  We formulate a topological theory in six dimensions with gauge group SO(3,3)
which reduces to gravity on a four dimensional defect if suitable boundary
conditions are chosen. In such a framework we implement the reflection
automorphism of SO(3,3) as a $\Z2$ symmetry which forbids the appearance of a
gravitational cosmological constant. Some temptative speculations are presented
also for the possible inclusion of the matter contribution at a full quantum
level.
