Inhibition of the receptor tyrosine kinase Axl impedes activation of the FLT3 internal tandem duplication in human acute myeloid leukemia: implications for Axl as a potential therapeutic target.

Approximately 20% to 25% of patients with acute myeloid leukemia (AML) have a constitutively activated FLT3-internal tandem duplication (FLT3-ITD), and these patients exhibit a poor prognosis. Here, we report that Axl, a receptor tyrosine kinase (RTK) overexpressed and constitutively active in human AML, targets the RTK FLT3 in FLT3-ITD(+) AML. Abrogation of Axl activation by soluble Axl chimeric protein (Axl-Fc) or small interfering RNA (siRNA) diminishes constitutive FLT3 phosphorylation in FLT3-ITD(+) AML. In addition, inhibition of Axl activation by Axl-Fc interferes with the physical interaction between Axl and FLT3. We found that Axl-Fc, a pharmacologic Axl inhibitor, or siRNA targeting Axl inhibits cell growth, induces cell-cycle arrest and apoptosis, and relieves a block in myeloid differentiation of FLT3-ITD(+) AML in vitro. Axl-Fc also suppresses the growth of human FLT3-ITD(+) AML in vivo. Collectively, our data suggest that Axl contributes to the pathogenesis of FLT3-ITD(+) AML through, at least in part, positive regulation of constitutive FLT3 activation. This also suggests that Axl should be pursued as a potential target for the treatment of FLT3-ITD(+) AML.

Axl/Gas6 pathway positively regulates FLT3 activation in human natural killer cell development.

Activation of the fibromyalgia syndrome-like tyrosine kinase 3 (FLT3) by its ligand, FLT3 ligand (FL), strongly augments the development of natural killer (NK) cells from human CD34⁺ hematopoietic progenitor cells (HPCs) in the presence of IL-15, compared with NK-cell development in the presence of IL-15 alone. In this study, we observed that blocking the receptor tyrosine kinase Axl/Gas6 pathway with a soluble Axl-IgG1 Fc fusion protein (Axl-Fc) in the presence of FL significantly diminished the absolute number of CD3⁻ CD56⁺ NK cells derived from human CD34⁺ HPCs. Axl-Fc reduced the expression levels of the IL-2/15 receptor ß chain (CD122) and γ chain (CD132) induced by activation of FLT3 and consequently reduced the frequency of NK precursor cells responding to IL-15. Furthermore, Axl-Fc diminished FL-induced FLT3 phosphorylation and impeded the physical interaction between Axl and FLT3 in CD34⁺ HPCs. Collectively, our data suggest that the Axl/Gas6 pathway contributes to normal human NK-cell development at least in part via its positive regulatory effect on FLT3 signaling in CD34⁺ HPCs.

CD94 surface density identifies a functional intermediary between the CD56bright and CD56dim human NK-cell subsets.

Human CD56(bright) natural killer (NK) cells possess little or no killer immunoglobulin-like receptors (KIRs), high interferon-gamma (IFN-gamma) production, but little cytotoxicity. CD56(dim) NK cells have high KIR expression, produce little IFN-gamma, yet display high cytotoxicity. We hypothesized that, if human NK maturation progresses from a CD56(bright) to a CD56(dim) phenotype, an intermediary NK cell must exist, which demonstrates more functional overlap than these 2 subsets, and we used CD94 expression to test our hypothesis. CD94(high)CD56(dim) NK cells express CD62L, CD2, and KIR at levels between CD56(bright) and CD94(low)CD56(dim) NK cells. CD94(high)CD56(dim) NK cells produce less monokine-induced IFN-gamma than CD56(bright) NK cells but much more than CD94(low)CD56(dim) NK cells because of differential interleukin-12-mediated STAT4 phosphorylation. CD94(high)CD56(dim) NK cells possess a higher level of granzyme B and perforin expression and CD94-mediated redirected killing than CD56(bright) NK cells but lower than CD94(low)CD56(dim) NK cells. Collectively, our data suggest that the density of CD94 surface expression on CD56(dim) NK cells identifies a functional and likely developmental intermediary between CD56(bright) and CD94(low)CD56(dim) NK cells. This supports the notion that, in vivo, human CD56(bright) NK cells progress through a continuum of differentiation that ends with a CD94(low)CD56(dim) phenotype.

The Axl/Gas6 pathway is required for optimal cytokine signaling during human natural killer cell development.

Interleukin-15 (IL-15) is essential for natural killer (NK) cell differentiation. In this study, we assessed whether the receptor tyrosine kinase Axl and its ligand, Gas6, are involved in IL-15-mediated human NK differentiation from CD34(+) hematopoietic progenitor cells (HPCs). Blocking the Axl-Gas6 interaction with a soluble Axl fusion protein (Axl-Fc) or the vitamin K inhibitor warfarin significantly diminished the absolute number and percentage of CD3(-)CD56(+) NK cells derived from human CD34(+) HPCs cultured in the presence of IL-15, probably resulting in part from reduced phosphorylation of STAT5. In addition, CD3(-)CD56(+) NK cells derived from culture of CD34(+) HPCs with IL-15 and Axl-Fc had a significantly diminished capacity to express interferon-gamma or its master regulator, T-BET. Culture of CD34(+) HPCs in the presence of c-Kit ligand and Axl-Fc resulted in a significant decrease in the frequency of NK precursor cells responding to IL-15, probably the result of reduced c-Kit phosphorylation. Collectively, our data suggest that the Axl/Gas6 pathway contributes to normal human NK-cell development, at least in part via its regulatory effects on both the IL-15 and c-Kit signaling pathways in CD34(+) HPCs, and to functional NK-cell maturation via an effect on the master regulatory transcription factor T-BET.

CD94 defines phenotypically and functionally distinct mouse NK cell subsets.

Understanding of heterogeneous NK subsets is important for the study of NK cell biology and development, and for the application of NK cell-based therapies in the treatment of disease. Here we demonstrate that the surface expression of CD94 can distinctively divide mouse NK cells into two approximately even CD94(low) and CD94(high) subsets in all tested organs and tissues. The CD94(high) NK subset has significantly greater capacity to proliferate, produce IFN-gamma, and lyse target cells than does the CD94(low) subset. The CD94(high) subset has exclusive expression of NKG2A/C/E, higher expression of CD117 and CD69, and lower expression of Ly49D (activating) and Ly49G2 (inhibitory). In vivo, purified mouse CD94(low) NK cells become CD94(high) NK cells, but not vice versa. Collectively, our data suggest that CD94 is an Ag that can be used to identify functionally distinct NK cell subsets in mice and could also be relevant to late-stage mouse NK cell development.

TGF-beta 1 inhibition of IFN-gamma-induced signaling and Th1 gene expression in CD4+ T cells is Smad3 independent but MAP kinase dependent.

In addition to classic Smad signaling pathways, the pleiotropic immunoregulatory cytokine TGF-beta1 can activate MAP kinases, but a role for TGF-beta1-MAP kinase pathways in T cells has not been defined heretofore. We have shown previously that TGF-beta1 inhibits Th1 development by inhibiting IFN-gamma's induction of T-bet and other Th1 differentiation genes, and that TGF-beta1 inhibits receptor-proximal IFN-gamma-Jak-Stat signaling responses. We now show that these effects of TGF-beta1 are independent of the canonical TGF-beta1 signaling module Smad3, but involve a specific MAP kinase pathway. In primary T cells, TGF-beta1 activated the MEK/ERK and p38 MAP kinase pathways, but not the JNK pathway. Inhibition of the MEK/ERK pathway completely eliminated the inhibitory effects of TGF-beta1 on IFN-gamma responses in T cells, whereas inhibition of the p38 pathway had no effect. Thus, TGF-beta1's inhibition of IFN-gamma signaling in T cells is mediated through a highly specific Smad3 independent, MEK/ERK-dependent signaling pathway.

GAS6 expression identifies high-risk adult AML patients: potential implications for therapy.

Emerging data demonstrate important roles for the TYRO3/AXL/MERTK receptor tyrosine kinase (TAM RTK) family in diverse cancers. We investigated the prognostic relevance of GAS6 expression, encoding the common TAM RTK ligand, in 270 adults (n=71 aged<60 years; n=199 aged ⩾60 years) with de novo cytogenetically normal acute myeloid leukemia (CN-AML). Patients expressing GAS6 (GAS6+), especially those aged ⩾60 years, more often failed to achieve a complete remission (CR). In all patients, GAS6+ patients had shorter disease-free (DFS) and overall (OS) survival than patients without GAS6 expression (GAS6-). After adjusting for other prognostic markers, GAS6+ predicted CR failure (P=0.02), shorter DFS (P=0.004) and OS (P=0.04). To gain further biological insights, we derived a GAS6-associated gene-expression signature (P<0.001) that in GAS6+ patients included overexpressed BAALC and MN1, known to confer adverse prognosis in CN-AML, and overexpressed CXCL12, encoding stromal cell-derived factor, and its receptor genes, chemokine (C-X-C motif) receptor 4 (CXCR4) and CXCR7. This study reports for the first time that GAS6 expression is an adverse prognostic marker in CN-AML. Although GAS6 decoy receptors are not yet available in the clinic for GAS6+ CN-AML therapy, potential alternative therapies targeting GAS6+-associated pathways, for example, CXCR4 antagonists, may be considered for GAS6+ patients to sensitize them to chemotherapy.

TGF-beta1 inhibits T-bet induction by IFN-gamma in murine CD4+ T cells through the protein tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1.

TGF-beta1 prevents the development of autoimmune disease by restraining the development of autoreactive Th1 cells. TGF-beta1 inhibits Th1 development in part by suppressing the expression of T-bet, an IFN-gamma-induced transcription factor that promotes Th1 differentiation, but how TGF-beta1 suppresses T-bet is not known. In this study we show that TGF-beta1 suppresses IFN-gamma-induced T-bet expression through the hemopoietic protein tyrosine phosphatase (PTP) Src homology region 2 domain-containing phosphatase-1 (Shp-1). In murine CD4+ T cells, IFN-gamma rapidly induced the expression of T-bet as well as of IFN regulatory factor-1, another transcription factor important for Th1 development. TGF-beta1 antagonized the effects of IFN-gamma, inhibiting IFN-gamma's induction of both Th1 transcription factors. In the presence of IFN-gamma, TGF-beta1 rapidly induced in Th cells the synthesis of the PTP Shp-1, but did not induce Shp-2 or several members of the suppressor of cytokine signaling family of Jak-Stat inhibitors. We tested the requirement for Shp-1 by using T cells from the Shp-1-deficient me(v)/me(v) mouse strain. Shp-1 was required for TGF-beta1's suppressive effects, because its suppression of T-bet and IFN regulatory factor-1 was completely abrogated in me(v)/me(v) CD4+ T cells. Receptor-proximal responses to IFN-gamma, such as the induction of Jak-Stat phosphorylation, were inhibited by TGF-beta1 in wild-type T cells, but not in me(v)/me(v) T cells. Consistent with a direct role for Shp-1, TGF-beta1's inhibition of IFN-gamma-induced Stat1 phosphorylation was sensitive to the general PTP inhibitor pervanadate. Together, these data show that TGF-beta1 suppresses IFN-gamma signaling and transcriptional responses in CD4+ T cells through the PTP Shp-1.

Necroinflammatory liver disease in BALB/c background, TGF-beta 1-deficient mice requires CD4+ T cells.

The etiology of autoimmune liver disease is poorly understood. BALB/c mice deficient in the immunoregulatory cytokine TGF-beta1 spontaneously develop necroinflammatory liver disease, but the immune basis for the development of this pathology has not been demonstrated. Here, we show that BALB/c-TGF-beta1(-/-) mice exhibit abnormal expansion in hepatic mononuclear cells (MNCs) compared with wild-type littermate control mice, particularly in the T cell and macrophage lineages. To test whether lymphocytes of the adaptive immune system are required for the spontaneous development of necroinflammatory liver disease, BALB/c-TGF-beta1(-/-) mice were rendered deficient in B and T cells by crossing them with BALB/c-recombinase-activating gene 1(-/-) mice. BALB/c-TGF-beta1(-/-)/recombinase-activating gene 1(-/-) double-knockout mice showed extended survival and did not develop necroinflammatory liver disease. The cytolytic activity of BALB/c-TGF-beta1(-/-) hepatic lymphocytes was assessed using an in vitro CTL assay. CTL activity was much higher in BALB/c-TGF-beta1(-/-) hepatic MNCs compared with littermate control hepatic MNCs and was particularly pronounced in the CD4(+) T cell subset. Experimental depletion of CD4(+) T cells in young BALB/c-TGF-beta1(-/-) mice prevented the subsequent development of necroinflammatory liver disease, indicating that CD4(+) T cells are essential for disease pathogenesis in vivo. These data definitively establish an immune-mediated etiology for necroinflammatory liver disease in BALB/c-TGF-beta1(-/-) mice and demonstrate the importance of CD4(+) T cells in disease pathogenesis in vivo. Furthermore, TGF-beta1 has a critical role in homeostatic regulation of the hepatic immune system, inhibiting the development or expansion of hepatic cytolytic CD4(+) T cells.