Biomedical subjects

Michael A Farrar

Publications and source records attributed to Michael A Farrar.

10 recordsLinked to original sources

PubMed2007

IL-7/STAT5 cytokine signaling pathway is essential but insufficient for maintenance of naive CD4 T cell survival in peripheral lymphoid organs.

Constitutive expression of suppressors of cytokine signaling (SOCS)1 in T lineage in vivo attenuated cytokine signaling and resulted in a dramatic reduction in the number of naive CD44(low)CD62L(high) CD4 T cells in the spleen. After adoptive transfer of thymocytes from SOCS1 transgenic mice into normal recipients, naive CD4 T cells rapidly disappeared from the spleen within 1 wk. Likewise, T cell-specific deletion of STAT5a/b in vivo resulted in a similar phenotype characterized by loss of naive CD4 T cells. Thus, STAT5-mediated signaling is crucial for promoting naive T cell survival. However, forced expression of constitutively active STAT5 failed to rescue CD4 T cells in SOCS1 transgenic mice, implying that STAT5 activation is necessary but not sufficient for naive CD4 T cell survival. Although blockade of the IL-7R, a SOCS1 target, resulted in clear inhibition of naive T cell survival, the effect occurred 3 wk after anti-IL-7R Ab treatment, but not at earlier time points. These results suggest that IL-7-mediated STAT5 activation is essential for long-term survival of naive CD4 cells after export from thymus, and that another SOCS1-sensitive cytokine is critical for short-term naive T cell survival.

Adoptive Transfer↗

PubMed2007

IL-2 receptor beta-dependent STAT5 activation is required for the development of Foxp3+ regulatory T cells.

IL-2(-/-) mice develop autoimmunity despite having relatively normal numbers of regulatory T cells (Tregs). In contrast, we demonstrate that IL-2(-/-) x IL-15(-/-) and IL-2Rbeta(-/-) mice have a significant decrease in Treg numbers. Ectopic expression of foxp3 in a subset of CD4(+) T cells rescued Treg development and prevented autoimmunity in IL-2Rbeta(-/-) mice, suggesting that IL-2Rbeta-dependent signals regulate foxp3 expression in Tregs. Subsequent analysis of IL-2Rbeta-dependent signal transduction pathways established that the transcription factor STAT5 is necessary and sufficient for Treg development. Specifically, T cell-specific deletion of STAT5 prevented Treg development; conversely, reconstitution of IL-2Rbeta(-/-) mice with bone marrow cells expressing an IL-2Rbeta mutant that exclusively activates STAT5 restored Treg development. Finally, STAT5 binds to the promoter of the foxp3 gene suggesting that IL-2Rbeta-dependent STAT5 activation promotes Treg differentiation by regulating expression of foxp3.

Animals↗

PubMed2006

Constitutive activation of STAT5 supersedes the requirement for cytokine and TCR engagement of CD4+ T cells in steady-state homeostasis.

The transcription factor STAT5 is one of several signaling mediators activated via common gamma-chain cytokine receptors. As such, it plays an important role in lymphocyte survival and proliferation during normal homeostasis as well as under lymphopenic conditions. Transgenic mice expressing a constitutively activated form of STAT5b have been shown previously to contain increased numbers of peripheral CD4+CD25- T cells. To define the mechanism(s) for this occurrence, we have used adoptive transfer studies to examine the effects of STAT5 activity on steady-state CD4+ T cell homeostasis. We observed that constitutive STAT5 signaling induced 4- to 7-fold increased levels of basal steady-state proliferation, which was accompanied by a comparable increase in T cell recovery. Most strikingly, steady-state CD4 T cell proliferation occurred independently of both MHC class II and IL-15. These observations demonstrate that the STAT5-driven pathway is important to lymphocyte homeostasis and can supersede the need for both TCR engagement and cytokine stimulation. This suggests that the need for TCR stimulation to induce common gamma-chain cytokine receptor expression, and thus STAT5 activation, is a key factor in maintaining normal CD4+ T cell homeostasis.

Adoptive Transfer↗

PubMed2006

Attenuation of IL-7 receptor signaling is not required for allelic exclusion.

Allelic exclusion prevents pre-B cells from generating more than one functional H chain, thereby ensuring the formation of a unique pre-BCR. The signaling processes underlying allelic exclusion are not clearly understood. IL-7R-dependent signals have been clearly shown to regulate the accessibility of the Ig H chain locus. More recent work has suggested that pre-BCR-dependent attenuation of IL-7R signaling returns the H chain loci to an inaccessible state; this process has been proposed to underlie allelic exclusion. Importantly, this model predicts that preventing pre-BCR-dependent down-regulation of IL-7R signaling should interfere with allelic exclusion. To test this hypothesis, we made use of transgenic mice that express a constitutively active form of STAT5b (STAT5b-CA). STAT5b-CA expression restores V(D)J recombination in IL-7R(-/-) B cells, demonstrating that IL-7 regulates H chain locus accessibility and V(D)J recombination via STAT5 activation. To examine the effects of constitutively active STAT5b on allelic exclusion, we crossed STAT5b-CA mice (which express the IgM(b) allotype) to IgM(a) allotype congenic mice. We found no difference in the percentage of IgM(a)/IgM(b)-coexpressing B cells in STAT5b-CA vs littermate control mice; identical results were observed when crossing STAT5b-CA mice with hen egg lysozyme (HEL) H chain transgenic mice. The HEL transgene enforces allelic exclusion, preventing rearrangement of endogenous H chain genes; importantly, rearrangement of endogenous H chain genes was suppressed to a similar degree in STAT5b-CA vs HEL mice. Thus, attenuation of IL-7R/STAT5 signaling is not required for allelic exclusion.

Alleles↗

PubMed2005

Restricted STAT5 activation dictates appropriate thymic B versus T cell lineage commitment.

The molecular mechanisms regulating lymphocyte lineage commitment remain poorly characterized. To explore the role of the IL7R in this process, we generated transgenic mice that express a constitutively active form of STAT5 (STAT5b-CA), a key downstream IL7R effector, throughout lymphocyte development. STAT5b-CA mice exhibit a 40-fold increase in pro-B cells in the thymus. As documented by BrdU labeling studies, this increase is not due to enhanced B cell proliferation. Thymic pro-B cells in STAT5b-CA mice show a modest increase in cell survival ( approximately 4-fold), which correlates with bcl-x(L) expression. However, bcl-x(L) transgenic mice do not show increases in thymic B cell numbers. Thus, STAT5-dependent bcl-x(L) up-regulation and enhanced B cell survival are not sufficient to drive the thymic B cell development observed in STAT5b-CA mice. Importantly, thymic pro-B cells in STAT5b-CA mice are derived from early T cell progenitors (ETPs), suggesting that STAT5 acts by altering ETP lineage commitment. Supporting this hypothesis, STAT5 binds to the pax5 promoter in ETPs from STAT5b-CA mice and induces pax5, a master regulator of B cell development. Conversely, STAT5b-CA mice exhibit a decrease in the DN1b subset of ETPs, demonstrating that STAT5 activation inhibits early T cell differentiation or lineage commitment. On the basis of these findings, we propose that the observed expression of the IL-7R on common lymphoid progenitors, but not ETPs, results in differential STAT5 signaling within these distinct progenitor populations and thus helps ensure appropriate development of B cells and T cells in the bone marrow and thymic environments, respectively.

Animals↗

PubMed2004

Distinct IL-2 receptor signaling pattern in CD4+CD25+ regulatory T cells.

Despite expression of the high-affinity IL-2R, CD4(+)CD25(+) regulatory T cells (Tregs) are hypoproliferative upon IL-2R stimulation in vitro. However the mechanisms by which CD4(+)CD25(+) T cells respond to IL-2 signals are undefined. In this report, we examine the cellular and molecular responses of CD4(+)CD25(+) Tregs to IL-2. IL-2R stimulation results in a G(1) cell cycle arrest, cellular enlargement and increased cellular survival of CD4(+)CD25(+) T cells. We find a distinct pattern of IL-2R signaling in which the Janus kinase/STAT pathway remains intact, whereas IL-2 does not activate downstream targets of phosphatidylinositol 3-kinase. Negative regulation of phosphatidylinositol 3-kinase signaling and IL-2-mediated proliferation of CD4(+)CD25(+) T cells is inversely associated with expression of the phosphatase and tensin homologue deleted on chromosome 10, PTEN.

Animals↗

PubMed2004

STAT5 activation underlies IL7 receptor-dependent B cell development.

Signals initiated by the IL7R are required for B cell development. However, the roles that distinct IL7R-induced signaling pathways play in this process remains unclear. To identify the function of the Raf and STAT5 pathways in IL7R-dependent B cell development, we used transgenic mice that express constitutively active forms of Raf (Raf-CAAX) or STAT5 (STAT5b-CA) throughout lymphocyte development. Both Raf-CAAX and STAT5b-CA mice exhibit large increases in pro-B cells. However, crossing the Raf-CAAX transgene onto the IL7R(-/-) background fails to rescue B cell development. In contrast, STAT5 activation selectively restores B cell expansion in IL7R(-/-) mice. Notably, the expansion of pro-B cells in STAT5b-CA mice correlated with an increase in cyclin D2, pim-1, and bcl-x(L) expression, suggesting that STAT5 directly affects pro-B cell proliferation and survival. In addition, STAT5 activation also restored B cell differentiation in IL7R(-/-) mice as determined by 1) the restoration of V(H) Ig gene rearrangement and 2) the appearance of immature and mature B cell subsets. These findings establish STAT5 as the key player entraining B cell development downstream of the IL7R.

Animals↗

PubMed2003

Distinct effects of STAT5 activation on CD4+ and CD8+ T cell homeostasis: development of CD4+CD25+ regulatory T cells versus CD8+ memory T cells.

Using transgenic mice that express a constitutively active version of STAT5b, we demonstrate that STAT5 plays a key role in governing B cell development and T cell homeostasis. STAT5 activation leads to a 10-fold increase in pro-B, but not pro-T, cells. Conversely, STAT5 signaling promotes the expansion of mature alphabeta T cells (6-fold increase) and gammadelta and NK T cells (3- to 4-fold increase), but not of mature B cells. In addition, STAT5 activation has dramatically divergent effects on CD8(+) vs CD4(+) T cells, leading to the selective expansion of CD8(+) memory-like T cells and CD4(+)CD25(+) regulatory T cells. These results establish that activation of STAT5 is the primary mechanism underlying both IL-7/IL-15-dependent homeostatic proliferation of naive and memory CD8(+) T cells and IL-2-dependent development of CD4(+)CD25(+) regulatory T cells.

Animals↗

PubMed2003

Membrane localization, oligomerization, and phosphorylation are required for optimal raf activation.

Activation of the serine/threonine kinase c-Raf-1 requires membrane localization, phosphorylation, and oligomerization. To study these mechanisms of Raf activation more precisely, we have used a membrane-localized fusion protein, myr-Raf-GyrB, which can be activated by coumermycin-induced oligomerization in NIH3T3 transfectants. By introducing a series of point mutations into the myr-Raf-GyrB kinase domain (S338A, S338A/Y341F, Y340F/Y341F, and T491A/S494A) we can separately study the role that membrane localization, phosphorylation, and oligomerization play in the process of Raf activation. We find that phosphorylation of Ser-338 plays a critical role in Raf activation and that this requires membrane localization but not oligomerization of Raf. Mutation of Tyr-341 had a limited effect, whereas mutation of both Ser-338 and Tyr-341 resulted in a synergistic loss of Raf activation following coumermycin-induced dimerization. Importantly, we found that membrane localization and phosphorylation of Ser-338 were not sufficient to activate Raf in the absence of oligomerization. Thus, our studies suggest that three key steps are required for optimal Raf activation: recruitment to the plasma membrane by GTP-bound Ras, phosphorylation via membrane-resident kinases, and oligomerization.

Aminocoumarins↗

PubMed2003

In vivo survival and homeostatic proliferation of natural killer cells.

While the specificity and development of natural killer (NK) cells have been intensely studied, little is known about homeostasis of the mature NK population. Here we show that mouse NK cells undergo homeostatic proliferation when transferred into NK-deficient Rag-/- gammaC-/- hosts. Normal NK functional activity is maintained during this process, although there are some changes in NK phenotype. Using cell sorting, we demonstrate that mature (Mac-1hi) NK cells undergo homeostatic proliferation in an NK-deficient environment, yet immature (Mac-1lo) NK cells also proliferate in such hosts. We find that mature NK cells survive but do not proliferate in hosts which possess an endogenous NK pool. However, we go on to show that mature NK survival is critically dependent on interleukin (IL)-15. Surprisingly, NK survival is also compromised after transfer of cells into IL-15Ralpha-/- mice, implying that IL-15 responsiveness by bystander cells is critical for NK maintenance. These data imply that, similar to T cells, homeostasis of the NK pool is much more dynamic than previously appreciated and this may be relevant to manipulation of NK cells for therapeutic purposes.

Adoptive Transfer↗