Human immune cell targeting of protein nanoparticles – caveospheres

Component: Nanoparticle Type; Description: Caveospheres — protein-lipid vesicles (45-50 nm) assembled from recombinant human caveolin-1 expressed in E. coli

Component: Core Structure; Description: Hollow, biconcave spherical architecture; ~43 nm diameter by EM; ~60-70 nm hydrodynamic diameter (DLS)

Component: Surface Engineering; Description: Caveolin-1 fused to minimal Z-domain of Staphylococcal Protein A (binds IgG Fc); N and C termini oriented on outer surface

Component: Antibody Functionalization; Description: Passive incubation of caveospheres with mouse anti-human IgG antibodies (1:2 mass ratio); ~10% of Z-domains occupied (~16 antibodies/particle)

Component: Targeting Antibodies; Description: • Anti-CD4 (OKT4, IgG2b) — CD4⁺ T cells<br>• Anti-CD20 (2H7, IgG2b) — B cells<br>• Anti-CCR5/CD195 (3A9, IgG2a) — CCR5⁺ memory T cells

Component: Fluorescent Labeling; Description: Alexa Fluor 647 succinimidyl ester (AF647) for tracking

Component: Cell Types; Description: • Cell lines: CEM.NKR-CCR5 (CD4⁺CCR5⁺ T cell line), 721.221 (CD20⁺ B cell line)<br>• Primary cells: Human PBMCs (CD4⁺ T cells, CD19⁺ B cells, NK cells, monocytes)

Component: Biological Context; Description: 10% FBS or 0.5% BSA (protein corona present); mixed primary cell populations

Parameter: Caveosphere Expression; Details: E. coli (Rosetta pLysS DE3) expressing caveolin-1-Z-domain fusion; IPTG induction (1 mM); cell disruptor lysis; amylose resin affinity chromatography

Parameter: Nanoparticle Characterization; Details: • Negative stain EM (size, morphology)<br>• DLS (hydrodynamic size)<br>• Flow cytometry (Ab-capture microbeads for functionalization confirmation)

Parameter: Cell Line Targeting; Details: CEM.NKR-CCR5 (CD4⁺) and 721.221 (CD20⁺); 1 μg NPs/10⁶ cells; 1 h on ice; flow cytometry and imaging flow cytometry

Parameter: Primary PBMC Targeting; Details: PBMCs from 9 healthy donors; phenotyping with 6-antibody cocktail; 1 μg NPs/10⁶ cells; 1 h on ice; flow cytometry; confocal microscopy on sorted populations

Parameter: Internalization Assay; Details: CEM.NKR-CCR5 or primary CD4⁺ T cells; 1 μg NPs; 90 min at 37°C or on ice; confocal microscopy z-stacks; surface vs. internal quantification

Parameter: Controls; Details: Untargeted caveospheres; irrelevant IgG-functionalized particles; temperature controls (ice, 37°C)

Parameter: Replicates; Details: n=3 independent experiments (cell lines); n=9 donors (PBMC targeting); >100 cells per condition for confocal quantification

Parameter: Statistical Tests; Details: Non-parametric one-way ANOVA (Friedman test) with Dunn's multiple comparisons test

Analysis Category: Nanoparticle Characterization; Methods: Negative stain TEM (JEOL 1011, 80 kV); DLS (Nanopartica SZ-100); flow cytometry (Ab-capture microbeads)

Analysis Category: Cell Line Targeting; Methods: Flow cytometry (BD LSR Fortessa); imaging flow cytometry (Amnis ImageStream); 30,000 cells/sample

Analysis Category: Primary PBMC Targeting; Methods: Flow cytometry panel: CD3-AF488, CD8-BV570, CD14-PE-Cy7, CD19-BV650, CD45-V500, CD56-PE; 9 donors

Analysis Category: Confocal Microscopy; Methods: Zeiss LSM710; 20× and 63× oil objectives; ZEN 2012 Black; Imaris 7 (3D renders); z-stack quantification (>100 cells/condition)

Analysis Category: Cell Sorting; Methods: BD FACS ARIA III (100 μm nozzle) for confocal imaging of sorted populations

Analysis Category: Internalization Quantification; Methods: Step-through z-stacks; count percentage of cells with surface-bound vs. internalized AF647-nanoparticles

Parameter: EM diameter (nm); Untargeted: 42.7 ± 8.3; αCD4: 41.3 ± 8.3; αCD20: 45.3 ± 6.8

Parameter: DLS hydrodynamic size (nm); Untargeted: 70.1 ± 18.7; αCD4: 62.1 ± 15.9; αCD20: 61.2 ± 19.8

Parameter: Morphology; Untargeted: Spherical, hollow; αCD4: Unchanged; αCD20: Unchanged

Parameter: Ab-capture microbead binding (anti-light chain); Result: Positive for Ab-functionalized caveospheres

Parameter: Control microbead binding (BSA only); Result: Negative

Parameter: Functionalization efficiency; Result: ~16 antibodies per particle (~10% of Z-domains occupied)

Target: αCD4 → CEM.NKR-CCR5; Cell Line: T cell line; % Association (Flow): 90.9%; % Association (Imaging Flow): 71.0%

Target: αCD20 → 721.221; Cell Line: B cell line; % Association (Flow): 90.6%; % Association (Imaging Flow): 50.0%

Target: Untargeted → CEM.NKR; Cell Line: T cell line; % Association (Flow): <1%; % Association (Imaging Flow): -

Target: Untargeted → 721.221; Cell Line: B cell line; % Association (Flow): <2%; % Association (Imaging Flow): -

Target: αCD4 → 721.221 (off-target); Cell Line: B cell line; % Association (Flow): <5%; % Association (Imaging Flow): -

Target: αCD20 → CEM.NKR (off-target); Cell Line: T cell line; % Association (Flow): <5%; % Association (Imaging Flow): -

Target: αCD4; Targeted Cell Type: CD4⁺ T cells; % Cells with Bound NPs: 76.0 ± 11.3%; MFI Fold Increase vs. Untargeted: 43.9 ± 12.7×

Target: αCD20; Targeted Cell Type: CD19⁺ B cells; % Cells with Bound NPs: 79.3 ± 23.6%; MFI Fold Increase vs. Untargeted: 6.6 ± 1.0×

Target: Untargeted; Targeted Cell Type: CD4⁺ T cells; % Cells with Bound NPs: Lower; MFI Fold Increase vs. Untargeted: 1×

Target: Untargeted; Targeted Cell Type: B cells; % Cells with Bound NPs: 48.3 ± 13.0%; MFI Fold Increase vs. Untargeted: 1×

Target: αCD4 (off-target); Targeted Cell Type: B cells; % Cells with Bound NPs: 51.6 ± 10.6%; MFI Fold Increase vs. Untargeted: Lower than αCD20

Target: αCD20 (off-target); Targeted Cell Type: CD4⁺ T cells; % Cells with Bound NPs: Lower; MFI Fold Increase vs. Untargeted: Lower

Target: Any caveosphere; Targeted Cell Type: NK cells; % Cells with Bound NPs: 28.8 ± 25.1% (low MFI); MFI Fold Increase vs. Untargeted: Low

Target: Any caveosphere; Targeted Cell Type: Monocytes; % Cells with Bound NPs: High association; MFI Fold Increase vs. Untargeted: Minimal targeting effect

Condition: αCCR5 → CEM.NKR (37°C); Cell Type: T cell line; % Cell Association: 98.6 ± 0.7%; % Cells with Internalized NPs: 43.3 ± 4.6%

Condition: αCCR5 → CEM.NKR (ice); Cell Type: T cell line; % Cell Association: 97.4 ± 0.2%; % Cells with Internalized NPs: 0.98 ± 0.98%

Condition: Untargeted → CEM.NKR (37°C); Cell Type: T cell line; % Cell Association: 5.4 ± 1.2%; % Cells with Internalized NPs: 0%

Condition: Untargeted → CEM.NKR (ice); Cell Type: T cell line; % Cell Association: 2.5 ± 0.4%; % Cells with Internalized NPs: 0%

Condition: αCCR5 → primary CD4⁺ T cells (37°C); Cell Type: Primary T cells; % Cell Association: High; % Cells with Internalized NPs: Up to 34%

Condition: αCCR5 → primary CD4⁺ T cells (ice); Cell Type: Primary T cells; % Cell Association: Surface only; % Cells with Internalized NPs: 0%

1. Protein A Z-domain limitations in vivo: The authors note that "the high concentration of IgG (with strong affinity to Protein A) in human blood might compete off targeting IgG and render this system less suitable in vivo." They are "actively investigating chemical conjugation of IgG, which are better suited to future in vivo studies."

2. Bacterial origin and immunogenicity: The authors acknowledge that "by nature of their bacterial origin, caveospheres contain immunostimulatory molecules. Delivering anti-HIV-1 therapeutics will require nonimmunostimulatory caveospheres and efforts are underway to overcome this potential problem."

3. In vitro only: All experiments were performed in vitro with primary PBMCs; no in vivo targeting, biodistribution, or therapeutic efficacy studies were conducted.

4. No cargo delivery demonstrated: While the authors propose caveospheres for therapeutic delivery (e.g., HIV siRNAs or latency-reacting agents), actual cargo loading and functional delivery were not demonstrated.

5. Loading limitations: The authors note that "to load larger molecules such as proteins or RNA, caveosphere production may require cell-free protein expression systems" because only small molecules can pass through the bacterial outer membrane during expression.

6. Limited internalization efficiency: Only 34% of primary CD4⁺ T cells internalized CCR5-targeted caveospheres, leaving room for improvement.

7. Targeting intensity difference: While αCD4 caveospheres showed 43.9-fold MFI increase, αCD20 only showed 6.6-fold, suggesting variable targeting efficiency depending on the receptor and antibody.

8. No evaluation of receptor downregulation: Targeting CCR5, which is known to internalize rapidly, could potentially downregulate the receptor and affect cell function—this was not assessed.

9. Donor variability: Data from 9 donors showed substantial variability (e.g., B cell binding SD of 23.6%), which may limit generalizability.

10. No active targeting to dendritic cells: The authors note their caveosphere platform "warrants investigation of targeting to more scarce dendritic cell populations" for vaccine applications, which was not performed.

11. Competition from serum proteins: While targeting was achieved in serum-containing media, the effect of a full protein corona on targeting efficiency was not systematically studied.

12. No therapeutic payload: The study focused solely on targeting and internalization; functional outcomes (e.g., gene silencing, drug delivery efficacy) were not demonstrated.

Report prepared based on the published Nanoscale article. For full experimental details, supplementary data, and complete references, please refer to the original publication.