A close developmental relationship between the lymphoid and myeloid lineages

Effectiveness of venetoclax and azacytidine against myeloid/natural killer cell precursor acute leukemia

Myeloid/natural killer (NK) cell precursor acute leukemia (MNKPL) is a rare leukemia subtype that possibly originates from precursor NK cells. The disease has a poor prognosis, and information on its treatment is lacking. We herein report the first case of a 46-year-old woman with MNKPL who was refractory to two lines of acute myeloid leukemia (AML)-type intensive chemotherapy but was successfully treated with venetoclax and azacytidine (VEN/AZA). She was diagnosed with MNKPL based on the conformations of immature lymphoblastoid morphology without myeloperoxidase reactivity that showed a CD7/CD33/CD34/CD56/HLA-DR positive phenotype and extramedullary regions. The disease was refractory to induction therapy with daunorubicin and cytarabine (DNR/Ara-C) and to reinduction therapy with mitoxantrone, etoposide, and cytarabine (MEC). After two lines of induction chemotherapy, massive pericardial and pleural effusion was found, and was suspected to be extramedullary lesions. The patient developed cardiac tamponade and required pericardiocentesis. Thus, VEN/AZA was administered as third-line therapy. After two cycles of VEN/AZA, the pericardial and pleural effusion disappeared, and complete remission was achieved. The patient received post-transplant cyclophosphamide-based haploidentical transplantation and has stayed relapse-free as of her last follow-up examination 2 years after diagnosis.

Tracing the evolutionary history of blood cells to the unicellular ancestor of animals

Blood cells are thought to have emerged as phagocytes in the common ancestor of animals followed by the appearance of novel blood cell lineages such as thrombocytes, erythrocytes, and lymphocytes, during evolution. However, this speculation is not based on genetic evidence and it is still possible to argue that phagocytes in different species have different origins. It also remains to be clarified how the initial blood cells evolved; whether ancient animals have solely developed de novo programs for phagocytes or they have inherited a key program from ancestral unicellular organisms. Here, we traced the evolutionary history of blood cells, and cross-species comparison of gene expression profiles revealed that phagocytes in various animal species and Capsaspora (C.) owczarzaki, a unicellular organism, are transcriptionally similar to each other. We also found that both phagocytes and C. owczarzaki share a common phagocytic program, and that CEBPα is the sole transcription factor highly expressed in both phagocytes and C. owczarzaki. We further showed that the function of CEBPα to drive phagocyte program in nonphagocytic blood cells has been conserved in tunicate, sponge, and C. owczarzaki. We finally showed that, in murine hematopoiesis, repression of CEBPα to maintain nonphagocytic lineages is commonly achieved by polycomb complexes. These findings indicate that the initial blood cells emerged inheriting a unicellular organism program driven by CEBPα and that the program has also been seamlessly inherited in phagocytes of various animal species throughout evolution.

Non-Coding RNAs in Normal B-Cell Development and in Mantle Cell Lymphoma: From Molecular Mechanism to Biomarker and Therapeutic Agent Potential

B-lymphocytes are essential for an efficient immune response against a variety of pathogens. A large fraction of hematologic malignancies are of B-cell origin, suggesting that the development and activation of B cells must be tightly regulated. In recent years, differentially expressed non-coding RNAs have been identified in mantle cell lymphoma (MCL) tumor samples as opposed to their naive, normal B-cell compartment. These aberrantly expressed molecules, specifically microRNAs (miRNAs), circular RNAs (circRNAs) and long non-coding RNAs (lncRNAs), have a role in cellular growth and survival pathways in various biological models. Here, we provide an overview of current knowledge on the role of non-coding RNAs and their relevant targets in B-cell development, activation and malignant transformation, summarizing the current understanding of the role of aberrant expression of non-coding RNAs in MCL pathobiology with perspectives for clinical use.

A microfibril-associated glycoprotein 4 (MFAP4) from Nile tilapia (Oreochromis niloticus) possesses agglutination and opsonization ability to bacterial pathogens

Microfibril-associated glycoprotein 4 (MFAP4), a pattern recognition-like molecule with a fibrinogen-like domain (FBG), has the ability to combine and agglutinate pathogens, playing an essential role in the first line of innate immune defense. In this study, the sequence of Nile tilapia (Oreochromis niloticus) microfibril-associated glycoprotein 4 (OnMFAP4) open reading frame (ORF) was amplified and identified. The ORF of OnMFAP4 is 720 bp of nucleotides and codes for 239 amino acids. Spatial mRNA encoding analysis indicated that OnMFAP4 was highly produced in liver, intestine and head kidney in healthy tilapia, and with the lowest expression in muscle. After challenges with Streptococcus agalactiae (S. agalactiae) and Aeromonas hydrophila (A. hydrophila), the expression of OnMFAP4 mRNA was prominently produced in the liver, spleen and head kidney. The up-regulation of OnMFAP4 expression was also presented in head kidney monocytes/macrophages (MO/MΦ) and hepatocytes. Recombinant OnMFAP4 ((r)OnMFAP4) could bind and agglutinate both bacterial pathogens. Moreover, (r)OnMFAP4 could take part in the modulation of inflammation and phagocytosis. In conclusion, this study revealed that OnMFAP4 might take effect in host defense against bacterial infection in Nile tilapia, with agglutination and opsonization capability to bacterial pathogens.

Sex steroids exert a suppressive effect on innate and cell mediated immune responses in fresh water teleost, Channa punctatus

The main objective of this study was to evaluate the potential role of two important sex steroids, crucial for reproductive success, on innate and cell mediated immune responses in a seasonally breeding, economically important fish, Channa punctatus. Intraperitoneal injections of testosterone and progesterone were given to different groups of fishes. Spleen and head kidney macrophages were isolated and studied for phagocytosis. Superoxide production and nitrite release by phagocytes were also investigated. Cell mediated immunity was measured by lymphocyte proliferation in presence of T and B cell mitogens. In vitro effect of steroids on mitogen induced lymphocyte proliferation was also analyzed. Results of the present investigation revealed the suppressive effects of testosterone and progesterone on immune responses of cells from spleen and head kidney. Concentration dependent effect of sex steroids were observed in vitro studies where phagocytosis and lymphocyte proliferation were suppressed. Immunosupression by these hormones may be the cost of reproduction and it is postulated that by suppressing immune responses, these steroids may, therefore, act as a physiological check regulating the relative amount of energy invested into either reproductive effort or immunocompetence.

Effects of Cell Differentiation on the Phagocytic Activities of IgM+ B Cells in a Teleost Fish

Teleost B cells have phagocytic activities for ingesting particulate antigens, such as bacteria, in addition to the functional secretion of immunoglobulins (Igs). In the present study, the phagocytic activities of IgM⁺ B cells under various differentiational conditions residing in peripheral blood leukocytes were investigated in a teleost fish Nile tilapia (Oreochromis niloticus). The IgM⁺ B cells were recognized as IgM^lo or IgM^hi subsets based on their membrane IgM (mIgM) levels. The mIgM, secreted IgM (sIgM), major histocompatibility complex class II and reactive oxygen species were detected. Expressions of transcription factors (Pax5 and Blimp-1) and B cell signaling molecules (CD79a, CD79b, BLNK, and LYN) suggested that IgM^lo B cells were resembling as plasma-like cells and IgM^hi resembling as naïve/mature B cells, respectively. Analysis of phagocytic activities demonstrated that both IgM^lo and IgM^hi B cells have a similar phagocytic ability (phagocytosis percentage); however, the phagocytic capacity [phagocytic index and the mean fluorescence intensity (MFI)] of IgM^hi B cells was significantly higher than that of IgM^lo B cells. Taken together, the results indicated that B cell differentiation may cause the decrease of phagocytic capacity but not phagocytic ability of phagocytic IgM⁺ B cells in teleost. The finding may provide an evolutionary evidence for understanding the greater specialization of the B cell in more sophisticated adaptive humoral immunity, by decreasing phagocytic activity in order to contribute its function more specifically into antibody-secreting.

To be red or white: lineage commitment and maintenance of the hematopoietic system by the "inner myeloid"

Differentiation of hematopoietic stem and progenitor cells is tightly regulated depending on environmental changes in order to maintain homeostasis. Transcription factors direct the development of hematopoietic cells, such as GATA-1 for erythropoiesis and PU.1 for myelopoiesis. However, recent findings obtained from single-cell analyses raise the question of whether these transcription factors are "initiators" or just "executors" of differentiation, leaving the initiation of hematopoietic stem and progenitor cell differentiation (i.e. lineage commitment) unclear. While a stochastic process is likely involved in commitment, it cannot fully explain the homeostasis of hematopoiesis nor "on-demand" hematopoiesis in response to environmental changes. Transcription factors BACH1 and BACH2 may regulate both commitment and on-demand hematopoiesis because they control erythroid-myeloid and lymphoid-myeloid differentiation by repressing the myeloid program, and their activities are repressed in response to infectious and inflammatory conditions. We summarize possible mechanisms of lineage commitment of hematopoietic stem and progenitor cells suggested by recent findings and discuss the erythroid and lymphoid commitment of hematopoietic stem and progenitor cells, focusing on the gene regulatory network composed of genes encoding key transcription factors. Surprising similarity exists between commitment to erythroid and lymphoid lineages, including repression of the myeloid program by BACH factors. The suggested gene regulatory network of BACH factors sheds light on the myeloid-based model of hematopoiesis. This model will help to understand the tuning of hematopoiesis in higher eukaryotes in the steady-state condition as well as in emergency conditions, the evolutional history of the system, aging and hematopoietic disorders.

Integrative genomic analysis of adult mixed phenotype acute leukemia delineates lineage associated molecular subtypes

Mixed phenotype acute leukemia (MPAL) is a rare subtype of acute leukemia characterized by leukemic blasts presenting myeloid and lymphoid markers. Here we report data from integrated genomic analysis on 31 MPAL samples and compare molecular profiling with that from acute myeloid leukemia (AML), B cell acute lymphoblastic leukemia (B-ALL), and T cell acute lymphoblastic leukemia (T-ALL). Consistent with the mixed immunophenotype, both AML-type and ALL-type mutations are detected in MPAL. Myeloid-B and myeloid-T MPAL show distinct mutation and methylation signatures that are associated with differences in lineage-commitment gene expressions. Genome-wide methylation comparison among MPAL, AML, B-ALL, and T-ALL sub-classifies MPAL into AML-type and ALL-type MPAL, which is associated with better clinical response when lineage-matched therapy is given. These results elucidate the genetic and epigenetic heterogeneity of MPAL and its genetic distinction from AML, B-ALL, and T-ALL and further provide proof of concept for a molecularly guided precision therapy approach in MPAL.

Mixed phenotype acute leukemia (MPAL) is a rare leukemia that presents both myeloid and lymphoid markers on blasts. Here the authors perform genomic analysis to show MPAL involves genetic and epigenetic heterogeneity and is genetically distinct from AML, B-ALL, and T-ALL.

Reemergence of translocation t(11;19)(q23;p13.1) in the absence of clinically overt leukemia

We report the case of a 10-year-old female with acute myeloid leukemia (AML) FAB M0 carrying a novel t(11;19)(q23;p13.1) MLL-ELL variant, in which intron 8 of MLL is fused to exon 6 of ELL. Complete remission, judged by morphology and cytogenetic analysis, was achieved after the conventional chemotherapy. Eight months after completion of therapy, the level of WT-1 in peripheral blood and the number of cells with the MLL-ELL fusion transcript resurged. However, the patient remained overtly healthy and the morphology in the bone-marrow smear was innocuous, with no sign of relapse or secondary leukemia. Without any evidence of relapse, the patient has been closely observed without any therapeutic intervention. For approximately 2 years after the completion of therapy, despite clonal proliferation of pre-leukemic cells with an MLL-ELL fusion gene, she has maintained complete remission. In this case, the rare variant form of MLL-ELL fusion that has been identified may be related to diminished leukemogenic capacity, resulting in the persistence of pre-leukemic status; an additional genetic abnormality may thus be necessary for full transformation of pre-leukemic cells.

Characterization of γδ T Cells from Zebrafish Provides Insights into Their Important Role in Adaptive Humoral Immunity

γδ T cells represent an evolutionarily primitive T cell subset characterized by distinct T cell receptors (TCRs) and innate and adaptive immune functions. However, the presence of this T cell subset in ancient vertebrates remains unclear. In this study, γδ T cells from a zebrafish (Danio rerio) model were subjected to molecular and cellular characterizations. The constant regions of zebrafish TCR-γ (DrTRGC) and δ (DrTRDC) were initially identified. Zebrafish γδ T cells accounted for 7.7–20.5% of the total lymphocytes in spleen, head kidney, peripheral blood, skin, gill, and intestine tissues. They possess typical morphological features of lymphocytes with a surface phenotype of γ⁺δ⁺CD4⁻CD8⁺. Zebrafish γδ T cells functionally showed a potent phagocytic ability to both soluble and particulate antigens. They can also act as an antigen-presenting cell to initiate antigen (KLH)-specific CD4⁺ T_KLH cell activation and to induce B cell proliferation and IgM production. Particularly, zebrafish γδ T cells also play a critical role in antigen-specific IgZ production in intestinal mucus. These findings demonstrated that γδ T cells had been originated as early as teleost fish, which providing valuable insights into the evolutionary history of T cell subset. It is anticipated that this study would be used as a guide to develop a zebrafish model for the cross-species investigation of γδ T cell biology.

Mixed Phenotypic Acute Leukemia Presenting as Mediastinal Mass-2 Cases

Mixed phenotype acute leukemia symbolizes a very small subset of acute leukemia that simply cannot be allocated as lymphoid or myeloid lineage. The 2008 World Health Organisation classification established stringent standard for diagnosis of mixed phenotype acute leukemia, accentuating myeloperoxidase for myeloid lineage, cytoplasmic CD3 for T lineage and CD19 with other B markers for B lineage obligation. Mixed phenotype leukemia is rare and 3-5 % of acute leukmias of all age groups, is associated with poor outcome with overall survival of 18 months. We wish to present two cases of mixed phenotypic acute leukemia who presented with mediastinal masses, were suspected to be T cell lymphoma/leukemia clinically and radiologically. In one case, tissue diagnosis was given as lymphoma for which treatment was given. These cases show that patients diagnosed as lymphoma on histopathology can be cases of mixed phenotype acute leukemia and varying specific treatment protocols and follow up are required. Awareness of these entities will help in proper diagnosis and treatment.

Evolutionary implication of B-1 lineage cells from innate to adaptive immunity

The paradigm that B cells mainly play a central role in adaptive immunity may have to be reevaluated because B-1 lineage cells have been found to exhibit innate-like functions, such as phagocytic and bactericidal activities. Therefore, the evolutionary connection of B-1 lineage cells between innate and adaptive immunities have received much attention. In this review, we summarized various innate-like characteristics of B-1 lineage cells, such as natural antibody production, antigen-presenting function in primary adaptive immunity, and T cell-independent immune responses. These characteristics seem highly conserved between fish B cells and mammalian B-1 cells during vertebrate evolution. We proposed an evolutionary outline of B cells by comparing biological features, including morphology, phenotype, ontogeny, and functional activity between B-1 lineage cells and macrophages or B-2 cells. The B-1 lineage may be a transitional cell type between phagocytic cells (e.g., macrophages) and B-2 cells that functionally connects innate and adaptive immunities. Our discussion would contribute to the understanding on the origination of B cells specialized in adaptive immunity from innate immunity. The results might provide further insight into the evolution of the immune system as a whole.

Mixed Phenotype Acute Leukemia with Two Immunophenotypically Distinct B and T Blasts Populations, Double Ph (+) Chromosome and Complex Karyotype: Report of an Unusual Case

Mixed phenotype acute leukemia (MPAL) is considered as a rare type of leukemia with an incidence of less than 4% of all acute leukemia based on the most recent 2008 WHO classification. Common subtypes are the B/myeloid and T/myeloid; B/T and trilineage MPAL being extremely rare. We present a case of a male in his 20s, whose peripheral blood smears showed 34% blast cells and bone marrow with 70% blasts. Immunophenotyping by multiparametric flow cytometry showed two populations of blasts, the major one with B-lineage and the minor one with T-lineage. Conventional karyotyping revealed complex karyotype with the presence of double Philadelphia chromosome (Ph⁺). BCR/ABL1 rearrangement was confirmed by fluorescent in situ hybridization (FISH) analysis. The BCR/ABL1 ES probe on interphase cells indicated p190 minor m-BCR/ABL fusion in 46% and a second abnormal clone with double Ph⁺ in 16% of the cells analyzed confirmed by reverse transcription-PCR (RT-PCR). The case was diagnosed as MPAL with double Philadelphia chromosome Ph⁺. The patient was treated with dasatinib, four cycle hyper CVAD/methotrexate cytarabin protocol, and allogeneic transplant. He is still alive in complete hematological, cytogenetic, and molecular remission. Mixed phenotype B/T acute leukemia is an extremely rare disease, particularly those with double Philadelphia chromosomes and clinically presents challenges in diagnosis and treatment.

Evolution of B cell immunity

Two types of adaptive immune strategies are known to have evolved in vertebrates: the VLR-based system, which is present in jawless organisms and is mediated by VLRA and VLRB lymphocytes, and the BCR/TCR-based system, which is present in jawed species and is provided by B and T cell receptors expressed on B and T cells, respectively. Here we summarize features of B cells and their predecessors in the different animal phyla, focusing the review on B cells from jawed vertebrates. We point out the critical role of nonclassical species and comparative immunology studies in the understanding of B cell immunity. Because nonclassical models include species relevant to veterinary medicine, basic science research performed in these animals contributes to the knowledge required for the development of more efficacious vaccines against emerging pathogens.

Functionally Distinct Subsets of Lineage-Biased Multipotent Progenitors Control Blood Production in Normal and Regenerative Conditions

Despite great advances in understanding the mechanisms underlying blood production, lineage specification at the level of multipotent progenitors (MPPs) remains poorly understood. Here, we show that MPP2 and MPP3 are distinct myeloid-biased MPP subsets that work together with lymphoid-primed MPP4 cells to control blood production. We find that all MPPs are produced in parallel by hematopoietic stem cells (HSCs), but with different kinetics and at variable levels depending on hematopoietic demands. We also show that the normally rare myeloid-biased MPPs are transiently overproduced by HSCs in regenerating conditions, hence supporting myeloid amplification to rebuild the hematopoietic system. This shift is accompanied by a reduction in self-renewal activity in regenerating HSCs and reprogramming of MPP4 fate toward the myeloid lineage. Our results support a dynamic model of blood development in which HSCs convey lineage specification through independent production of distinct lineage-biased MPP subsets that, in turn, support lineage expansion and differentiation.

Challenges and opportunities of microRNAs in lymphomas

MicroRNAs (miRNAs) are small non-coding RNAs that control the expression of many target messenger RNAs (mRNAs) involved in normal cell functions (differentiation, proliferation and apoptosis). Consequently their aberrant expression and/or functions are related to pathogenesis of many human diseases including cancers. Haematopoiesis is a highly regulated process controlled by a complex network of molecular mechanisms that simultaneously regulate commitment, differentiation, proliferation, and apoptosis of hematopoietic stem cells (HSC). Alterations on this network could affect the normal haematopoiesis, leading to the development of haematological malignancies such as lymphomas. The incidence of lymphomas is rising and a significant proportion of patients are refractory to standard therapies. Accurate diagnosis, prognosis and therapy still require additional markers to be used for diagnostic and prognostic purpose and evaluation of clinical outcome. The dysregulated expression or function of miRNAs in various types of lymphomas has been associated with lymphoma pathogenesis. Indeed, many recent findings suggest that almost all lymphomas seem to have a distinct and specific miRNA profile and some miRNAs are related to therapy resistance or have a distinct kinetics during therapy. MiRNAs are easily detectable in fresh or paraffin-embedded diagnostic tissue and serum where they are highly stable and quantifiable within the diagnostic laboratory at each consultation. Accordingly they could be specific biomarkers for lymphoma diagnosis, as well as useful for evaluating prognosis or disease response to the therapy, especially for evaluation of early relapse detection and for greatly assisting clinical decisions making. Here we summarize the current knowledge on the role of miRNAs in normal and aberrant lymphopoiesis in order to highlight their clinical value as specific diagnosis and prognosis markers of lymphoid malignancies or for prediction of therapy response. Finally, we discuss their controversial therapeutic role and future applications in therapy by modulating miRNA.

Langerhans cell histiocytosis in acute leukemias of ambiguous or myeloid lineage in adult patients: support for a possible clonal relationship

Four patients presented with acute leukemia of ambiguous or myeloid lineage in association with Langerhans cell histiocytosis and provide evidence suggesting a common origin of the two neoplasms. One patient had a non-constitutional trisomy 21 in both the leukemic blasts and the Langerhans cells indicative of a clonal relationship. A second case expressed CD2, CD13, and CD117 on both the Langerhans cells and the blasts suggesting a possible clonal relationship. All four cases exhibited geographic intermingling of the Langerhans cell histiocytosis and acute leukemia and shared unique features including extramedullary leukemia involving lymph nodes in all cases with Langerhans cell histiocytosis only present in sites involved by acute leukemia. T-cell antigen expression was present in all cases with one meeting criteria for mixed phenotype acute leukemia, T/myeloid, not otherwise specified. These findings support the concept that coexistent Langerhans cell histiocytosis and acute leukemia is clonally related in some cases. Furthermore, these cases of acute myeloid or acute leukemia of ambiguous lineage with Langerhans cell histiocytosis share some unique features suggesting a common underlying neoplastic hematopoietic stem cell.

Dysregulated signaling pathways in the development of CNTRL-FGFR1-induced myeloid and lymphoid malignancies associated with FGFR1 in human and mouse models

Myeloid and lymphoid neoplasm associated with FGFR1 is an aggressive disease, and resistant to all the current chemotherapies. To define the molecular etiology of this disease, we have developed murine models of this disease, in syngeneic hosts as well as in nonobese diabetic/severe combined immunodeficiency/interleukin 2Rγ(null) mice engrafted with transformed human CD34+ hematopoietic stem/progenitor cells. Both murine models mimic the human disease with splenohepatomegaly, hypercellular bone marrow, and myeloproliferative neoplasms that progresses to acute myeloid leukemia. Molecular genetic analyses of these model mice, as well as primary human disease, demonstrated that CNTRL-FGFR1, through abnormal activation of several signaling pathways related to development and differentiation of both myeloid and T-lymphoid cells, contribute to overt leukemogenesis. Clonal evolution analysis indicates that myeloid related neoplasms arise from common myeloid precursor cells that retain potential for T-lymphoid differentiation. These data indicate that simultaneously targeting these pathways is essential to successfully treating this almost invariably lethal disease.

Mixed phenotypic acute leukemia with two immunophenotypically distinct blast populations: report of an unusual case

Mixed phenotypic acute leukemia (MPAL) is a rare disorder with an incidence of less than 2% of all acute leukemia using the recent 2008 WHO criteria. Common subtypes encountered are the B/myeloid and T/myeloid; B/T or trilineage MPAL being an exception. We discuss here a case of 20-year-male patient who presented with pallor and generalised lymphadenopathy. Peripheral blood smear examination showed presence of 61% blasts of lymphoid morphology. Immunophenotyping by multicolor flow cytometry showed two distinct populations of blasts with T and B phenotype respectively. He was diagnosed as MPAL with two distinct blast lineages. Conventional karyotyping done on bone marrow sample showed t(9;22)(q34;q11)(Ph +). Induction was started using ALL based protocol. The patient is on follow up with post induction marrow being in morphological remission.

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.

Novel functions of murine B1 cells: active phagocytic and microbicidal abilities

B1 cells are evolutionarily conserved innate-like cells that share many features with macrophages. It has also been established that B1 cells have a close developmental relationship with macrophages. However, whether B1 cells are able to act as professional phagocytic cells is not clear. In this study, we report that mouse peritoneal cavity (PerC) B cells demonstrate in vivo and in vitro phagocytic activities for Staphylococcus aureus, Escherichia coli, and polystyrene fluorescent microspheres. Approximately 5% of PerC B cells, mainly B1b cells, showed phagocytic activity. Ingested microbes were killed efficiently in the phagolysosome. The antigen-specific B-cell antigen receptor promoted B-cell phagocytosis, resulting in antigen presentation to T cells after uptake of bacteria. Our results reveal for the first time that mouse B1 cells have active phagocytic capabilities and thereby act as a bridge linking innate and adaptive immunity.

Bone marrow-derived IL-13Rα1-positive thymic progenitors are restricted to the myeloid lineage

The earliest thymic progenitors (ETPs) were recently shown to give rise to both lymphoid and myeloid cells. Whereas the majority of ETPs are derived from IL-7Rα-positive cells and give rise exclusively to T cells, the origin of the myeloid cells remains undefined. In this study, we show both in vitro and in vivo that IL-13Rα1(+) ETPs yield myeloid cells with no potential for maturation into T cells, whereas IL-13Rα1(-) ETPs lack myeloid potential. Moreover, transfer of lineage-negative IL-13Rα1(+) bone marrow stem cells into IL-13Rα1-deficient mice reconstituted thymic IL-13Rα1(+) myeloid ETPs. Myeloid cells or macrophages in the thymus are regarded as phagocytic cells whose function is to clear apoptotic debris generated during T cell development. However, the myeloid cells derived from IL-13Rα1(+) ETPs were found to perform Ag-presenting functions. Thus, IL-13Rα1 defines a new class of myeloid restricted ETPs yielding APCs that could contribute to development of T cells and the control of immunity and autoimmunity.

Pivotal advance: peritoneal cavity B-1 B cells have phagocytic and microbicidal capacities and present phagocytosed antigen to CD4+ T cells

Breaking the long-held paradigm that primary B cells are not phagocytic, several studies have demonstrated recently that B cells from fish, amphibians, and reptilians have a significant phagocytic capacity. Whether such capacity has remained conserved in certain mammalian B cell subsets is presently an enigma. Here, we report a previously unrecognized ability of PerC B-1a and B-1b lymphocytes to phagocytose latex beads and bacteria. In contrast, B-2 lymphocytes had an almost negligible ability to internalize these particles. Upon phagocytosis, B-1a and B-1b cells were able to mature their phagosomes into phagolysosomes and displayed the ability to kill internalized bacteria. Importantly, B-1a and B-1b cells effectively present antigen recovered from phagocytosed particles to CD4(+) T cells. However, these cells showed a much lower competence to present soluble antigen or antigen from large, noninternalized particles. B-1 B cells presented particulate and soluble antigen to CD4(+) T cells more efficiently than macrophages, whereas DCs were the most potent APCs. The novel phagocytic and microbicidal abilities identified in B-1 B lymphocytes strengthen the innate nature that has long been attributed to these cells. In the context of adaptive immunity, we show that these innate immune processes are relevant, as they enable B-1 B cells to present phagocytosable particulate antigen. These capacities position these cells at the crossroads that link innate with adaptive immune processes. In a broader context, these newly identified capacities of B-1 B cells further support the previously recognized functional, developmental, and evolutionary relationships between these cells and macrophages.

A map for lineage restriction of progenitors during hematopoiesis: the essence of the myeloid-based model

Most hematology and immunology textbooks describe that the first branch point from the hematopoietic stem cells (HSCs) produces two progenitors, one for myelo-erythroid cells and the other for lymphoid cells including T and B cells. This model is based on the concept that the blood cell family can be subdivided into two major lineages, a myelo-erythroid lineage and a lymphoid lineage. Several alternative models have been proposed during the last three decades. We proposed the myeloid-based model in 2001, in which myeloid potential is retained in an early stage of branches toward erythroid, T-, and B-cell lineages. In this review, we focus on the point that cell differentiation models have two different facets: as a map of developmental potential and a cell fate map. These two are expressed in other words as a map for lineage restriction and a map for physiological production routes. We argue that a map of potential is first and foremost essential for the study of molecular mechanisms of lineage commitment, which is the least clarified aspect of cell differentiation. The validity of the myeloid-based model of hematopoiesis will be discussed in reference to these two issues, developmental potential and cell fate.

Mixed-phenotype acute leukemia: clinical and laboratory features and outcome in 100 patients defined according to the WHO 2008 classification

The features of 100 mixed-phenotype acute leukemias (MPALs), fulfilling WHO 2008 criteria, are documented. Myeloid and T-lineage features were demonstrated by cytoplasmic myeloperoxidase and CD3; B-lineage features were demonstrated by at least 2 B-lymphoid markers. There were 62 men and 38 women; 68% were adults. Morphology was consistent with acute lymphoblastic leukemia (ALL; 43%), acute myeloid leukemia (AML; 42%), or inconclusive (15%). Immunophenotyping disclosed B + myeloid (59%), T + myeloid (35%), B + T (4%), or trilineage (2%) combinations. Cytogenetics evidenced t(9;22)/(Ph(+)) (20%), 11q23/MLL rearrangements (8%), complex (32%), aberrant (27%), or normal (13%) karyotypes. There was no correlation between age, morphology, immunophenotype, or cytogenetics. Response to treatment and outcome were available for 67 and 70 patients, respectively; 27 received ALL, 34 AML, 5 a combination of ALL + AML therapy, and 1 imatinib. ALL treatment induced a response in 85%, AML therapy in 41%; 3 of 5 patients responded to the combination therapy. Forty (58%) patients died, 33 of resistant disease. Overall median survival was 18 months and 37% of patients are alive at 5 years. Age, Ph(+), and AML therapy were predictors for poor outcome (P < .001; P = .002; P = .003). MPAL is confirmed to be a poor-risk disease. Adults and Ph(+) patients should be considered for transplantation in first remission.

Revised map of the human progenitor hierarchy shows the origin of macrophages and dendritic cells in early lymphoid development

The classical model of hematopoiesis posits the segregation of lymphoid and myeloid lineages as the earliest fate decision. The validity of this model in the mouse has been questioned; however, little is known about the lineage potential of human progenitors. Here we provide a comprehensive analysis of the human hematopoietic hierarchy by clonally mapping the developmental potential of seven progenitor classes from neonatal cord blood and adult bone marrow. Human multilymphoid progenitors, identified as a distinct population of Thy-1(neg-lo)CD45RA(+) cells in the CD34(+)CD38(-) stem cell compartment, gave rise to all lymphoid cell types, as well as monocytes, macrophages and dendritic cells, which indicated that these myeloid lineages arise in early lymphoid lineage specification. Thus, as in the mouse, human hematopoiesis does not follow a rigid model of myeloid-lymphoid segregation.

A revised scheme for developmental pathways of hematopoietic cells: the myeloid-based model

Blood cells comprise very diverse cell types with a wide range of crucial functions; however, they share a common progenitor cell type-the hematopoietic stem cell (HSC). Clarifying how HSCs differentiate into these diverse cell types is important for understanding how they attain their various functions and offers the potential for therapeutic manipulation. Various theories exist about how HSCs diversify; in particular, one model (the 'classical' model) proposes that lymphocytes and myelo-erythroid lineages branch separately at an early stage of hematopoiesis, whereas another model (the 'myeloid-based' model) proposes that the myeloid potential is retained for much longer among cells that can become lymphocytes. This article describes and compares these models and outlines recent evidence supporting the myeloid-based model.

Evolution of genetic networks underlying the emergence of thymopoiesis in vertebrates

About 500 million years ago, a new type of adaptive immune defense emerged in basal jawed vertebrates, accompanied by morphological innovations, including the thymus. Did these evolutionary novelties arise de novo or from elaboration of ancient genetic networks? We reconstructed the genetic changes underlying thymopoiesis by comparative genome and expression analyses in chordates and basal vertebrates. The derived models of genetic networks were experimentally verified in bony fishes. Ancestral networks defining circumscribed regions of the pharyngeal epithelium of jawless vertebrates expanded in cartilaginous fishes to incorporate novel genes, notably those encoding chemokines. Correspondingly, novel networks evolved in lymphocytes of jawed vertebrates to control the expression of additional chemokine receptors. These complementary changes enabled unprecedented Delta/Notch signaling between pharyngeal epithelium and lymphoid cells that was exploited for specification to the T cell lineage. Our results provide a framework elucidating the evolution of key features of the adaptive immune system in jawed vertebrates.

A functional gammadeltaTCR/CD3 complex distinct from gammadeltaT cells is expressed by human eosinophils

BACKGROUND

Eosinophils are effector cells during parasitic infections and allergic responses. However, their contribution to innate immunity has been only recently unravelled.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that human eosinophils express CD3 and gammadelta T Cell Receptor (TCR) but not alphabeta TCR. Surface expression of gammadeltaTCR/CD3 is heterogeneous between eosinophil donors and inducible by mycobacterial ligands. Surface immunoprecipitation revealed expression of the full gammadeltaTCR/CD3 complex. Real-time PCR amplification for CD3, gamma and delta TCR constant regions transcripts showed a significantly lower expression in eosinophils than in gammadeltaT cells. Limited TCR rearrangements occur in eosinophils as shown by spectratyping analysis of CDR3 length profiles and in situ hybridization. Release by eosinophils of Reactive Oxygen Species, granule proteins, Eosinophil Peroxidase and Eosinophil-Derived Neurotoxin and cytokines (IFN-gamma and TNF-alpha) was observed following activation by gammadeltaTCR-specific agonists or by mycobacteria. These effects were inhibited by anti-gammadeltaTCR blocking antibodies and antagonists. Moreover, gammadeltaTCR/CD3 was involved in eosinophil cytotoxicity against tumor cells.

CONCLUSIONS/SIGNIFICANCE

Our results provide evidence that human eosinophils express a functional gammadeltaTCR/CD3 with similar, but not identical, characteristics to gammadeltaTCR from gammadeltaT cells. We propose that this receptor contributes to eosinophil innate responses against mycobacteria and tumors and may represent an additional link between lymphoid and myeloid lineages.

The adaptive phenotype of cortical thymic epithelial cells

Epithelial cells in the thymus are required for positive and negative selection of developing thymocytes. Although medullary epithelial cells play a major role in negative selection owing to their facility of expressing peripheral self-antigens, the adaptive features of cortical epithelial cells are largely unknown. A paper in this issue of the European Journal of Immunology shows that the putative serine protease Prss16 affects positive selection of a subset of CD4(+) T cells. A survey of chordate genomes indicates that the Prss16 gene emerged in vertebrates as a paralogue of evolutionarily older members of the serine carboxypeptidase 28 family; thus, Prss16 is not associated with the appearance of the adaptive immune system and the thymus in jawed vertebrates. Nevertheless, it appears that Prss16 has later evolved as an essential contributor to the MHC class II peptide/ligand repertoire.

A new paradigm for hematopoietic cell lineages: revision of the classical concept of the myeloid-lymphoid dichotomy

The concept that blood cells arising from hematopoietic stem cells (HSC) can be subdivided into two major lineages, a myelo-erythroid and a lymphoid lineage, has long persisted. Indeed, it has become almost axiomatic that the first branch point from the HSC produces two progenitors, one for myelo-erythroid cells and the other for lymphoid cells. However, recent studies have provided a battery of findings that cannot be explained by this classical model. We will outline how this classical model arose before describing how we came to propose an alternative 'myeloid-based model', in which myeloid potential is retained in erythroid, T, and B cell branches even after these lineages have segregated from each other.

Genome-wide lineage-specific transcriptional networks underscore Ikaros-dependent lymphoid priming in hematopoietic stem cells

The mechanisms regulating lineage potential during early hematopoiesis were investigated. First, a cascade of lineage-affiliated gene expression signatures, primed in hematopoietic stem cells (HSCs) and differentially propagated in lineage-restricted progenitors, was identified. Lymphoid transcripts were primed as early as the HSC, together with myeloid and erythroid transcripts. Although this multilineage priming was resolved upon subsequent lineage restrictions, an unexpected cosegregation of lymphoid and myeloid gene expression and potential past a nominal myeloid restriction point was identified. Finally, we demonstrated that whereas the zinc finger DNA-binding factor Ikaros was required for induction of lymphoid lineage priming in the HSC, it was also necessary for repression of genetic programs compatible with self-renewal and multipotency downstream of the HSC. Taken together, our studies provide new insight into the priming and restriction of lineage potentials during early hematopoiesis and identify Ikaros as a key bivalent regulator of this process.

Adult T-cell progenitors retain myeloid potential

During haematopoiesis, pluripotent haematopoietic stem cells are sequentially restricted to give rise to a variety of lineage-committed progenitors. The classical model of haematopoiesis postulates that, in the first step of differentiation, the stem cell generates common myelo-erythroid progenitors and common lymphoid progenitors (CLPs). However, our previous studies in fetal mice showed that myeloid potential persists even as the lineage branches segregate towards T and B cells. We therefore proposed the 'myeloid-based' model of haematopoiesis, in which the stem cell initially generates common myelo-erythroid progenitors and common myelo-lymphoid progenitors. T-cell and B-cell progenitors subsequently arise from common myelo-lymphoid progenitors through myeloid-T and myeloid-B stages, respectively. However, it has been unclear whether this myeloid-based model is also valid for adult haematopoiesis. Here we provide clonal evidence that the early cell populations in the adult thymus contain progenitors that have lost the potential to generate B cells but retain substantial macrophage potential as well as T-cell, natural killer (NK)-cell and dendritic-cell potential. We also show that such T-cell progenitors can give rise to macrophages in the thymic environment in vivo. Our findings argue against the classical dichotomy model in which T cells are derived from CLPs; instead, they support the validity of the myeloid-based model for both adult and fetal haematopoiesis.

Launching the T-cell-lineage developmental programme

Multipotent blood progenitor cells enter the thymus and begin a protracted differentiation process in which they gradually acquire T-cell characteristics while shedding their legacy of developmental plasticity. Notch signalling and basic helix-loop-helix E-protein transcription factors collaborate repeatedly to trigger and sustain this process throughout the period leading up to T-cell lineage commitment. Nevertheless, the process is discontinuous with separately regulated steps that demand roles for additional collaborating factors. This Review discusses new evidence on the coordination of specification and commitment in the early T-cell pathway; effects of microenvironmental signals; the inheritance of stem-cell regulatory factors; and the ensemble of transcription factors that modulate the effects of Notch and E proteins, to distinguish individual stages and to polarize T-cell-lineage fate determination.

Plasmacytoid dendritic cells and type I IFN: 50 years of convergent history

It has been 50 years since the initial descriptions of what are now known as plasmacytoid dendritic cells (pDC) and type I IFN. pDC, which are infrequent cells found in the peripheral blood and lymphoid organs, are the most potent producers of type I and type III IFNs in the body. pDC produce IFN-alpha in response to both DNA and RNA enveloped viruses by virtue of their ribonucleic acids signaling in the endosome through TLR9 and TLR7, respectively. This stimulation, which also occurs with DNA or RNA-containing immune complexes and synthetic TLR7 and -9 agonists, is dependent upon the transcription factor IRF-7, which is expressed at high constitutive levels in pDC. In addition to releasing as much as 3-10pg of IFN-alpha/cell, pDC are also potent modulators of the immune response. In this review, we discuss the signaling pathways in pDC, their roles in linking innate and adaptive immunity, and their roles in infectious disease and autoimmunity.

T cell lineage determination precedes the initiation of TCR beta gene rearrangement

Loss of dendritic cell potential is one of the major events in intrathymic T cell development, during which the progenitors become determined to the T cell lineage. However, it remains unclear whether this event occurs in synchrony with another important event, TCRbeta chain gene rearrangement, which has been considered the definitive sign of irreversible T cell lineage commitment. To address this issue, we used transgenic mice in which GFP expression is controlled by the lck proximal promoter. We found that the double-negative (DN) 2 stage can be subdivided into GFP- and GFP+ populations, representing functionally different developmental stages in that the GFP-DN2, but not GFP+DN2, cells retain dendritic cell potential. The GFP+DN2 cells were found to undergo several rounds of proliferation before the initiation of TCRbeta rearrangement as evidenced by the diversity of D-Jbeta rearrangements seen in T cells derived from a single GFP+DN2 progenitor. These results indicated that the determination step of progenitors to the T cell lineage is a separable event from TCRbeta rearrangement.

The sequential determination model of hematopoiesis

Analysis of hematopoietic development has for decades been central to understanding lineage diversification. Some models consider hematopoietic commitment to be random, and branching lineage maps often include an early myeloid or lymphoid bifurcation. However, the existence of joint lymphoid or myeloid intermediate progenitors argues against both. One of us earlier proposed the sequential determination (SD) model, which features a limited and stepwise set of binary choices across the full hematopoietic spectrum. This model arose from observations that hematopoietic progenitors show preferences for particular associations of lineage potentials--indicating that these linked fates are neighbours developmentally. An updated SD model complemented by several recently recognized processes--spatiotemporal fluctuations in transcription factor concentrations, asymmetric cell division, and Notch signalling--still offers a sound summary of hematopoiesis.

Regulatory factors for initial T lymphocyte lineage specification

PURPOSE OF REVIEW

Initiation of T lymphocyte development depends on balanced regulatory inputs from multiple essential transcription factors. This review highlights contributions of E2A, hematopoietic transcription factor PU.1, growth factor independence (Gfi)-1, T cell factor (TCF)-1, and Runx factors and their interactions with the Notch pathway to promote T cell development.

RECENT FINDINGS

E2A and Runx family factors have been implicated in establishing competent precursors in which Notch signaling can induce the T cell program. An early role was also indicated for PU.1. Later PU.1 activities are antagonistic to pro-T cell factors, however, including E proteins, Myb, Gfi-1, and TCF-1. Diversion to a non-T lineage can be promoted by PU.1, CCAAT/enhancer binding protein, or even GATA and TCF, but these diversion mechanisms are blocked by Notch signaling. An emergent gene network summarizes the cross-regulatory relationships among these factors.

SUMMARY

Entry into the T-cell pathway is controlled by a dynamic balance among essential regulatory factors that depend on Notch signaling not only to trigger initiation of the T-cell program but also to maintain the lineage fidelity of their collective action.

Acute bilineal leukemia: a rare disease with poor outcome

Most cases of acute leukemia can be assigned to the myeloid, B or T lineage. In a few cases, definitive assignment cannot be achieved because blasts express antigens of more than one lineage. A subset of these, referred to as acute bilineal leukemias (aBLLs), is characterized by the presence of more than one population of blasts, each comprising a single lineage. We identified 19 cases of aBLL, including 10 mixed T and myeloid (T-My) and nine mixed B and myeloid (B-My); no mixed B and T cases were identified. Cytogenetic data were available for 16 patients. Three of seven patients with B-My had a t(9;22)(q34q11.2), two had 11q23 translocations and one had del(9). Two of nine patients with T-My had 2p13 translocations; five had other unrelated abnormalities. Of 16 patients with outcome data, only six achieved complete remission and only two remain free of disease 2.5 and 4.5 years after chemotherapy or stem cell transplantation. aBLL is a rare disease that combines B or T and myeloid blasts. Cytogenetic abnormalities of t(9;22) and 11q23 are common in, and may be restricted to, B-My cases, while T-My cases have frequent but generally non-recurring abnormalities. Both types of aBLL are associated with poor outcome.

The role of type I interferon production by dendritic cells in host defense

Type I interferons (IFN) and dendritic cells (DC) share an overlapping history, with rapidly accumulating evidence for vital roles for both production of type 1 IFN by DC and the interaction of this IFN both with DC and components of the innate and adaptive immune responses. Within the innate immune response, the plasmacytoid DC (pDC) are the "professional" IFN producing cells, expressing specialized toll-like receptors (TLR7 and -9) and high constitutive expression of IRF-7 that allow them to respond to viruses with rapid and extremely robust IFN production; following activation and production of IFN, the pDC subsequently mature into antigen presenting cells that help to shape the adaptive immune response. However, like most cells in the body, the myeloid or conventional DC (mDC or cDC) also produce type I IFNs, albeit typically at a lower level than that observed with pDC, and this IFN is also important in innate and adaptive immunity induced by these classic antigen presenting cells. These two major DC subsets and their IFN products interact both with each other as well as with NK cells, monocytes, T helper cells, T cytotoxic cells, T regulatory cells and B cells to orchestrate the early immune response. This review discusses some of the converging history of DC and IFN as well as mechanisms for IFN induction in DC and the effects of this IFN on the developing immune response.

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.

Steady-state and inflammatory dendritic-cell development

The developmental pathways that lead to the production of antigen-presenting dendritic cells (DCs) are beginning to be understood. These are the last of the pathways of haematopoiesis to be mapped. The existence of many specialized subtypes of DC has complicated this endeavour, as has the need to distinguish the DCs formed in steady state from those produced during an inflammatory response. Here we review studies that lead to the concept that different types of DC develop through different branches of haematopoietic pathways that involve different immediate precursor cells. Furthermore, these studies show that many individual tissues generate their own DCs locally, from a reservoir of immediate DC precursors, rather than depending on a continuous flux of DCs from the bone marrow.

Primitive lymphoid progenitors in bone marrow with T lineage reconstituting potential

Multiple subsets of the bone marrow contain T cell precursors, but it remains unclear which is most likely to replenish the adult thymus. Therefore, RAG-1+ early lymphoid progenitors (RAG-1+ ELP), and CD62L/L-selectin+ progenitors (LSP), as well as common lymphoid progenitors from C57BL6-Thy1.1-RAG-1/GFP mouse bone marrow were directly compared in transplantation assays. The two c-Kit(high) populations vigorously regenerated the thymus and were superior to common lymphoid progenitors in magnitude and frequency of thymic reconstitution. Regeneration was much faster than the 22 days described for transplanted stem cells, and RAG-1+ ELP produced small numbers of lymphocytes within 13 days. As previously reported, LSP were biased to a T cell fate, but this was not the case for RAG-1+ ELP. Although RAG-1+ ELP and LSP had reduced myeloid potential, they were both effective progenitors for T lymphocytes and NK cells. The LSP subset overlapped with and included most RAG-1+ ELP and many RAG-1- TdT+ ELP. LSP and RAG-1+ ELP were both present in the peripheral circulation, but RAG-1+ ELP had no exact counterpart among immature thymocytes. The most primitive of thymocytes were similar to Lin- c-Kit(high) L-selectin+ TdT+ RAG-1- progenitors present in the marrow, suggesting that this population is normally important for sustaining the adult thymus.

B lymphocytes from early vertebrates have potent phagocytic and microbicidal abilities

The present paradigm dictates that phagocytosis is accomplished mainly by 'professional' phagocytes (such as macrophages and monocytes), whereas B cells lack phagocytic capabilities. Here we demonstrate that B cells from teleost fish have potent in vitro and in vivo phagocytic activities. Particle uptake by B cells induced activation of 'downstream' degradative pathways, leading to 'phagolysosome' formation and intracellular killing of ingested microbes. Those results indicate a previously unknown function for B cells in the innate immunity of these primitive animals. A considerable proportion of Xenopus laevis B cells were also phagocytic. Our findings support the idea that B cells evolved from an ancestral phagocytic cell type and provide an evolutionary framework for understanding the close relationship between mammalian B lymphocytes and macrophages.