4. Host anti-infective responses
Several biotechnological advancements have made possible the
characterisation of signalling pathways that are conserved across
infection types39,40. Profiling global gene expression
and sequence alignment to reference genomes enable isolation of
differentially expressed genes pre- and
post-infection41,42. Selected genes are assessed
against repositories and online databases to probe enrichment of
functional biological pathways, and subnetworks are constructed by
comparing and connecting identified genes to curated protein-protein
interaction databases43. Traditional monolayer cell
cultures are also being supplanted by human in vitro 3D models
which probe functional multicellular interactions of epithelial and
immune cells (dendritic cells, neutrophils)44.
Detailed mapping of host anti-infective responses in this way has led to
the emergence of key signalling pathways that may be targeted by both
existing and future pan-pathogen antimicrobials, such as STING and MAPK.
The first line of host defence against infectious agents involves
activation of innate immune signalling pathways that recognise specific
pathogen-associated molecular patterns (PAMPs)45,46.
For example, RIG-I-like receptors (RLRs) have evolved to detect viral
RNA species and to activate the production of host defence molecules and
cytokines that stimulate adaptive immune responses; their regulation by
host-derived ncRNAs is of particular interest47. In
addition, host defence countermeasures, including the production of type
I interferons (IFNs), can also be triggered by microbial DNA from
bacteria, viruses and perhaps parasites and are regulated by the
cytosolic sensor, stimulator of interferon genes
(STING)48,49. The discovery of the STING signalling
pathway has provided considerable insight into microbial pathogenesis,
mechanisms of host defence, and causes of inflammatory disease and even
cancer50. Regulation of the STING pathway has
therefore been suggested as a pan-pathogen antimicrobial
strategy51. Given the importance of STING as a
mediator of both antiviral and pro-inflammatory responses to viral
infection, it is interesting to consider last year it was shown to have
a crucial role in replication of RV-A and RV-C
rhinoviruses52. STING is relatively highly expressed
in lung tissue and thus may contribute to protection against both
bacterial and viral respiratory tract infection53.
Considering azithromycin’s ability to upregulate virus-induced type I
interferon responses, its use as an antibiotic for pulmonary bacterial
infections, and the fact that it has been described as a ‘holy grail’ to
prevent exacerbations in chronic respiratory disease, a molecular
mechanism of azithromycin and other macrolides via STING is
possible54,55.
The MAP kinases (MAPKs), which include ERK, JNK, and p38 families,
constitute an integral part of the host intracellular signalling
network, essential for signal transduction from receptors and stimuli to
biological reaction56-59. Appropriate functioning of
MAPK signalling is thus critical to mount effective immune responses,
and presents a broad-spectrum therapeutic target across pathogen
classes, which drugs such as macrolides may
exploit60,61. Macrolides are a class of diverse
compounds which include antibiotics, antifungals, prokinetics, and
immunosuppressants. The non-antimicrobial properties of macrolides have
been suspected as far back as the 1960s and their successful treating of
hyperinflammatory diseases such as diffuse panbronchiolitis (DPB) has
served to extend their use to a number of chronic inflammatory
diseases62. Macrolides have been shown to modulate
intracellular MAPK, especially ERK1/2, and the NF-kB pathway downstream
of ERK63. Due to the fact that these pathways exert
plethoric cellular functions, including inflammatory cytokine
production, cell proliferation, and mucin secretion, modulation of
ERK1/2 and NF-kB can explain the majority of the reported
immunomodulatory effects of macrolides64,65.
Intriguingly, however, specific proteins and receptors targeted by
macrolides that affect MAPK/NF-kB signalling have not yet been
identified, offering an avenue for experimental verification. Indeed,
putative binding molecule(s) may have multiple mechanisms of action.
Overall, macrolide treatment of DPB, asthma, bronchiectasis,
rhinosinusitis, and CF is made possible by polymodal modulation exerted
at different levels of cellular signalling, yet among these, modulation
of ERK1/2 and transcription factors is prominent, consistent, and
clearly unrelated to antimicrobial properties66.
Due to its broad-spectrum anti-infective effect against bacteria,
parasites, and viruses, several studies have sought to delineate the
underlying molecular mechanism of nitazoxanide, a thiazolide
drug67. Tizoxanide, the main active metabolite of
nitazoxanide, exerts anti-inflammatory effects by inhibiting the
production of pro-inflammatory cytokines and suppressing activation of
the NF-kB and the MAPK signalling pathways in LPS-treated macrophage
cells68. Similarly, niclosamide, a potential
pan-pathogen antimicrobial, was found to inhibit MAPK/ERK in human
glioblastoma studies, indicative of crosstalk between anti-infectives
and anti-cancer therapeutics69. Moreover, ivermectin,
a potential treatment for COVID-19, reverses drug resistance in cancer
cells via the EGFR/ERK/Akt/NF-kB pathway70. During
viral infection, signalling pathways that govern essential physiological
roles, such as apoptosis, mitogenesis, cell proliferation, metabolism,
and cytoskeletal reorganisation, can be usurped to the benefit of the
virus. Considering the vital role played by the ERK/MAPK pathway in
controlling diverse host physiological processes, it is not surprising
that many viruses co-opt the pathway for their own biologic
needs71. Development of new antiviral therapeutics
based on clinical trials of ERK/MAPK inhibitors has been suggested for
both DNA and RNA viruses, including SARS-CoV-2
recently72,73.
Autophagy signalling has also emerged as a host pharmacological target
with broad-spectrum anti-infective potential. Recently, the Centers of
Excellence for Translational Research (CETR) Program were founded to
develop host-directed broad-spectrum anti-infective agents against
pathogens with pandemic potential. According to their grant proposal,
later funded by the National Institute of Allergy and Infectious
Diseases (NIAID), ‘broad-spectrum host-directed therapeutics, once
approved for clinical use, can be deployed for emerging pathogens, new
outbreaks, and pathogens engineered with
ill-intent’74. The goal of this proposal is to
generate autophagy pathway-directed compounds that are active against a
range of taxonomically-unrelated pathogens. To accomplish this, several
strategies are being employed including targeting Beclin 1 complexes,
genes and pathways for autophagy-dependent inhibition of bacterial
infection, and Atg gene-dependent immunity75,76.
Virulence factors secreted by pathogens have co-evolved to manipulate
host signalling pathways via a range of mechanisms, including
constitutive pathway activation and subversion of critical signalling
molecules. A major challenge is to determine enzymatic activities and
host substrates for pathogen virulence factors that show no clear
homology to eukaryotic proteins. Following from this, an even more
complex challenge is to glean an understanding of the orchestra of
factors within the host-pathogen interactome involved in successful
infection. Both temporal and spatial considerations are essential for
regulating host cells during infection, justifying the employment of
model organisms to understand system-level effects of therapeutic
intervention within a physiological context. Ultimately, the discovery
of conserved anti-infective pathways is a landmark discovery, not only
to incite unification of microbiological disciplines first envisioned by
Casadevall and Pirofski, but also to mechanistically confirm the
therapeutic success of existing antimicrobials which treat diseases
pertaining to multiple pathogen classes.