Recent work has identified a new subset of effector T cells that produces interleukin (IL)-17 known as T helper 17 (Th17) cells, which is involved in the pathophysiology of inflammatory diseases and is thought to be developmentally related to regulatory T (Treg) cells. Because of its importance for Treg cells, we examined the role of IL-2 in Th17 generation and demonstrate that a previously unrecognized aspect of IL-2 function is to constrain IL-17 production. Genetic deletion or antibody blockade of IL-2 promoted differentiation of the Th17 cell subset. Whereas STAT3 appeared to be a key positive regulator of RORgammat and IL-17 expression, absence of IL-2 or disruption of its signaling by deletion of the transcription factor STAT5 resulted in enhanced Th17 cell development. We conclude that in addition to the promotion of activation-induced cell death of lymphocytes and the generation of Treg cells, inhibition of Th17 polarization appears to be an important function of IL-2.

Studies have focused on the events that influence the development of interleukin 17 (IL-17)-producing T helper cells (T(H)-17 cells) associated with autoimmunity, such as experimental autoimmune encephalitis, but relatively little is known about the cytokines that antagonize T(H)-17 cell effector responses. Here we show that IL-27 receptor-deficient mice chronically infected with Toxoplasma gondii developed severe neuroinflammation that was CD4+ T cell dependent and was associated with a prominent IL-17 response. In vitro, treatment of naive primary T cells with IL-27 suppressed the development T(H)-17 cells induced by IL-6 and transforming growth factor-beta, which was dependent on the intracellular signaling molecule STAT1 but was independent of inhibition of IL-6 signaling mediated by the suppressor protein SOCS3. Thus IL-27, a potent inhibitor of T(H)-17 cell development, may be a useful target for treating inflammatory diseases mediated by these cells.

Prolactin (PRL) induces mammary gland development (defined as mammopoiesis) and lactogenesis. Binding of PRL to its receptor leads to the phosphorylation and activation of STAT (signal transducers and activators of transcription) proteins, which in turn promote the expression of specific genes. The activity pattern of two STAT proteins, StatSa and StatSb, in mammary tissue during pregnancy suggests an active role for these transcription factors in epithelial cell differentiation and milk protein gene expression. To investigate the function of StatSa in mammopoiesis and lactogenesis we disrupted this gene in mice by gene targeting. StatSa-deficient mice developed normally and were indistinguishable from hemizygous and wild-type littermates in size, weight, and fertility. However, mammary lobuloalveolar outgrowth during pregnancy was curtailed, and females failed to lactate after parturition because of a failure of terminal differentiation. Although StatSb has a 96% similarity with StatSa and a superimposable expression pattern during mammary gland development it failed to counterbalance for the absence of StatSa. These results document that StatSa is the principal and an obligate mediator of mammopoietic and lactogenic signaling.

Cre-mediated excision of exon 11 of the breast-tumour suppressor gene Brca1 in mouse mammary epithelial cells causes increased apoptosis and abnormal ductal development. Mammary tumour formation occurs after long latency and is associated with genetic instability characterized by aneuploidy, chromosomal rearrangements or alteration of Trp53 (encoding p53) transcription. To directly test the role of p53 in Brca1-associated tumorigenesis, we introduced a Trp53-null allele into mice with mammary epithelium-specific inactivation of Brca1. The loss of p53 accelerated the formation of mammary tumours in these females. Our results demonstrate that disruption of Brca1 causes genetic instability and triggers further alterations, including the inactivation of p53, that lead to tumour formation.

To delete genes specifically from mammary tissue using the Cre-lox system, we have established transgenic mice expressing Cre recombinase under control of the WAP gene promoter and the MMTV LTR. Cre activity in these mice was evaluated by three criteria. First, the tissue distribution of Cre mRNA was analyzed. Second, an adenovirus carrying a reporter gene was used to determine expression at the level of single cells. Third, tissue specificity of Cre activity was determined in a mouse strain carrying a reporter gene. In adult MMTV-Cre mice expression of the transgene was confined to striated ductal cells of the salivary gland and mammary epithelial cells in virgin and lactating mice. Expression of WAP-Cre was only detected in alveolar epithelial cells of mammary tissue during lactation. Analysis of transgenic mice carrying both the MMTV-Cre and the reporter transgenes revealed recombination in every tissue. In contrast, recombination mediated by Cre under control of the WAP gene promoter was largely restricted to the mammary gland but occasionally observed in the brain. These results show that transgenic mice with WAP-Cre but not MMTV-Cre can be used as a powerful tool to study gene function in development and tumorigenesis in the mammary gland.

This study explored the functions of the signal transducers and activators of transcription 5a and 5b (referred to as Stat5 here) during different stages of mouse mammary gland development by using conditional gene inactivation. Mammary gland morphogenesis includes cell specification, proliferation and differentiation during pregnancy, cell survival and maintenance of differentiation throughout lactation, and cell death during involution. Stat5 is activated by prolactin, and its presence is mandatory for the proliferation and differentiation of mammary epithelium during pregnancy. To address the question of whether Stat5 is also necessary for the maintenance and survival of the differentiated epithelium, the two genes were deleted at different time points. The 110-kb Stat5 locus in the mouse was bracketed with loxP sites, and its deletion was accomplished by using two Cre-expressing transgenic lines. Loss of Stat5 prior to pregnancy prevented epithelial proliferation and differentiation. Deletion of Stat5 during pregnancy, after mammary epithelium had entered Stat5-mediated differentiation, resulted in premature cell death, indicating that at this stage epithelial cell proliferation, differentiation, and survival require Stat5.The signal transducer and activator of transcription (STAT) family of transcription factors conveys cytokine signals from the respective membrane receptors to the nucleus, where they activate diverse genetic programs (11, 12). The two highly conserved Stat5 proteins (Stat5a and Stat5b) are activated by many cytokines, including interleukins, erythropoietin, and prolactin, as well as growth hormone. Mice in which either one or both Stat5 genes were inactivated have revealed unique and redundant roles of the two Stat5 isoforms. Stat5a deficiency results in the loss of prolactin-dependent mammary gland development (15) but does not affect body growth. In contrast, inactivation of Stat5b does not adversely affect mammary development and function but leads to severe growth retardation (34). Mice lacking Stat5a and -5b show defects in multiple organs (33). T-cell proliferation is severely compromised in the absence of Stat5 (22,33), and mammary alveolar epithelium fails to develop during pregnancy (20). Moreover, the multilineage reconstitution potential of hematopoietic stem cells is highly dependent on Stat5 (2).Expansion and differentiation of the mouse mammary alveolar compartment during pregnancy are controlled through the prolactin receptor (23), Jak2 (29), and Stat5 (15,20). Inactivation of the Stat5 locus by using conventional gene targeting resulted in the complete lack of mammary alveolar epithelium, suggesting that the progenitor cells were unable to proliferate. However, those studies could not address the potential roles of Stat5 in the physiology of the epithelial cell beyond its initial proliferation, i.e., whether Stat5 is required for epithelial proliferation, differentiation, and survival throughout pregnancy and for maintenance of epithelial function during lactation. These qu...

IntroductionThe development and differentiation of immune cells is carefully orchestrated by an array of cytokines. Signal transducers and activators of transcription (Stats) represent a small but critical family of transcription factors that play important roles in transmitting cytokine signals. Consequently, Stats are critical for immunoregulation and the development of immune cells. 1,2 Stat5a and Stat5b are two closely related proteins that have overlapping functions with respect to lymphoid development and differentiation. 3,4 Gene targeting of Stat5a and Stat5b (collectively referred to as Stat5), results in impairment in the development of T, B, and natural killer (NK) cells. [5][6][7] In mice in which the amino termini of Stat5a and Stat5b are deleted (denoted as Stat5 ⌬N mice), major disruption of various immune cell parameters was noted. 8,9 However, residual Stat5 function permits T cell development, albeit suboptimally. 10 This contrasts with the complete absence of Stat5a/b, which results in dramatic reduction in thymocyte numbers, in part due to effects on lymphoid stem cell function. 5 T regulatory (Treg) cells comprise a population of cells enriched in CD4 ϩ CD25 ϩ T cells that suppresses T-cell proliferation and function and attenuates immune responses against self-or nonself-antigens. [11][12][13] Naturally arising Treg cells are produced in the thymus as a functionally distinct T-cell subpopulation, whereas adaptive Treg cells are induced from naive T cells after antigen exposure in the periphery. [14][15][16][17] In classic studies, mice develop organ-specific autoimmune disease following neonatal thymectomy, which is corrected by reconstitution with CD4 ϩ CD25 ϩ T cells. 13 The essential role of Treg cells in maintaining tolerance has been confirmed by findings that defective function of this subset is a feature of many models of autoimmunity. 18 More recently, it was discovered independently by several groups that a subset of CD4 ϩ CD25 ϩ T cells expresses the transcription factor Foxp3, which is necessary and sufficient for Treg cell development and function. [19][20][21][22] Foxp3 is highly conserved in mice and humans. Mutation of Foxp3 in mice (scurfy) results in early autoimmune disease, 23 whereas mutations of human Foxp3 are associated with a disorder known as immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX). 24 In mice, Foxp3 is a reliable marker for the Treg lineage.Multiple lines of evidence have indicated that IL-2 is an important growth factor for Treg development and maintenance. Mice lacking IL-2 or its receptor subunits, IL-2R␣ (CD25) and IL-2R␤ (CD122), have deficits in CD4 ϩ CD25 ϩ Treg cells and develop autoimmune disease similar to Foxp3 Ϫ/Ϫ mice. [25][26][27] However, IL-2 is dispensable for Treg cell development, as some Foxp3-expressing cells are present in Il2 Ϫ/Ϫ and Il2ra Ϫ/Ϫ mice, suggesting the involvement of other cytokines. 28 In vitro culture of CD4 ϩ T cells with transforming growth factor-␤1 (TGF-␤1) can promote the generation o...

Suppressor of cytokine signaling (Socs) 3 is a cytokine-inducible inhibitor with critical but selective cell-specific effects. We show that deficiency of Socs3 in T cells had minimal effects on differentiation of T cells to the T helper (Th) 1 or Th2 subsets; accordingly, Socs3 had no effect on IL-12-dependent signal transducer and activator of transcription (Stat) 4 phosphorylation or IL-4-dependent Stat6 phosphorylation. By contrast, Socs3 was found to be a major regulator of IL-23-mediated Stat3 phosphorylation and Th17 generation, and Stat3 directly binds to the IL-17A and IL-17F promoters. We conclude that Socs3 is an essential negative regulator of IL-23 signaling, inhibition of which constrains the generation of Th17 differentiation.signal transducer and activator of transcription 3 ͉ T lymphocytes