This review focuses on recent progress in our understanding of how mast cells can contribute to the initiation, development, expression, and regulation of acquired immune responses, both those associated with IgE and those that are apparently expressed independently of this class of Ig. We emphasize findings derived from in vivo studies in mice, particularly those employing genetic approaches to influence mast cell numbers and/or to alter or delete components of pathways that can regulate mast cell development, signaling, or function. We advance the hypothesis that mast cells not only can function as proinflammatory effector cells and drivers of tissue remodeling in established acquired immune responses, but also may contribute to the initiation and regulation of such responses. That is, we propose that mast cells can also function as immunoregulatory cells. Finally, we show that the notion that mast cells have primarily two functional configurations, off (or resting) or on (or activated for extensive mediator release), markedly oversimplifies reality. Instead, we propose that mast cells are "tunable," by both genetic and environmental factors, such that, depending on the circumstances, the cell can be positioned phenotypically to express a wide spectrum of variation in the types, kinetics, and/or magnitude of its secretory functions.

Mast cells can function as effector and immunoregulatory cells in IgE-associated allergic disorders, as well as in certain innate and adaptive immune responses. This review will focus on exciting new developments in the field of mast cell biology published within the last year. It will highlight advances in the understanding of FcεRI-mediated signaling and mast cell activation events, as well as in the use of genetic models to study mast cell function in vivo. Finally, we will discuss newly identified roles of mast cells or individual mast cell products, such as proteases and IL-10, in host defense, cardiovascular disease and tumor biology, and in settings in which mast cells have anti-inflammatory or immunosuppressive functions.

Although IgE binding to mast cells is thought to be a passive presensitization step, we demonstrate herein that monomeric IgE (mIgE) in the absence of antigen (Ag) stimulates multiple phosphorylation events in normal murine bone marrow-derived mast cells (BMMCs). While mIgE does not induce degranulation or leukotriene synthesis, it leads to a more potent production of cytokines than IgE + Ag. Moreover, mIgE prevents the apoptosis of cytokine-deprived BMMCs, likely by maintaining Bcl-X(L) levels and producing autocrine-acting cytokines. The addition of Ag does not increase this IgE-induced survival. Since IgE concentrations as low as 0.1 microg/ml enhance BMMC survival, elevated plasma IgE levels in humans with atopic disorders may contribute to the elevated mast cell numbers seen in these individuals.

Allergic contact dermatitis, such as in response to poison ivy or poison oak, and chronic low-dose ultraviolet B irradiation can damage the skin. Mast cells produce proinflammatory mediators that are thought to exacerbate these prevalent acquired immune or innate responses. Here we found that, unexpectedly, mast cells substantially limited the pathology associated with these responses, including infiltrates of leukocytes, epidermal hyperplasia and epidermal necrosis. Production of interleukin 10 by mast cells contributed to the anti-inflammatory or immunosuppressive effects of mast cells in these conditions. Our findings identify a previously unrecognized function for mast cells and mast cell-derived interleukin 10 in limiting leukocyte infiltration, inflammation and tissue damage associated with immunological or innate responses that can injure the skin.

An initial exposure to lipopolysaccharide (LPS) induces a transient state of hyporesponsiveness to a subsequent challenge with LPS. The mechanism underlying this phenomenon, termed endotoxin tolerance, remains poorly understood despite a recent resurgence of interest in this area. We demonstrate herein that SHIP(-/-) bone marrow-derived macrophages (BMmphis) and mast cells (BMMCs) do not display endotoxin tolerance. Moreover, an initial LPS treatment of wild-type BMmphis or BMMCs increases the level of SHIP, but not SHIP2 or PTEN, and this increase is critical for the hyporesponsiveness to subsequent LPS stimulation. Interestingly, this increase in SHIP protein is mediated by the LPS-induced production of autocrine-acting TGFbeta and neutralizing antibodies to TGFbeta block LPS-induced endotoxin tolerance. In vivo studies with SHIP(+/+) and SHIP(-/-) mice confirm these in vitro findings and show a correlation between the duration of endotoxin tolerance and elevated SHIP levels.

IntroductionMyeloid cell leukemia sequence 1 (Mcl-1) 1 has been identified as an intracellular antiapoptotic factor in a variety of hematopoietic cells, both in vitro and in vivo. [2][3][4][5][6] Human mast cells express 7,8 and Mcl-1 can promote the survival of some populations of human neoplastic mast cells in vitro. 7 Basophils, granulocytes with many characteristics and functions that partially overlap with those of tissue mast cells, [9][10][11][12] can also express Mcl-1. 13 However, it is not clear to what extent Mcl-1 is important in the development and/or survival of mast cells or basophils in vivo.Opferman et al showed that the genetic manipulation of Mcl-1 can be used to delete individual hematopoietic cell populations in mice. 4 We therefore used this approach to examine the effects of reducing expression of Mcl-1 in the mast cell lineage in vivo. To attempt to delete Mcl-1 selectively in mast cells, we used the promoter for the peptidase carboxypeptidase A3 (CPA3; originally named mast cell carboxypeptidase A 14 ). CPA3 is highly expressed in mast cells, 15 but is also expressed in basophils 16 and can be expressed in some populations of T-cell progenitors and thymic T cells 17,18 and in certain hematopoietic progenitor cells. 19 We generated C57BL/6 mice in which a segment of the Cpa3 promoter drives expression of Cre recombinase, and then mated these Cpa3-Cre transgenic mice to mice bearing a floxed allele of Mcl-1. 4 We found that C57BL/6-Cpa3-Cre; Mcl-1 fl/fl mice are severely deficient in mast cells and have a marked deficiency in basophils, and also exhibit striking impairment in mast cell-or basophil-and IgE-dependent biologic responses. Methods MiceAll animal experiments were carried out following protocols approved by the Stanford University Administrative Panel on Laboratory Animal Care. B6-Tg(Cpa3-cre)3Glli (Cpa3-Cre-transgenic mice) were generated by microinjecting the Cpa3-Cre transgene into embryonic stem cells in the B6 background (Stanford University). Gt(ROSA)26Sor tm4(ACTB-tdTomato,-EGFP)Luo /J(mT/mG) mice, obtained from The Jackson Laboratory, were crossed to Cpa3-Cre mice for Cre expression analysis. Mcl-1 ϩ/fl (B6;129-Mcl1 tm3sjk J) animals were obtained from The Jackson Laboratory. Mcl-1 ϩ/fl mice were bred to progeny from 2 Cpa3-Cre founder lines (founder lines #4 and #5) to obtain Cpa3-Cre; Mcl-1 ϩ/ϩ , Cpa3-Cre; Mcl-1 ϩ/fl , and Cpa3-Cre; Mcl-1 fl/fl animals, but only the Cpa3-Cre; Mcl-1 fl/fl mice derived from founder line #4 exhibited a substantial mast cell deficiency. Therefore, the mice used were derived from crosses between founder line #4 (subsequently referred to as Cpa3-Cre mice) and Mcl-1 fl animals, and these mice had been intercrossed a minimum of 6 generations into the C57BL/6 background. Heterozygous Cpa3-Cre mice were determined to have 5 copies of the Cpa3-Cre transgene by real-time PCR. To emphasize that Cpa3-Cre; Mcl-1 fl/fl mice have deficiencies in mast cells and basophils that are independent of mutations affecting Kit, we call them informally in our labo...

We used mast cell-engrafted genetically mast cell-deficient C57BL/6-Kit W-sh/W-sh mice to investigate the roles of mast cells and mast cell-derived tumor necrosis factor in two models of severe bacterial infection. In these mice, we confirmed findings derived from studies of mast cell-deficient WBB6F 1 -Kit W/W-v mice indicating that mast cells can promote survival in cecal ligation and puncture (CLP) of moderate severity. However, we found that the beneficial role of mast cells in this setting can occur independently of mast cell-derived tumor necrosis factor. By contrast, using mast cell-engrafted C57BL/6-Kit W-sh/W-sh mice, we found that mast cell-derived tumor necrosis factor can increase mortality during severe CLP and can also enhance bacterial growth and hasten death after intraperitoneal inoculation of Salmonella typhimurium. In WBB6F 1 -Kit W-sh/W-sh mice, mast cells enhanced survival during moderately severe CLP but did not significantly change the survival observed in severe CLP. Our findings in three types of genetically mast celldeficient mice thus support the hypothesis that, depending on the circumstances (including mouse strain background, the nature of the mutation resulting in a mast cell deficiency, and type and severity of infection), mast cells can have either no detectable effect or opposite effects on survival during bacterial infections, eg, promoting survival during moderately severe CLP associated with low mortality but, in C57BL/ 6-Kit W-sh/W-sh mice, increasing mortality during severe CLP or infection with S. typhimurium. (Am J Pathol

The hematopoietic-restricted protein Src homology 2-containing inositol-5-phosphatase (SHIP) blunts phosphatidylinositol-3-kinase-initiated signaling by dephosphorylating its major substrate, phosphatidylinositol-3,4,5-trisphosphate. As SHIP(-/-) mice contain increased numbers of osteoclast precursors, that is, macrophages, we examined bones from these animals and found that osteoclast number is increased two-fold. This increased number is due to the prolonged life span of these cells and to hypersensitivity of precursors to macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappa B ligand (RANKL). Similar to pagetic osteoclasts, SHIP(-/-) osteoclasts are enlarged, containing upwards of 100 nuclei, and exhibit enhanced resorptive activity. Moreover, as in Paget disease, serum levels of interleukin-6 are markedly increased in SHIP(-/-) mice. Consistent with accelerated resorptive activity, 3D trabecular volume fraction, trabecular thickness, number and connectivity density of SHIP(-/-) long bones are reduced, resulting in a 22% loss of bone-mineral density and a 49% decrease in fracture energy. Thus, SHIP negatively regulates osteoclast formation and function and the absence of this enzyme results in severe osteoporosis.