Mucins in the host defence against Naegleria fowleri and mucinolytic activity as a possible means of evasion

Abstract

Naegleria fowleri is the aetiological agent of primary amoebic meningoencephalitis (PAM). This parasite invades its host by penetrating the olfactory mucosa. During the initial stages of infection, the host response is initiated by the secretion of mucus that traps the trophozoites. Despite this response, some trophozoites are able to reach, adhere to and penetrate the epithelium. In the present work, we evaluated the effect of mucins on amoebic adherence and cytotoxicity to Madin-Darby canine kidney (MDCK) cells and the MUC5AC-inducing cell line NCI-H292. We showed that mucins inhibited the adhesion of amoebae to both cell lines; however, this inhibition was overcome in a time-dependent manner. N. fowleri re-established the capacity to adhere faster than N. gruberi. Moreover, mucins reduced the cytotoxicity to target cells and the progression of the illness in mice. In addition, we demonstrated mucinolytic activity in both Naegleria strains and identified a 37 kDa protein with mucinolytic activity. The activity of this protein was inhibited by cysteine protease inhibitors. Based on these results, we suggest that mucus, including its major mucin component, may act as an effective protective barrier that prevents most cases of PAM; however, when the number of amoebae is sufficient to overwhelm the innate immune response, the parasites may evade the mucus by degrading mucins via a proteolytic mechanism.

Pathogens Penetrating the Central Nervous System: Infection Pathways and the Cellular and Molecular Mechanisms of Invasion

“…In the mucin-producing cell line NCI-H292, N. fowleri induced the release of reactive oxygen species, which led to the activation of epidermal growth factor receptor, which in turn stimulated production of the mucin MUC5A and IL-8 secretion (314). Despite this host response, by 12 h, N. fowleri was observed adhering to and invading the olfactory epithelium in mice, suggesting that the amoeba may have pathogenic mechanisms to efficiently penetrate the mucous layer (392). In vitro, it was shown that both live trophozoites and crude total N. fowleri extracts demonstrated mucinolytic activity.…”

“…In vivo studies have shown that N. fowleri induces mucus production within the nasal cavity as early as 1 h postinfection and that, by 6 h, amoebae are covered by mucus and surrounded by neutrophils (392). In the mucin-producing cell line NCI-H292, N. fowleri induced the release of reactive oxygen species, which led to the activation of epidermal growth factor receptor, which in turn stimulated production of the mucin MUC5A and IL-8 secretion (314).…”

“…In vitro, it was shown that both live trophozoites and crude total N. fowleri extracts demonstrated mucinolytic activity. A 37-kDa cysteine protease of N. fowleri was subsequently identified as being responsible for this mucinolytic activity (392); this most likely enabled the amoeba to penetrate the nasal mucosa and to invade the neuroepithelium. Penetration of the olfactory epithelium in mice occurred without cellular disruption or damage (390), and microscopy studies have shown that N. fowleri may be ingested by sustentacular cells within the epithelium or may migrate between the sustentacular cells (393).…”

Disruption of MDCK cell tight junctions by the free-living amoeba Naegleria fowleri

“…In the early stages of infection, N. fowleri trophozoites are embedded in a large amount of mucus and surrounded by an inflammatory infiltrate composed mainly of neutrophils (Cervantes-Sandoval et al, 2008a;RojasHernández et al, 2004). However, N. fowleri is able to degrade the mucus by secreting a 37 kDa cysteine protease (Cervantes-Sandoval et al, 2008b), which most likely allows the amoeba to evade the mucus and adhere to the olfactory epithelium (Cervantes-Sandoval et al, 2008a). For many micro-organisms, adherence is one of the most important features observed during the early invasion of different tissues or organs (Kucknoor et al, 2005;McCoy et al, 1994;Panjwani, 2010).…”

“…Immunohistochemical studies of the early events of infection using a murine model have shown that the amoebae induce intense mucus secretion and an inflammatory reaction in the nasal cavity (Cervantes-Sandoval et al, 2008a;Rojas-Hernández et al, 2004). However, N. fowleri is able to evade host innate defence mechanisms by mucous degradation that occurs via a 37 kDa cysteine protease (Cervantes-Sandoval et al, 2008b), allowing the amoebae to adhere to the neuroolfactory epithelium. A striking observation is that N. fowleri transits the epithelium (Cervantes-Sandoval et al, 2008a;Rojas-Hernández et al, 2004) by traversing cell junctions without causing any apparent damage.…”