Defensins are a family of cysteine-rich vertebrate antimicrobial peptides
which can be divided into two main families, a-defensins, found only in mammals,
and {J-defensins, identified in all vertebrate species. In chickens, only {J-defensins
have been reported, ana are now named avian {J-defensins (AvBD). The project aim
was to compare AvBDs mRNA levels in different chicken lines considered as
susceptible and resistant to different Salmonella serotypes. Indeed, a previous study
showed that a-defensins expressed in Paneth cells were inhibited ~fter Salmonella
infection in mice suggesting that the inhibition defensins is a virulence strategy of the
intestine pathogen. Therefore, we wished to test the hypothesis that the susceptibility
of chicken lines to Salmonella infection correlated with decreased AvBD. transcript
levels, as previously shown in mice.
To date, thirteen AvBDs have been already described in the literature. Here l
describe a novel avian {J-defensin, named AvBD14. The AvBD14 has two exons and
one intron and is only expressed at the mRNA level in the skin and spleen. I also
propose that two {j-defensins, originally described as gallinacin 1 and 1ex which differ
by only three amino acids due to three nucleotide substitutions, actually represent
polymorphic variants ofthe same gene, named AvBDl.
Because of their differential expression profiles as previously reported,
AvBDs 1 and 2, originally isolated from heterophils, and AvBDs 3, 4 and 5,
previously described as peptides expressed by epithelial tissues, were chosen to study
their expression in a variety of in vitro and in vivo systems. In order to develop new
bioreagents, I attempted to express AvBDs using the Baculovirus system.
Unfortunately, the specific physicochemical characteristics of these antimicrobial
peptides made them difficult to purify and they were, therefore, examined by
measuring their mRNA expression levels.
In this study, inbred chicken lines 61 and N, previously characterized both for
their resistance to systemfc Salmonella disease and their levels of Salmonella
colonization, were selected to analyze the expression ofthe AvBD panel chosen. Line
61 and line N chickens are resistant and susceptible to Salmonella serovar
Typhimurium colonisation respectively and, interestingly, mRNAs for AvBDs 2, 3
and 5 were undetectable at 7 dpi in the caecal tonsil of line N chickens infected with
S. Typhimurium. AvBDl mRNA expression was also down-regulated soon after
infection suggesting thatline N susceptibility is a deficiency in innate immunity.
In addition, the differential responses of inbred lines to Salmonella serovars
indicate the involvement of a common mechanism of resistance. For this purpose,
AvBD expression was also analysed in the resistant line, line 61, and the susceptible
line, line 72, following infection with different Salmonella serovars. In resistant. and
susceptible chickens infected with host-specific or broad host range Salmonella
serovars, mRNA level ofAvBDs was differentially expressed, but not inhibited.
In conclusion, we have demonstrated .that the level of expression of AvBDs
did not detennine the resistance or susceptibility pattern of chicken line 61 and 72 to systemic salmonellosis. However, the expression of several of AvBDs may regulate
the resistance of chicken line 61to S.Typhimurium colonisation.