCLUSION

The present findings demonstrate that CX3CR1 plays a pivotal role in recruitment of T cells into the bone marrow in acquired AA and is a potential therapeutic target for treatment of this disorder.

Nonregenerative anemia: mechanisms of decreased or ineffective erythropoiesis

In veterinary medicine, anemia without an appropriate compensatory hematopoietic response is termed nonregenerative. Nonregenerative anemia is a common clinical entity, occurring as a result of diminished or ineffective erythropoiesis in association with many types of pathology. This article reviews nonregenerative anemia in domestic animals, emphasizing mechanisms of disease, and also covers other conditions associated with nonregenerative anemia in people. Many aspects of nonregenerative anemia in animals are worthy of further investigation, from molecular mechanisms of disease to epidemiologic impacts.

Final results of the phase II study of rabbit anti-thymocyte globulin, ciclosporin, methylprednisone, and granulocyte colony-stimulating factor in patients with aplastic anaemia and myelodysplastic syndrome

This report describes the final results of a Phase II clinical trial investigating the efficacy of rabbit antithymocyte globulin (rATG), ciclosporin, steroids, and granulocyte colony-stimulating factor (GCSF) in patients with untreated aplastic anaemia (AA), or low to intermediate-risk and hypocellular myelodysplastic syndrome (MDS). We treated 24 patients each with AA and MDS with rATG (3·5 mg/kg/d × 5; reduced to 2·5 mg/kg/d × 5 in patients with MDS ≥ 55 years), ciclosporin (5 mg/kg orally daily × 6 months), steroids (1 mg/kg daily, tapered off over 1 month), and GCSF. The overall response rate in AA patients was 64% compared to 25% in MDS patients. The median time to response was 3 months in AA patients and 4 months in MDS patients. Pretreatment clinical characteristics, such as age, sex, blood counts, cellularity, cytogenetics, or HLA-DR15 status, did not predict for response. Response to therapy, however, predicted for improved overall survival (OS), with a 3-year OS of 89% vs. 43% in responders versus non-responders, respectively (P < 0·001). Infusion reactions occurred in about half the patients and were manageable. Myelosuppression, elevation in liver enzymes, and infections were common. The early mortality in MDS patients was 13% vs. 0% in AA patients.

Failure of hypomethylating agent-based therapy in myelodysplastic syndromes

Hypomethylating agents such as 5-azacytidine or decitabine have been a major breakthrough in the treatment of patients with myelodysplastic syndromes (MDS). They have been shown to improve transfusion requirements and to change the natural history of the disease. However, with increasing cumulative clinical experience, it has become apparent that these agents are not curative and have their own shortcomings. There is a subgroup of patients who do not respond to frontline therapy and a large, growing cohort of patients that lose response or progress while on hypomethylating agent-based therapy. There are no standard treatment options in this arena and it is therefore a focus of significant research interest. Since the mechanisms of resistance to hypomethylating agents are not known, selection of therapy is largely empiric but must take into account the age, comorbidities, and performance status of the patient, as well as the characteristics of the disease at the time of treatment failure. Higher intensity approaches and allogeneic stem cell transplant can yield improved response rates and long-term disease control but should be limited to a selected cohort of patients who can tolerate the treatment-related morbidities. For the majority of patients who likely will be better candidates for lower intensity therapy, several novel, investigational approaches are becoming available. Among these are newer nucleoside analogues, inhibitors of protein tyrosine kinases, molecules that interact with redox signaling within the cell, immunotherapy approaches, and others. Patients with MDS whose disease has failed to respond to hypomethylating agent therapy should be referred for clinical trials when available. As we learn more about the patterns and mechanisms of failure, the next challenge will be to determine which therapies are suitable for each individual patient.

Tcf-1 gene silence suppresses downstream gene expression in CD4(+) T cells from bone marrow of aplastic anemia patients

CD4(+) T cells play a crucial role in the pathogenesis of acquired aplastic anemia (AA). Tcf-1 gene regulates T cell development and function, and it is significantly upregulated in the bone marrow CD4(+) T cells from patients with acquired AA. To explore the role of Tcf-1 in the pathogenesis of AA, we knocked down Tcf-1 gene in CD4(+) T cells of AA patients and studied the effects of Tcf-1 silencing on its downstream gene expression. Upon transfection of psiRNA into marrow CD4(+) T cells from bone marrow of aplastic anemia patients, the expression of Tcf-1 was significantly knocked down; consequently, expressions of c-Myc and CD44 were also significantly reduced. Our results suggest that Tcf-1 may contribute to pathogenesis of AA by regulating downstream gene expression such as c-myc and CD44.

Comparison of immunological abnormalities of lymphocytes in bone marrow in myelodysplastic syndrome (MDS) and aplastic anemia (AA)

OBJECTIVE

The subsets and the polarization of lymphocytes in bone marrow from low-risk myelodysplastic syndrome (MDS) were studied and compared with those from patients with aplastic anemia (AA).

METHODS

A total of 34 patients with low-risk MDS (IPSS score< or =1.0) who presented abnormal chromosomes and 16 patients with AA were enrolled in this study. We determined T lymphocyte subsets, T cells polarization status, and the percentages of NK cells and of B lymphocytes in bone marrow and compared these parameters between the two groups of patients. As controls, 24 patients with high-risk MDS (IPSS score>1.0) presenting abnormal chromosomes and 22 healthy/benign hematologic disease subjects were used.

RESULTS

In low-risk MDS/AA patients, the percentage of CD3+ lymphocytes was significantly increased compared to controls (p=0.006 and p=0.001), while the percentage of CD19+ lymphocytes was significantly decreased (p<0.001 and p=0.002); there were no significant differences between MDS/AA and normal controls in other parameters; For low-risk MDS patients, the polarization status of bone marrow CD4+ cells toward Th1 (Th1/Th2) and of CD8+ cells toward Tc1 (Tc1/Tc2) was stronger than that for AA patients (p=0.05 and p<0.001). Other parameters did not show significant differences; Regardless of the predominance of CD4 or CD8 T cells, all patients with low-risk MDS were accompanied with elevated Tc1 polarization (Tc1/Tc2).

CONCLUSION

In both AA and MDS, the number of total T lymphocytes increased. However, polarization towards Th1 and Tc1 was obviously stronger in MDS patients than in AA patients. This might be related to T cell stimulation from the clones of malignant hematopoietic cells.

Abnormal immunity and stem/progenitor cells in acquired aplastic anemia

Acquired aplastic anemia (AA) is considered as an immune-mediated bone marrow failure syndrome, characterized by hypoplasia and pancytopenia with fatty bone marrow. Abnormal immunity is the major factor mediating the pathogenesis of acquired AA. Activated DCs might promote the polarization to Th1 cells, and activate CD8(+) T cells. A variety of immune molecules including IFN-gamma, TNF-alpha, MIP-1alpha and IL-2, 8, 12, 15, 17, 23, produced by them and stromal cells, compose a cytokine network to destruct stem/progenitor cells as well as hematopoietic stem/progenitor cells, mesenchymal stem cells (MSCs) and angioblasts/endothelial progenitor cells. Inversely, deficient MSCs, CD4(+)CD25(+) T cells, NK cells, NKT cells and early hematopoietic growth factors diminish the capacity of immune regulation and the support of hematopoiesis. As a result, stem/progenitor cells are significantly impaired to be disabled cells with markedly deficient proliferation, differentiation, induced apoptosis and dysfunctional response to growth factor stimuli, together with rare normal ones. Although some patients can be ameliorated by stem-cell transplantation or immunosuppressive therapy, more effective and convenient therapies such as patient-specific pluripotent iPS cells based on definite pathogenesis are expected.

Acquired aplastic anemia in childhood

In comparison to past decades, children who have acquired aplastic anemia (AA) enjoy excellent overall survival that reflects improvements in supportive care, more accurate exclusion of children who have alternate diagnoses, and advances in transplantation and immunosuppressive therapy (IST). Matched sibling-donor hematopoietic stem cell transplants (HSCT) routinely provide long-term survival in the range of 90%, and 75% of patients respond to IST. In this latter group, the barriers to overall and complication-free survival include recurrence of AA, clonal evolution with transformation to myelodysplasia/acute myelogenous leukemia, and therapy-related toxicities. Improvements in predicting responses to IST, in alternative-donor HSCT, and in rationalizing therapy by understanding the pathophysiology in individual patients are likely to improve short- and long-term outcomes for these children.

Clinical impact of HLA-DR15, a minor population of paroxysmal nocturnal haemoglobinuria-type cells, and an aplastic anaemia-associated autoantibody in children with acquired aplastic anaemia

Aplastic anaemia (AA) is defined as a pancytopenia caused by bone marrow failure, and its pathogenesis is thought to involve autoimmune processes. Several predictive markers of the response to immunosuppressive therapy (IST) have been proposed, which appear to reflect the immune pathophysiology. We prospectively investigated the presence of human leucocyte antigen (HLA)-DR15, a minor population of paroxysmal nocturnal haemoglobinuria (PNH)-type cells, and antibodies to the recently identified autoantigen postmeiotic segregation increased 1 (PMS1) in 103 children with AA enrolled in a multicentre study. In contrast to adults, children with AA did not show an increased frequency of HLA-DR15. In addition, a sensitive flow cytometric assay revealed that children with AA have a much lower prevalence of PNH-type cells (21.4%) than reported for adults with this disease. An immunoblotting assay detected anti-PMS1 antibody in 15 of 103 (14.6%) of the children. Finally, the response rate to IST was not significantly different between patients with and without DR15 (45.5% vs. 54.0%), PNH-type cells (68.2% vs. 53.1%) or anti-PMS1 antibody (40.0% vs. 59.1%). The current study did not confirm a correlation between these markers and the response to IST, suggesting that there is a difference in the pathophysiologies of adult and paediatric AA.

Prevalence and clinical association of clonal T-cell expansions in Myelodysplastic Syndrome

Selected patients with Myelodysplastic Syndromes (MDS) are responsive to immunosuppressive therapy, suggesting that hematopoietic suppressive T cells have a pathogenic role in ineffective hematopoiesis. We assessed T-cell receptor (TCR) clonality through combined flow cytometry and molecular analysis of the complementarity determining region (CDR)-3 of the T-cell receptor-Vbeta gene. We identified clonal T cells in 50% of MDS patients (n=52) compared to 5% of age-matched normal controls (n=20). The presence of T-cell clones was not associated with features linked previously to immunosuppression response, including WHO diagnostic category, karyotype, marrow cellularity, IPSS category, sex or age <or=60. Using flow cytometry to identify expanded Vbeta-families, we found that T cells showed greater expansion in the bone marrow compared with peripheral blood, and were characterized as CD8(+)/CD57(+)/CD28(-) effector T cells. Expanded effector T cell were CD62L negative and expressed the natural killer C-lectin-family receptor NKG2D and CD244 (2B4). We conclude that clonal T-cell expansion is common among all MDS prognostic subgroups.

Expression of CD28 and CTLA4 on T cells in bone morrow of immune-mediated aplastic anemia mice

To investigate the expression and significance of CD28 and CTLA4 on T cells in bone marrow of aplastic anemia (AA) mice, in vitro bone marrow mononuclear cells (BMMNCs) were activated through being incubated with PHA (15 microg/mL). The expression of CD28 and CTLA4 on T cells incubated with or without PHA was detected by two-color flow cytometry. The expression of CD28 and CTLA4 was significantly increased after PHA stimulation. In the AA mice, the expression of CD28 with or without PHA stimulation was both higher than that in the normal mice (both P < 0.01), but the expression of CTLA4 with or without PHA stimulation showed no significant difference in comparison to that in the normal mice (both P > 0.05). In the AA mice, there were more activation and activated potential of T cells than the normal, and the abnormal expression of CD28 and CTLA4 may participate in immunological disorder mediated by T cells.

Antigen-recognition sites of micromanipulated T cells in patients with acquired aplastic anemia

OBJECTIVE

Acquired aplastic anemia (AA) is a rare disorder characterized by pancytopenia and hypocellular bone marrow. Though experimental and clinical data suggest that AA represents a T cell-mediated disease, neither the immune response nor the nature of inciting antigen(s) have been characterized so far. The identification of a restricted T cell repertoire by PCR techniques in total lymphocyte populations supports an antigen-driven T cell response. In order to investigate the clonal composition, we analyzed the gene rearrangements of the T cell receptor (TCR) variable beta chain (Vbeta) at the single-cell level.

PATIENTS AND METHODS

CD3(+) T lymphocytes were micromanipulated from peripheral blood and bone marrow samples of 8 AA patients and healthy controls. Subsequently amplified VDJ gene segments of the TCRVbeta chain were analyzed for functional rearrangements. More than 500 functionally rearranged TCR loci were studied for Vbeta/Jbeta gene segment usage and molecular composition of the complementary-determining region 3 (CDR3).

RESULTS

In comparison to healthy controls, the Vbeta sequences confirmed a highly restricted T cell repertoire in AA patients at the single-cell level. Both in bone marrow and peripheral blood a predominance of Vbeta13 and Jbeta2S7 was observed. Furthermore, individual clonal T-cell expansion was identified in the majority of patients. However, deduced CDR3 amino acid sequences revealed a high variability without common motifs among the 8 patients.

CONCLUSION

Individual clonal T-cell expansion with high diversity of the antigen-binding sites among the analyzed patients argues for the predominance of private inciting epitopes in AA.

Hemangiopoietin, a novel human growth factor for the primitive cells of both hematopoietic and endothelial cell lineages

The cells of hematopoietic and vascular endothelial cell lineages are believed to share a common precursor, termed hemangioblast. However, the existence of a growth factor acting relatively specifically on hemangioblasts remains unclear. Here we report the identification of hemangiopoietin (HAPO), a novel growth factor acting on both hematopoietic and endothelial cell lineages. In vitro in the human system, recombinant human HAPO (rhHAPO) significantly stimulated the proliferation and hematopoietic and/or endothelial differentiation of human bone marrow mononuclear cells and of purified CD34+, CD133+, kinase domain receptor-positive (KDR+), or CD34+/KDR+ cell populations. In the murine system, rhHAPO stimulated the proliferation of long-term culture-initiating cells (LTC-ICs) as well as CD34+ and stem cell antigen-1 (Sca-1+) cell subsets. In vivo, subcutaneous injection of rhHAPO into normal mice resulted in a significant increase in bone marrow hematopoietic cells. Furthermore, irradiated mice injected with rhHAPO had an enhanced survival rate and accelerated hematopoiesis. Our data suggest that HAPO is a novel growth factor acting on the primitive cells of both hematopoietic and endothelial cell lineages and that HAPO may have a clinical potential in the treatment of various cytopenias and radiation injury and in the expansion of hematopoietic and endothelial stem/progenitor cells.

Autoantibodies frequently detected in patients with aplastic anemia

Although accumulating evidence strongly suggests that aplastic anemia (AA) is a T cell-mediated autoimmune disease, no target antigens have yet been described for AA. In autoimmune diseases, target autoantigens frequently induce not only cellular T-cell responses but also humoral B-cell responses. We hypothesized that the presence of antigen-specific autoantibodies could be used as a "surrogate marker" for the identification of target T-cell autoantigens in AA patients. We screened a human fetal liver library for serologic reactivity against hematopoietic stem/progenitor cell antigens and isolated 32 genes. In 7 of 18 AA patients, an immunoglobulin G (IgG) antibody response was detected to one of the genes, kinectin, which is expressed in all hematopoietic cell lineages tested including CD34+ cells. No response to kinectin was detected in healthy volunteers, multiply transfused non-AA patients, or patients with other autoimmune diseases. Epitope mapping of IgG autoantibodies against kinectin revealed that the responses to several of the epitopes were shared by different AA patients. Moreover, CD8+ cytotoxic T cells raised against kinectin-derived peptides suppressed the colony formation of granulocyte macrophage colony-forming units (CFU-GMs) in an HLA class I-restricted fashion. These results suggest that kinectin may be a candidate autoantigen that is involved in the pathophysiology of AA.

Persistent questions about the treatment of severe aplastic anemia in children as illustrated by five cases

We present our recent experience treating five children with severe aplastic anemia (SAA). One patient was transplanted and the remaining 4 patients were treated with immunosuppression of various types and duration. These five cases illustrate the many outstanding points which remain to be clarified regarding the care of children with SAA. We have outlined the treatment options available to each of these patients and the path that we followed along with their outcomes. Additional research and consideration is necessary to arrive at definitive answers to the questions posed by these five cases.