An anti-PD-1-GITR-L bispecific agonist induces GITR clustering-mediated T cell activation for cancer immunotherapy.

Costimulatory receptors such as glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) play key roles in regulating the effector functions of T cells. In human clinical trials, however, GITR agonist antibodies have shown limited therapeutic effect, which may be due to suboptimal receptor clustering-mediated signaling. To overcome this potential limitation, a rational protein engineering approach is needed to optimize GITR agonist-based immunotherapies. Here we show a bispecific molecule consisting of an anti-PD-1 antibody fused with a multimeric GITR ligand (GITR-L) that induces PD-1-dependent and FcγR-independent GITR clustering, resulting in enhanced activation, proliferation and memory differentiation of primed antigen-specific GITR+PD-1+ T cells. The anti-PD-1-GITR-L bispecific is a PD-1-directed GITR-L construct that demonstrated dose-dependent, immunologically driven tumor growth inhibition in syngeneic, genetically engineered and xenograft humanized mouse tumor models, with a dose-dependent correlation between target saturation and Ki67 and TIGIT upregulation on memory T cells. Anti-PD-1-GITR-L thus represents a bispecific approach to directing GITR agonism for cancer immunotherapy.

Epitope and Fc-Mediated Cross-linking, but Not High Affinity, Are Critical for Antitumor Activity of CD137 Agonist Antibody with Reduced Liver Toxicity.

CD137 (TNFRSF9, 4-1BB) agonist antibodies (mAb) have demonstrated potent antitumor activity with memory response while causing hepatotoxicity in mouse models. In clinical trials, the degrees of liver toxicity of anti-CD137 vary from grade 4 transaminitis (urelumab) to nonexistent (utomilumab). To exploit the antitumor potential of CD137 signaling, we identified a new class of CD137 agonist mAbs with strong antitumor potency without significant transaminitis in vivo compared with CD137 agonists previously reported. These mAbs are cross-reactive to mouse and cynomolgus monkey and showed cross-linking-dependent T-cell costimulation activity in vitro Antitumor efficacy was maintained in Fc gamma receptor (FcγR) III-deficient mice but diminished in FcγRIIB-deficient mice, suggesting the critical role for FcγRIIB to provide cross-linking in vivo Interestingly, a single dose of an affinity-reduced variant was sufficient to control tumor growth, but a higher affinity variant did not improve efficacy. These observations suggest that binding epitope and FcγR interaction, but not necessarily high affinity, are important for antitumor efficacy and reduced liver toxicity of CD137 mAb. Our study suggests the possibility of CD137 agonist therapy with improved safety profile in humans.

A Bispecific Molecule Targeting CD40 and Tumor Antigen Mesothelin Enhances Tumor-Specific Immunity.

Agonistic CD40 monoclonal antibodies (mAb) have demonstrated some clinical activity, but with dose-limiting toxicity. To reduce systemic toxicity, we developed a bispecific molecule that was maximally active in the presence of a tumor antigen and had limited activity in the absence of the tumor antigen. LB-1 is a bispecific molecule containing single-chain Fv domains targeting mouse CD40 and the tumor antigen mesothelin. LB-1 exhibited enhanced activity upon binding to cell-surface mesothelin but was less potent in the absence of mesothelin binding. In a mouse model implanted with syngeneic 4T1 tumors expressing cell-surface mesothelin, LB-1 demonstrated comparable antitumor activity as an agonistic CD40 mAb but did not cause elevation of serum cytokines and liver enzymes, as was observed in anti-CD40-treated mice. The results from our study of LB-1 were used to develop a human cross-reactive bispecific molecule (ABBV-428) that targeted human CD40 and mesothelin. ABBV-428 demonstrated enhanced activation of antigen-presenting cells and T cells upon binding to cell-surface mesothelin, and inhibition of cultured or implanted PC3 tumor cell growth after immune activation. Although expression of cell-surface mesothelin is necessary, the bispecific molecules induced immune-mediated antitumor activity against both mesothelin+ and mesothelin- tumor cells. ABBV-428 represents a class of bispecific molecules with conditional activity dependent on the binding of a tumor-specific antigen, and such activity could potentially maximize antitumor potency while limiting systemic toxicity in clinical studies.

LRRC15 Is a Novel Mesenchymal Protein and Stromal Target for Antibody-Drug Conjugates.

Progress in understanding tumor stromal biology has been constrained in part because cancer-associated fibroblasts (CAF) are a heterogeneous population with limited cell-type-specific protein markers. Using RNA expression profiling, we identified the membrane protein leucine-rich repeat containing 15 (LRRC15) as highly expressed in multiple solid tumor indications with limited normal tissue expression. LRRC15 was expressed on stromal fibroblasts in many solid tumors (e.g., breast, head and neck, lung, pancreatic) as well as directly on a subset of cancer cells of mesenchymal origin (e.g., sarcoma, melanoma, glioblastoma). LRRC15 expression was induced by TGFß on activated fibroblasts (αSMA+) and on mesenchymal stem cells. These collective findings suggested LRRC15 as a novel CAF and mesenchymal marker with utility as a therapeutic target for the treatment of cancers with LRRC15-positive stromal desmoplasia or cancers of mesenchymal origin. ABBV-085 is a monomethyl auristatin E (MMAE)-containing antibody-drug conjugate (ADC) directed against LRRC15, and it demonstrated robust preclinical efficacy against LRRC15 stromal-positive/cancer-negative, and LRRC15 cancer-positive models as a monotherapy, or in combination with standard-of-care therapies. ABBV-085's unique mechanism of action relied upon the cell-permeable properties of MMAE to preferentially kill cancer cells over LRRC15-positive CAF while also increasing immune infiltrate (e.g., F4/80+ macrophages) in the tumor microenvironment. In summary, these findings validate LRRC15 as a novel therapeutic target in multiple solid tumor indications and support the ongoing clinical development of the LRRC15-targeted ADC ABBV-085.Significance: These findings identify LRRC15 as a new marker of cancer-associated fibroblasts and cancers of mesenchymal origin and provide preclinical evidence for the efficacy of an antibody-drug conjugate targeting the tumor stroma. Cancer Res; 78(14); 4059-72. ©2018 AACR.

Enavatuzumab, a Humanized Anti-TWEAK Receptor Monoclonal Antibody, Exerts Antitumor Activity through Attracting and Activating Innate Immune Effector Cells.

Enavatuzumab is a humanized IgG1 anti-TWEAK receptor monoclonal antibody that was evaluated in a phase I clinical study for the treatment of solid malignancies. The current study was to determine whether and how myeloid effector cells were involved in postulated mechanisms for its potent antitumor activity in xenograft models. The initial evidence for a role of effector cells was obtained in a subset of tumor xenograft mouse models whose response to enavatuzumab relied on the binding of Fc of the antibody to Fcγ receptor. The involvement of effector cells was further confirmed by immunohistochemistry, which revealed strong infiltration of CD45+ effector cells into tumor xenografts in responding models, but minimal infiltration in nonresponders. Consistent with the xenograft studies, human effector cells preferentially migrated toward in vivo-responsive tumor cells treated by enavatuzumab in vitro, with the majority of migratory cells being monocytes. Conditioned media from enavatuzumab-treated tumor cells contained elevated levels of chemokines, which might be responsible for enavatuzumab-triggered effector cell migration. These preclinical studies demonstrate that enavatuzumab can exert its potent antitumor activity by actively recruiting and activating myeloid effectors to kill tumor cells. Enavatuzumab-induced chemokines warrant further evaluation in clinical studies as potential biomarkers for such activity.

Nuclear Factor κB is Required for Tumor Growth Inhibition Mediated by Enavatuzumab (PDL192), a Humanized Monoclonal Antibody to TweakR.

TweakR is a TNF receptor family member, whose natural ligand is the multifunctional cytokine TWEAK. The growth inhibitory activity observed following TweakR stimulation in certain cancer cell lines and the overexpression of TweakR in many solid tumor types led to the development of enavatuzumab (PDL192), a humanized IgG1 monoclonal antibody to TweakR. The purpose of this study was to determine the mechanism of action of enavatuzumab's tumor growth inhibition and to provide insight into the biology behind TweakR as a cancer therapeutic target. A panel of 105 cancer lines was treated with enavatuzumab in vitro; and 29 cell lines of varying solid tumor backgrounds had >25% growth inhibition in response to the antibody. Treatment of sensitive cell lines with enavatuzumab resulted in the in vitro and in vivo (xenograft) activation of both classical (p50, p65) and non-classical (p52, RelB) NFκB pathways. Using NFκB DNA binding functional ELISAs and microarray analysis, we observed increased activation of NFκB subunits and NFκB-regulated genes in sensitive cells over that observed in resistant cell lines. Inhibiting NFκB subunits (p50, p65, RelB, p52) and upstream kinases (IKK1, IKK2) with siRNA and chemical inhibitors consistently blocked enavatuzumab's activity. Furthermore, enavatuzumab treatment resulted in NFκB-dependent reduction in cell division as seen by the activation of the cell cycle inhibitor p21 both in vitro and in vivo. The finding that NFκB drives the growth inhibitory activity of enavatuzumab suggests that targeting TweakR with enavatuzumab may represent a novel cancer treatment strategy.

Expression of TweakR in breast cancer and preclinical activity of enavatuzumab, a humanized anti-TweakR mAb.

BACKGROUND: The receptor for the cytokine TWEAK (TweakR) is a cell surface member of the tumor necrosis factor receptor superfamily with diverse biological roles. TNFRSF family members are appealing therapeutic targets in oncology due to their aberrant expression and function in tumor cells. The goal of the current study was to examine the potential of TweakR as a therapeutic target in breast cancer. METHODS: Expression of TweakR in primary breast cancer tissues and metastases was characterized using immunohistochemistry. To determine the functional relevance of TweakR, breast cancer cell lines were treated in vitro and in vivo with enavatuzumab, a humanized mAb against TweakR. RESULTS: Overexpression of TweakR was observed in infiltrating tumors compared to normal adjacent breast tissues, and strong staining of TweakR was observed in all subtypes of invasive ductal breast cancer. In addition, a positive correlation of TweakR and HER2 expression and co-localization were observed, irrespective of ER status. TweakR expression was also observed in bone metastasis samples from primary breast cancer but rarely in benign tumors. Enavatuzumab inhibited the in vitro growth of TweakR-expressing breast cancer cell lines, and this activity was augmented by cross-linking the mAb. In addition, enavatuzumab significantly inhibited the in vivo growth of multiple breast cancer xenograft models including a model of metastasis. CONCLUSIONS: TweakR is highly expressed in all subtypes of invasive ductal breast cancer, and enavatuzumab administration exhibited a dose-dependent inhibition of primary tumor growth and lung metastasis and enhanced the antitumor activity of several chemotherapy agents currently used to treat breast cancer. These data provide the rationale to evaluate enavatuzumab as a potential therapy for the treatment of breast cancer.

Hyperglycemia limits experimental aortic aneurysm progression.

OBJECTIVE: Diabetes mellitus (DM) is associated with reduced progression of abdominal aortic aneurysm (AAA) disease. Mechanisms responsible for this negative association remain unknown. We created AAAs in hyperglycemic mice to examine the influence of serum glucose concentration on experimental aneurysm progression. METHODS: Aortic aneurysms were induced in hyperglycemic (DM) and normoglycemic models by using intra-aortic porcine pancreatic elastase (PPE) infusion in C57BL/6 mice or by systemic infusion of angiotensin II (ANG) in apolipoprotein E-deficient (ApoE(-/-)) mice, respectively. In an additional DM cohort, insulin therapy was initiated after aneurysm induction. Aneurysmal aortic enlargement progression was monitored with serial transabdominal ultrasound measurements. At sacrifice, AAA cellularity and proteolytic activity were evaluated by immunohistochemistry and substrate zymography, respectively. Influences of serum glucose levels on macrophage migration were examined in separate models of thioglycollate-induced murine peritonitis. RESULTS: At 14 days after PPE infusion, AAA enlargement in hyperglycemic mice (serum glucose ≥ 300 mg/dL) was less than that in euglycemic mice (PPE-DM: 54% ± 19% vs PPE: 84% ± 24%, P < .0001). PPE-DM mice also demonstrated reduced aortic mural macrophage infiltration (145 ± 87 vs 253 ± 119 cells/cross-sectional area, P = .0325), elastolysis (% residual elastin: 20% ± 7% vs 12% ± 6%, P = .0209), and neovascularization (12 ± 8 vs 20 ± 6 vessels/high powered field, P = .0229) compared with PPE mice. Hyperglycemia limited AAA enlargement after ANG infusion in ApoE(-/-) mice (ANG-DM: 38% ± 12% vs ANG: 61% ± 37% at day 28). Peritoneal macrophage production was reduced in response to thioglycollate stimulation in hyperglycemic mice, with limited augmentation noted in response to vascular endothelial growth factor administration. Insulin therapy reduced serum glucose levels and was associated with AAA enlargement rates intermediate between euglycemic and hyperglycemic mice (PPE: 1.21 ± 0.14 mm vs PPE-DM: 1.00 ± 0.04 mm vs PPE-DM + insulin: 1.14 ± 0.05 mm). CONCLUSIONS: Hyperglycemia reduces progression of experimental AAA disease; lowering of serum glucose levels with insulin treatment diminishes this protective effect. Identifying mechanisms of hyperglycemic aneurysm inhibition may accelerate development of novel clinical therapies for AAA disease.

Hyperglycemia modulates plasminogen activator inhibitor-1 expression and aortic diameter in experimental aortic aneurysm disease.

BACKGROUND: Extracellular matrix degradation is a sentinel pathologic feature of abdominal aortic aneurysm (AAA) disease. Diabetes mellitus, a negative risk factor for AAA, may impair aneurysm progression through its influence on the fibrinolytic system. We hypothesize that hyperglycemia limits AAA progression through effects on endogenous plasminogen activator inhibitor-1 (PAI-1) levels and subsequent reductions in plasmin generation. METHODS: Experimental AAAs were induced in diabetic and control mice via the intra-aortic elastase infusion method. Serial transabdominal high-frequency ultrasound examinations were performed to monitor aortic diameter following elastase infusion. Circulating PAI-1 and plasmin alpha2-antiplasmin (PAP) complex concentrations were determined by ELISA and local expression of PAI-1 levels was examined by RT-PCR and immunohistochemistry. RESULTS: Hyperglycemia was associated with reduced AAA diameter, increased plasma PAI-1 concentration and reduced plasmin generation. Aneurysmal aortic PAI-1 gene expression increased in parallel with plasma concentration, with peak expression occurring early after aneurysm initiation. CONCLUSION: Hyperglycemia increases PAI-1 expression and attenuates AAA diameter in experimental AAA disease. These results emphasize the role of the fibrinolytic pathway in AAA pathophysiology, and suggest a candidate mechanism for hyperglycemic inhibition of AAA disease.

Antibodies to TWEAK receptor inhibit human tumor growth through dual mechanisms.

PURPOSE: Targeted therapeutics have significantly changed the outcome for patients diagnosed with cancer. Still, effective therapeutic intervention does not exist for many cancers and much remains to be done. The objective of this study was to identify novel genes that potentially regulate tumor growth, to target these gene products with monoclonal antibodies, and to examine the therapeutic potential of these antibodies. EXPERIMENTAL DESIGN: Using cDNA microarray analysis, we identified genes overexpressed in several solid malignancies. We generated a mouse monoclonal antibody, 19.2.1, and its humanized counterpart, PDL192, to one such target, TweakR (TWEAK receptor, Fn14, TNFRSF12A, CD266), and characterized the antitumor activities in vitro and in mouse xenograft models. RESULTS: Both 19.2.1 (mouse IgG2a) and PDL192 (human IgG1), like TWEAK, the natural ligand of TweakR, inhibited the growth of several TweakR-expressing cancer cell lines in anchorage-dependent and anchorage-independent assays in vitro. Both antibodies showed significant antitumor activity in multiple mouse xenograft models. PDL192 and 19.2.1 also induced antibody-dependent cellular cytotoxicity (ADCC) of cancer cell lines in vitro. A chimeric version of 19.2.1 containing the mouse IgG1 Fc region (19.2.1 x G1) exhibited significantly less ADCC than 19.2.1. However, 19.2. 1x G1 showed differential activity in vivo, with activity equivalent to 19.2.1 in one model, but significantly less efficacy than 19.2.1 in a second model. These results indicate that PDL192 and 19.2.1 mediate their antitumor effects by signaling through TweakR, resulting in reduced tumor cell proliferation, and by ADCC.

Analysis of in situ and ex vivo vascular endothelial growth factor receptor expression during experimental aortic aneurysm progression.

OBJECTIVE: Mural inflammation and neovascularization are characteristic pathological features of abdominal aortic aneurysm (AAA) disease. Vascular endothelial growth factor receptor (VEGFR) expression may also mediate AAA growth and rupture. We examined VEGFR expression as a function of AAA disease progression in the Apolipoprotein E-deficient (Apo E(-/-)) murine AAA model. METHODS AND RESULTS: Apo E(-/-) mice maintained on a high-fat diet underwent continuous infusion with angiotensin II at 1000 ng/kg/min (Ang II) or vehicle (Control) via subcutaneous osmotic pump. Serial transabdominal ultrasound measurements of abdominal aortic diameter were recorded (n=16 mice, 3 to 4 time points per mouse) for up to 28 days. Near-infrared receptor fluorescent (NIRF) imaging was performed on Ang II mice (n=9) and Controls (n=5) with scVEGF/Cy, a single-chain VEGF homo-dimer labeled with Cy 5.5 fluorescent tracer (7 to 18 microg/mouse IV). NIRF with inactivated single chain VEGF/Cy tracer (scVEGF/In, 18 microg/mouse IV) was performed on 2 additional Ang II mice to control for nonreceptor-mediated tracer binding and uptake. After image acquisition and sacrifice, aortae were harvested for analysis. An additional AAA mouse cohort received either an oral angiogenesis inhibitor or suitable negative or positive controls to clarify the significance of angiogenesis in experimental aneurysm progression. Aneurysms developed in the suprarenal aortic segment of all Ang II mice. Significantly greater fluorescent signal was obtained from aneurysmal aorta as compared to remote, uninvolved aortic segments in Ang II scVEGF/Cy mice or AAA in scVEGF/In mice or suprarenal aortic segments in Control mice. Signal intensity increased in a diameter-dependent fashion in aneurysmal segments. Immunostaining confirmed mural VEGFR-2 expression in medial smooth muscle cells. Treatment with an angiogenesis inhibitor attenuated AAA formation while decreasing mural macrophage infiltration and CD-31(+) cell density. CONCLUSIONS: Mural VEGFR expression, as determined by scVEGF/Cy fluorescent imaging and VEGFR-2 immunostaining, increases in experimental AAAs in a diameter-dependent fashion. Angiogenesis inhibition limits AAA progression. Clinical VEGFR expression imaging strategies, if feasible, may improve real-time monitoring of AAA disease progression and response to suppressive strategies.

Intermittent short-duration exposure to low wall shear stress induces intimal thickening in arteries exposed to chronic high shear stress.

We sought to determine whether intermittent short-duration exposure to low wall shear stress could induce intimal thickening in arteries chronically exposed to high shear stress. An arteriovenous fistula (AVF) was created between the left common carotid artery and the corresponding external jugular vein in 20 Japanese white male rabbits. After 4 weeks, blood flow was increased 10-fold to 182 +/- 39 ml/min and shear stress was increased to 33.4 +/- 13 dyn/cm(2). The AVF was then occluded for 1 h by finger compression with an 85% reduction in carotid artery blood flow (27 +/- 7 ml/min) and a reduction in wall shear stress to 4.9 +/- 1.7 dyn/cm(2) (P < 0.0001). Release of finger compression restored flow to the AVF and high shear stress to the carotid artery. This procedure was repeated at weekly intervals with a cumulative total of 4 h of low shear stress exposure. Arteries exposed to intermittent low shear stress developed a layer of intimal thickening which consisted of 3-4 layers of smooth muscle cells lined with thin elastic fibers and medial hyperplasia. Control arteries exposed to 8 weeks of continuous high shear had no intimal thickening. Transient exposure to low shear stress upregulated TGF-beta1, MMP-2, -14, and TIMP-2 gene expression while MMP-9 expression was downregulated. We conclude that repeated, intermittent short-duration exposure to low shear stress in the setting of high flow and high shear stress can induce arterial intimal thickening. Short-duration alterations in hemodynamic forces can induce rapid vascular cell message expression, which may effect arterial remodeling. This experiment suggests that a threshold value of 5 dyn/cm(2) may be needed in order to initiate and sustain the intimal thickening response.

Comparison of cell-type-specific vs transmural aortic gene expression in experimental aneurysms.

OBJECTIVE: Abdominal aortic aneurysm (AAA) progression and disease resistance are related to mural cellularity; adventitial macrophages and neocapillaries predominate in larger, advanced aneurysms, whereas smaller AAAs have fewer macrophages and retain more medial smooth muscle cells (SMCs). Expression analysis of mRNA derived from the entire aorta may mask the role that specific cell types play in modulating disease progression. We used laser capture microdissection (LCM) to isolate SMC and macrophage-predominant mural cell populations for gene expression analysis in variable-flow AAA. METHODS: Rat AAAs were created via porcine pancreatic elastase (PPE) infusion. Aortic flow was increased via femoral arteriovenous fistula creation (HF-AAA) or reduced via unilateral iliac ligation (LF-AAA) in selected cohorts. SMC and macrophage-predominant cell populations were isolated via LCM and analyzed for expression of pro-inflammatory transcription factors and chemokines, cytokines, and proteolytic enzymes via real-time polymerase chain reaction. RESULTS: Aortic PPE infusion precipitated endothelial cell (EC) denudation, SMC apoptosis, and elastic lamellar degeneration. Increased aortic flow (HF > NF > LF) stimulated restorative EC and SMC proliferation (45.8 +/- 6.6 > 30.5 +/- 2.1 > 21 +/- 3.6 and 212.2 +/- 9.8 > 136.5 +/- 8.9 > 110 +/- 13.5, respectively, for both cell types; P < .05) at 5 days after PPE infusion, while simultaneously reducing medial SMC apoptosis and transmural macrophage infiltration. Expression of nuclear factor kappa B (NF-kappab), granulocyte macrophage-colony stimulating factor (GM-CSF), macrophage migration inhibitory (MIF), heparin-binding EGF-like factor (HB-EGF) and inducible nitric oxide synthase (iNOS) varied between cell types and flow conditions at all time points examined. Gelatinolytic protease expression varied by cell type in response to flow loading (eg, increased in SMCs, decreased in macrophages), consistent with observed patterns of elastolysis and SMC proliferation reported in prior experiments. CONCLUSIONS: Flow differentially regulates cell-specific AAA gene expression. Whole-organ analysis of AAA tissue lysates obscures important cellular responses to inflammation and flow, and may explain previous seemingly contradictory observations regarding proteolysis and cell proliferation. Cell-type specific expression and functional analyses may substantially clarify the pathophysiology of AAA disease. CLINICAL RELEVANCE: Understanding aneurysmal aortic degeneration at the most fundamental level is a critical precursor to the development of next-generation therapies such as drug-eluting endografts and/or medical therapies to limit expansion of preclinical AAA in high-risk or elderly patients. Although animal modeling is necessary to gain insight into the early initiating events of AAA disease, the methods used in such analyses have critical bearing on the conclusions drawn regarding pathogenesis and potential therapeutic derivations. By analyzing cell-type-specific gene expression rather than whole-organ tissue lysates, the precise roles of important mediators such as metalloproteinases can be placed in the appropriate context. Further refinement of these techniques may allow cell-specific therapies to be applied at defined time points in disease progression with improved patient outcome and reduced procedural morbidity.

Hemodynamic regulation of CD34+ cell localization and differentiation in experimental aneurysms.

OBJECTIVES: Bone marrow-derived vascular progenitor cells (CD34+) are present in human and animal models of abdominal aortic aneurysm (AAA) disease. These preterminally differentiated cells may modulate disease resistance. We examined the influence of variable hemodynamic conditions on progenitor cell localization and differentiation in experimental AAAs. METHODS AND RESULTS: Murine AAAs were created via porcine pancreatic elastase (PPE) infusion. AAA blood flow was increased by aortocaval fistula (ACF) formation (HF-AAA), decreased via left iliac ligation (LF-AAA), or left unchanged (NF-AAA). ACF creation increased flow by 1700%, whereas iliac ligation decreased flow 79% compared with baseline (0.6+/-0.1 mL/min). Wall shear stress (WSS) increased or decreased accordingly, and remained elevated (9.2+/-2.0 dynes/cm2) in HF-AAA 14 days after PPE infusion. CD34+ cells were identified throughout the aortic wall in all flow conditions. Seven days after PPE infusion, HF-AAAs had more CD34+ cells than LF-AAA (187+/-10 versus 155+/-7 CD34+ cells/cross sectional, P<0.05), more medial smooth muscle cells, fewer infiltrative macrophages, and a smaller diameter than LF-AAA. LF-AAAs also contained more adventitial capillaries (CD34+ capillaries 181+/-12 versus 89+/-32/cross-sectional area in HF-AAA, P<0.05). The total progenitor cell/capillary index (CD34+ capillary plus CD31+ capillary/cross sectional area) was higher in LF-AAA (282+/-31 versus 129+/-47, P<0.05). Vascular endothelial (VEGF) and platelet-derived growth factor (PDGF) expression varied directly with capillary density between groups. Increased granulocyte-macrophage colony-stimulating factor (GM-CSF) expression was also present in LF-AAAs. CONCLUSIONS: Hemodynamic conditions influence CD34+ cell localization and differentiation in experimental AAA. Adventitial capillary angiogenesis may augment inflammation and disease progression. Modulating cell lineage differentiation of mature progenitor cells may represent a novel therapeutic strategy to maintain medial cellularity and extracellular matrix integrity in AAA disease.

Continuous periaortic infusion improves doxycycline efficacy in experimental aortic aneurysms.

OBJECTIVE: We created a novel continuous infusion system to evaluate the efficacy of juxta-aortic doxycycline delivery as a transitional step toward developing hybrid drug/device treatment strategies for abdominal aortic aneurysm (AAA) disease. METHODS: Controlled comparison of treatment outcomes was studied in animal models with molecular and morphologic tissue analysis in a collaboration between university and corporate research laboratories. Rat AAAs were created via porcine pancreatic elastase (PPE) infusion and grouped and analyzed by subsequent treatment status (either doxycycline in vehicle or vehicle alone) and drug delivery method (continuous infusion via periaortic delivery system [PDS] or twice-daily subcutaneous injection). The main outcome measures were AAA diameter via direct measurement, medial elastin lamellar preservation via light microscopy, mural smooth muscle cell (SMC) proliferation and SMC and macrophage density via immunostaining and counting, expression of matrix metalloproteinases 2, 9, and 14 and tissue inhibitors of metalloproteinases 1 and 2 via real-time reverse transcriptase-polymerase chain reaction, and enzymatic activity via substrate zymography. Serum drug levels were analyzed via liquid chromatography/mass spectroscopy. RESULTS: PDS (1.5 mg/kg/day) and subcutaneous (60 mg/kg/day) delivery methods caused comparable reductions in AAA diameter during the period of 14 days after PPE infusion. PDS rats gained more weight during the postoperative period (P <.001), possibly as a result of reduced serum drug levels and systemic toxicity. Doxycycline treatment reduced AAA macrophage infiltration and SMC proliferation significantly. Despite reduced diameter, circumferential elastic lamellar preservation was not apparent in doxycycline-treated AAAs. CONCLUSIONS: Continuous periaortic infusion lowers the effective doxycycline dose for experimental AAA limitation. Alternative biologic inhibition strategies might also be amenable to direct intra-aortic or juxta-aortic delivery. Periaortic infusion might improve the clinical outcome of minimally invasive AAA treatment strategies. Clinical relevance Aneurysm remodeling may continue after successful endovascular AAA exclusion. Continued proteolytic activity within the aneurysm wall potentiates late graft migration and failure. The doxycycline infusion system developed in these experiments may serve as a prototype for adjuvant treatment modalities that complement endovascular AAA exclusion. Local delivery of doxycycline or other agents active in AAA disease, either continuously or at selected intervals after graft implantation, may stabilize the wall and aid in maintaining aneurysm exclusion. Alternative delivery methods could include passive diffusion from either the graft material itself or treatment reservoirs incorporated into endografts. Given the recognized limitations of current technologies, adjuvant biologic therapies have the potential to improve long-term patient outcome significantly after endovascular exclusion.

Hemodynamic forces regulate mural macrophage infiltration in experimental aortic aneurysms.

Blood flow (BF) and wall shear stress (WSS) influence reactive oxygen species production and oxidative stress in abdominal aortic aneurysm (AAA) disease. To gain further insight into the mechanisms of hemodynamic influences on AAA inflammation, we examined aneurysm macrophage density, chemotaxis and survival under varying aortic flow conditions. Rat AAAs were created via porcine pancreatic elastase (PPE) infusion. In selected cohorts, AAA flow was increased via left common femoral arteriovenous fistula (AVF) creation (HF-AAA) or decreased by left common iliac ligation (LF-AAA). WSS was highest in HF-AAA (10.4 +/- 2.3 dyn/cm(2) vs. 2.4 +/- 0.4 and 0.5 +/- 0.2 for NF- and LF-AAA, respectively, P < 0.001) 7 days after PPE infusion, with reduced medial macrophage density and increased apoptosis. Adventitial macrophage density was not significantly influenced by flow. Monocyte chemoattractant protein-1 (MCP-1) and granulocyte-macrophage colony-stimulating factor (GM-CSF) gene expression correlated with observed macrophage densities in the media and adventitia. Luminal flow conditions regulate AAA inflammation in part via influences on medial macrophage density. Hemodynamic forces may modulate AAA inflammation and diameter enlargement via direct regulation of intimal macrophage adhesion, transmural migration or survival.

Arterial enlargement, tortuosity, and intimal thickening in response to sequential exposure to high and low wall shear stress.

We investigated the effects of sequential and prolonged exposure to high and low wall shear stress on arterial remodeling using a rabbit arteriovenous fistula (AVF) model. Blood flow was increased by approximately 17-fold to 20-fold when the AVF was open, and returned to normal when the AVF was occluded. Repeated opening and closing of the AVF resulted in sequential exposure of the artery to high and low wall shear stress. High flow and high wall shear stress induced arterial dilatation, elongation, and tortuosity, without intimal thickening. The common carotid artery was elongated 37% after 4 weeks of high flow, and was shortened 10% after 6 weeks of normal flow. Subsequent cycles of high flow induced less elongation, with less shortening after return to normal flow. Enlargement of the distal segment was more dramatic than in the proximal segment, despite exposure to the same volume of flow and the same initial high wall shear stress after creation of the AVF. The distal carotid segment enlarged more than did the proximal segment during each exposure to high flow. In segments of carotid artery exposed to low wall shear stress (<5 dynes/cm(2)) intimal thickening developed. These changes were maximal in the distal carotid segment, just before the AVF. Each cycle of low wall shear stress induced intimal thickening accompanied by medial hyperplasia. Intimal thickening was inhibited during periods of high flow when wall shear stress was high. Three cycles of flow alteration induced three layers of intimal thickening in the distal arterial segment, two layers of intimal thickening in the middle segment, and one layer of intimal thickening in the proximal segment. Long-term exposure to low wall shear stress induced severe intimal thickening and medial hyperplasia in different segments. Thus the response of the carotid artery afferent to an AVF varies along the length of the artery, with maximum enlargement, elongation, and tortuosity in the distal segment, just proximal to the AVF. Similarly, intimal thickening in response to low wall shear stress is maximal in the distal carotid artery. It appears that intimal thickening is related to local levels of low wall shear stress, and occurs when wall shear stress chronically falls to less than 5 dynes/cm(2).

High flow drives vascular endothelial cell proliferation during flow-induced arterial remodeling associated with the expression of vascular endothelial growth factor.

Endothelial cell activation and proliferation are the essential steps in flow-induced arterial remodeling. We investigated endothelial cell turnover in the early stages of high-flow in the rabbit common carotid arteries using an arteriovenous fistula (AVF) model by kinetic investigation of cell proliferation and cell molecular analysis. BrdU was administrated to label endothelial cells (ECs) in DNA synthetic phase (S-phase) of the cell mitotic cycle. Pulse labeling revealed that ECs entered S-phase at 1.5 days of AVF (0.93 +/- 0.19%). Endothelial cell labeling index (EC-LI) peaked at 2 days of AVF (8.90 +/- 0.87%) with a high index of endothelial cell mitosis (EC-MI, 1.67 +/- 0.47%). Endothelial cell density increased remarkably at 3 days of AVF with a significant decrease in EC-LI (54%) and EC-MI (60%). Study of kinetics of EC proliferation revealed that endothelial cells took 16-24 h to finish one cycle of cell mitosis. Tracking investigation of pulse BrdU-labeled endothelial cells at 1.5 days showed that more than 66% of endothelial cells were BrdU-labeled 1.5 days after labeling. VEGF, integrin alphanubeta3, PECAM-1, and VE-cadherin were upregulated significantly preceding endothelial cell proliferation and kept at high levels during endothelial cell proliferation. These data suggest that endothelial cell proliferation is the initial step in flow-induced arterial remodeling. Hemodynamic forces may drive endothelial cell downstream migration. Expression of VEGF and cell junction molecules contribute to flow-induced arterial remodeling.

Wall shear stress and strain modulate experimental aneurysm cellularity.

OBJECTIVE: Clinical evidence indicates that hemodynamic conditions influence abdominal aortic aneurysm (AAA) disease. We modified blood flow to evaluate the effects of wall shear stress (WSS) and relative wall strain (RWS) on aneurysm structure and cellularity. METHODS: Rodent AAAs were created with porcine pancreatic elastase infusion. In group 1 AAA WSS was increased with left femoral arteriovenous fistula creation, whereas in group 2 AAA WSS was decreased with left iliac artery ligation. Aortic flow, wall motion, and blood pressure were recorded in both groups. AAA diameter, endothelial and smooth muscle cellularity (CD31 and alpha-smooth muscle actin immunostaining), markers for cell proliferation (5-bromodeoxyuridine), endothelial and smooth muscle cell growth factor production (vascular endothelial growth factor-D and platelet-derived growth factor-beta, respectively), and apoptosis (deoxyuridine triphosphate nick end-labeled [TUNEL] stain) were compared between groups when the animals were killed. RESULTS: Arteriovenous fistula creation increased WSS (high-flow AAA) by 300% and RWS by 150%. Iliac ligation reduced WSS (low-flow AAA) by 60%. Neither procedure significantly altered systolic, diastolic, or mean aortic pressure. When the animals were killed 7 days after elastase infusion, low-flow AAAs were significantly larger than high-flow AAAs. High-flow AAAs also contained more endothelial cells and smooth muscle cells, and evidence of increased growth factor production, cell proliferation, and decreased apoptosis. No difference in type or severity of AAA inflammatory cell infiltrate was noted between groups. CONCLUSIONS: High flow conditions stimulate endothelial cell and smooth muscle cell proliferation in experimental aneurysms. Enhanced cellularity may stabilize aortic integrity, limiting aneurysm growth. Increased lower extremity activity may prevent or retard AAA disease through salutary effects on aortic remodeling mediated by endothelial cells and smooth muscle cells.

Flow loading induces macrophage antioxidative gene expression in experimental aneurysms.

OBJECTIVE: Reactive oxygen species may act as proinflammatory mediators in abdominal aortic aneurysm (AAA) disease. Flow loading increases antioxidative enzyme expression and limits reactive oxygen species production in vascular smooth muscle cells in vitro, limits experimental AAA enlargement in rodent models, and is indirectly associated with reduced clinical AAA risk. We attempted to determine the mechanism or mechanisms by which flow loading limits AAA enlargement. METHODS AND RESULTS: Rodent AAAs were flow loaded via femoral arteriovenous fistula creation. Aortic wall shear stress and relative wall strain were significantly higher in flow-loaded rodents. Flow loading reduced AAA diameter by 26% despite evidence of flow-mediated aortic enlargement proximal to the aneurysmal segment. Messenger RNA from AAA tissue was harvested for cDNA labeling and hybridization to a 384-clone DNA microarray. Twenty-nine genes were differentially expressed (relative intensity/relative intensity of control ratio >1.5 and <0.67) in flow-loaded compared with normal flow AAA tissue, including heme oxygenase 1 (HO-1). Increased HO-1 expression was confirmed via reverse transcriptase-polymerase chain reaction. Immunohistochemistry localized HO-1 expression to infiltrative macrophages. alpha-Tocopherol was found to be as effective as flow loading in limiting AAA enlargement. Flow loading and alpha-tocopherol therapy reduced AAA reactive oxygen species production. CONCLUSIONS: Flow loading may attenuate AAA enlargement via wall shear or strain-related reductions in oxidative stress.