Many tick-borne pathogens hijack host proteins to establish infection in their tick vector or mammalian host. Previous work by this group, for example, showed that the Lyme disease bacterium Borrelia burgdorferi induces the expression of the salivary protein Salp15 in infected Ixodes scapularis ticks as they feed. The bacterium then coats itself with Salp15 as it exits the tick, creating a shield against the destructive antibodies encountered in the mammalian host.
Here, Sukumaran and colleagues used a similar approach to study the obligate intracellular bacterium A. phagocytophilum, which causes a common and sometimes deadly tick-borne disease in humans called anaplasmosis (formerly granulocytic ehrlichiosis). They found that if I. scapularis ticks fed on A. phagocytophilum–infected mice, the ticks selectively increased their expression of the salivary protein Salp16.
But in this case, Salp16 was used for protection in the tick, not the mammalian host. Inhibiting Salp16 expression in the tick (using RNAi) reduced the number of bacteria in the insect's salivary glands, suggesting that Salp16 was needed for the bacterium to establish a foothold in the salivary gland where it resides after migrating from the gut. Once there, however, the bug no longer required Salp16 to maintain its residence. Inhibiting Salp16 had no effect on transmission of the bug to the mouse reservoir or on the initial uptake of the bug (with the blood meal) into the tick gut.
The authors are now trying to determine how Salp16 helps A. phagocytophilum to colonize the salivary gland. In the meantime, blocking Salp16 in ticks might provide a way to disrupt the transmission cycle of the pathogen and thus lower the prevalence of disease.