Lactoferrin (Lf), a multifunctional molecule present in mammalian secretions and blood, plays important roles in host defense and cancer. Indeed, Lf has been reported to inhibit the proliferation of cancerous mammary gland epithelial cells and manifest a potent antiviral activity against human immunodeficiency virus and human cytomegalovirus. The Lf‐binding sites on the cell surface appear to be proteoglycans and other as yet undefined protein(s). Here, we isolated a Lf‐binding 105 kDa molecular mass protein from cell extracts and identified it as human nucleolin. Medium–affinity interactions (≈ 240 nm) between Lf and purified nucleolin were further illustrated by surface plasmon resonance assays. The interaction of Lf with the cell surface‐expressed nucleolin was then demonstrated through competitive binding studies between Lf and the anti‐human immunodeficiency virus pseudopeptide, HB‐19, which binds specifically surface‐expressed nucleolin independently of proteoglycans. Interestingly, binding competition studies between HB‐19 and various Lf derivatives in proteoglycan‐deficient hamster cells suggested that the nucleolin‐binding site is located in both the N‐ and C‐terminal lobes of Lf, whereas the basic N‐terminal region is dispensable. On intact cells, Lf co‐localizes with surface nucleolin and together they become internalized through vesicles of the recycling/degradation pathway by an active process. Morever, a small proportion of Lf appears to translocate in the nucleus of cells. Finally, the observations that endocytosis of Lf is inhibited by the HB‐19 pseudopeptide, and the lack of Lf endocytosis in proteoglycan‐deficient cells despite Lf binding, point out that both nucleolin and proteoglycans are implicated in the mechanism of Lf endocytosis.

Lactoferrin (Lf) is an iron binding glycoprotein of the transferrin family that is expressed in most biological fluids and is a major component of mammals' innate immune system. Its protective effect ranges from direct antimicrobial activities against a large panel of microorganisms, including bacteria, viruses, fungi, and parasites, to anti-inflammatory and anticancer activities. This plethora of activities is made possible by mechanisms of action implementing not only the capacity of Lf to bind iron but also interactions of Lf with molecular and cellular components of both host and pathogens. This chapter summarizes our current understanding of the Lf structure-function relationships that explain the roles of Lf in host defense.

Cyclophilins A and B (CyPA and CyPB) are cyclosporin A-binding proteins that are involved in inflammatory events. We have reported that CyPB interacts with two types of cell-surface-binding sites. The first site corresponds to a functional receptor and requires interaction with the central core of CyPB. This region is highly conserved in cyclophilins, suggesting that CyPA and CyPB might share biological activities mediated by interaction with this receptor. The second site is identified with glycosaminoglycans (GAGs), the binding region located in the N terminus of CyPB. The difference in the N-terminal extensions of CyPA and CyPB suggests that a unique interaction with GAGs might account for selective activity of CyPB. To explore this hypothesis, we analyzed the lymphocyte responses triggered by CyPA, CyPB, and CyPB KKK؊, a mutant unable to interact with GAGs. The three ligands seemed capable enough to elicit calcium signal and chemotaxis by binding to the same signaling receptor. In contrast, only CyPB enhanced firm adhesion of T cells to the extracellular matrix. This activity depended on the interactions with GAGs and signaling receptor. CyPB-mediated adhesion required CD147 presumably because it was a costimulatory molecule and was related to an activation of ␣4␤1 and ␣4␤7 integrins. Finally, we showed that CyPB was capable mainly to enhance T cell adhesion of the CD4 ؉ CD45RO ؉ subset. The present data indicate that CyPB rather than CyPA is a proinflammatory factor for T lymphocytes and highlight the crucial role of CyPB-GAG interaction in the chemokine-like activity of this protein.