Therapeutic glycan-specific antibody binding mediates protection during primary amoebic meningoencephalitis
Authors: Moseman Annie Park , Chen Ching-wen , Liang Xiaoe , Liao Dongmei , Kuraoka Masayuki , Moseman E. Ashley 
Published: September 2024

Naegleria fowleri
(
N. fowleri
) infection
via
the upper respiratory tract causes a fatal CNS disease known as primary amoebic meningoencephalitis (PAM). The robust
in vivo
immune response to
N. fowleri infection
underlies the immunopathology that characterizes the disease. However, little is known about why this pathogen evades immune control. Infections occur in seemingly healthy individuals and effective clinical options are lacking, thus a nearly 98% fatality rate. It is unclear how or if host factors may contribute to susceptibility or disease exacerbation, yet mechanistic studies of the
in vivo
immune response and disease progression are hampered by a lack of tools. In this study, we have generated monoclonal antibodies to
N. fowleri
surface antigens and shown them to be excellent tools for studying the
in vivo
immune response. We also identified one monoclonal, 2B6, with potent inherent anti-amoebastatic activity
in vitro
. This antibody is also able to therapeutically prolong host survival
in vivo
and furthermore, recombinant antibodies with an isotype more capable of directing immune effector activity further improved survival when given therapeutically. Thus, we report the generation of a novel monoclonal antibody to
N. fowleri
that can enhance beneficial immune functions, even when given therapeutically during disease. We believe this provides evidence for the potential of therapeutic antibody treatments in PAM.

IMPORTANCE

Naegleria fowleri
(
N. fowleri
) is a free-living amoeba that is found ubiquitously in warm freshwater. While human exposure is common, it rarely results in pathogenesis. However, when
N. fowleri
gains access to the upper airway, specifically the olfactory mucosa, infection leads to a lethal disease known as primary amoebic meningoencephalitis (PAM). As a free-living amoeba,
N. fowleri
does not need a mammalian host; indeed, it can be accurately described as an accidental opportunistic pathogen. While most opportunistic infections occur in humans who are immunocompromised, there are no reported immune dysfunctions associated with
N. fowleri
infection. Therefore, the basis for
N. fowleri
opportunism is not known, and the reasons why some humans develop PAM while others do not are simply not well understood. It is reasonable to speculate that local or acute immune failures, potentially even a lack of prior adaptive immunity, are related to disease susceptibility. Careful immune profiling and characterization of the
in vivo
immune response to
N. fowleri
in a mammalian host are desperately needed to understand which host factors are critical to defense, and how these responses might be compromised in a way that results in lethal infection. To identify genes and pathways that provide resistance against
in vivo N. fowleri
infection, we generated surface reactive monoclonal antibodies (Abs) that provide rapid amoeba detection and quantification
in vivo
. Interestingly,
N. fowleri
binding Abs have been readily detected in the serum and saliva of humans and animals suggesting that non-lethal exposure drives a humoral immune response against the amoeba. Yet, how Abs might interact with
Naegleria in vivo
or contribute to preventing lethal infection is not well understood. In this study, we have generated and characterized a monoclonal antibody (Ab), Clone 2B6, that recognizes a glycosylated surface antigen present in cultured
in vitro N. fowleri
as well as mouse passaged
N. fowleri
. When clone 2B6 binds to
N. fowleri
, it inhibits amoeba motility and feeding behavior, leading to strong growth inhibition. Mice treated systemically and intracerebrally with Ab displayed a delayed disease onset and prolonged survival. In addition, we found that enhancing immune-directed effector activity
via
antibody isotype could further enhance survival without obvious immunopathogenic side effects. These findings show the potential for antibody treatment as an additional therapeutic to those used currently in PAM.