A λ-Carrageenan-Enriched Sulfated Galactan from Gigartina radula Attenuates Atopic Dermatitis via Coordinated Anti-Inflammatory and Immunomodulatory Mechanisms

Atopic dermatitis (AD) is a chronic, relapsing inflammatory skin disease driven by immune dysregulation and epidermal barrier dysfunction. Current therapeutic options are often limited by safety concerns or suboptimal tolerability. In this study, we isolated and structurally characterized GRB-H—a λ-carrageenan-enriched sulfated hybrid galactan from the marine red alga Gigartina radula—as a complex polysaccharide containing κ-, ι-, μ-, ν-, and λ-carrageenan structural units, and systematically evaluated its anti-AD potential using both in vitro and in vivo models. In vitro, GRB-H significantly suppressed lipopolysaccharide (LPS)-induced nitric oxide (NO), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) in RAW 264.7 macrophages, and reduced 2,4-dinitrochlorobenzene (DNCB)-evoked TNF-α and IL-1β expression in HaCaT keratinocytes. In a DNCB-induced murine model of AD, topical application of GRB-H markedly ameliorated skin inflammation, epidermal hyperplasia, and dermal immune cell infiltration. GRB-H treatment lowered total serum immunoglobulin E (IgE) levels, restored the imbalanced Th1/Th2 cell ratio in the spleen, and downregulated the mRNA expression of key inflammatory cytokines—including TNF-α, IL-4, IL-5, IL-31, and interferon-γ (IFN-γ)—in lesional skin. Collectively, these findings demonstrate that GRB-H alleviates AD symptoms through coordinated local anti-inflammatory and systemic immunomodulatory actions, highlighting its promise as a marine-derived candidate for the topical management of AD.