Introduction
Dengue viral infections is one of the most rapidly emerging mosquitos
borne infections which is estimated to infect 390 million individuals
annually 1. Although the age standardized infection
rates, disability adjusted life years and mortality rates have increased
over the last 30 years 2, there is no specific
treatment for dengue. Due to climate change and rapid urbanization, the
incidence of dengue is predicted to further increase in future3. Many dengue endemic countries experience seasonal
outbreaks every year with the health care facilities becoming
overwhelmed with the large number of dengue patients. Although most
individuals infected with the dengue virus (DENV) develop mild illness,
a significant proportion develop complications such as dengue
haemorrhagic fever (DHF), organ dysfunction and bleeding4. However, as there are no prognostic markers to
predict who is likely to develop severe disease and due to the
non-availability of specific treatment, all dengue infected patients are
serially monitored for early detection of complications for timely fluid
management. Therefore, there is an urgent need to development of
therapeutics for dengue.
Vascular leak is the hallmark of severe dengue, which leads to plasma
leakage with fluid accumulation in pleural and peritoneal cavities,
hypotension leading to shock and poor organ perfusion that contributes
to organ dysfunction 5. Endothelial dysfunction that
leads to vascular leakage has shown to occur due to viral factors such
as the secretory protein NS1 directly acting on the endothelial
glycocalyx and activating immune cells to produce inflammatory mediators
and inflammatory lipid mediators such as secretory phospholipase A2
(sPLA2s) and cytoplasmic phospholipase A2 enzymes 6-8.
In addition, a dysfunctional host innate immune response and
pre-existing poorly neutralizing DENV specific antibodies also lead to
endothelial dysfunction by acting on many different immune cells such as
mast cells, monocytes and neutrophils. This in turn induces production
of chymase, tryptase, leukotrienes, platelet activating factor (PAF) and
sPLA2, which have shown to induce vascular leak 9-12.
There are several clinical trials that have been completed and several
ongoing trials that have used repurposed drugs to inhibit PAF and those
that stabilize mast cells 13,14. Apart from the above
completed and ongoing trials, many patients in dengue endemic countries
use herbal medicines to treat patients who present with fever.
Tragia hispida, which belongs to the plant family Euphorbiaceae,
is a medicinal plant (known as “Welkahambiliya” in Sinhala) is used in
the Sri Lankan traditional medicine to treat fever. Although the
properties of Tragia hispida have not been characterized, a
similar plant in the same family (Tragia involucrate ) was shown
to inhibit prostaglandin induced pain, have hepatoprotective effects
against chemical induced hepatotoxicity in rats and antibacterial
activity against many opportunistic bacteria 15.Justicia adathoda is another plant used for treatment of fever
and many inflammation related diseases in Sri Lanka and many other
countries. This plant has shown to reduce carrageenan-induced
inflammation in rats and had anti-pyrexic properties16. Cyperus rotundus too is included in many
traditional medicines which is used to treat pain and many gynecological
problems. This plant too has shown to have many anti-inflammatory and
anti-pyretic properties 17. However, the use and
potential action of these plant extracts in treating patients with
dengue has not been explored previously.
As many plant extracts are given in the aqueous form traditional
medicine in Sri Lanka and many countries for treatment of fever, we
sought to explore if any of these plant extracts were able to inhibit
inflammatory mediators that associate with endothelial dysfunction in
dengue. We previously showed that PAF was an important cause of vascular
leakage and that phospholipase enzymes are important in the generation
of PAF 18. It was shown that the activity of the
inflammatory lipid enzyme sPLA2 was significantly increased during the
early illness in those who progressed to develop DHF9,10. DENV NS1 protein was shown to induce sPLA2
activity and cPLA2 activity 6. These phospholipase A2
enzymes hydrolyze membrane phospholipids thereby generating fatty acids,
lysolipds and generate PAF. Therefore, we proceeded to explore if some
of the plant extracts used in traditional medicine had any sPLA2
inhibitory activity.