Trefoil Factor Family (TFF) Peptides and their Different Roles in the Mucosal Innate Immune Defense and More: An Update

Abstract

Mucous epithelia are protected by complex mucus barrier layers, which are part of the innate immune defense. Trefoil factor family peptides TFF1, TFF2, and TFF3 have lectin activities and are predominantly co-secreted together with mucins from these epithelia. TFF1 and TFF2 are mainly expressed in the gastric mucosa, whereas TFF3 is widely secreted from most mucous epithelia and their glands. TFF1 and TFF3 consist of a single TFF domain and an additional free 7th cysteine residue, whereas TFF2 contains two TFF domains. Systematic analyses of the molecular forms of TFFs gave new insights into their diverse molecular functions. TFF1 mainly exists as a monomer with an unusual free thiol group and only minor amounts form a disulfide-linked homodimer as well as heterodimers with gastrokine-2 and IgG-Fc-binding protein (FCGBP). TFF3 mainly forms a heterodimer with FCGBP in vivo, but also binds Deleted in Malignant Brain Tumors/gp340 (DMBT1^gp340) in vitro. In contrast, TFF2 binds as a lectin to a conserved O-linked carbohydrate moiety of the mucin MUC6. Both FCGBP and DMBT1^gp340 are secreted by most mucous epithelia and their glands and are involved in mucosal innate immunity. Thus, a new picture emerged pointing to functions of TFF3-FCGBP (and TFF1-FCGBP) for mucosal innate immune defense, e.g. supporting the clearing of the microorganisms. Such a function could be well be supported by DMBT1^gp340. In contrast, the TFF2/MUC6 lectin complex probably physically stabilizes the inner adherent gastric mucus layer. Furthermore, there are indications that TFF3- FCGBP might also play a role in the blood vessels.

Keywords: DMBT1; FCGBP; Innate immunity; SARS-CoV-2.; inflammation; lectin; mucin; trefoil factor.