As important pattern recognition receptors (PRRs), C-type lectins play crucial roles in the crustacean innate immune system. In this study, a novel C-type lectin, designated as MnLec1, was obtained from the exoskeleton of the oriental river prawn Macrobrachium nipponense for the first time. The full-length cDNA of MnLec1 was 1329 bp with an open reading frame of 774 bp. The predicted MnLec1 protein contains a single carbohydrate-recognition domain with an EPN/LND motif and one Ca 2+ binding site-2. MnLec1 transcripts were widely detected in the tested tissues of M. nipponense and significantly up-regulated after Aeromonas hydrophila challenge.The recombinant MnLec1 protein was found to have a wide spectrum of binding activities towards various microorganisms, agglutinate two kinds of Gram-negative bacteria (Escherichia coli and A. hydrophila) in a Ca 2+ -independent manner. What's more, the survivability of prawns was significantly down-regulated after RNAi of MnLec1 when infected with A. hydrophila. Collectively, these findings suggest that MnLec1 from the exoskeleton might function as a PRR and play a crucial role in immune defense against invading pathogens in M. nipponense.

The exoskeleton of a few crustacean species possesses additional antimicrobial activities to defend invading pathogens than merely a simple physical barrier. However, the genetic links underlying are apparently still lacking. Aiming at establishing the link between the genetic cause and the antimicrobial function, the transcriptomes of exoskeleton of Macrobrachium nipponense, coupled with or without the challenge of Aeromonas hydrophila, were sequenced and analysed. Using the Illumina RNA‐Seq method, 41,261 unigenes were obtained in total, 6540 of which were differentially expressed. All differentially expressed unigenes (DEGs) were analysed by GO and KEGG. Interestingly, a total of 131 immune‐related genes were spotted and grouped into 12 varied functions and 3 different KEGG pathways, among which the pattern recognition receptors (PRRs) make up the largest population. Furthermore, five c‐type lectins (CTLs) were selected from DEGs for further analysis and it was found that all CTLs are closely correlated with moult cycle and constitutively expressed in tested samples except haemocytes. Upon the challenge of A.hydrophila, their transcript levels (CTLs) were found highly dynamic, reduced in the beginning but increased later on. Together, our results suggest that cuticular CTLs is most likely involved in the defence of A.hydrophila, which establish the foundation for investigating immune response of crustacean exoskeleton and constructing possible strategies to prevent the spread of A. hydrophila in aquaculture of crustaceans.