Vitellogenin (Vg), the precursor of yolk proteins, was traditionally regarded as the energy reserve for nourishment of the developing embryos. However, its roles to extend beyond the nutrient function. Here we discuss recent developments in the understanding of Vg. Accumulating data have demonstrated that Vg fulfils important roles in innate immune responses. It acts as a multivalent pattern recognition receptor capable of binding to lipopolysaccharide, lipoteichoic acid, peptidolycan, glucan and virons. It is also a bactericidal molecule capable of damaging bacterial cell walls. Moreover, it is an acute phase protein with bacterial-binding and inhibiting activities, and possibly functions in the immune responses of host in vivo. Further understanding of Vg and its derived yolk proteins should provide new insights into the mechanisms of host defense, and reveal if they can be used as alternative strategies promoting the immunity of cultured fish as well as developing embryos.
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