Positive Quaternionic Kaehler manifolds and symmetry rank: II
  Let M be a positive quaternionic Kaehler manifold of dimension 4m. If the isometry group Isom(M) has rank at least m/2 +3, then M is isometric to HP^m or Gr_2(C^{m+2}). The lower bound for the rank is optimal if m is even. 
Fuquan Fang