Inmunoferon is an immunomodulatory drug whose active principle (AM3) is a glycoconjugate of natural origin composed of a glucomannan polysaccharide from Candida utilis and a storage protein from nongerminated seeds of Ricinus communis (49). In vivo, AM3 enhances lymphocyte proliferation, interleukin-2 (IL-2) production, and NK activity (37); functions as an adjuvant to hepatitis B revaccination in nonresponder healthy persons (32, 38); and partially rescues the defective natural killer and phagocytic activities seen in chronic obstructive pulmonary disease patients (34). Besides, oral administration of AM3 increases IL-10 and reduces lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-␣), IL-1, and inducible nitric oxide synthase; and thus, AM3 acts as a modulator of the innate immune system by acting on peripheral blood mononuclear cells (5, 25).Dendritic cells (DCs) are professional antigen-presenting cells which link the innate and adaptive branches of the immune response by virtue of their capacity to recognize pathogen-associated structures and promote the initiation of Tcell-dependent immunity (3). In the steady state, immature myeloid DCs display a potent antigen uptake ability and contribute to the establishment of peripheral tolerance (41), whereas mature DCs display a strong capacity for T-cell stimulation and polarization of the immune response. Pathogen recognition by immature DCs is carried out by a number of cell surface molecules named pathogen-associated molecular pattern receptors, which include the Toll-like receptor (TLR) family (44) and a large number of lectins and lectin-like molecules (48), including the DC-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin (DC-SIGN; CD209) lectin. DC-SIGN is a type II membrane C-type lectin (13,16,21) which recognizes a large array of viral, bacterial, fungal, and parasite pathogens (1, 7, 9, 12, 22-24, 33, 40, 43, 45) in a mannan-and a Lewis oligosaccharide-dependent manner (17, 18) and which mediates DC interactions with naïve T lymphocytes, endothelial cells, and neutrophils by recognition of ICAM-3 (21), ICAM-2 (19), and Mac-1 (46), respectively.Given its adjuvant and immunomodulatory effects, we have hypothesized that AM3 might have a direct effect on DCs. In fact, AM3 triggers DC maturation and promotes the preferential release of IL-10 from mature human monocyte-derived DCs (P. Majano et al., submitted for publication). Since fungus-derived mannans are capable of inhibiting the lymphoproliferative responses of human mononuclear leukocytes (4, 40) and modulate pathogen recognition by human DCs (40), the effects of the polysaccharide moiety of AM3 (IF-S) on the effector functions of immature human monocyte-derived DCs were analyzed. In the present report we present evidence that the polysaccharide moiety of AM3 directly influences pathogen