Comparison of lipooligosaccharides from human challenge strains of Neisseria gonorrhoeae

Cyclization increases bactericidal activity of arginine-rich cationic cell-penetrating peptide for Neisseria gonorrhoeae

We previously reported that a linear cationic 12-amino acid cell-penetrating peptide (CPP) was bactericidal for Neisseria gonorrhoeae. In this study, our objectives were to determine the effect of cyclization of the linear CPP on its antibacterial activity for N. gonorrhoeae and cytotoxicity for human cells. We compared the bactericidal effect of 4-hour treatment with the linear CPP to that of CPPs cyclized by a thioether or a disulfide bond on human challenge and multi-drug resistant (MDR) strains of N. gonorrhoeae grown in cell culture media with 10% fetal bovine serum (FBS). The effect of lipooligosaccharide (LOS) sialylation on bactericidal activity was analyzed. We determined the ability of the CPPs to treat human cells infected in vitro with N. gonorrhoeae, to reduce the inflammatory response of human monocytic cells to gonococci, to kill strains of three commensal Neisseria species, and to inhibit gonococcal biofilms. The cyclized CPPs killed 100% of gonococci from all strains at 100 µM and >90% at 20 µM and were more potent than the linear form. The thioether-linked but not the disulfide-linked CPP was less cytotoxic for human cervical cells compared to the linear CPP. LOS sialylation had minimal effect on bactericidal activity. In treating infected human cells, the thioether-linked CPP at 20 µM killed >60% of extra- and intracellular bacteria and reduced TNF-α expression by THP-1 cells. The potency of the CPPs for the pathogenic and the commensal Neisseria was similar. The thioether-linked CPP partially eradicated gonococcal biofilms. Future studies will focus on determining efficacy in the female mouse model of gonorrhea.IMPORTANCENeisseria gonorrhoeae remains a major cause of sexually transmitted infections with 82 million cases worldwide in 2020, and 710,151 confirmed cases in the US in 2021, up 25% from 2017. N. gonorrhoeae can infect multiple tissues including the urethra, cervix, rectum, pharynx, and conjunctiva. The most serious sequelae are suffered by infected women as gonococci ascend to the upper reproductive tract and cause pelvic inflammatory disease, chronic pelvic pain, and infertility in 10%-20% of women. Control of gonococcal infection is widely recognized as increasingly challenging due to the lack of any vaccine. N. gonorrhoeae has quickly developed resistance to all but one class of antibiotics and the emergence of multidrug-resistant strains could result in untreatable infections. As such, gonorrhea is classified by the Center for Disease Control (CDC) as an urgent public health threat. The research presented herein on new therapeutics for gonorrhea has identified a cyclic cell-penetrating peptide (CPP) as a potent molecule targeting N. gonorrhoeae.

Neisseria gonorrhoeae scavenges host sialic acid for Siglec-mediated, complement-independent suppression of neutrophil activation

Gonorrhea, caused by the bacterium Neisseria gonorrhoeae (Gc), is characterized by neutrophilic influx to infection sites. Gc has developed mechanisms to resist killing by neutrophils that include modifications to its surface lipooligosaccharide (LOS). One such LOS modification is sialylation: Gc sialylates its terminal LOS sugars with cytidine-5'-monophosphate-N-acetylneuraminic acid, which is scavenged from the host using LOS sialyltransferase (Lst) since Gc cannot make its sialic acid. Sialylation enables sensitive strains of Gc to resist complement-mediated killing in a serum-dependent manner. However, little is known about the contribution of sialylation to complement-independent, direct Gc-neutrophil interactions. In the absence of complement, we found sialylated Gc expressing opacity-associated (Opa) proteins decreased the oxidative burst and granule exocytosis from primary human neutrophils. In addition, sialylated Opa+ Gc survived better than vehicle treated or Δlst Gc when challenged with neutrophils. However, Gc sialylation did not significantly affect Opa-dependent association with or internalization of Gc by neutrophils. Previous studies have implicated sialic acid-binding immunoglobulin-type lectins (Siglecs) in modulating neutrophil interactions with sialylated Gc. Blocking neutrophil Siglecs with antibodies that bind to their extracellular domains eliminated the ability of sialylated Opa+ Gc to suppress the oxidative burst and resist neutrophil killing. These findings highlight a new role for sialylation in Gc evasion of human innate immunity, with implications for the development of vaccines and therapeutics for gonorrhea.

IMPORTANCE

Neisseria gonorrhoeae, the bacterium that causes gonorrhea, is an urgent global health concern due to increasing infection rates, widespread antibiotic resistance, and its ability to thwart protective immune responses. The mechanisms by which Gc subverts protective immune responses remain poorly characterized. One way N. gonorrhoeae evades human immunity is by adding sialic acid that is scavenged from the host onto its lipooligosaccharide, using the sialyltransferase Lst. Here, we found that sialylation enhances N. gonorrhoeae survival from neutrophil assault and inhibits neutrophil activation, independently of the complement system. Our results implicate bacterial binding of sialic acid-binding lectins (Siglecs) on the neutrophil surface, which dampens neutrophil antimicrobial responses. This work identifies a new role for sialylation in protecting N. gonorrhoeae from cellular innate immunity, which can be targeted to enhance the human immune response in gonorrhea.

Neisseria gonorrhoeae scavenges host sialic acid for Siglec-mediated, complement-independent suppression of neutrophil activation

Gonorrhea, caused by the bacterium Neisseria gonorrhoeae (Gc), is characterized by neutrophil influx to infection sites. Gc has developed mechanisms to resist killing by neutrophils that include modifications to its surface lipooligosaccharide (LOS). One such LOS modification is sialylation: Gc sialylates its terminal LOS sugars with cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-NANA) scavenged from the host using LOS sialyltransferase (Lst), since Gc cannot make its own sialic acid. Sialylation enables sensitive strains of Gc to resist complement-mediated killing in a serum-dependent manner. However, little is known about the contribution of sialylation to complement-independent, direct Gc-neutrophil interactions. In the absence of complement, we found sialylated Gc expressing opacity-associated (Opa) proteins decreased the oxidative burst and granule exocytosis from primary human neutrophils. In addition, sialylated Opa+ Gc survived better than vehicle treated or Δlst Gc when challenged with neutrophils. However, Gc sialylation did not significantly affect Opa-dependent association with or internalization of Gc by neutrophils. Previous studies have implicated sialic acid-binding immunoglobulin-type lectins (Siglecs) in modulating neutrophil interactions with sialylated Gc. Blocking neutrophil Siglecs with antibodies that bind to their extracellular domains eliminated the ability of sialylated Opa+ Gc to suppress oxidative burst and resist neutrophil killing. These findings highlight a new role for sialylation in Gc evasion of human innate immunity, with implications for the development of vaccines and therapeutics for gonorrhea.