GM-CSF-loaded chitosan hydrogel as an immunoadjuvant enhances antigen-specific immune responses with reduced toxicity

Purpose: Conventional adjuvants such as Complete Freund’s Adjuvant (CFA) and Incomplete Freund’s Adjuvant (IFA) are potent but toxic, limiting clinical use. A safer, biodegradable adjuvant system was needed to enhance antigen-specific humoral and cellular immunity without severe local or systemic side effects.

Hypothesis: If ovalbumin (OVA) and GM-CSF are co-delivered in a thermosensitive chitosan hydrogel (CH-HG), then the formulation will act as a safe, local antigen-delivery adjuvant that enhances OVA-specific antibody and T cell responses with lower toxicity than CFA or IFA.

Aims: Evaluate the safety, local tolerability, and biodegradation of CH-HG after subcutaneous injection in mice. - Determine whether CH-HG containing OVA ± GM-CSF induces OVA-specific IgG, IgG1, and IgG2a responses. - Assess antigen-specific CD4⁺ and CD8⁺ T cell immune responses after immunization. - Compare the immune response and toxicity profile of CH-HG/OVA + GM-CSF with CFA- and IFA-based adjuvants.

Delivery system: Platform: Thermosensitive chitosan hydrogel (CH-HG), formed with glycerol 2-phosphate disodium salt hydrate (β-GP). - Polymer: Chitosan, medium molecular weight 161 kDa, viscosity 200,000 cps, 80% deacetylation. - Formulation: Chitosan dissolved in 0.1 M HCl; β-GP solution containing OVA and/or GM-CSF added at 4°C; hydrogel forms at body temperature and physiological pH after subcutaneous injection. - Payload: OVA as model antigen (50 µg) and GM-CSF (50 ng in the immune-response experiments; the methods section contains an inconsistent “50 mg” notation, likely a typo). - Targeting ligand: None. - Dose/volume: 50 µL injected subcutaneously; booster injection at 7 days.

Approach: Model: Female C57BL/6 mice, 6 weeks old, ~20 g; n = 5 per group. - Safety study: 50 µL CH-HG, CFA, or IFA injected subcutaneously; injection-site morphology, adjuvant volume, and body weight monitored for 14 days. - Immunization study: Mice immunized with OVA solution, OVA + GM-CSF, CH-HG/OVA, CH-HG/OVA + GM-CSF, or IFA/OVA + GM-CSF; boosted at 7 days. - Comparison study: CH-HG/OVA + GM-CSF vs CFA/OVA + GM-CSF vs IFA/OVA + GM-CSF. - Readout: Serum collected 2 weeks after last immunization for antibody ELISA; splenocytes harvested for intracellular cytokine flow cytometry.

Key methods: Injection-site morphology and adjuvant volume by calipers. - Body-weight monitoring for systemic toxicity. - ELISA for OVA-specific IgG, IgG1, and IgG2a in serum. - Flow cytometry for intracellular IFN-γ and IL-4 in splenocytes after stimulation with MHC class I OVA peptide (aa 257–264) or MHC class II OVA peptide (aa 323–339). - Statistical analysis by Student’s t-test, ANOVA, or Mann–Whitney rank sum test; p < 0.05 considered significant.

Key results: Safety: CH-HG formed a local hydrogel after subcutaneous injection, caused no pus, discharge, or scab, and was largely degraded by 2 weeks. CH-HG-injected mice maintained body weight similar to controls, whereas CFA- and IFA-injected mice weighed less. At day 14, CH-HG volume was markedly lower than CFA/IFA (approximately 15 mm³ vs ~50 mm³). - Humoral immunity: CH-HG/OVA induced OVA-specific IgG ~10-fold higher than soluble OVA alone (p < 0.001). Adding GM-CSF further increased IgG titer ~10³-fold compared with CH-HG/OVA without GM-CSF (p < 0.001). CH-HG/OVA + GM-CSF produced significantly higher OVA-specific IgG and IgG1 than CFA/OVA + GM-CSF or IFA/OVA + GM-CSF (p < 0.01). IgG2a titers were similar across adjuvant groups. - Cellular immunity: CH-HG/OVA + GM-CSF significantly increased OVA-specific IFN-γ⁺ CD4⁺, IL-4⁺ CD4⁺, and IFN-γ⁺ CD8⁺ T cells compared with OVA or CH-HG/OVA (p < 0.001). CD8⁺ T cell responses were higher than in other groups (p < 0.001). - Immune bias: Higher IgG1 than IgG2a suggested a strong Th2-type humoral response, while antigen-specific CD8⁺ IFN-γ⁺ T cells also indicated cellular immune activation.

Interpretation: The authors conclude that CH-HG loaded with GM-CSF is a safe and effective immunoadjuvant that enhances antigen-specific humoral and cellular immune responses with reduced toxicity compared with CFA or IFA. The injectable, biodegradable hydrogel system may broaden clinical adjuvant use by reducing concerns about toxic side effects.

Limitations: Only a mouse OVA immunization model was used; no tumor challenge, infection model, or protective efficacy data. - No survival or long-term immune-memory analysis. - Small group sizes (n = 5) and short follow-up (up to ~5 weeks in some antibody measurements). - Mechanistic details of GM-CSF release, hydrogel degradation kinetics, and APC recruitment were not deeply dissected. - No large-animal or human validation. - The methods section reports “50 mg” GM-CSF in one place while results use “50 ng,” an inconsistency that should be noted.