Inhibitory effects of T cells on in vitro granulopoiesis, erythropoiesis, and immunoglobulin production in patients with aplastic anaemia

Regulation of hematopoiesis by T lymphocytes and natural killer cells

T lymphocytes and natural killer (NK) cells exert both stimulatory and suppressive effects that regulate growth and differentiation of hematopoietic cells. Activated T and NK cells have been demonstrated in different pathological states of bone marrow failure and are proposed to play a role in the pathogenesis of the disease. T and NK cells have also been shown to be responsible for bone marrow graft rejection in both allogeneic and syngeneic donor/recipient combinations. Lymphocytes can regulate hematopoietic cell growth by direct cellular contact or by releasing soluble factors, such as colony-stimulating factors, immune interferon, lymphotoxin, and tumor necrosis factor, active on hematopoietic precursor cells.

Gamma interferon and lymphotoxin, released by activated T cells, synergize to inhibit granulocyte/monocyte colony formation

We have shown that lymphocytes stimulated by PHA produce colony-forming unit of granulocyte/monocyte (CFU-GM)-stimulating and -inhibiting activities, IFN-gamma, and lymphotoxin (LT). IFN-gamma is necessary for inhibition of CFU-GM by PHA-conditioned medium (CM), as shown by experiments in which removal of IFN-gamma from PHA-CM abrogated inhibition. However, experiments in which rIFN-gamma was added to IFN-gamma-depleted PHA-CM revealed the presence, in PHA-CM, of other factors that act in synergy with IFN-gamma to inhibit CFU-GM. Fractionation of PHA-CM on a Sephadex G-100 column was used to separate IFN-gamma and LT. Colony-inhibiting activity was eluted in fractions that contained both IFN-gamma and LT activities, identifying LT as a factor present in PHA-CM that synergizes with IFN-gamma to inhibit CFU-GM. Treatment of PHA-CM with mAb against either IFN-gamma or LT completely abrogated the colony-inhibiting activity, demonstrating a requirement for both lymphokines in PHA-CM-induced inhibition of CFU-GM. Experiments using rIFN-gamma and preparations of purified LT confirmed that neither lymphokine alone, when added to bone marrow cells at the concentrations present in PHA-CM, strongly inhibited day 7 or day 14 CFU-GM, but that the two lymphokines, added together, behaved synergistically to inhibit CFU-GM by up to 70%. The inhibition observed using purified preparations of lymphokines shows that synergy between IFN-gamma and LT is sufficient to explain PHA-CM-induced inhibition of CFU-GM. Our findings suggest that activated T cells regulate hematopoiesis through the release of inhibitory as well as stimulatory factors, and that the simultaneous production of IFN-gamma and LT may represent a mechanism of suppression of hematopoiesis in the cases of bone marrow failure associated with the presence of activated T cells.

Hypocellular bone marrow with increased blasts

Twenty patients with hypocellular bone marrow and increased blasts (HBMIB) were reviewed. The median age was 60 years with a male:female ratio of 17:3. History of alcohol abuse was noted in 30%, potential exposure to toxic chemicals in 20%, second malignancies in 20%, and aplastic anemia in 25%. Pancytopenia with marrow hypocellularity and increased marrow blast cells were characteristic hematopathologic features. Marrow hypocellularity was moderate to severe (less than or equal to 25%) in over half of the cases and mild to moderate (greater than 25, less than or equal to 35%) in the remainder. Blast cells were the predominant cellular elements in the marrow displaying scanty to moderate amounts of cytoplasm, round to oval nuclei, and one or more nucleoli. Special stains were performed in 19 cases. Blast cells morphologically displayed myeloid features, but Sudan black B and/or peroxidase positivity was noted in only ten patients. The overall mortality was high, especially in patients undergoing chemotherapy. At 1 year follow-up, 11 patients had received chemotherapy and eight of these eleven were dead compared to three of nine patients dead in those not receiving chemotherapy. Only two patients developed "overt" leukemia evidenced by hypercellular marrow and over 30% blast cells in the peripheral blood. HBMIB is a distinct clinicopathologic entity characterized by severe marrow failure and a low response rate to chemotherapy.

Inhibitory effect of PWM-stimulated OKT4+ subsets on erythro-, granulo- and megakaryocytopoiesis in vitro

Normal human peripheral blood T cells and T-cell subsets defined by monoclonal antibodies of the OKT series were pretreated with pokeweed mitogen (PWM). Their effects on the haematopoietic precursors, erythroid (BFU-E, CFU-E), granulocyte-macrophage (CFU-GM) and megakaryocyte (CFU-M) colony forming cells were evaluated by coculture. While unstimulated T cells and T-cell subsets enhanced growth of autologous blood BFU-E, PWM-stimulated T and OKT4+ cells suppressed it, also inhibiting proliferation of both autologous and allogeneic bone marrow BFU-E, CFU-E, CFU-GM and CFU-M. PWM-stimulated OKT8+ cells had little effect on the growth of any of the precursors at the cell concentration at which growth was completely inhibited by PWM-stimulated OKT4+ cells. Irradiation of T or OKT4+ cells with 3000 rad before PWM stimulation completely abrogated the inhibition. These observations might be related to the mechanism of pancytopenia in some cases of immune-mediated aplastic anaemia.

T-lymphocyte subpopulations in the peripheral blood and bone marrow of patients with aplastic anemia

A decrease in the absolute number of total lymphocytes, OKT3+ and OKT4+ lymphocytes, and a normal number of OKT8+ lymphocytes were found in the peripheral blood of patients with aplastic anemia. The OKT4:OKT8 ratio was decreased in patients due to a reduction in the percentage of OKT4+ cells and 3 out of 18 patients had a ratio less than 1. The values of the OKT4:OKT8 ratio were not associated either with the severity of the disease or with treatment with androgens. There was no correlation between the OKT4:OKT8 ratio and the number of transfusions received by patients. On the other hand, studies performed with bone marrow lymphocytes showed that the OKT4:OKT8 ratio for both patients and controls was lower than that of the peripheral blood. Since the ratio of OKT4:OKT8 cells in aplastic and control bone marrow was similar no direct pathogenic role can be assigned to the marrow for the imbalance detected in the peripheral blood.

Comparison between bone marrow transplantation and antithymocyte globulin in treatment of young patients with severe aplastic anemia

Fifty-seven patients younger than 25 years with severe aplastic anemia underwent either bone marrow transplantation or antithymocyte globulin therapy (ATG) to ascertain which approach should be used in young patients. Thirty-five patients who had an HLA-identical sibling donor underwent bone marrow transplantation after conditioning with cyclophosphamide and low-dose total-body radiation. Twenty-two patients who did not have an HLA-identical donor received ATG. The 2-year actuarial survival of patients after transplant is 72% (95%, CI 64% to 80%), versus 45% (95%, CI 29% to 61%) in those given ATG therapy (P = 0.18). In those patients surviving 6 months after treatment, return of peripheral blood counts to normal values was more common in patients who received marrow transplant compared with those given ATG therapy (P less than 0.001). Furthermore, 24 of 26 transplant survivors had Karnofsky performance scores greater than 90%, compared with only five of 13 ATG survivors. These data suggest that bone marrow transplantation is the preferred therapy for severe aplastic anemia in young patients who have an HLA-identical sibling donor. ATG should be reversed for those young patients with severe aplastic anemia who do not have a histocompatible marrow donor.