  Enforcing the spacetime of supermassive cosmic strings to satisfy the
symmetry of a gravitational topological defect, that is a spacetime kink, it is
shown that the energy of these strings becomes quantized so that only defects
whose internal geometry is that of a hemisphere (critical string) and that of a
sphere (extreme string) are allowed. In the latter case, the exterior conical
singularity becomes an event-horizon singularity which can be removed by using
the Kruskal technique in the kink metric. It is also shown that the
geodesically complete metric of the extreme string can accommodate closed
timelike curves which are compatible with quantum theory, but cannot be noticed
by any observers. The quantum string can also drive an essentially unique
process of inflationary expansion in its core, without any fine tuning of the
initial conditions.
