We have investigated the expression of a calcium-binding protein, the S100
protein, in Eimeria brunetti and Eimeria acervulina stages. For this purpos
e, paraffin sections of distal ileum and bursa of Fabricius or duodenum fro
m experimentally infected chickens were treated with anti-alpha-S100 (anti-
alpha subunit of S100 protein) and anti-beta-S100 (anti-beta subunit of S10
0 protein) monoclonal antibodies and anti-S100 whole molecule polyclonal an
tibody. The avidin-biotin peroxidase method was used to demonstrate immunor
eactivity. In the ileum, our results reveal a positive immunoreaction for t
he beta subunit and S100 whole molecule within the macrogametes of E. brune
tti, whereas they were devoid of immunostaining after treatment of the para
ffin sections with the anti-alpha-S100 antiserum. Schizonts and oocysts of
E. brunetti and all the E. acervulina stages gave a negative reaction after
treatment with any of the three antiserum used in the study. This result i
ndicated that the S100 protein molecules within these stages were not recog
nized by the antibodies, suggesting that these molecules are different from
those identified in macrogametes of E. brunetti. By contrast, in the epith
elial cells, lining the lumen of the bursa of Fabricius, macrogametes of E.
brunetti were stained by the three antibodies used. These results may indi
cate the existence of metabolic adaptations that enable the parasite to inv
ade tissue sites different from those where the parasite usually develops.
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