KK-42 pretreatment can promote the survival rate of Macrobrachium nipponense infected with Aeromonas hydrophila.In order to investigate the possible molecular mechanism of KK-42 action,the partial sequence of α2M gene was first cloned and the spatio-temporal expression of the gene as well as the effects of KK-42 on its gene expression and activity were measured.The prawns,3.5-5.0 cm long,were soaked for 1 min in KK-42 solution at a concentration of 1.95×10^-4 mol/L(KK-42 treatment)or 0(KK-42 control),respectively.The KK-42 treatment group was injected individually with A.hydrophila suspension(KK-42 treatment-A.hydrophila)or saline(treatment-1)into the ventral sinus at 12 h after KK-42 treatment and the KK-42 control group was divided into groups as control-A.hydrophila and control-1,according to the same way.Then the survival rate was surveyed at different time points,and the α2M mRNA level and activity derived from different tissues were measured.Results showed that the survival rate of KK-42 treatment-A.hydrophila dramatically increased by 133% than that of the control-A.hydrophila group.Sequence comparison indicated that the α2M deduced amino acid sequence of M.nipponense had an overall similarity of 90% to that of Macrobrachium rosenbergii.Real-time PCR analysis revealed that α2M was mainly expressed in haemocytes,and higher at premolt.Compared with the KK-42 control group,the changes of α2M mRNA level and activity from KK-42 treatment group were not statistically significant.In control-A.hydrophila group,the α2M gene mRNA level and its activity in haemocytes obviously increased by 580% and 47.5%,respectively,only at 3 and 24 h post-injection compared to 0 h.In KK-42 treatment-A.hydrophila group,the α2M mRNA transcripts were significantly higher at 6-48 h and reached the maximum at 12 h with an increase of 511% than that of control-A.hydrophila group.The change trend of α2M activity was basically similar to that of its expression.In conclusion,our present results indicated that KK-42 pretreatment could significantly up-regulate the α2M gene expression as well as its activity in haemocytes of M.nipponens infected with A.hydrophila.This might lead to the enhancement of immunity in the prawns,which was likely one of the molecular mechanisms of KK-42 acting for increasing survival rate of the prawns infected with A.hydrophila.