In the late 70's MacDowell and Mansouri introduced a new approach to
gravity based on broken symmetry of the gauge theory. Recent interest in
his formalism has been renewed in the form of constrained topological BF
theory. The equations of motion of this theory turn out to be the vacuum
Einstein equations. By solving the B field equations one finds that the
action of this theory contains not only the standard Einstein-Hilbert
action with cosmological constant term, but also the Holst term
proportional to the inverse of the Immirzi parameter, as well as a
combination of topological invariants (Euler, Pontryagin, and Nieh-Yan).
This model easily provides supersymmetric extension of the theory and
offers other interesting features.