The transferrin–iron import system from pathogenic
Nicholas Noinaj,1Susan K. Buchanan1* and
Cynthia Nau Cornelissen2*
1National Institute of Diabetes and Digestive and Kidney
Diseases, National Institutes of Health, Bethesda, MD
2Department of Microbiology and Immunology, Virginia
Commonwealth University Medical Center, PO Box
980678, Richmond, VA 23298, USA.
Two pathogenic species within the genus Neisseria
cause the diseases gonorrhoea and meningitis. While
vaccines are available to protect against four N. men-
ingitidis serogroups, there is currently no commercial
vaccine to protect against serogroup B or against
N. gonorrhoeae. Moreover, the available vaccines
have significant limitations and with antibiotic resist-
ance becoming an alarming issue, the search for
effective vaccine targets to elicit long-lasting protec-
tion against Neisseria species is becoming more
urgent. One strategy for vaccine development has
targeted the neisserial iron import systems. Without
iron, the Neisseriae cannot survive and, therefore,
these iron import systems tend to be relatively well
conserved and are promising vaccine targets, having
the potential to offer broad protection against both
gonococcal and meningococcal infections. These
efforts have been boosted by recent reports of the
crystal structures of the neisserial receptor proteins
TbpA and TbpB, each solved in complex with human
transferrin, an iron binding protein normally respon-
sible for delivering iron to human cells. Here, we
review the recent structural reports and put them into
perspective with available functional studies in order
to derive the mechanism(s) for how the pathogenic
Neisseriae are able to hijack human iron transport
systems for their own survival and pathogenesis.
Pathogenic Neisseria species
While at least 10 Neisseria species are associated with
humans, only N. gonorrhoeae and N. meningitidis are
pathogenic to humans (Marri et al., 2010). N. gonor-
rhoeae causes the sexually transmitted infection gonor-
rhoea. By contrast N. meningitidis is a frequent colonizer
of the human oropharynx, but can also cause invasive
disease manifested as meningitis or septicemia. The
reported incidence of gonorrhoea in USA is over 300 000
cases per year, in contrast to the incidence of invasive
meningococcal disease, which has been decreasing and
is currently below 1000 cases per year (CDC, 2012).
N. meningitidis may be carried asymptomatically by up to
10% of healthy humans, but in rare cases, the pathogen
can disseminate to cause rapidly progressing septicemia
as well as meningitis, both of which are potentially lethal
infections (Stephens et al., 2007). In contrast, gonococcal
infections are rarely life-threatening. Nonetheless, signifi-
cant morbidity is associated with gonococcal infections as
many are asymptomatic, particularly in women, which
facilitates ascension into the upper reproductive tract,
leading to salpingitis, pelvic inflammatory disease, infer-
tility, and ectopic pregnancy (Sparling, 1990). Ascending
gonococcal infections in men are uncommon but can lead
to prostatitis, epididymitis and infertility (Sparling, 1990).
Despite the distinct diseases caused by the pathogenic
Neisseria species, there are very few differences between
the pathogens at the genomic level. The primary virulence
factor employed by N. meningitidis, which is lacking in
N. gonorrhoeae, is the polysaccharide capsule. This
surface structure protects the meningococcus from des-
iccation, enhances serum resistance, and elicits a protec-
tive immune response (for review see Virji, 2009). The
gonococcus, which lacks a polysaccharide capsule, is
exquisitely sensitive to drying, leading to the necessity for
intimate contact for transmission. While occasional dis-
semination to the bloodstream occurs as a consequence
of gonococcal infections, serum resistance is mediated by
Accepted 15 August, 2012. *For correspondence. E-mail skbuchan@
helix.nih.gov; Tel. (+1) 301 594 9222; Fax (+1) 301 480 0597 or
E-mail firstname.lastname@example.org; Tel. (+1) 804 827 1754; Fax (+1) 804
Molecular Microbiology (2012) 86(2), 246–257 ?
First published online 7 September 2012
© 2012 Blackwell Publishing Ltd
factors other than encapsulation, including sialylation of
the outer membrane-localized lipooligosaccharide (LOS)
(Gulati et al., 2005). The polysaccharide capsule of
N. meningitidis is a protective antigen; the efficacious
vaccine that protects against meningococcal disease con-
tains capsular material from four of the 13 serogroups of
N. meningitidis. The capsule from serogroup B N. menin-
gitidis is a self-antigen, and thus not a component of the
current vaccine. However, a vaccine against serogroup B
N. meningitidis, employing sub-capsular protein antigens,
is in development (Gossger et al., 2012). In stark contrast,
N. gonorrhoeae lacks a capsule; therefore, this structure
cannot be utilized for vaccine development. Moreover,
many surface antigens, including LOS, the proteinaceous
pilus, and surface-deployed invasins called Opa proteins,
are subject to high-frequency phase and antigenic varia-
tion, making these targets unacceptable vaccine antigens
(Virji, 2009; Zhu et al., 2011). Even with many years of
effort, no successful vaccine has yet been developed to
prevent gonococcal infections.
Treatment of invasive meningococcal disease requires
N. meningitidis has yet to develop high-level resistance to
this front line, but still effective, antibiotic (Stephens et al.,
2007). In contrast, N. gonorrhoeae has evolved resist-
ance to every antimicrobial agent used to treat these
infections. In 2007, ciprofloxacin was removed from the
list of approved drugs for treatment of gonococcal infec-
tions (CDC, 2007), leaving only extended-spectrum
cephalosporins as the treatment of choice. By 2011,
however, resistance to the last line of defence, ceftriax-
one, had emerged (CDC, 2011). N. gonorrhoeae is now
recognized as a ‘superbug’ with an enormous capacity for
antigenic variation, against which there is no means of
A primary focus of current therapeutic design has been
towards vaccine development to protect against infections
by the pathogenic Neisseria species. Given the reported
limitations of the existing vaccines, lack of a gonococcal
vaccine, and the emergence of antibiotic resistant strains,
there is an immediate need for rapid development of
protective vaccines to protect against neisserial infec-
tions. Since Neisseria species cannot survive without iron,
recent studies have targeted the iron import systems,
which tend to be relatively well conserved and are prom-
ising vaccine targets, having the potential to offer broad
protection against both species.
Iron import systems in pathogenic Neisseria
Most bacterial pathogens must compete with their hosts
for iron, an essential nutrient for survival. For many patho-
gens, this process involves secretion of low-molecular
weight chelators called siderophores, which sequester
and solublize otherwise inaccessible ferric iron from the
environment within the host (for recent review see Braun
and Hantke, 2011). The ability to secrete siderophores
and subsequently to internalize ferric–siderophore com-
plexes is critical for the virulence of many bacterial patho-
gens (reviewed recently in Saha et al., 2012). In Gram-
negative bacteria, ferric–siderophores are internalized in
a conserved fashion utilizing a family of outer membrane
similarity, called TonB-dependent transporters (TBDTs)
(Noinaj et al., 2011). The crystal structures of several of
these transporters have been reported (reviewed in
Noinaj et al., 2011), all sharing a TBDT fold characterized
by an N-terminal plug domain of ~ 160 residues (plug
domain) folded inside a C-terminal 22-stranded beta-
barrel domain (beta-domain). The plug domain prevents
entry of noxious substances into the periplasm until the
appropriate ligand is bound; subsequently, the transporter
is energized by TonB and the rest of the Ton system,
which includes ExbB and ExbD (for a recent review, see
Krewulak and Vogel, 2011). Although the precise details
are not known, the plug is proposed to undergo a confor-
mational change that leads to either partial or full ejection
of the plug domain into the periplasm, thereby forming an
entry pathway for the iron cargo directly through the outer
The pathogenic Neisseria species are somewhat
unusual in that they do not have the capacity to secrete
siderophores. Despite this, they do express TBDTs of
unknown function (TdfF, TdfG, TdfH and TdfJ; Turner
et al., 2001; Hagen and Cornelissen, 2006; Cornelissen
and Hollander, 2011) in addition to transporters such as
FetA that enable the bacteria to utilize siderophores pro-
duced by neighbouring bacteria (Carson et al., 1999;
Hollander et al., 2011); however, the contribution of these
transporters to neisserial pathogenesis has not been
tested (Fig. 1A). The pathogenic Neisseria species addi-
tionally express surface receptors that mediate direct
extraction and import of iron from the human host iron
binding proteins haemoglobin, lactoferrin and transferrin
(Cornelissen and Hollander, 2011). Haemoglobin is pre-
dominantly sequestered within red blood cells and is a
tetrameric protein with each subunit capable of binding
one molecule of haem. Lactoferrin can be found in secre-
tions, in milk, and in polymorphonuclear leucocytes and
is a glycoprotein composed of two structurally similar
domains (also called lobes), each of which has the capac-
ity to bind a single iron atom. Transferrin can be found
predominantly in serum and on inflamed mucosal sur-
faces and is structurally very similar to lactoferrin, binding
one iron atom per lobe. All strains of N. meningitidis have
the capacity to utilize haemoglobin, lactoferrin and trans-
ferrin (Marri et al., 2010). In contrast, approximately half of
gonococcal isolates have undergone a large deletion in
Iron import in pathogenic Neisseria
© 2012 Blackwell Publishing Ltd, Molecular Microbiology, 86, 246–257
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Iron import in pathogenic Neisseria
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