Identification of B/macrophage progenitors in adult bone marrow

Immune escape of B-cell lymphoblastic leukemic cells through a lineage switch to acute myeloid leukemia

Acute leukemia (AL) with a lineage switch (LS) is associated with poor prognosis. The predisposing factors of LS are unknown, apart from KMT2A rearrangements that have been reported to be associated with LS. Herein, we present two cases and review all 104 published cases to identify risk factors for LS. Most of the patients (75.5%) experienced a switch from the lymphoid phenotype to the myeloid phenotype. Eighteen patients (17.0%) experienced a transformation from acute myelogenous leukemia (AML) to acute lymphoblastic leukemia (ALL). Forty-nine (46.2%) patients carried a KMT2A rearrangement. Most of the cases involved LS from B-cell ALL (B-ALL) to AML (59.4%), and 49 patients (46.2%) carried KMT2A-rearrangements. Forty patients (37.7%) received lineage-specific immunotherapy. Our findings suggest that the prevalence of KMT2A rearrangements together with the lineage-specific immunotherapy may trigger LS, which supports the thesis of the existence of leukemia stem cells that are capable of lymphoid or myeloid differentiation.

The Cebpa +37-kb enhancer directs transgene expression to myeloid progenitors and to long-term hematopoietic stem cells

C/EBPα is expressed preferentially in myeloid compared with lymphoid or erythroid cells and directs myeloid lineage specification. C/EBPα is also expressed at lower levels in HSCs and in several nonhematopoietic tissues. The Cebpa gene has a conserved, 450-bp segment at +37 kb that harbors enhancer-specific epigenetic marks and is activate in a myeloid cell line. Herein, we characterize transgenic C57BL/6 mice, in which the Cebpa enhancer and 845-bp promoter regulate a hCD4 reporter. FACS analysis, in vitro colony assays, and in vivo competitive and secondary transplantation revealed that myeloid but not MEPs or lymphoid progenitors and also functional LT-HSCs are found almost exclusively in the Cebpa-hCD4(+) compared with hCD4(-) marrow population. hCD4(+) CMP yielded predominantly myeloid, whereas hCD4(-) CMP generated mainly Meg/E colonies. Providing insight into control of CMP maturation, Cebpa and Pu.1 RNAs were preferentially expressed in hCD4(+) CMP, Scl, Gata2, Gata1, Klf1, Ets1, and Fli1 predominated in hCD4(-) CMP, and Runx1, Myb, HoxA9, and Erg levels were similar in both. Cebpa-hCD4 transgene expression was lacking in multiple nonhematopoietic tissues. In summary, the +37-kb Cebpa enhancer and promoter are sufficient for marrow myeloid progenitor and LT-HSC-specific expression.

Lineage switch in childhood acute leukemia: an unusual event with poor outcome

Although rarely, switches between lymphoid and myeloid lineages may occur during treatment of acute leukemias (AL). Correct diagnosis relies upon confirmation by immunophenotyping of the lineage conversion and certification that the same cytogenetic/molecular alterations remain despite the phenotypic changes. From a total of 1,482 AL pediatric patients, we report nine cases of lineage conversion (0.6%), seven from lymphoid (four Pro-B, two Pre-B, one Common) to myelo-monocytic, and two from myeloid (bilineal, with myeloid predominance) to Pro-B. Eight patients were infants. Switches were suggested by morphology and confirmed with a median of 15 days (range: 8 days-6 months) from initiation of therapy. Of note, in five cases switches occurred before day 15. Stability of the clonal abnormalities was assessed by cytogenetic, RT-PCR/Ig-TCR rearrangement studies in all patients. Abnormalities in 11q23/MLL gene were detected in seven cases. Treatment schedules were ALL (two pts), Interfant-99 (five pts) and AML (two pts) protocols. Despite changing chemotherapy according to the new lineage, all patients died. Our findings support the association of lineage switches with MLL gene alterations and the involvement of a common lymphoid B-myeloid precursor. New therapies should be designed to address these rare cases. Possible mechanisms implicated are discussed.

The histopathologic and molecular basis for the diagnosis of histiocytic sarcoma and histiocyte-associated lymphoma of mice

Histiocytic sarcoma (HS) and histiocyte-associated lymphoma (HAL) of mice are difficult to distinguish histologically. Studies of multiple cases initially diagnosed as HS or HAL allowed us to define HS as round, fusiform, or mixed cell types that were F4/80+, Mac-2+, and PAX5-; that lacked markers for other sarcomas; and that had immune receptor genes in germline configuration. Two other subsets had clonal populations of lymphocytes. The first, HAL, featured malignant lymphocytes admixed with large populations of normal-appearing histiocytes. The second appeared to be composites of lymphoma and HS. Several cases suggestive of B myeloid-lineage plasticity were also observed.

Resolution of unique Sca-1highc-Kit- lymphoid-biased progenitors in adult bone marrow

We have identified a distinctive lymphoid-restricted progenitor population in adult mouse bone marrow based on a unique c-Kit(-)Sca-1(high)Flt3(+) AA4(+) surface phenotype. These cells are highly lymphoid biased and rapidly generate B and T cells after adoptive transfer. However, whereas previously described lymphoid progenitors such as common lymphoid progenitors express TdT and relatively high levels of RAG2, and are enriched for cells with an active V(D)J recombinase, Flt3(+) AA4(+) cells within the c-Kit(-)Sca-1(high) bone marrow fraction are TdT(-), are RAG2(low), and do not display evidence for ongoing or past recombinase activity. Furthermore, unlike common lymphoid progenitors that readily generate B cells upon stimulation with IL-7, c-Kit(-)Sca-1(high)Flt3(+) precursors do not express abundant levels of the IL-7R, and require costimulation with Flt3 ligand and IL-7 to generate B cells in vitro. Moreover, these findings suggest that hematopoietic stem cells in adults generate an array of lymphoid-biased progenitor populations characterized by distinct gene expression and cytokine response profiles.

The Protean Nature of Cells in the B Lymphocyte Lineage

The subdivision of bone marrow (BM) with surface markers and reporter systems and the use of multiple culture and transplantation assays to assess differentiation potential have led to extraordinary progress in defining stages of B lymphopoiesis between the hematopoietic stem cell and B cell receptor (BCR)-expressing lymphocytes. Despite the lack of standard nomenclature and a series of technical issues that still need to be resolved, there seems to be a general consensus regarding the major route to becoming a B cell. Nevertheless, evidence that additional, minor pathways through which B lineage cells are generated exists, and a new appreciation that lymphoid progenitors are protean and able to alter their differentiation potential during embryogenesis and after birth in response to infections suggests that a full understanding of B cell development and how it is regulated has not yet been attained.

IL-7 induces myelopoiesis and erythropoiesis

IL-7 administration to mice was previously reported to increase the mobilization of progenitor cells from marrow to peripheral sites. We now report that IL-7 increases the number of mature myeloid and monocytic cells in spleen and peripheral blood. This effect required T cells, and we show that IL-7 treatment in vivo induced GM-CSF and IL-3 production by T cells with memory phenotype. However, additional myelopoietic cytokines were shown to be involved because mice deficient in both GM-CSF and IL-3 also responded to IL-7 with increased myelopoiesis. Candidate cytokines included IFN-gamma and Flt3 ligand, which were also produced in response to IL-7. Because IFN-gamma-deficient mice also increased myelopoiesis, it was suggested that IL-7 induced production of redundant myelopoietic cytokines. In support of this hypothesis, we found that the supernatant from IL-7-treated, purified T cells contained myelopoietic activity that required a combination of Abs against GM-CSF, IL-3, and anti-Flt3 ligand to achieve maximum neutralization. IL-7 administration increased the number of splenic erythroid cells in either normal, Rag1 or GM-CSF-IL-3-deficient mice, suggesting that IL-7 might directly act on erythroid progenitors. In support of this theory, we detected a percentage of TER-119(+) erythroid cells that expressed the IL-7Ralpha-chain and common gamma-chain. Bone marrow cells expressing IL-7R and B220 generated erythroid colonies in vitro in response to IL-7, erythropoietin, and stem cell factor. This study demonstrates that IL-7 can promote nonlymphoid hemopoiesis and production of cytokines active in the host defense system in vivo, supporting its possible clinical utility.

Interleukin-7 partially rescues B-lymphopoiesis in osteopetrotic oc/oc mice through the engagement of B220+ CD11b+ progenitors

OBJECTIVE

We recently identified in the mouse bone marrow a B-lymphoid/myeloid B220+ CD11b+ progenitor population. This population is accumulated in the osteopetrotic oc/oc mouse, which suggests that it could be controlled by bone marrow factors whose expression varies in this pathologic bone environment. Among the possible factors, interleukin (IL)-7 is involved in the control of B lymphopoiesis and osteoclastogenesis. Therefore, we hypothesized that IL-7 could regulate the accumulation of the B220+ CD11b+ population in oc/oc mice.

METHODS

B220+ CD11b+ cells sorted from oc/oc mice were treated with IL-7 and their phenotype was analyzed by flow cytometry and real-time reverse transcriptase polymerase chain reaction (RT-PCR). In vivo, IL-7 was injected in oc/oc mice, and B220+ CD11b+ and B cells, as well as B-cell proliferation and apoptosis, were analyzed by flow cytometry. The expression of B lymphopoiesis and myelopoiesis markers was analyzed by real-time RT-PCR.

RESULTS

In vitro, IL-7 induced the differentiation of B220+ CD11b+ cells into B lymphocytes through the induction of Pax5 and the inhibition of myeloid markers. In vivo, IL-7 injections in oc/oc mice induced a decrease of the B220+ CD11b+ population and the partial restoration of B-cell population, which was reduced in oc/oc mice. In parallel, upon IL-7 injections, Pax5 expression was induced in B220+ cells and B-cell apoptosis was reduced.

CONCLUSIONS

Our results demonstrate that IL-7 injection can partially rescue B lymphopoiesis in oc/oc mice through the engagement of the B220+ CD11b+ population in the B-lymphoid pathway. Therefore, IL-7 delivery could represent a new therapeutic perspective to circumvent the lymphopenia observed in infantile malignant osteopetrosis patients.

Identification of a human B-cell/myeloid common progenitor by the absence of CXCR4

CXCR4 is a chemokine receptor required for hematopoietic stem cell engraftment and B-cell development. This study found that a small fraction of primitive CD34(+)/CD19(+) B-cell progenitors do not express CXCR4. These CD34(+)/CD19(+)/CXCR4(-) cells were also remarkable for the relative lack of primitive myeloid or lymphoid surface markers. When placed in B-lymphocyte culture conditions these cells matured to express CXCR4 and other surface antigens characteristic of B cells. Surprisingly, when placed in a myeloid culture environment, the CXCR4(-) B-cell progenitors could differentiate into granulocyte, macrophage, and erythroid cells at a high frequency. These data define a novel B-cell/myeloid common progenitor (termed the BMP) and imply a less restrictive pathway of myeloid versus lymphoid development than previously postulated.

Mature B-cell acute lymphoblastic leukemia with t(9;11) translocation: a distinct subset of B-cell acute lymphoblastic leukemia

Mature B-cell acute lymphoblastic leukemia (ALL) is typically associated with the FAB-L3 morphology and rearrangement of the MYC gene, features characteristic of the leukemic phase of Burkitt's lymphoma. However, the term 'mature' has also been used to describe other rare cases of B-ALL with light-chain surface immunoglobulin expression. In contrast, infantile B-cell ALL is generally characterized by rearrangement of the MLL gene, an immature pro-B-cell phenotype, and CD10 negativity. We describe two unusual cases of infantile B-ALL with non-L3 morphology, expressing a mature B-cell phenotype (lambda sIg+, CD19+, CD10-, TdT-, and CD34-), and showing MLL rearrangement without MYC rearrangement at presentation. Both infants relapsed after months of morphologic and genetic remission. At relapse, the t(9;11) translocation was detected in both cases by spectral karyotyping. After the initial relapse, both cases followed a rapid and aggressive course. Literature search identified few similar cases, all expressed lambda surface immunoglobulin and showed MLL rearrangement (majority with the t(9;11) translocation). These cases show that B-ALL with MLL rearrangement, especially the t(9;11) translocation, can express a 'mature' B-cell phenotype and may represent a distinct subset. Identification of additional cases will further clarify the significance of MLL rearrangements in mature B-ALL.