Tyrosine kinase inhibition increases the cell surface localization of FLT3-ITD and enhances FLT3-directed immunotherapy of acute myeloid leukemia

The fms-related tyrosine kinase 3 (FLT3) receptor has been extensively studied over the past two decades with regard to oncogenic alterations that do not only serve as prognostic markers but also as therapeutic targets in acute myeloid leukemia (AML). Internal tandem duplications (ITDs) became of special interest in this setting as they are associated with unfavorable prognosis. Because of sequence-dependent protein conformational changes FLT3-ITD tends to autophosphorylate and displays a constitutive intracellular localization. Here, we analyzed the effect of tyrosine kinase inhibitors (TKIs) on the localization of the FLT3 receptor and its mutants. TKI treatment increased the surface expression through upregulation of FLT3 and glycosylation of FLT3-ITD and FLT3-D835Y mutants. In T cell-mediated cytotoxicity (TCMC) assays, using a bispecific FLT3 × CD3 antibody construct, the combination with TKI treatment increased TCMC in the FLT3-ITD-positive AML cell lines MOLM-13 and MV4-11, patient-derived xenograft cells and primary patient samples. Our findings provide the basis for rational combination of TKI and FLT3-directed immunotherapy with potential benefit for FLT3-ITD-positive AML patients.

Blockade of the PD-1/PD-L1 axis augments lysis of AML cells by the CD33/CD3 BiTE antibody construct AMG 330: reversing a T-cell-induced immune escape mechanism

Bispecific T-cell engagers (BiTEs) are very effective in recruiting and activating T cells. We tested the cytotoxicity of the CD33/CD3 BiTE antibody construct AMG 330 on primary acute myeloid leukemia (AML) cells ex vivo and characterized parameters contributing to antileukemic cytolytic activity. The E:T ratio and the CD33 expression level significantly influenced lysis kinetics in long-term cultures of primary AML cells (n=38). AMG 330 induced T-cell-mediated proinflammatory conditions, favoring the upregulation of immune checkpoints on target and effector cells. Although not constitutively expressed at the time of primary diagnosis (n=123), PD-L1 was strongly upregulated on primary AML cells upon AMG 330 addition to ex vivo cultures (n=27, P<0.0001). This phenomenon was cytokine-driven as the sole addition of interferon (IFN)-γ and tumor necrosis factor-α also induced expression. Through blockade of the PD-1/PD-L1 interaction, AMG 330-mediated lysis (n=9, P=0.03), T-cell proliferation (n=9, P=0.01) and IFN-γ secretion (n=8, P=0.008) were significantly enhanced. The combinatorial approach was most beneficial in settings of protracted AML cell lysis. Taken together, we have characterized a critical resistance mechanism employed by primary AML cells under AMG 330-mediated proinflammatory conditions. Our results support the evaluation of checkpoint molecules in upcoming clinical trials with AMG 330 to enhance BiTE antibody construct-mediated cytotoxicity.

Hepatocyte-specific deletion of the antiapoptotic protein myeloid cell leukemia-1 triggers proliferation and hepatocarcinogenesis in mice

Regulation of hepatocellular apoptosis is crucial for liver homeostasis. Increased sensitivity of hepatocytes toward apoptosis results in chronic liver injury, whereas apoptosis resistance is linked to hepatocarcinogenesis and nonresponsiveness to therapy-induced cell death. Recently, we have demonstrated an essential role of the antiapoptotic Bcl-2 family member Myeloid cell leukemia-1 (Mcl-1) in hepatocyte survival. In mice lacking Mcl-1 specifically in hepatocytes (Mcl-1(Deltahep)), spontaneous apoptosis caused severe liver damage. Here, we demonstrate that chronically increased apoptosis of hepatocytes coincides with strong hepatocyte proliferation resulting in hepatocellular carcinoma (HCC). Liver cell tumor formation was observed in >50% of Mcl-1(Deltahep) mice already by the age of 8 months, whereas 12-month-old wild-type (wt) and heterozygous Mcl-1(flox/wt) mice lacked tumors. Tumors revealed a heterogenous spectrum ranging from small dysplastic nodules to HCC. The neoplastic nature of the tumors was confirmed by histology, expression of the HCC marker glutamine synthetase and chromosomal aberrations. Liver carcinogenesis in Mcl-1(Deltahep) mice was paralleled by markedly increased levels of Survivin, an important regulator of mitosis which is selectively overexpressed in common human cancers.

CONCLUSION

This study provides in vivo evidence that increased apoptosis of hepatocytes not only impairs liver homeostasis but is also accompanied by hepatocyte proliferation and hepatocarcinogenesis. Our findings might have implications for understanding apoptosis-related human liver diseases.

The regulatory protein PhoP controls susceptibility to the host inflammatory response in Shigella flexneri

The PhoP/PhoQ two-component regulatory system controls transcription of several key virulence genes essential for Salmonella survival in the host cell phagosome. Here, we determine that the PhoP/PhoQ system also regulates virulence in the aetiological agent of bacillary dysentery, Shigella flexneri, even though this pathogen escapes from the phagosome into the cytoplasm of the host cell. A phoP mutant of Shigella established infections and induced an acute inflammatory response in two different animal models. However, infections with phoP mutant bacteria were resolved more rapidly than infections with wild-type Shigella. Moreover, the Shigella phoP mutant was more sensitive than the wild-type strain to killing by polymorphonuclear leucocytes (PMNs), cationic polypeptides extracted from PMNs and other animal-derived antimicrobial peptides. The phoP mutant, however, invaded epithelial cells, spread intercellularly, induced apoptosis in macrophages and tolerated extreme acid pH as efficiently as the wild-type strain. PhoP appears to regulate Shigella susceptibility to PMNs and antimicrobial molecules that are important for the late stages of infection with this enteric bacterium.

Fatty Acid synthesis in endosperm of young castor bean seedlings

Enzyme assays on organelles isolated from the endosperm of germinating castor bean (Ricinus communis) by sucrose density gradient centrifugation showed that fatty acid synthesis from [(14)C]malonyl-CoA was localized exclusively in the plastids. The optimum pH was 7.7 and the products was mainly free palmitic and oleic acids. Both NADH and NADPH were required as reductants for maximum activity. Acetyl-CoA, and acyl-carrier protein from Escherichia coli increased the rate of fatty acid synthesis, while low O(2) levels suppressed synthesis. In the absence of NADPH or at low O(2) concentration, stearic acid became a major product at the expense of oleic acid. Fatty acid synthesis activity was highest during the first 3 days of germination, preceding the maximum development of mitochondria and glyoxysomes. It is proposed that the plastids are the source of fatty acids incorporated into the membranes of developing organelles.

Phosphatidic Acid synthesis in castor bean endosperm

Enzyme assays on organelles isolated from the endosperm of castor bean (Ricinus communis var. Hale) by sucrose density gradient centrifugation showed that palmitoyl-CoA:sn-glycerol 3-phosphate acyltransferase (EC 2.3.1.15) was localized in the membranes of the endoplasmic reticulum. Mn(2+) was required for activity, but Ca(2+) and Mg(2+) could substitute for Mn(2+) at higher concentrations. The apparent Km was 170 mum for sn-glycerol 3-phosphate and approximately 8 mum for palmitoyl-CoA. The optimum pH range was 7 to 7.5 and the principal reaction product was diacyl-sn-glycerol 3-phosphate (phosphatidic acid). Monoacyl-sn-glycerol 3-phosphate (lysophosphatidic acid) was not released as a free intermediate in the reaction. The maximum activity of the enzyme occurred immediately after imbibition, preceding the development of mitochondria and glyoxysomes.