Abstract:
Dengue viruses are arthropod-borne pathogens that cause significant morbidity & mortality in many parts of the world and are often associated with explosive outbreaks. The World Health Organization estimates that about 2.5 billion people are at risk from dengue and approximately 50 million dengue infections are occurring worldwide every year. Dengue hemorrhagic fever and dengue shock syndrome constitutes the severe manifestation of dengue virus (DENV) disease. Its severity correlates with the presence of pre-existing antibodies that augment instead of neutralize infections in fragment, crystallizable region (Fc) receptor bearing cells. These antibodies arise either from a preceding infection by a different dengue serotype, or from declining antibody titers seen in infants and young children weaned from breast milk containing protective dengue-specific antibodies. Overcoming the burden of dengue is hampered by several challenges, including a poor understanding of the pathophysiological process, lack of specific therapy against the virus, and the technical difficulties in developing a vaccine. There remains no cure for dengue and as such, management is confined to supportive measures. Not surprisingly, medications using natural sources or herbal remedies are often resorted to when allopathic medical treatment is unsuccessful. The challenges above have underscored the importance of development of antiviral therapies, particularly by using natural sources to find lead compounds that act against the virus. Prompted by the popular use of crude preparation from Carica papaya leaf (PLE) for the treatment of dengue infections, the objective of this study is to determine if water and methanolic fractions of C. papaya leaf extract inhibited DENV activity. We conducted a time-of-addition and dose-dependent cell viability assay to determine if the water fraction of C. papaya leaf extract protected human-derived monocytes and elucidated its potential antiviral mechanisms. Real time quantitative PCR (RTqPCR) was also performed to quantify intracellular and extracellular viral RNA loads in Vero, mosquito C6/36 and glial cell lines between treatment and control groups. C. papaya (crude) causes a dose-dependent increase in cell viability in dengue-infected monocytes with increasing viability seen at lower doses of the crude C. papaya leaf extract. Time-of-addition studies revealed significantly higher viability when the crude extract was incubated with the virus for 1 hour prior to delivery to cells. RTqPCR demonstrated a dose-dependent lower RNA load in Vero cell lines particularly with the water fraction. In mosquito C6/36 cell line, a similar inhibitory effect was observed with the water fraction but the methanolic fraction demonstrated a mixed response whereby lower doses increased the viral load. In glial cells however, there was no expression of virus leading us to hypothesize that glial cells may not be permissible to dengue infection. In conclusion, our multi-parameter evaluation of anti-DENV 1 activity of PLE indicates the potential for it being developed as a dengue suppressant agent
