T cell-mediated inhibition of haematopoiesis in aplastic anaemia: serial assay of inhibitory activities of T cells to autologous CFU-E during immunosuppressive therapy

HLA-DR2 Frequency Increase in Severe Aplastic Anemia Patients is Mainly Attributed to the Prevalence of DR15 Subtype

The association between severe aplastic anemia (AA) and DR2 antigen seems to be well established. However, since discrimination between two DR2-associated splits, namely DR15 and DR16, rarely was performed, it remains unclear whether one or both of these subvariants are responsible for AA susceptibility. In this study, we have analyzed the HLA-DR allelic distribution in a group of 37 AA patients of slavic origin from North-Western Russia. The experimental design included PCR-based amplification of DRB-specific sequences, followed by reverse dot-blot hybridization of the biotinylated PCR-product with the set of sequence-specific oligonucleotide probes. HLA-DRB alleles were identified by non-radioactive enzymatic reaction, then standard serological specificities of HLA-DR antigen were estimated according to the WHO nomenclature. Whereas DR15 subtype occurred more often in the patients (23.0% vs. 13.3%, p< 0.05), DR16 split did not show the same tendency. The results, show the overall predominance of HLA-DR2 specificity (DR15+DR16) did not reach statistical significance (24.4% vs.17.5%, p<0.2). Thus, we conclude that repeatedly reported DR2 frequency increase in AA patients is mainly attributed to the prevalence of DR15 subtype.

Regulation of autologous T-lymphocyte clones on in vitro growth of BFU-e: enhancing effects of both CD4- and CD8-positive clones

In order to characterize the effect of T lymphocytes on the in vitro growth of erythroid progenitor cells (EPC), we have optimized an assay for evaluating the influence of autologous T-lymphocyte clones on BFU-e, grown from PBMNCs depleted of T lymphocytes. Testing a large number of T-lymphocyte clones from 2 individuals we found that, irrespective of immunological phenotype, all clones tested were found to enhance the growth of BFU-e. Thus, no evidence for a functional dichotomy between CD4+ and CD8+ T lymphocytes was found in this system. The enhancing effect worked across a histocompatibility barrier and was, at least in part, mediated by soluble factors. We therefore conclude that the role of the majority of cloned normal T lymphocytes in the regulation of the in vitro growth of EPC is a supportive one.

Pathophysiology of aplastic anaemia

No single pathophysiological phenomenon--neither the intrinsic defect of haemopoiesis nor any of the described immune effects--explains aplastic anaemia. Since the intrinsic defect is compatible with near normal haemopoietic function, as seen in autologous bone marrow reconstitution, it cannot be the cause of severe pancytopenia. On the other hand, immune mechanisms cannot be the primary cause of the disease, otherwise haemopoietic function would recover to complete normality after immunosuppressive therapy. From these observations we deduce that the intrinsic defect, a premalignant haemopoietic disorder, can either be clinically quiescent by virtue of repair mechanisms, or induce auto-reactivity of the immune system against the abnormal haemopoietic tissue, drugs, chemicals and viruses acting as non-specific triggers or amplifiers. In this sense, aplastic anaemia could be interpreted as an attempt to 'self-cure' from a variant type of preleukaemia. This means that the original concept of aplastic anaemia being a hypoplastic variant of leukaemia may be true. The fact that aplastic anaemia can present either as acute severe bone marrow failure, as chronic mild pancytopenia or as a myelodysplasia-like syndrome does not imply that the underlying pathophysiological mechanisms are basically different. Variations of the clinical course and the response to immunosuppressive treatment could be explained by variations in the balance between the primary defect and the secondary immune reaction; the co-involvement of accessory cells in the primary disease; the relative time course of the two components and the efficiency of repair mechanisms. From repeated in vitro studies in a large group of aplastic anaemia patients at various stages of disease this concept can be applied to the majority of cases, including chloramphenicol- and virus-induced aplastic anaemia. In a small proportion of patients with pancytopenia occurring after exposure to certain drugs other than chloramphenicol, aplastic anaemia is rapidly and completely reversible after withdrawal of the drug. These patients probably have truly benign aplastic anaemia and thus differ from the majority of patients who are left with a permanently fragile bone marrow once they have acquired aplastic anaemia.

Aplastic anemia associated with in vitro inhibition of erythropoiesis by bone marrow-adherent cells

A patient with aplastic anemia that evolved following pure red cell aplasia is described. Cultures of the patient's marrow cells revealed greatly reduced numbers of primitive (BFU-E) and relatively mature (CFU-E) erythroid progenitors, but normal numbers of multipotential (CFU-GEMM) precursors. The BFU-E/CFU-GEMM and CFU-E/BFU-E ratios in the patient's marrow cell cultures were also reduced. T cell- or antibody-mediated inhibition of in vitro erythropoiesis could not be demonstrated in this patient. However, the patient's marrow-adherent cells suppressed the growth of autologous and allogeneic BFU-E and CFU-E, without influencing the growth of CFU-GEMM. Medium conditioned by the patient's adherent cells failed to inhibit the growth of normal erythroid precursors. Our findings suggest a role for marrow-adherent cells in the pathogenesis of aplastic anemia in this patient.

No evidence for gamma-interferon mediated haematopoietic inhibition by T cells in aplastic anaemia: an observation in the course of immunosuppressive therapy

Two patients with aplastic anaemia were treated with immunosuppressive agents and peripheral blood T cells were cryopreserved serially. Inhibitory activity of T cells to autologous CFU-E, gamma-IFN production by T cells and T cell subpopulations were assayed after remission. Inhibitory activity to autologous CFU-E was not correlated with the numbers or ratios of T cell subpopulations. gamma-IFN production by T cells were within the normal range when inhibitory activity was found. In addition, gamma-IFN production increased after haematopoietic recovery. These findings suggest that gamma-IFN is not a soluble mediator of T cell-mediated haematopoietic inhibition in aplastic anaemia.