== Development of skin pathology and expansion of myeloid cells in eosinophil-deficientRabgef1/mice

== Development of skin pathology and expansion of myeloid cells in eosinophil-deficientRabgef1/mice.Rabgef1+/mice were bred withGATA1dblmice to generate eosinophil-deficientRabgef1+/mice, and mice with each of the four possible genotypes were analyzed.A:Representative microscopic images of histological sections of abdominal skin stained with hematoxylin and eosin; Scale bar = 50 m. required for the development of epidermal hyperplasia but contribute to the striking myeloid hyperplasia and overproduction of immunoglobulins observed in these animals. Our study shows that RabGEF1 can negatively regulate TSLP productionin vivoand that excessive production of TSLP contributes to many of the phenotypic abnormalities inRabgef1/mice. However, the marked epidermal hyperplasia, cutaneous inflammation, and increased numbers of dermal mast cells associated with RabGEF1 deficiency can develop via a TSLPR-independent pathway, as well as in the absence of Rag1 or eosinophils. Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-7like cytokine initially identified as a growth and differentiation factor for lymphocytes, and later implicated as an important regulator of allergic inflammation.1,2,3,4,5TSLP binds to the TSLP receptor complex, which consists of an IL-7 receptor chain (IL-7R) and a TSLP receptor (TSLPR) chain.1,2,3TSLPR is an atypical type I cytokine receptor that binds TSLP with low affinity, but the binding affinity with TSLP is markedly enhanced when TSLPR is incorporated into the heterodimeric IL-7R/TSLPR receptor complex.5TSLP treatment results in the expansion and maturation of human myeloid dendritic cells and the induction of OX40L expression in these cells.3,6TSLP-treated myeloid dendritic cells can promote inflammatory responses by inducing OX40-OX40Ldependent Th2 cytokine production in nave CD4 T cells and by secreting Propiolamide chemokines that recruit Th2 cells and granulocytes.3,7,8Moreover, TSLP can induce mast cells to produce Th2 cytokines independently of IgE stimulation.9Recently, TSLP has been shown to promote survival, cytokine and chemokine release, and up-regulation of adhesion molecules in human eosinophils.10These observations suggest that TSLP can orchestrate allergic inflammation by activating cells of both the adaptive and the innate immune systems. Propiolamide In accord with this notion, transgenic mice expressing TSLP under the regulation of the keratinocyte-specific promoters for keratin-1411or keratin-512develop atopic dermatitis-like skin lesions, with inflammatory infiltrates including eosinophils and lymphocytes, and also exhibit pronounced splenomegaly, lymphadenopathy, and elevated serum levels of IgE and IgG1 antibodies.11,12In addition to the development of a disorder resembling atopic dermatitis, mice that overproduce TSLP in the skin are also more susceptible to antigen induced airway inflammation and hyperresponsiveness, suggesting that TSLP contributes to the so-called atopic march which is hypothesized to link atopic dermatitis with the later onset of asthma.13,14 The Propiolamide mouse ortholog of RabGEF1 (also known as Rabex-5, Rabaptin-5 associated exchange factor for Rab5) was first identified in a survey of differentially expressed mRNA transcripts in mouse mast cells Propiolamide activated by aggregation of high affinity IgE receptors (FcRI).15RabGEF1-deficient (Rabgef1/) mast cells exhibit delayed receptor internalization, elevated and BSP-II prolonged intracellular signaling events, and enhanced cytokine and mediator release in response to FcRI- or Kit-dependent activation.15,16,17In addition to its role as a negative regulator of mast Propiolamide cell activation, RabGEF1 can contribute to intracellular vesicular trafficking by catalyzing the conversion of GDP to GTP on members of the Rab5 family of small GTPases, molecules known to be important for early endosome fusion during receptor internalization and for endocytic vesicular trafficking.18,19,20 We previously reported that deletion of RabGEF1 protein in the mouse results in substantial morbidity, including skin lesions characterized by marked epidermal hyperplasia and hyperkeratosis, infiltration of lymphocytes, eosinophils, and other inflammatory cells, and increased numbers of dermal mast cells.15Also, the serum of RabGEF1-deficeint mice contains high concentrations of IgE and histamine.15Notably, phenotypic abnormalities very similar to those ofRabgef1/mice also have been observed in transgenic mice which overexpress TSLP in the skin.11,12In this study, we show thatRabgef1/mice exhibit markedly elevated levels of TSLP and IL-4, develop splenomegaly, lymphadenopathy, and expansion of myeloid cells in the blood, bone marrow, and spleen, and produce large numbers of B cells in the spleen. To investigate the contribution of TSLP to the dermatitis and other aspects of the pathology ofRabgef1/mice, we disrupted TSLP signaling in these mice by breedingRabgef1+/mice with TSLP receptor-deficient (TSLPR/) mice, which lack expression of TSLPR and are unresponsive to TSLP.21Our data indicate that TSLPR-mediated signaling contributes substantially to the development of myeloid hyperplasia, splenomegaly, and high levels of serum IL-4, IgE, and IgG1 inRabgef1/mice. By contrast, we show that the spontaneous cutaneous inflammation, increased numbers of dermal mast cells, and marked epidermal hyperplasia.