Nanoparticle Uptake by Human Corneal Epithelial Cells

Study Background and Research Question

Topically applied ophthalmic drugs are convenient and widely used, but their ocular bioavailability is often limited by rapid precorneal clearance and restricted penetration through the tear film and cornea. The reference study describes the tear film as a dynamic barrier containing lipid, aqueous, and mucin-rich components that can entrap foreign materials and promote their clearance. The corneal epithelium is especially important: although it represents approximately 10% of total corneal thickness, it accounts for about 90% of the tissue’s barrier function, according to the reference study.

Nanoparticle formulations can potentially address these limitations by increasing surface residence, protecting unstable payloads, supporting controlled release, and improving epithelial penetration. However, nanoparticle uptake is not determined by size alone. Surface charge, polymer chemistry, interactions with mucins, and the endocytic machinery of epithelial cells can all influence whether particles remain near the cell surface or enter the intracellular compartment.

Marjan Azadi and Allan E. David therefore asked a focused mechanistic question: how do nanoparticle size and surface chemistry alter interactions with human corneal epithelial cells, and which endocytic pathways account for those differences? This question connects formulation design with cell biology rather than treating cellular uptake as a purely descriptive endpoint.

Key Innovation from the Reference Study

The principal innovation was the use of a common poly(lactic-co-glycolic acid), or PLGA, nanoparticle framework modified with polymers that represent different strategies for navigating ocular barriers. Alginate and chitosan were used as mucoadhesive surface materials, whereas polyethylene glycol was used to create a mucopenetrative formulation. This design allowed the investigators to compare particle size and interfacial chemistry within a more coherent experimental system than comparisons across unrelated nanoparticle technologies.

A second strength was the inclusion of a simulated mucosal environment in an in vitro human corneal epithelial cell model. The study did not simply measure uptake in an unconditioned cell suspension. Instead, it used a cell monolayer integrated with simulated mucosal solution, creating a more relevant setting for evaluating how particles encounter both the mucosal barrier and epithelial membrane.

Finally, the work combined physicochemical characterization, cytotoxicity testing, uptake analysis, and inhibitor-based pathway studies. This progression helped distinguish three questions that are often conflated: whether particles are well formed, whether they are tolerated, and how they enter cells.

Methods and Experimental Design Insights

PLGA nanoparticles were prepared using the emulsion-solvent evaporation method and then surface modified with alginate, chitosan, or polyethylene glycol. The resulting particles were described as spherical and monodisperse. Their hydrodynamic sizes covered approximately 100–250 nm, with zeta potentials ranging from −25 to +15 mV and polydispersity indices below 0.2, as reported in the published study. These measurements are important because broad size distributions can obscure biological comparisons, while changes in surface charge can alter mucin binding and membrane interactions.

Cell compatibility was assessed with the MTT assay. After 24 hours of exposure to nanoparticle concentrations up to 100 μg/mL, reported viability generally remained between 70% and 100%. This result supports the use of the selected formulations for uptake experiments, although an MTT signal should be interpreted as a metabolic readout rather than a complete assessment of cell health.

For uptake measurements, the authors used human corneal epithelial cells in a monolayer model supplemented with simulated mucosal solution. They then evaluated uptake in the presence of inhibitors associated with distinct internalization routes. This inhibitor strategy provided functional evidence about pathway involvement, while the comparison of unmodified and polymer-coated PLGA particles connected those pathways to formulation properties.

Protocol Parameters

• Particle composition: Use PLGA as the base material and compare mucoadhesive alginate or chitosan surfaces with a PEG-modified, mucopenetrative surface, following the formulation logic of the reference study.
• Particle size and dispersity: The literature-backed panel covered approximately 100–250 nm with a polydispersity index below 0.2. Treat this as the tested design space rather than a universal optimum.
• Surface charge: The reported zeta-potential range was −25 to +15 mV. Measure charge after the final surface modification and in a relevant dispersion medium because ionic conditions can alter the value.
• Compatibility screen: The study evaluated 24-hour exposure at concentrations up to 100 μg/mL and observed 70–100% viability by MTT. A reproduction workflow should include untreated controls and an independent morphology or membrane-integrity check.
• Uptake model: Use a human corneal epithelial monolayer with simulated mucosal solution when the goal is to approximate the first epithelial encounter more closely than a simple suspension assay.
• Mechanism assignment: Use pathway inhibitors comparatively and interpret them as evidence of pathway contribution, not as absolute proof of exclusive uptake through one route.

Core Findings and Why They Matter

The central finding was that nanoparticle internalization by human corneal epithelial cells is primarily energy dependent, supporting an endocytic mechanism rather than passive diffusion across the plasma membrane. This distinction matters for ocular delivery because endocytosis can determine intracellular trafficking, retention, degradation, and the eventual availability of a nanoparticle-associated payload.

Among the formulations tested, 100 nm unmodified PLGA nanoparticles and PEG-PLGA particles with a nominal size of 150 nm produced the highest uptake levels. The result does not establish that either size is universally optimal. Instead, it shows that particle size and surface chemistry operate together: a PEG surface did not simply suppress interaction with cells, and a smaller particle was not automatically superior in every formulation context.

Inhibitor experiments indicated that macropinocytosis and caveolae-mediated endocytosis were the dominant pathways under the study conditions. Clathrin-mediated endocytosis also contributed partially, whereas phagocytosis did not appear to play a meaningful role within the investigated size and surface-chemistry ranges. These observations refine the usual assumption that all epithelial nanoparticle uptake follows a single canonical route.

For formulation scientists, the practical implication is that particle engineering should consider the sequence of barriers. A mucoadhesive surface may increase residence in the mucosal layer but can also influence diffusion toward the epithelial membrane. A PEG coating may improve movement through mucus while preserving efficient entry for selected particle sizes. The most useful design target is therefore not maximum cell-associated fluorescence alone, but a balance among mucosal transport, epithelial uptake, intracellular fate, tolerability, and payload release.

Comparison with Existing Internal Articles

The internal overview Nanoparticle Uptake Mechanisms in Human Corneal Epithelial Cells reaches a similar high-level conclusion: macropinocytosis and caveolae-mediated endocytosis are prominent routes for nanoparticle internalization in HCECs. Its emphasis is on summarizing uptake mechanisms, whereas the ACS study provides the experimental basis for relating those mechanisms to a defined PLGA formulation set, particle size range, polymer coatings, and a simulated mucosal model.

This distinction is useful when reading the two resources together. The internal article provides a mechanism-oriented entry point, while the reference paper supplies the comparative formulation evidence needed to understand why two particles entering through related pathways may nevertheless show different uptake levels.

Limitations and Transferability

The study offers a well-controlled in vitro comparison, but several limitations constrain direct translation to clinical ocular delivery. A corneal epithelial monolayer cannot reproduce the full architecture of the stratified cornea, the continuous dynamics of human tear secretion, blinking, lipid-layer behavior, or interactions with deeper ocular tissues. Simulated mucosal solution improves experimental relevance but remains an approximation of the human tear film.

Uptake also should not be equated with successful transcorneal delivery. Internalized particles may remain in epithelial cells, undergo intracellular degradation, or fail to reach the intended tissue compartment. The reference work did not establish in vivo pharmacokinetics, tissue distribution, therapeutic payload release, or disease-specific efficacy.

Generalization is also limited by the tested physicochemical window. The conclusions apply most directly to spherical PLGA particles spanning approximately 100–250 nm and zeta potentials from −25 to +15 mV. Particles outside that range, with different shapes, degradable matrices, ligands, or payloads may engage different barriers and trafficking routes. Inhibitor-based studies add mechanistic value, but pathway inhibitors can have off-target effects and may alter cell physiology independently of endocytosis. For this reason, future validation should combine inhibitor data with imaging, temperature dependence, colocalization, and orthogonal uptake assays.

Research Support Resources

Why this cross-domain matters, maturity, and limitations

Researchers testing whether actin-dependent membrane remodeling contributes to nanoparticle uptake can use Cytochalasin D (SKU B6645), a selective actin polymerization inhibitor, as a pharmacological perturbation in related cellular workflows. The product information reports an IC₅₀ of 25 nM and typical cell-culture use at 0.2–0.5 μg/mL; these are product-level specifications rather than parameters established by the ocular nanoparticle paper. Vehicle controls, viability measurements, and concentration-response testing are essential because actin disruption can broadly affect cell shape, adhesion, cytokinesis, and transport.

The same product information describes cell cycle arrest at G1-S transition, tumor cell proliferation inhibition, apoptosis induction in cancer cells, and viral transcription inhibition in other experimental models. Those findings are mechanistically adjacent to cytoskeletal perturbation but are not outcomes measured in the reference ocular study. They should not be used to infer antiviral, anticancer, or ocular therapeutic efficacy for a nanoparticle formulation without separate validation.