Limitations of a mouse model of sickle cell anemia

Pathophysiological characterization of the Townes mouse model for sickle cell disease

A deeper pathophysiologic understanding of available mouse models of sickle cell disease (SCD), such as the Townes model, will help improve preclinical studies. We evaluated groups of Townes mice expressing either normal adult human hemoglobin (HbA), sickle cell trait (HbAS), or SCD (HbS), comparing younger versus older adults, and females versus males. We obtained hematologic parameters in steady-state and hypoxic conditions and evaluated metabolic markers and cytokines from serum. Kidney function was evaluated by measuring the urine protein/creatinine ratio and urine osmolality. In vivo studies included von Frey assay, non-invasive plethysmography, and echocardiography. Histopathological evaluations were performed in lung, liver, spleen, and kidney tissues. HbS mice displayed elevated hemolysis markers and white blood cell counts, with some increases more pronounced in older adults. After extended in vivo hypoxia, hemoglobin, platelet counts, and white blood cell counts decreased significantly in HbS mice, whereas they remained stable in HbA mice. Cytokine analyses showed increased TNF-alpha in HbS mice. Kidney function assays revealed worsened kidney function in HbS mice. The von Frey assay showed a lower threshold to response in the HbS mice than controls, with more noticeable differences in males. Echocardiography in HbS mice suggested left ventricular hypertrophy and dilatation. Plethysmography suggested obstructive lung disease and inflammatory changes in HbS mice. Histopathological studies showed vascular congestion, increased iron deposition, and disruption of normal tissue architecture in HbS mice. These data correlate with clinical manifestations in SCD patients and highlight analyses and groups to be included in preclinical therapeutic studies.

G-CSF induces membrane expression of a myeloperoxidase glycovariant that operates as an E-selectin ligand on human myeloid cells

The host defense response critically depends on the production of leukocytes by the marrow and the controlled delivery of these cells to relevant sites of inflammation/infection. The cytokine granulocyte-colony stimulating factor (G-CSF) is commonly used therapeutically to augment neutrophil recovery following chemo/radiation therapy for malignancy, thereby decreasing infection risk. Although best known as a potent inducer of myelopoiesis, we previously reported that G-CSF also promotes the delivery of leukocytes to sites of inflammation by stimulating expression of potent E-selectin ligands, including an uncharacterized ∼65-kDa glycoprotein. To identify this ligand, we performed integrated biochemical analysis and mass spectrometry studies of G-CSF-treated primary human myeloid cells. Our studies show that this novel E-selectin ligand is a glycoform of the heavy chain component of the enzyme myeloperoxidase (MPO), a well-known lysosomal peroxidase. This specialized MPO glycovariant, referred to as "MPO-E-selectin ligand" (MPO-EL), is expressed on circulating G-CSF-mobilized leukocytes and is naturally expressed on blood myeloid cells in patients with febrile leukocytosis. In vitro biochemical studies show that G-CSF programs MPO-EL expression on human blood leukocytes and marrow myeloid cells via induction of N-linked sialofucosylations on MPO, with concomitant cell surface display of the molecule. MPO-EL is catalytically active and mediates angiotoxicity on human endothelial cells that express E-selectin. These findings thus define a G-CSF effect on MPO chemical biology that endows unsuspected functional versatility upon this enzyme, unveiling new perspectives on the biology of G-CSF and MPO, and on the role of E-selectin receptor/ligand interactions in leukocyte migration and vascular pathology.

Drug-loaded sickle cells programmed ex vivo for delayed hemolysis target hypoxic tumor microvessels and augment tumor drug delivery

Selective drug delivery to hypoxic tumor niches remains a significant therapeutic challenge that calls for new conceptual approaches. Sickle red blood cells (SSRBCs) have shown an ability to target such hypoxic niches and induce tumoricidal effects when used together with exogenous pro-oxidants. Here we determine whether the delivery of a model therapeutic encapsulated in murine SSRBCs can be enhanced by ex vivo photosensitization under conditions that delay autohemolysis to a time that coincides with maximal localization of SSRBCs in a hypoxic tumor. Hyperspectral imaging of 4T1 carcinomas shows oxygen saturation levels <10% in a large fraction (commonly 50% or more) of the tumor. Using video microscopy of dorsal skin window chambers implanted with 4T1 tumors, we demonstrate that allogeneic SSRBCs, but not normal RBCs (nRBCs), selectively accumulate in hypoxic 4T1 tumors between 12 and 24h after systemic administration. We further show that ex vivo photo-oxidation can program SSRBCs to postpone hemolysis/release of a model therapeutic to a point that coincides with their maximum sequestration in hypoxic tumor microvessels. Under these conditions, drug-loaded photosensitized SSRBCs show a 3-4 fold greater drug delivery to tumors compared to non-photosensitized SSRBCs, drug-loaded photosensitized nRBCs, and free drug. These results demonstrate that photo-oxidized SSRBCs, but not photo-oxidized nRBCs, sequester and hemolyze in hypoxic tumors and release substantially more drug than photo-oxidized nRBCs and non-photo-oxidized SSRBCs. Photo-oxidation of drug-loaded SSRBCs thus appears to exploit the unique tumor targeting and carrier properties of SSRBCs to optimize drug delivery to hypoxic tumors. Such programmed and drug-loaded SSRBCs therefore represent a novel and useful tool for augmenting drug delivery to hypoxic solid tumors.