Abstract:【Objective】 To research the in vitro and in vivo anti -RSV activity of isoorientin isolated from the leaves of Lophatherum gracile Brongn

a traditional Chinese medicine (TCM) used for the treatment of fever and inflammation

and the preliminary mechanism of isoorientin against RSV infection. 【Methods】 MTT (3 -[4

5 -dimethylthiazol -2yl] -2

5 -diphenyl tetrazolium bromide) assay was adopted to evaluate the cytotoxicity of isoorientin. The anti -RSV activity of isoorientin against A2 and Long strains was investigated by plaque reduction assay. Plaque reduction assay was also performed to evaluate the inhibitory effect of isoorientin on the extracellular and intracellular stages of RSV replication cycle

as well as the inhibitory effects of isoorientin on viral penetration. Quantitative real -time PCR assay was conducted to determine mRNA expression of RSV proteins relating to RSV intracellular replication. Male BALB/c mice of 3 week old were used to evaluate the anti -RSV activity of isoorientin in vivo. Mice were lightly anesthetized with diethyl ether then intranasally challenged with RSV Long strain at 4th day after isoorientin treatment. Isoorientin was continuously treated by intragastric administration for 3 days. The mice were sacrificed

and its lungs were removed

weighed and fixed with methanol for H&E staining. The viral loads in the lungs were tested by plaque reduction assay. 【Results】 Isoorientin could dose -dependently inhibit the RSV -induced plaque formation on HEp -2 cells and the IC50 values against RSV A2 and Long strains were (3.6 ± 0.8)μg/mL and (2.4 ± 0.2)μg/mL

respectively

which were almost equal to that of ribavirin. But isoorientin showed lower cytotoxcity with much higher selectivity index (SI) values than that of ribavirin. The preliminary investigation on anti -RSV mechanism showed that isoorientin mainly inhibited replication of RSV at the stage of intracellular

even though it slightly stopped RSV from penetrating into the host infection at the stage of extracellular. Moreover

quantitative real -time PCR showed that the mRNA levels of RSV F

P and L protein were inhibited by isoorientin treatment at different magnitudes. In the RSV infected mice

the levels of viral loads and inflammatory response in lung tissues were reduced in the presence of isoorientin. 【Conclusions】 Isoorientin isolated from the leaves of Lophatherum gracile showed significant antiviral activity against RSV in vitro and in vivo. This compound could alleviate the inflammation in the lungs caused by RSV infection. These effects may be attributed to its significant inhibition of mRNA expression of RSV F

P and L at the stages of intracellular.