Investigating highly specific innate immune responses in shrimp
connected with the diversified IgSF molecule, Dscam
KC Han-Ching Wang
Institute of Biotechnology, National Cheng Kung University, Tainan 701, Taiwan
wanghc@mail.ncku.edu.tw
Although adaptive immune systems and antibodies are not found in invertebrates,
some arthropods have exhibited innate immune responses supplemented by a novel
immune-system that exhibits both specificity and memory. The mechanisms
underlying this unique immune system are still unclear. Some evidence suggests that
the arthropod Dscam molecule (Down syndrome cell adhesion molecule) may
potentially function as a specific receptor for pathogen recognition. Arthropod Dscams
are generated through mutually exclusive alternative splicing, creating a striking
hypervariability. This seems to be related to its antibody-like function in immune
defense. These molecules consist of a hypervariable extracellular region which
functions in pathogen recognition, and a cytoplasmic tail related to subsequent signal
transduction. After isolating the first Dscams from shrimp (LvDscam and PmDscam
isolated from Litopenaeus vannamei and Penaeus monodon), we found that these
Dscams shared extracellular domain architecture typical of arthropod Dscams
(9Ig-4FNIII-Ig-TM). In shrimp Dscams, variable alternatively spliced events were
produced by mutually exclusive alternative splicing. These involved, variously, the
N-terminal Ig2, N-terminal Ig3, the entire Ig7 or the transmembrane domain located in
the extracellular region as well as exon inclusion/exclusion in the cytoplasmic tail.
Given this variability, LvDscam can potentially encode over than 54,000 unique
isoforms. We demonstrated LvDscam expression after shrimp were injected with white
spot syndrome virus (WSSV) or Vibrio harveyi. After pathogenic challenge, a greater
number of LvDScam isoforms was expressed. It appears that the Ig2 and Ig3 variable
regions are the primary loci of host specific immune response against pathogens.
Intriguingly, phylogenetic analysis places LvDscam isoforms into two clades. Those
expressed after V. harveyi challenge are separate from isoforms expressed in
unchallenged shrimp. Future research may reveal possible specific V. harveyi inhibition
activity of these induced LvDscam isoforms.
Keywords: shrimp、highly specific innate immune responses、Dscam、WSSV、Vibrio
harveyi