In the Release-8/9 of 3GPP, X2 interface between Small Cells and between Small Cells and Macro cells was not available. In Release-10 and 11, this was made available as shown in the picture above.
X2 is not only useful for lossless handovers in LTE/LTE-A but is also very useful for Interference management using eICIC (enhanced Inter Cell Interference Management) feature introduced in Rel-10.
More details on this enhancement is available in 3GPP TS 36.300 here.