Dietary boron: An overview of the evidence for its role in immune function

This review summarizes the evidence for boron essentiality across the biological spectrum with special focus on biochemical pathways and biomolecules relevant to immune function. Boron is an essential trace element for at least some organisms in each of the phylogenetic kingdoms Eubacteria, Stramenopila (brown algae and diatoms), Viridiplantae (green algae and familiar green plants), Fungi, and Animalia. Discovery of several of the currently recognized boron-containing biomolecules was achieved because the bound boron formed four coordinate covalent bonds with the ligand, creating a thermodynamically stable complex that is almost undissociable in water. Boron is a constitutive element in three antibiotics and a quorum-sensing signal in bacteria. It enhances Fe receptor expression and interleukin-6 production in cultured mammalian macrophages. Boron binds tightly to the diadenosine polyphosphates and inhibits the in vitro activities of various serine protease and oxidoreductase enzymes. Physiological amounts of dietary boron decrease skinfold thickness after antigen injection in gilts and elevated circulating natural killer cells after adjuvant injection in rats. It is predicted that several boron biomolecules waiting discovery are signaling molecules that interact with the cell surface and are probably composed of two mirror or near-mirror halves stabilized by a single boron atom to form a large circular biomolecule.