Lactel® 50:50 PLGA was purchased from DURECT Corporation (DURECT Corporation, Cupertino, CA, USA). Lipids, including 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-2000] (ammonium salt) (DSPE-PEG2000), and 1,2-diphytanoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt) (NBD PE), were purchased from Avanti Polar Lipids, Inc. (Avanti Polar Lipids, Inc., Alabaster, AL, USA). KLH, poly(vinyl alcohol) (PVA; Mw 89,000 to 98,000), dichloromethane, rhodamine B, sodium deoxycholate (DOC), trichloroacetic acid (TCA), sodium dodecyl sulfate (SDS), paraformaldehyde, and Triton™ X-100 were purchased from Sigma-Aldrich Inc. (Sigma-Aldrich Inc., Saint Louis, MO, USA). 1-Ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride (EDC) was purchased from Thermo Fisher Scientific Inc. (Thermo Fisher Scientific Inc., Waltham, MA, USA). JAWSII (ATCC® CRL-11904™) immature DCs were purchased from ATCC (Manassas, VA, USA). FBS, granulocyte-macrophage colony-stimulating factor (GM-CSF) recombinant mouse protein, minimum essential medium (MEM) α, trypsin/ethylenediaminetetraacetic acid (EDTA), and HCS CellMask™ Blue Stain were purchased from Life Technologies Corporation (Life Technologies Corporation, Grand Island, NY, USA).
Fabrication of PLGA-KLH (PK) nanocomplex
PLGA-KLH nanocomplex was prepared using double emulsion solvent evaporation method . Briefly, PLGA of 200 mg was dissolved in 5 mL dichloromethane, followed by mixing with 300 μL of 10 mg/mL KLH using a vortex mixer for 2 min. The resulting mixture emulsified via sonication at 20% amplitude for 20 s using a sonic dismembrator (Model 500; Fisher Scientific, Pittsburgh, PA, USA). The primary emulsion was added dropwise into 200 mL 1% (w/v) PVA and stirred for 10 min at 500 rpm. The above suspension was emulsified through sonication at 50% amplitude for 120 s. The secondary emulsion was stirred overnight to allow organic solvent to evaporate. After settling at room temperature for 30 min, precipitant was removed. NPs in suspension were collected by centrifuge at 20,000 g, 4°C for 30 min (Beckman Coulter Avanti J-251, Brea, CA, USA). Pellet was washed using ultrapure water for three times. The final suspension was freeze dried (LABCONCO FreeZone 4.5, Kansas City, MO, USA) and stored at 2°C for later use.
Assembly of liposome-PK (LPK) nanocomplex
Lipid film of 20 mg with various lipid compositions was hydrated with 15 mL hydration buffer (0.9% saline, 5% dextrose, and 10% sucrose). After vigorous mixing with vortex for 2 min, the resulting solution was incubated in a 55°C water bath for 5 min and cooled to room temperature. PK NPs of 200 mg were added into liposome solution and pre-homogenized for 15 min using Branson 2510 bath sonicator (Branson Ultrasonics Corporation, Danbury, CT, USA), followed by sonication in ice bath at 15% amplitude for 5 min (pulse on 20 s, pulse off 50 s) using a sonic dismembrator (Model 500; Fisher Scientific, Pittsburgh, PA). The formed LPK NPs were collected by centrifuge at 20,000 g, 4°C for 30 min and stored at 2°C after being lyophilized.
Labeling KLH with rhodamine B fluorescence
Ten milligrams of EDC dissolved in 700 μL ultrapure water (pH 6.8) was mixed with 300 μL of 2 mg/mL rhodamine B. After incubation at 0°C for 10 min, the mixture was added with 10 mg KLH (10 mg/mL) and stirred in darkness at room temperature for 12 h. Fluorescently labeled KLH was purified using Microcon centrifugal filter units (50,000 MWCO) from EMD Millipore (EMD Millipore, Billerica, MA, USA) and stored at 2°C after freeze dry.
Physicochemical property characterization of NPs
Five milligrams of NPs was dispersed in 20 mL ultrapure water (pH 7.0) using a water bath sonicator for 5 min. Each sample was diluted by ten folds using ultrapure water. Particle size (diameter, nm) and surface charge (zeta potential, mV) were measured using a Malvern Nano-ZS zetasizer (Malvern Instruments Ltd, Worcestershire, UK) at room temperature.
Imaging of NPs using a transmission electrical microscope (TEM)
NPs suspended in ultrapure water (5 mg/mL) were dropped onto a 300-mesh Formvar (Agar Scientific, Essex, UK)-coated copper grid. After 10 min standing, the remaining suspension was carefully removed with wipes, and the samples were negatively stained using fresh 1% phosphotunstic acid for 60 s and washed by ultrapure water twice. The dried samples were imaged on a JEOL JEM 1400 Transmission Electron Microscope (JEOL Ltd., Tokyo, Japan).
Confocal imaging of LPK NPs
Fluorescent LPK NPs were formed using the above-described methods, except that KLH were labeled with rhodamine B and 0.5 mg of NBD PE was added into existing lipids (DOPC:DSPE-PEG = 16 mg:4 mg). One hundred microliters of NP suspension (1 mg/mL) was placed onto a glass slide and covered with a cover glass (thickness 0.16 to 0.19 mm) from Fisher Scientific (Pittsburgh, PA). The sample was imaged using a Zeiss LSM 510 Laser Scanning Microscope (LSM) (Carl Zeiss, Oberkochen, Germany).
In vitro stability of NPs
Twenty milligrams of NPs was suspended in 20 mL 10% (v/v) human serum (pH 7.4), 10% (v/v) FBS, and 10 mM PBS, respectively. The suspensions were constantly mixed on a shaker at room temperature for 9 days. One hundred fifty microliter samples were diluted in 2 mL ultrapure water at different time points, and the particle size was measured by Malvern Nano-ZS zetasizer. The measurements were performed in triplicate at room temperature.
Determination of KLH content in NPs
KLH in NPs was quantified using a modified method . Briefly, 10 mg of NPs was dissolved in 1 mL of 0.1 M NaOH solution and incubated at 2°C for 12 h. The solution pH was adjusted to 7.0 using 1 M HCl. Two hundred microliters of DOC (0.15, w/v) was added and the final volume was adjust to 2 mL using ultrapure water. After sitting at room temperature for 15 min, the mixture was added with 200 μL of TCA (80%, w/v) and incubated for 5 min. Samples were vortexed for 2 min and centrifuged at 5,000 g for 20 min at room temperature. Pellets were dissolved in 500 μL of SDS (5%, w/v) containing 0.01 M NaOH. Following the protocol from the supplier, KLH concentration was determined using Micro BCA Protein Assay Kit (Thermo Fisher Scientific Inc., Waltham, MA, USA).
In vitro release of KLH from NPs in human plasma
Five milligrams of NPs containing rhodamine B-labeled KLH was suspended in 1 mL of 10% (v/v) human serum (pH 7.4) and incubated in darkness (covered by foil) at 37°C. Samples were centrifuged at 10,000 g for 15 min at determined time points. The supernatant (200 μL) was added into a blank 96-well plate (Thermo Fisher Scientific Inc., Waltham, MA, USA) and measured using Synergy HT Multi-Mode Microplate Reader (BioTek Instruments, Inc., Winooski, VT, USA) with excitation at 530 nm and emission at 590 nm. The pellets were resuspended in 1 mL of 10% (v/v) human serum. Release of KLH at certain time points was calculated by using the following equation: KLH release% = Absorbance at certain time point/Total absorbance × 100.
Flow cytometry measurement of endocytosis of NPs by DCs
JAWSII (ATCC® CRL-11904™) immature DCs from ATCC were cultured with alpha MEM (80%v) including ribonucleosides, deoxyribonucleosides, 4 mM l-glutamine, 1 mM sodium pyruvate and 5 ng/mL murine GM-CSF, and FBS (20%v) at 37°C, 5% CO2 in 24-well plates (CORNING, Tewksbury, MA, USA). NPs were assembled according to the above-mentioned method, except that KLH was labeled with rhodamine B and 0.5 mg of NBD PE was added to existing lipids. One milligram of NPs suspended in 2 mL complete medium with a final concentration of 0.5 mg/mL was added into each well containing 106 cells and incubated for 1, 2, and 3 h, respectively. After incubation, the medium was immediately removed and cells were washed with ultrapure water for five times. Cells were detached from culture plate using trypsin/EDTA solution and centrifuged at 200 g for 10 min, and cell pellets were resuspended in 10 mM PBS (pH 7.4). Cell samples were immediately analyzed using flow cytometer (BD FACSAriaI, BD, Franklin Lakes, NJ, USA).
LSM imaging of endocytosis of NPs by DCs
Cells were cultured in a four-well chamber slide (Thermo Fisher Scientific Inc., Waltham, MA, USA) using the same method described above. NPs (0.1 mg) suspended in 500 μL complete medium with a final concentration of 0.2 mg/mL were incubated with 105 cells for certain times (1, 2, and 3 h) at 37°C, 5% CO2. After incubation, medium was immediately removed and cells were washed with ultrapure water for five times. Freshly prepared 4% (w/v) paraformaldehyde (500 μL) was added into each well, and cells were fixed for 15 min and washed three times using PBS (10 mM, pH 7.4). Fixed cells were permeabilized using 500 μL of 0.1% (v/v) Triton™ X-100 for 15 min at room temperature and washed three times using PBS (10 mM, pH 7.4). Cells were stained using 500 μL of freshly diluted 1X HCS CellMask™ Blue Stain for 15 min and washed three times using PBS (10 mM, pH 7.4). Cell samples were covered with a glass cover and sealed by nail polish. Images were acquired using a Zeiss LSM 510 Laser Scanning Microscope (Carl Zeiss, Germany). Each step was carried out in darkness as much as possible to avoid fluorescence quenching.
All experiments were performed in at least triplicate. Results were expressed as mean ± standard deviation. Different treatment groups in stability test were compared by one-way ANOVA following Tukey test using the JMP pro 10 (SAS, Cary, NC, USA). Differences were considered significant at p values that were less than or equal to 0.05.