Pathogenic neisseriae have a repertoire of high-affinity iron uptake system
s to facilitate acquisition of this essential element in the human host, Th
ey possess surface receptor proteins that directly bind the extracellular h
ost iron-binding proteins transferrin and lactoferrin. Alternatively, they
have siderophore receptors capable of scavenging iron when exogenous sidero
phores are present. Released intracellular haem iron present in the form of
haemoglobin, haemoglobin-haptoglobin or free haem can be used directly as
a source of iron for growth through direct binding by specific surface rece
ptors. Although these receptors may vary in complexity and composition, the
key protein involved in the transport of iron (as iron, haem or iron-sider
ophore) across the outer membrane is a Tons-dependent receptor with an over
all structure presumably similar to that determined recently for Escherichi
a coli FhuA or FepA. The receptors are potentially ideal vaccine targets in
view of their critical role in survival in the host. Preliminary pilot stu
dies indicate that transferrin receptor-based vaccines may be protective in
humans.