phagocytosis [34e36]. ProPO1 and 2 were used to evaluate the host
immune response under different infection conditions in our previous
study [1]. In single infection group by V. anguillarum, the
transcription levels of proPO1 and 2 were up-regulated from 12 to
48 hpi, and finally returned to the baseline level. Similarly, Yeh et al.
[37] reported that the mRNA expression levels of proPO1 and 2
increased in a short time after injection with Vibrio alginolyticus and
then decreased to the baseline level similar to the control PS injection
group. The expression of genes proPO1 and 2 was mostly
inhibited under the multiple infection conditions. In addition, the
transcription level of PPAF was also inhibited in the multipleinfection
groups. ProPOs and PPAF were functioned to clear the
pathogens from circulation after infection in proPO system [38].
This down-regulation of proPOs and PPAF might reduce the total PO
activity, hemocyte count or mediate the lower transcripts of antimicrobial
peptide gens, which will make shrimp be susceptible to
pathogens.