The importance of the JAK/STAT signaling pathway in
antiviral immunity has been addressed extensively in
mammals [1], and increasing evidence has suggested that
this pathway also plays a role in invertebrate organisms
during virus infection [2–4]. Activation of the JAK/STATyrosine residue near the carboxyl terminus. The tyrosinephosphorylated
STATs then dimerize and translocate to the
nucleus, where they transcriptionally activate appropriate
genes to mediate various responses, including cell growth,
cell differentiation and immune responses [5].
The immune responses mediated by the JAK/STAT pathway
have been widely studied in model organisms. Genetic knockout
studies of mice suggested that the mammalian JAK/STAT
signaling pathway is predominant in controlling immune
responses, especially the antiviral response [6]. In insects,
the first evidence of the JAK/STAT pathway being involved in
immune responses was in the mosquito Anopheles gambiae,
whereas in mammals, AgSTAT translocated into the nucleus of
the fat body cell upon bacterial infection [7]. When Drosophila
is challenged by bacteria, the JAK/STAT pathway activates at
least two gene families, TEP and TOT, that are involved in
innate immunity [8,9]. The transcriptional profile of Drosophila
C Virus-infected Drosophila further suggests that the JAK/STAT
pathway might also respond to viral infection [10]. Taken
together, these data suggest that in addition to the Toll and
Imd pathways, the JAK/STAT pathway could be a third
evolutionarily conserved pathway that supports innate
immunity in insects. Because the JAK/STAT pathway plays a
significant antiviral role, it follows that this pathway might be
targeted by a virus that is attempting to evade the host’s
immune responses; and in fact, several viruses have been
shown to counter their hosts’ antiviral mechanisms by
inhibiting the JAK/STAT signaling pathway. For example, the
V protein of simian virus targets its host’s STAT1 for
degradation [11], while the human cytomegalovirus targets
JAK1 for degradation [12]. Infection with the varicella-zoster
virus inhibits the expression of both STAT1 and JAK2 [13].
Hepatitis C virus also inhibits host immunity by disrupting the
crosstalk between the MAPK and JAK/STAT pathways [14].
WSSV also targets STAT, but instead of inhibiting or
disrupting its activity, WSSV exploits the host STAT by using
it to bind to the promoter region of the WSSV immediate
early gene ie1 and thus enhance ie1 transcription [2]. To
better understand how the JAK/STAT pathway generally
responds to pathogens in shrimp, in the present study, we
first cloned and characterized the full-length cDNA sequence
of shrimp STAT from Penaeus monodon. Next, real-time RTPCR
was used to investigate whether the transcription level
of STAT was changed after Litopenaeus vannamei were
challenged with WSSV and the immune stimulants LPS and
PGN (lipopolysacharide and peptidoglycan). A previous study
had used EMSA to show that WSSV infection led to increased
levels of activated STAT, so when our RT-PCR results showed
that STAT transcription was reduced after challenge with
WSSV, we used two other more direct methods (a phosphorylation
status analysis and an immunostaining assay) to
reconfirm the activation status of STAT in lymphoid organ
cells of WSSV-infected shrimp.