Materials
Manganese chloride tetrahydrate and ammonium ceric nitrate were obtained from Aladdin (Shanghai, China). Polyvinylpyrrolidone (PVP) and N, N-dimethylformamide (DMF, ≥ 99.5%) were obtained from Macklin (Shanghai, China). 2′,7′-Dichlorofluorescin diacetate (DCFH-DA) was provided by Beyotime (Shanghai, China). Dulbecco’s modified eagle medium (DMEM), penicillin, streptomycin, trypsin and cell counting kit-8 (CCK-8) were purchased from Meilunbio (Dalian, China). Foetal bovine serum (FBS) was obtained from Gibco (Guangzhou, China). Anti-CD86, anti-CD206, anti-CD3, anti-CD4, F4/80, and Foxp3 were obtained from BioLegend (San Diego, CA, USA). Lipopolysaccharide (LPS) and Interferon-γ (IFN-γ) were purchased from Sigma-Aldrich (Santa Barbara, CA, USA). The hematoxylin and eosin (HE) staining kit, catalase (CAT) activity assay kit, and superoxide dismutase (SOD) activity assay kit were purchased from Solarbio (Beijing, China). All chemicals and reagents were used as received without additional purification. All solutions were prepared with deionised (DI) water, which was purified through an 18-MΩ system (Millipore, USA).
Preparation and characterization of BSA@NPs-MTX
According to a previously reported method(33), DMF (9 mL) and methanol (1.2 mL) were firstly added to a three-neck flask. Secondly, 0.08 g of 2-aminoterephthalic acid, 0.12 g of cerium ammonium nitrate, 0.22 g of manganese chloride tetrahydrate, and 0.25 g of PVP were added in sequence. The mixed solution was then transferred to a 50 mL stainless-steel autoclave lined with polytetrafluoroethylene and heated in a vacuum drying oven at 150°C for 4 h. After the reactor cooled to room temperature, the solution was centrifuged at 11,000 × rpm for 5 minutes, and the resulting precipitate was washed once with DI water and once with anhydrous ethanol. And then the supernatant was collected after centrifuging at 2000 × rpm for 3 minutes. Finally, the obtained sample was suspended in anhydrous ethanol.
1 mL of MTX solution (0.2 mg/mL in DI water) was combined with 1 mL of NPs and stirred overnight under ambient conditions. The unloaded MTX was removed by multiple washes with DI water. Subsequently, MTX-loaded NPs (NPs-MTX) were introduced into 1 mL of BSA solution (60 mg/mL) and allowed to react for 24 h under ambient conditions. Finally, the BSA@NP-MTX complex was isolated by centrifugation and purified by three successive washes with DI water.
The morphology of NPs was observed using transmission electron microscopy (TEM; FEI Talos F200S G2, USA). The hydrodynamic size and zeta potential of the NPs were determined by dynamic light scattering (DLS; Zetasizer Nano ZS ZEN3600, Malvern, UK) at room temperature. Fourier transform infrared (FTIR) spectra were acquired using an FTIR spectrometer (Tensor II, Bruker, Germany), and the absorption spectrum of the NPs was documented using ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometry (CARY 5000, USA). To evaluate the loading capacity of MTX, BSA@NPs-MTX was dissolved in 2 mL of 1 × PBS solution. Following agitation and centrifugation, the resultant supernatant was isolated to quantify the residual MTX. The loading capacity and efficiency of MTX incorporation were ascertained using UV-vis-NIR spectrophotometry.
To measure the in vitro release profile of MTX in BSA@NPs-MTX, 5 mL of BSA@NPs-MTX was dialysed against 30 mL of PBS (pH 7.4) and incubated at 37°C with constant stirring at 120 × rpm. Periodically, 5 mL of the surrounding buffer was extracted and promptly replenished with fresh PBS. The MTX concentrations in these aliquots were quantified using UV-vis-NIR spectrophotometry.
Antioxidant properties of the NPs
Briefly, different concentrations of NPs were incubated with 2,2′-amino-di (2-ethylbenzothiazoline sulphonic acid-6) ammonium salt (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) for 30 min in darkness(34). Subsequently, the absorbances at the characteristic wavelengths of ABTS (734 nm) and DPPH (517 nm) were quantified using a UV-vis-NIR spectrophotometer. The ABTS and DPPH scavenging rates were calculated.
A hydroxyl free radical-scavenging capacity assay kit was used to assess the hydroxyl radical-scavenging capabilities of the NPs. The Fenton reaction, initiated by H2O2/Fe2+, produces hydroxyl radicals that subsequently oxidise Fe2+ to Fe3+ within the orthophenanthroline-Fe2+ aqueous solution, decreasing the absorbance at 536 nm. The ability to scavenge hydroxyl radicals is related to the absorbance at 536 nm (35).
To evaluate the H2O2 neutralised by the NPs, various concentrations of NPs were incubated in 2 mL of PBS solution fortified with 400 µM H2O2 at 37°C for 24 h. The residual H2O2 concentration was determined using the H2O2 detection kit by measuring the absorbance at 405 nm, from which the H2O2 scavenging proficiency was derived(36).
In addition, the ability of NPs to neutralise superoxide anions was evaluated. In the designated assays, different NPs concentrations were exposed to superoxide anions. The residual superoxide anion concentration was determined using a superoxide anion-scavenging capacity assay kit. Superoxide anions oxidised hydroxylamine hydrochloride to produce nitrite. Upon interaction with aminobenzenesulfonic acid and α-naphthylamine, the nitrite formed a red azo compound, exhibiting a characteristic absorption peak at 530 nm. The absorbance at 530 nm was used for quantitative detection of superoxide anions.
The SOD-mimetic potential of the NPs was assessed using an SOD activity assay kit according to the manufacturer’s protocol. In essence, the superoxide radical anion (O2•−) was produced through the xanthine and xanthine oxidase reaction system. O2•− reduced nitro blue tetrazolium to form blue formazan, which was absorbed at 560 nm. SOD catalysed the dismutation of O2•− to produce H2O2 and O2, thereby inhibiting formazan formation. The deeper the blue colour of the reaction solution, the lower the activity of SOD. And the CAT mimetic potential of the NPs was assessed using a CAT activity assay kit according to the manufacturer’s protocol. The decomposition of H2O2 by the NPs was rapidly arrested using ammonium molybdate. The remaining H2O2 interacted with ammonium molybdate to yield a yellow compound. The absorbance of this compound was measured at 405 nm using spectrophotometry and served as an indicator of CAT activity.
Cell viability and cellular uptake
RAW264.7 cells were cultured on circular microscope slides situated in the 24-well plates at a density of 2.0 × 105 cells per well in DMEM supplemented with LPS (100 ng/mL) and IFN-γ (20 ng/mL). After incubation for 24 h, these cells were exposed to rhodamine B, NPs loading rhodamine B, and BSA@NPs loading rhodamine B for 4 h. After three washes with PBS, cells were fixed with 4% paraformaldehyde for 10 min and blocked with 1% BSA for an additional 30 min. Nuclear staining was accomplished with 4′,6-diamidino-2-phenylindole (DAPI) for 10 min. Fluorescence microscopy (Axio Vert.A1; Carl Zeiss, Oberkochen, Germany) was used to visualise the cells. Cells after the same treatment were isolated, rinsed three times with 1 × PBS, and prepared for flow cytometry.
In vitro ROS scavenging
RAW264.7 cells were cultured on circular microscope slides within a 24-well plate at a density of 2.0 × 105 cells per well in DMEM supplemented with LPS (100 ng/mL) and IFN-γ (20 ng/mL). After 24 h, the cells were exposed to various concentrations of NPs for another 24 h. Subsequently, the cells were stained with DCFH-DA (Beyotime, Beijing, China) at 37°C for 30 min and with DAPI for 10 min. Cellular imaging was performed using an inverted fluorescence microscope. Concurrently, for flow cytometric assessments, RAW264.7 cells were placed in a 24-well plate at a density of 2.0 × 105 cells per well. After 24 h exposure to various concentrations of NPs, the cells were stained with DCFH-DA, and ROS generation was quantified by flow cytometry.
The effects of NPs on macrophages
RAW264.7 cells were cultured in a 24-well plate at a density of 2.0 × 105 cells per well using a medium supplemented with LPS (100 ng/mL) and IFN-γ (20 ng/mL). The cells were incubated at 37°C for 24 h before exposure to various formulations for another 24 h.
The phenotypic switching of macrophages was examined using flow cytometry. Following the treatment, the cells were harvested by trypsinisation and blocked with 1% BSA for 30 min. The cells were washed with PBS wash once and labelled with PE-conjugated F4/80 (BD pharmingen, USA), PB450-conjugated CD86 (Thermo Fisher Scientific, USA), and APC-conjugated CD206 (Thermo Fisher Scientific, USA) for 40 min at 4°C. For APC-conjugated CD206 staining, the cells were fixed and permeabilised using the cytofix/cytoperm fixation/permeabilisation kit (BD Pharmingen, USA) prior to staining. Stained cells were washed with PBS supplemented with 1% FBS and analysed using a flow cytometer (CytoFLEX, USA). Immunofluorescence analysis was performed on the cells subjected to the same treatment regimen.
To further investigate the mRNA expression of M1 and M2 macrophage markers, the cells were subjected to various treatments for an additional 24 h. Total RNA was extracted by the RNA-Quick Purification Kit (Yishan Bio, China). The extracted mRNA was reverse-transcribed using a PrimeScript RT reagent kit (TaKaRa Bio, Shiga, Japan). Quantitative real-time polymerase chain reaction (qRT-PCR) was conducted to assess the mRNA levels using SYBR Premix Ex Taq (TaKaRa Bio) with specific primers. The expression levels of the target genes were standardised against that of the housekeeping gene GAPDH. The primer sequences for the genes of interest were listed in Table S1.
To evaluate anti-inflammation effects of NPs in vitro, the cells were subjected to various treatments for an additional 24 h. Thereafter, the concentrations of specific inflammatory cytokines, including TNF-α and IL-1β, in the culture supernatant were quantified using ELISA kits (Peprotech, Thermo Fisher Scientific, USA).
To assess the expression of HIF-1α, RAW264.7 cells were cultured in DMEM supplemented with LPS and subjected to hypoxic conditions for 4 h. Then, cells were treated with a fresh medium containing 100 µg/mL of NPs for an additional 4 h. Following the treatment, cells were fixed with paraformaldehyde and permeabilised with 0.1% Triton X-100. Blocking was achieved by incubating the cells in 10% serum for 45 min at room temperature. The cells were then probed with primary antibodies, namely anti-HIF-1α (BF8002) and anti-beta tubulin (AF7011), for 1 h at 37°C. An AlexaFluor594-conjugated goat anti-mouse IgG (red) was used as a secondary antibody.
Preparation and characterization of MNs
A polydimethylsiloxane (PDMS) mould procured from Taizhou Microchip Pharmaceutical Technology Co., Ltd. (Taizhou, China) with a length of 1 mm (array size: 15 × 15; bottom dimensions: 0.45 mm × 0.45 mm) was employed for MN fabrication. A 10% (w/v) HA solution was prepared by dissolving HA powder in DI water. Then, MTX and BSA@NPs were added to the HA solution. The mixture was introduced into a PDMS mould, followed by centrifugation at 5000 × rpm for 5 min. The mould was then desiccated at an ambient temperature for 24 h. A 25 wt.% PVP aqueous solution was subsequently added to the mould and centrifuged at 5000 × rpm for 5 min to construct the base layer. The PDMS mould was carefully removed to retrieve MNs, and the MNs were stored at ambient temperature for further analysis. MNs loaded with rhodamine B were synthesised using a similar method. The architecture and morphology of the MNs were examined by field-emission scanning electron microscopy (FE-SEM; SU8010, HITACHI, Japan), optical microscopy, and confocal laser scanning microscopy (CLSM; Nikon A1, Japan).
To determine the drug content within the MNs, the MNs were separated and dissolved in PBS. The absorbance of the resulting solution was quantified using a UV-vis-NIR spectrophotometer at 302 nm, and the MTX concentration was deduced from a reference standard curve.
The mechanical properties of the MNs were assessed using an electronic universal material-testing machine (Instron 5944). The preliminary gap between the MN tips and the upper plate was 2 mm, and the sensor motion speed for the top plate was 0.01 mm/s. The acquired force and displacement parameters were subsequently plotted to generate a force-displacement graph.
Skin penetration test in vivo
The skin penetration efficacy of the MNs was evaluated in murine skin. Before applying MNs, the dorsal region of the mouse was depilated under anaesthesia. After 5 min, the MN array was carefully removed, and the treated skin area was visualised using an optical microscope. The skin sample was excised, preserved in 4% paraformaldehyde for 24 h, processed for embedding, sectioned, and subsequently stained with hematoxylin and eosin (H&E). The histological examination of skin penetration was performed using an Olympus CKX53 microscope.
In vivo dissolution of MNs
The HA MNs were applied to the hairless dorsal skin and subsequently removed at specific intervals (0, 3, 5, 10, 15, and 20 min). After removal, the MNs were sectioned into strips. The dissolution of the MNs were imaged under an Olympus SZ61 microscope. The dissolution kinetics was represented by plotting the residual height percentages of the HA MNs at specified time points.
In vitro drug release
MNs were submerged in PBS in centrifuge tubes, and the tubes were subsequently incubated at 37°C with agitation. At specific time points, 1 mL of the suspension was replaced with an equivalent volume of fresh PBS. The drug release profiles from the MNs were quantified using a UV-vis-NIR spectrophotometer.
For in vitro transdermal absorption studies, a Franz diffusion cell apparatus with a 10 mL receptor chamber was employed. The MNs were embedded in Parafilm M, and the receptor chamber was filled with 10 mL of PBS (pH 7.4) as the receiving medium. At the predetermined time points, 1 mL of the solution was extracted and replaced with a fresh aliquot of PBS. The drug concentrations in the solutions were assessed using a UV-vis-NIR spectrophotometer at a wavelength of 302 nm.
Therapeutic efficacy evaluations in vivo
All experimental procedures involving animals followed the guidelines stipulated in the Guidance Suggestions for the Care and Use of Laboratory Animals. The study protocol was approved by the Animal Research and Ethics Committee of the Wenzhou Institute of the University of the Chinese Academy of Sciences (Approval Reference: WIUCAS23020207). Mice were maintained in a specific pathogen-free facility at 25°C, following a 12-h light/dark regimen.
Male DBA/1 mice aged 6–8 weeks were purchased from Beijing Vital River Laboratory Animal Technology Co. Ltd. (Beijing, China). The initial immunisation involved intradermal injection of an equal-volume mixture of bovine type-II collagen solution (2 mg/mL) and complete Freund’s adjuvant (4 mg/mL) into the tail base. Booster immunisation was administered after 21 d using bovine type-II collagen solution emulsified in incomplete Freund’s adjuvant to intensify disease manifestation.
After the second immunisation, the collagen-induced arthritis (CIA) model mice were randomly divided into five groups (n = 6): normal group, model group, MTX MNs group, BSA@NPs MNs group, and BSA@NPs-MTX MNs group. Mice were treated every two days, and the clinical scores and paw thickness were observed and recorded. After 20 d of treatment, the mice were sacrificed, and their hind limbs were collected. After fixation in 4% paraformaldehyde, the knee joints and paws were imaged with Micro‑computed tomography (micro-CT; Bruker, Germany). Bone parameters, including bone surface area to bone volume ratio (BS/BV), bone volume to tissue volume ratio (BV/TV), and trabecular number (Tb.N), were quantitatively assessed using proprietary analysis software (CTAn, Version 1.15.4.0, Germany). Serum concentrations of TNF-α, IL-1β, interleukin-6 (IL-6), interleukin-10 (IL-10), and transforming growth factor-β (TGF-β) were quantified utilising ELISA assays. Histopathological analyses of knee specimens included H&E staining, safranin-O staining, immunohistochemical staining for TNF-α and IL-1β, and immunofluorescent staining targeting CD86, CD206, and HIF-1α. Concurrently, key mouse organs were subjected to histological examination using H&E staining.
Histopathological evaluations were conducted to delineate the characteristic RA markers, including hypoxia, M1/M2 macrophage polarization, synovial inflammation, and cartilage degradation. Knee joint specimens were preserved in 4% paraformaldehyde and decalcified in a solution from Elabscience (Wuhan, China) with agitation for 28 d at ambient temperature. The decalcified tissues were then embedded in paraffin and sectioned into 10 µm slices using a microtome.
For immunohistochemical analysis, sections were dewaxed and incubated with primary antibodies against HIF-1α (Abcam), CD86 (ABclonal), and CD206 (Abcam). Subsequently, secondary antibodies were applied, including goat anti-rabbit Alexa Fluor 568 (Abcam) for HIF-1α and CD86 and goat anti-rabbit Alexa Fluor 488 (Abcam) for CD206. After antibody incubation, sections were counterstained with DAPI mounting medium and visualised by CLSM.
Statistical analysis
Data analysis was performed using GraphPad Prism 8 software (GraphPad Software, USA). The results were presented as mean ± standard error of the mean. Differences among multiple groups were determined using one-way analysis of variance (ANOVA). Comparisons between two distinct groups were performed using unpaired two-tailed t-tests. The statistical significance thresholds were set at p < 0.05 (*), 0.01 (**) and 0.001 (***).
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