  We set up a mesoscopic theory for interacting Brownian particles embedded in
a nonequilibrium environment, starting from the microscopic interacting
many-body theory. Using nonequilibrium linear response theory, we characterize
the effective dynamical interactions on the mesoscopic scale and the statistics
of the nonequilibrium environmental noise, arising upon integrating out the
fast degrees of freedom. As hallmarks of nonequilibrium, the breakdown of the
fluctuation-dissipation and action-reaction relations for Brownian degrees of
freedom are exemplified with two prototypical models for the environment,
namely, active Brownian particles and stirred colloids.
