On page 1691, Carlin et al. uncover a new tool of bacterial deception.
Bacteria wear camouflage to creep through our bodies undetected. Some adorn their surfaces with sugary structures tipped with sialic acids that engage inhibitory receptors on white blood cells. Normally, these receptors, called Siglecs (Sia-recognizing Ig superfamily lectins), help the body recognize itself. Because sialic acids decorate our own cells, the receptors may help prevent inflammatory self-attack. Some bacteria have tapped into this pathway, impairing phagocytosis and cytokine production by immune cells, and thus helping to ensure their own survival.
Now, Carlin and colleagues show that the cell wall β protein from group B Streptococcus (GBS) can do what sialic acid does—making this the first known protein to bind Siglecs. β protein engaged Siglec-5 on human monocytes and neutrophils, triggering an inhibitory cascade that shut off immune cell defenses and promoted bacterial survival.
But why would the bacteria turn to proteins with sialic acids already in hand? As certain Siglecs evolve quickly—particularly in sialic acid-binding sites—a protein-based trigger that targets other parts of the Siglec molecule might provide bacteria with longer lasting protection. β protein is already used for protection in other ways, one of which is to inhibit the complement cascade. The protein's skill for multitasking may explain why strains expressing high levels of β protein are the most virulent. AM