Abstract
Embryos of most fish develop externally and are exposed to an aquatic environment full of potential pathogens, whereas they have little or only limited ability to mount an efficient and protective response. How fish embryos survive pathogenic attacks remains poorly defined. Here we demonstrate that the maternal immunization of female zebrafish with formalin-killed Aeromonas hydrophila causes a significant increase in C3 and Bf contents in the mother, a corresponding rise in the offspring, and induces a remarkable increase in the hemolytic activities in both the mother and offspring. In addition, the embryos derived from the immunized mother are significantly more tolerant to A. hydrophila challenge than those from the unimmunized fish, and blocking C3 and Bf activities by injection of the antibodies against C3 and Bf into the embryos render them more susceptible to A. hydrophila. These results clearly show that the protection of zebrafish embryos against A. hydrophila can be achieved by the maternally-transferred immunity of the complement system operating via the alternative pathway. This appears to be the first report providing in vivo evidences for the protective role of the alternative complement components in the early embryos of zebrafish, paving the way for insights into the in vivo function of other maternally-transferred factors in fish.
Highlights
Eggs of most fish are released and fertilized externally, and the resulting embryos and larvae are exposed to an aquatic environment full of potential pathogens capable of causing various types of diseases
Our previous study showed that the key components, complement component 3 (C3) and factor B (Bf), functioning in the alternative pathway (AP), are present in the early embryos of D. rerio, and the complement system operating via the AP is largely attributable to the bacteriolytic activity in the early embryos, the contents of C3 and Bf were determined and compared between the whole body homogenates (WBHs) and egg extracts
The maternal immunization caused a marked corresponding rise in C3 and Bf levels in the egg extracts, and the fluctuation profile of the C3 and Bf contents in the egg extracts generally coincided with that in the WBHs (Fig. 1B and D). This indicated that the maternal immunization induced a significant increase of C3 and Bf levels in the mothers, which in turn resulted in a marked increase in C3 and Bf levels in their eggs
Summary
Eggs of most fish are released and fertilized externally, and the resulting embryos and larvae are exposed to an aquatic environment full of potential pathogens capable of causing various types of diseases. Immunization of parents results in a significant increase in IgM levels [6,11] and anti-protease and lysozyme activities [6] in their eggs compared to controls These transferred maternal molecules have been proposed to be involved in the early defense against pathogens in developing fish embryos and larvae. Wang et al [16] have recently demonstrated by an in vitro assay system of complement activity that the the protection of early embryos of zebrafish Danio rerio against microbial attack can be attributed to maternal complement components operating via the alternative pathway (AP) It remains unknown whether these alternative complement components function in vivo during the early developmental stages. The objectives of this study were to examine if the maternal alternative complement components are transferred from the immunized female D. rerio to offspring, and if so, to test if these components transferred affect the offspring immunity
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