Immunological Mechanisms Underlying Delayed Adverse Reactions to Injectable Aesthetic Treatments

Abstract

Delayed inflammatory reactions (DARs) following injectable aesthetic procedures, including hyaluronic acid (HA) fillers and botulinum toxin type A (BoNT-A), represent an emerging clinical challenge in aesthetic medicine. These reactions present as inflammatory nodules, granulomas, or edema, many weeks to years after administration. The pathophysiology remains incompletely understood, particularly in heterogeneous populations, despite the rising number of cases. These reactions involve T-cell hypersensitivity, cytokine dysregulation, and exogenous antigens. This study aimed to characterize these immunological pathways to guide prevention and management strategies. The study is a multicenter prospective cohort (n=218 adults; mean age=41.7 ± 9.2 years; 81% female) based on the U.S. sites recruiting subjects with DARs (onset ≥14 days post-injection) and then followed them over time. Comprehensive assessments included clinical grading (0–4 scale), skin biopsies (n=187) with immunohistochemistry (CD3, CD4, CD8, CD68, FoxP3), multiplex cytokine profiling (IL-1β, IL-4, IL-6, IL-17, TNF-α, IFN-γ), T-cell subset analysis by flow cytometry, serum immunoglobulin measurement, PCR for biofilm detection, and HLA typing were done. There were documents on triggers (e.g., vaccinations, infections) that were filled in through structured questionnaires. Data analysis was done in SPSS v28 and R v4.3 in multivariate logistic regression as predictors (p<0.05). Principally, DARs were type IV hypersensitivity reactions (72%), dominated by CD4+ T-cell infiltrates (68%; mean CD4/CD8 ratio 3.1:1) and non-caseating granulomas (41%). Th1/Th17 cytokines were significantly elevated (TNF-α: 52.4 pg/mL; IL-17: 21.3 pg/mL; p<0.0001 vs. controls) and correlated with severity (r=0.62, p<0.005). In severe cases, there was marked depletion of regulatory T cells (mean 3.8% vs. 9.2% in mild cases; p=0.002). Biofilms were detected in 17% of recurrent cases (OR 4.2, 95% CI 2.1–8.4; p<0.001). MRNA COVID-19 vaccination (29%), infections (24%), and dental procedures (11%) were the triggers. The results of our observations indicate that there might be differences between Hispanic (48%) and African American (45%) subgroups, and the granuloma rates were higher (p=0.012); nevertheless, these results have to be investigated in greater and more varied populations. Multivariate analysis has revealed vaccination (OR 2.4, 95% CI 1.3-4.5; p=0.008) and HA fillers (OR 1.8, 95% CI 1.1-3.0; p=0.02) to be independent predictors. A longitudinal follow up showed that 68% of the cases were resolved in 12 months, and there were better DLQI scores (12.3 to 3.8; p<0.001). T-cell hypersensitivity and Th1/Th17 dominate DARs, accompanied by Treg dysfunction, which is enhanced by immunizations and biofilms. The results suggest the use of immunological screening before the procedure, ethnicity-specific risk stratification, and focused immunomodulation to promote safety in aesthetic medicine. Future studies need to consider Treg-modulating therapy and worldwide populations to be widely applicable.