Marine algae-derived sulfated polymannuroguluronate (SPMG), a candidate drug for AIDS treatment, was intraperitoneally injected into normal mice for 6 weeks, and the in vivo and in vitro mechanisms of SPMG for immunomodulation were investigated in isolated lymphocytes by MTT assay, flow cytometry, and surface plasmon resonance assay. SPMG treatment at 5 and 10 mg/kg enhanced concanavalin A (ConA)-induced T cell proliferation, cellular levels of CD69, interleukin-2 (IL-2), and interferon-gamma (IFN-gamma), as well as CD4/CD8 ratio, while decreasing tumor necrosis factor-alpha (TNF-alpha) level in T cells of peripheral blood mononuclear cells. In addition, 1 molecule of SPMG bound to 2/3 molecules of IL-2 with a K(D) of 9.53 x 10(-7) M. Heparin prevented SPMG binding to IL-2 by 72.2%; thus, to a large extent, SPMG and heparin share common binding sites on IL-2. In contrast, other glycosaminoglycans (e.g., chondroitin sulfate and dermatan sulfate) had little effect on SPMG and IL-2 interaction, suggesting the requirement of a defined sequence within the sugar chain for specific recognition of IL-2. Concomitant treatment of IL-2 and SPMG augmented lymphocyte proliferation, compared with IL-2 alone; in contrast, SPMG alone had no proliferative effect. Taken together, our findings demonstrated for the first time that SPMG exerted its immunomodulation by direct activation of T cell function, accompanied by simultaneous modulation of cytokine function, which suggests that SPMG would show great promise for use in anti-AIDS therapy.