Immunoglobulin-mediated signal transduction in B cells from CD45-deficient mice.

CD45 expression is essential for immunoglobulin (Ig)-mediated B cell activation. Treatments with either anti-Ig or anti-CD45 suggest that CD45 may facilitate early signaling events such as calcium mobilization, and phosphoinositide hydrolyis as well as later events leading to transcription of genes such as c-myc. To examine the role of CD45 more extensively, CD45-deficient mice were generated by disruption of exon 6. Although normal numbers of B cells were found in peripheral lymphoid tissues, CD45-deficient cells failed to proliferate upon IgM crosslinking. In the present study, we demonstrate that the fraction of high buoyant density B cells is reduced while low buoyant density cells are increased. Moreover, there is a significant decline in the number of splenic B cells of the mature IgDhi, IgMlo phenotype. Although both the basal and anti-Ig-induced levels of phosphorylation of Ig-alpha and phospholipase C gamma 2 are indistinguishable from that observed in CD45+ control B cells, a major distinction was found in Ca2+ mobilization. While anti-Ig-induced mobilization of intracellular Ca2+ stores was normal, influx from extracellular sources was abrogated. This finding reveals a novel pathway of regulating B cell responses mediated by CD45.

Identification of a novel developmental stage marking lineage commitment of progenitor thymocytes.

Bipotent progenitors for T and natural killer (NK) lymphocytes are thought to exist among early precursor thymocytes. The identification and functional properties of such a progenitor population remain undefined. We report the identification of a novel developmental stage during fetal thymic ontogeny that delineates a population of T/NK-committed progenitors (NK1. 1(+)/CD117(+)/CD44(+)/CD25(-)). Thymocytes at this stage in development are phenotypically and functionally distinguishable from the pool of multipotent lymphoid-restricted (B, T, and NK) precursor thymocytes. Exposure of multipotent precursor thymocytes or fetal liver- derived hematopoietic progenitors to thymic stroma induces differentiation to the bipotent developmental stage. Continued exposure to a thymic microenvironment results in predominant commitment to the T cell lineage, whereas coculture with a bone marrow-derived stromal cell line results in the generation of mature NK cells. Thus, the restriction point to T and NK lymphocyte destinies from a multipotent progenitor stage is marked by a thymus-induced differentiation step.

Deregulated T cell activation and autoimmunity in mice lacking interleukin-2 receptor beta.

In mice lacking the interleukin-2 receptor beta chain (IL-2R beta), T cells were shown to be spontaneously activated, resulting in exhaustive differentiation of B cells into plasma cells and the appearance of high serum concentrations of immunoglobulins G1 and E as well as autoantibodies that cause hemolytic anemia. Marked infiltrative granulocytopoiesis was also apparent, and the animals died after about 12 weeks. Depletion of CD4+ T cells in mutant mice rescued B cells without reversion of granulocyte abnormalities. T cells did not proliferate in response to polyclonal activators, nor could antigen-specific immune responses be elicited. Thus, IL-2R beta is required to keep the activation programs of T cells under control, to maintain homeostasis, and to prevent autoimmunity.

Differentiation and characterization of B-cell precursors detected in the yolk sac and embryo body of embryos beginning at the 10- to 12-somite stage.

The embryonic sites in which progenitors of the hematopoietic lineages first emerge are ideal regions to characterize both the cells and environment needed to initiate blood cell development. For a number of years both the murine yolk sac and embryo have been recognized to contain progenitors of B lymphocytes. However, clonal, quantitative in vitro assays, which allow precise observation of precursors and their progeny, have been lacking. Moreover, the site of origin of the initial events remains controversial. In this report we document the presence of B-cell progenitors in yolk sac and embryonic tissue obtained from mouse fetuses beginning at the 10-somite stage, day 8.5. We determine the frequency, cell-surface phenotype, and growth properties of these progenitors. We show that these cells can differentiate into immunoglobulin-secreting cells and that the progeny derived from single progenitors are diverse with respect to immunoglobulin heavy-chain allotype expression, diversity-joining region use, and heavy-chain variable-region utilization.

Hemokinin is a hematopoietic-specific tachykinin that regulates B lymphopoiesis.

We report here the molecular cloning of a newly identified preprotachykinin gene, Pptc, which specifies the sequence for a new preprotachykinin protein and bioactive peptide designated hemokinin 1 (HK-1). PPT-C mRNA was detected primarily in hematopoietic cells in contrast to the previously described Ppta and Pptb genes, which are predominantly expressed in neuronal tissues. HK-1 has several biological activities that are similar to the most studied tachykinin, substance P, such as induction of plasma extravasation and mast cell degranulation. However, HK-1 also has properties that are indicative of a critical role in mouse B cell development. HK-1 stimulated the proliferation of interleukin 7-expanded B cell precursors, whereas substance P had no effect. HK-1, but not substance P, promoted the survival of freshly isolated bone marrow B lineage cells or cultured, lipopolysaccharide-stimulated pre-B cells. N-acetyl-L-trytophan-3,5-bistrifluromethyl benzyl ester, a tachykinin receptor antagonist, increased apoptosis of these cells and in vivo administration of this antagonist led to specific reductions of the B220lowCD43 population (the pre-B cell compartment) in the bone marrow and the IgMhighIgDlow population (the newly generated B cells) in the spleen. Thus, HK-1 may be an autocrine factor that is important for the survival of B cell precursors at a critical phase of development.

Characterization of thymic stromal-derived lymphopoietin (TSLP) in murine B cell development in vitro.

B cell development is dependent on both direct interactions with stromal cells and their secreted cytokines. The precise mechanisms by which these interactions regulate B cell differentiation are currently unknown. We report here that a novel growth factor thymic stromal-derived lymphopoietin (TSLP) can replace the activity of interleukin-7 (IL-7) in supporting B cell development in vitro. TSLP was found to promote the proliferation and differentiation of committed B220+ B cell progenitors from day 15 fetal liver. Phenotypic analysis of these cells revealed that they are at the pro-B cell stage of differentiation and express cell surface markers characteristic of pro-B cells cultured in IL-7. TSLP can replace the activity of IL-7 in supporting the progression of B lymphocytes from uncommitted bipotential precursors. In the absence of either TSLP or IL-7, the progeny of cells that give rise to mature B lymphocytes fail to develop from these bipotential precursors. Moreover, TSLP can substitute for IL-7 in supporting the sustained proliferative response exhibited by B cell progenitors from CBA/N mice. Together these results show that TSLP can replace the requirement for IL-7 during in vitro B cell development.

Flt3 ligand supports the differentiation of early B cell progenitors in the presence of interleukin-11 and interleukin-7.

B cell development is influenced by interactions between B cell progenitors and stromal cells. The precise mechanisms by which these interactions regulate B cell differentiation are currently unknown. Flt3 ligand (FL) is a growth factor which stimulates the proliferation of stem cells and early progenitors. Mice deficient for the FLT3 receptor exhibit severe reductions in early B lymphoid progenitors. We have previously described a clonal assay in vitro which allows us to follow the entire B cell differentiation pathway from uncommitted progenitors to mature, immunoglobulin-secreting plasma cells. The growth factor combination of interleukin (IL)-11, mast cell growth factor (MGF) and IL-7 was shown to maintain the differentiation of these hematopoietic precursors into B cell progenitors capable of giving rise to functionally mature B cells in secondary cultures. Here, we show that FL in combination with IL-11 and IL-7 is sufficient to support the differentiation of uncommitted progenitors from day 10 yolk sac (AA4.1+) or day 12 fetal liver (AA4.1+ B220- Mac-1- Sca-1+) into the B lineage. The frequency of B cell progenitors obtained in these conditions was similar, if not better, than the frequency of B cell precursors that arose when cultured in IL-11+MGF+IL-7. Furthermore, the growth factor combination of IL-11+FL+ IL-7 was able to maintain the potential of bipotent precursors giving rise to both the B and myeloid lineages in secondary cultures. We also show that FL synergizes with IL-7 in the proliferation of committed B220+ pro-B cells and may contribute to the maintenance of an earlier pro-B cell population. Together, these results show that FL is important in supporting the differentiation and proliferation of early B cell progenitors in vitro.

Expansion of myelopoietic precursors and inhibition of B-cell precursors in mice that express a T-cell receptor gamma (V gamma 1.1J gamma 4C gamma 4) transgene.

Knowledge of the genetic determinants that can affect renewal of multipotential stem cells and their commitment to specific cell lineages is essential to our understanding of multicellular development. However, despite the vast amount of accumulated knowledge in this area, genetic determinants that affect renewal and commitment of precursor cells are unknown. In this study, we demonstrate that three independently derived founder mouse strains, transgenic for the TcR V gamma 1.1J gamma 4C gamma 4 (TcR gamma 4) chain gene, differed significantly from normal mice in their development of T and B cells as well as myelopoietic precursor cells. Ontogenic programs consistent with an acceleration of T-cell development and a delayed appearance and suppressed levels of pre-B- and B-cell precursors were evident in these transgenic mice. In addition, TcR gamma 4 transgenic mice possessed a significantly elevated level of myelopoietic pluripotential precursors. 3H-thymidine cell suicide studies suggest that higher percentages of pluripotent precursors from the bone marrow of the TcR gamma 4 transgenic mice were in the S phase of the cell cycle. These modulations of the lymphoid and myelopoietic compartments, however, were not found in other T-cell receptor transgenic mice (e.g., TcR V gamma 1.2J gamma 2C gamma 2, TcR gamma 2; or V beta 8.1D beta J beta 2.4C beta 2, TcR beta) constructed with the same or similar cDNA expression vector. The results suggest that the expression of a specific T-cell receptor gamma chain gene, and/or an elevated level of particular subset of TcR gamma delta cells, may affect the proliferation and relative proportions of haemopoietic and lymphoid precursors.

A role for CD4+ T cells in the pathogenesis of skin fibrosis in tight skin mice.

The tight skin (Tsk/+) mouse represents a murine model of heritable fibrosis with some similarities to the skin fibrosis seen in human scleroderma. Tsk/+ animals display alterations in connective tissue in some internal organs. Skin fibrosis can be adoptively transferred to normal recipients with Tsk/+ bone marrow or spleen cells and older Tsk/+ animals develop autoantibodies against topoisomerase suggesting that some of the pathogenesis in the Tsk/+ mouse may be mediated by autoimmunity. To determine the role of T cell subsets in the pathogenesis of fibrotic disease, Tsk/+ mice were bred with CD4- and CD8-deficient (CD4-/- and CD8-/-) mice. Tsk/+ CD4-/- mice showed a marked reduction in skin fibrosis as well as decreased cellularity and only mild collagen disorganization as compared to Tsk/+ CD4+ CD8+ control mice yet did not differ from Tsk controls in the level of serum anti-topoisomerase activity. In contrast, Tsk/+ CD8-/- mice exhibited the same histology in the skin as Tsk/+ controls yet had significantly reduced levels of serum anti-topoisomerase activity. Lung pathology, i.e. emphysema, was unaffected by both the CD4 or CD8 mutations. These data show that only some of the pathological effects of the Tsk mutation are T cell dependent and that different T cell subsets affect different parameters in this multi-organ model of fibrotic disease.

Stromal cell-independent maturation of IL-7-responsive pro-B cells.

The proliferation, survival, and differentiation of B cell progenitors in primary hematopoietic tissues depends on extracellular signals produced by stromal cells within the microenvironment. IL-7 is a stromal-derived growth factor that plays a crucial role in B lineage development. We have shown that in the presence of IL-7, pro-B cells proliferate and differentiate to a stage in which they are responsive to stromal cells and LPS, leading to terminally differentiated IgM-secreting plasma cells. In this report, we examine in detail the role of stromal cells in the transition from the IL-7-responsive pro-B cell stage to the mature LPS-responsive B cell stage. We demonstrate that this transition fails to occur, even in the presence of stromal cells and LPS, if constant exposure to IL-7 is maintained. The transition from the large pro-B cell stage to the small cmu+ pre-B cell stage occurs independent of stromal cells. Moreover, the "stromal cell-dependent" maturation that occurs subsequent to the expression of surface IgM leading to responsiveness to B cell mitogens can also be accomplished in the absence of stromal cells if pre-B cells are cultured in proximity to each other or at high cell concentrations. Together these results suggest that stromal cells mediate B cell differentiation by providing the necessary growth requirements (i.e., IL-7) to sustain the development of pre-B cells. The progeny of these pre-B cells can then differentiate through as yet unidentified homotypic interactions, leading to the production of LPS-responsive B cells.

CD44 regulates hematopoietic progenitor distribution, granuloma formation, and tumorigenicity.

CD44 is expressed in various isoforms on numerous cell types and tissues during embryogenesis and in the mature organism. CD44 may also be involved in tumor growth. To study the multiple roles of CD44, we abolished expression of all known isoforms of CD44 in mice by targeting exons encoding the invariant N-terminus region of the molecule. Surprisingly, mice were born in Mendelian ratio without any obvious developmental or neurological deficits. Hematological impairment was evidenced by altered tissue distribution of myeloid progenitors with increased levels of colony-forming unit-granulocyte-macrophage (CFU-GM) in bone marrow and reduced numbers of CFU-GM in spleen. Fetal liver colony-forming unit-spleen and granulocyte colony-stimulating factor mobilization assays, together with reduced CFU-GM in peripheral blood, suggested that progenitor egress from bone marrow was defective. In what was either a compensatory response to CD44 deficiency or an immunoregulatory defect, mice also developed exaggerated granuloma responses to Cryotosporidium parvum infection. Finally, tumor studies showed that SV40-transformed CD44-deficient fibroblasts were highly tumorigenic in nude mice, whereas reintroduction of CD44s expression into these fibroblasts resulted in a dramatic inhibition of tumor growth.

TRAF6 deficiency results in osteopetrosis and defective interleukin-1, CD40, and LPS signaling.

Bone resorption and remodeling is an intricately controlled, physiological process that requires the function of osteoclasts. The processes governing both the differentiation and activation of osteoclasts involve signals induced by osteoprotegerin ligand (OPGL), a member of tumor necrosis factor (TNF) superfamily, and its cognate receptor RANK. The molecular mechanisms of the intracellular signal transduction remain to be elucidated. Here we report that mice deficient in TNF receptor-associated factor 6 (TRAF6) are osteopetrotic with defects in bone remodeling and tooth eruption due to impaired osteoclast function. Using in vitro assays, we demonstrate that TRAF6 is crucial not only in IL-1 and CD40 signaling but also, surprisingly, in LPS signaling. Furthermore, like TRAF2 and TRAF3, TRAF6 is essential for perinatal and postnatal survival. These findings establish unexpectedly diverse and critical roles for TRAF6 in perinatal and postnatal survival, bone metabolism, LPS, and cytokine signaling.