Bone marrow from a patient with aplastic anemia was shown by multiple criteria to have a block in early myeloid differentiation. This block was overcome in vitro by elimination of marrow lymphocytes. Furthermore, this differentiation block was transferred in vitro to normal marrow by coculturing with the patient's marrow. We suggest that some cases of aplastic anemia may be due to an immunologically based suppression of marrow cell differentiation rather than to a defect in stem cells or their necessary inductive environment.The hematopoietic system in man is thought to develop from a common stem cell analogous to the spleen colony forming unit in mice (CFU-s) (1), which then differentiates into committed progenitor cells of the granulocytic and monocytic series (CFU-c) (2), megakaryocytic, lymphoid, and erythroid lines, and then passes through several more differentiation steps into mature effector cells. Aplastic anemia is a disease characterized by a marked decrease in the production of erythrocytes, leukocytes, and platelets. The etiology of this kind of bone marrow failure is complex, and about 50% of all cases are idiopathic. There has been considerable speculation about the possible role of an immunologic mechanism in the pathogenesis of some cases (3, 4), but this mechanism has not yet been fully documented. We present here evidence for such a mechanism. The number of thymus-derived (T) lymphocytes was determined by spontaneous rosette formation with SRBC (8). Fifty microliters of bone marrow cells at 3 X 106 cells per ml were mixed with 50 Ml of washed SRBC at 2 X 108 cells per ml and 25 ,l of heat-inactivated SRBC-absorbed fetal calf serum. This mixture was incubated for 5 min at 37?, centrifuged at 200 X g for 5 min at 24°, and then incubated for 3-6 hr at 4°. Four fields of 100 cells were counted in a hemocytometer, and cells were scored as positive if they bound four or more SRBC. CASE REPORTSurface Marker Induction. Separated bone marrow cell fractions were incubated at 1.5 X 106 cells per ml in RPMI-1640 containing 5% fetal calf serum for 8 hr at 37°in humidified 5% CO2 and 95% air. These cultures contained either medium alone (control), or ubiquitin (0.5 ,g/ml). After incubation, the cells were washed twice, resuspended in RPMI-1640 to 3 X 106 cells per ml, and assayed for surface markers. Ubiquitin was prepared as described (9).Spontaneous DNA Synthesis. In the course of studying the response of marrow cells in mixed leukocyte culture, the spontaneous DNA synthesis of cells was measured by substituting irradiated autologous marrow cells for irradiated allogenic peripheral blood lymphocytes.Cells were resuspended at 1 X 106 cells per ml in RPMI-1640 with penicillin (50 units/ml), streptomycin (50 Mug/ml) and 15% 2890Abbreviations: CFU-s, colony forming unit (spleen); CFU-c, colony forming unit (culture); SRBC, sheep erythrocytes; T cell, thymusderived lymphocyte.

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The fitness effects of all possible mutations available to an organism largely shape the dynamics of evolutionary adaptation. Yet, whether and how this adaptive landscape changes over evolutionary times, especially upon ecological diversification and changes in community composition, remains poorly understood. We sought to fill this gap by analyzing a stable community of two closely related ecotypes (“L” and “S”) shortly after they emerged within the E. coli Long-Term Evolution Experiment (LTEE). We engineered genome-wide barcoded transposon libraries to measure the invasion fitness effects of all possible gene knockouts in the coexisting strains as well as their ancestor, for many different, ecologically relevant conditions. We find consistent statistical patterns of fitness effect variation across both genetic background and community composition, despite the idiosyncratic behavior of individual knockouts. Additionally, fitness effects are correlated with evolutionary outcomes for a number of conditions, possibly revealing shifting patterns of adaptation. Together, our results reveal how ecological and epistatic effects combine to shape the adaptive landscape in a nascent ecological community.

Understanding how dynamical responses of biological networks are constrained by underlying network topology is one of the fundamental goals of systems biology. Here we employ monotone systems theory to formulate a theorem stating necessary conditions for non-monotonic time-response of a biochemical network to a monotonic stimulus. We apply this theorem to analyze the non-monotonic dynamics of the σB-regulated glyoxylate shunt gene expression in Mycobacterium tuberculosis cells exposed to hypoxia. We first demonstrate that the known network structure is inconsistent with observed dynamics. To resolve this inconsistency we employ the formulated theorem, modeling simulations and optimization along with follow-up dynamic experimental measurements. We show a requirement for post-translational modulation of σB activity in order to reconcile the network dynamics with its topology. The results of this analysis make testable experimental predictions and demonstrate wider applicability of the developed methodology to a wide class of biological systems.