  Ab initio QED calculations of the nuclear magnetic shielding constant in
helium-like ions are presented. We combine the nonrelativistic QED approach
based on an expansion in powers of the fine-structure constant $\alpha$ and the
so-called ``all-order'' QED approach which includes all orders in the parameter
$Z\alpha$ but uses a perturbation expansion in the parameter $1/Z$ (where $Z$
is the nuclear charge number). The combination of the two complementary methods
makes our treatment applicable both to low-$Z$ and high-$Z$ ions. Our
calculations confirm the presence of a rare antiscreening effect for the
relativistic shielding correction and demonstrate the importance of the
inclusion of the negative-energy part of the Dirac spectrum.
