(gonococcus) is one of the most common sexually transmitted bacterial pathogens. Worldwide, N. gonorrhoeae
accounts for an estimated 60 million cases of urethritis and cervicitis each year (1
). During gonococcal infection, there is a local inflammatory response to mucosal invasion by the organism. In women, N. gonorrhoeae
infections also lead to major complications including pelvic inflammatory disease, infertility (via inflammatory scarring of the fallopian tubes), and neonatal disease. The host innate immune responses to N. gonorrhoeae
are critical in dictating the local inflammatory response to gonococcus, which, in turn, mediate many of the complications of infection by this organism.
Despite the initial innate immune response, most patients develop little adaptive immunity to N. gonorrhoeae
and re-exposure frequently results in recurrent infection (2
). The mechanisms leading to this poor adaptive immune response are unknown. N. gonorrhoeae
is known to engage immunosuppressive signaling pathways in B and T lymphocytes (3
). However, there have been no reports of such immunosuppressive signaling in antigen presenting cells, which act as the bridge between the innate and adaptive immune system.
Several cytokines have been implicated in mediating inflammation associated with gonococcal infection. Experimental infection with N. gonorrhoeae
in human subjects has been shown to result in measurable increases in systemic and urethral proinflammatory cytokine levels, including IL-1β (5
). In women with naturally acquired gonococcal cervicitis, the levels of systemic inflammatory cytokines are not significantly elevated except in the presence of co-infection with other sexually transmitted infections (7
). The source of these cytokines is likely to include local epithelial cells, resident phagocytes, and recruited immune cells. Various immortalized epithelial cell lines have been shown to increase expression of IL-1β and other cytokines after exposure to N. gonorrhoeae
). Peripheral blood mononuclear cells (primarily lymphocytes) exposed to N. gonorrhoeae
produce a number of T-cell-associated cytokines, including IL-2, 4, 8, 10, and 12 (10
Macrophages and other phagocytes are critical cells in the innate immune response to pathogens that are sensed in the environment and phagocytized. In addition to living freely in the extracellular space, N. gonorrhoeae has the capacity to penetrate the cytoplasm of these phagocytes, posing additional difficulties to its detection and elimination by the innate immune system.
There are now at least four known families of signaling effectors involved in the innate recognition of pathogens: Toll-like receptors (TLRs), NLRs, RIG-I-like helicases (RLHs), and the C-lectin receptors (CLRs) (11
). While the RLHs are thought to play roles primarily in innate recognition of viral pathogens, the TLRs, CLRs, and NLRs have all been implicated in recognizing or responding to various bacterially derived compounds. Recognition of extracellular gonococcal lipooligosaccharide (LOS) can be mediated by TLR4 and the C-lectin receptor, DC-SIGN (14
). Additionally, innate recognition of several N. gonorrhoeae
-derived proteins, including Porin and Lip, induces lymphocyte activation that is dependent on TLR2 (16
). To date, the role of intracellular NLR proteins in the recognition of N. gonorrhoeae
The NLR gene product NLRP3 (also known as cryopyrin, NALP3, Pypaf1 and CLR1.1), is required for IL-1β induction by the innate immune system in response to a large number of bacterial pathogens and proinflammatory substances (18
). NLRP3 can also be activated by mutations in its nucleotide-binding domain (24
). In humans, these mutations are associated with the periodic fever syndrome CAPS (cryopyrin-associated periodic syndrome) (25
). Upon activation, NLRP3 assembles with caspase-1 (the protease responsible for cleaving pro-IL-1β to its mature form), ASC1, Cardinal/TUCAN, and potentially other proteins to produce one of several IL-1β-processing complexes known as “inflammasomes” (26
). The activation of the NLRP3 inflammasome has recently been shown to occur both in the setting of exposure to Pathogen Associated Molecular Patterns (PAMPS) and other molecular signs of danger, known as Danger Associated Molecular Patterns (DAMPS). In addition to production of IL-1β, activation of NLRP3 by mutation or pathogens also initiates a pro-inflammatory, necrotic cell death program in monocyte-derived cell lines (27
We now demonstrate that N. gonorrhoeae potently activates NLRP3-dependent signaling pathways to elicit IL-1β secretion. Our studies suggest that the NLRP3/inflammasome signaling pathway is critical to the secretion of mature IL-1β, which has been observed in humans infected with gonococci. Additionally, we have demonstrated that gonococcus causes the activation of the cysteine protease, Cathepsin B. Inhibition of this protease reduced NLRP3-mediated pyronecrosis and IL-1β secretion in monocyte-derived cells. Isolated gonococcal lipooligosaccharide (LOS), which is shed through membrane blebbing by this organism, also elicited both of these NLRP3-mediated signaling responses. Lipooligosaccharide induced activation of this signaling system likely represents a major component of the inflammatory signaling involved in the pathogenesis of infections caused by N. gonorrhoeae.