A cutter assembly for a tunnel boring machine comprising: a shaft; a cutter ring assembly rotatably mounted on the shaft; a first end retainer attached to one end of the shaft and a second end retainer attached on an opposite end of the shaft, the first end retainer having a movable piston portion with an outer surface subject to a local hydrostatic pressure; a first rotary seal group having a first portion engaging the cutter ring assembly and a second portion engaging the first end retainer; and a second rotary seal group having a first portion engaging the cutter ring assembly and a second portion engaging the second end retainer; wherein a portion of the first rotary seal group is immersed in a lubricant; and wherein the movable piston portion is configured to increase the pressure of the lubricant in response to an increase in the local hydrostatic pressure, such that the pressure differential between the external hydrostatic pressure acting on the cutter assembly and the pressure of the lubricant acting on the portion of the seal group that is immersed in lubricant remains small; wherein the first end retainer comprises: a fixed retainer having at least one pressure port therethrough that is in fluid communication with the lubricant; and the movable piston portion of the first end retainer comprises a floating retainer that slidably and sealingly engages the fixed retainer such that a gap, defined between the fixed retainer and the floating retainer, is in fluid communication with the at least one pressure port, and wherein the gap is also filled with the lubricant; characterized in that the floating retainer further comprises a through port to facilitate injecting lubricant into the gap and a plug that sealingly closes the through port. a shaft; a cutter ring assembly rotatably mounted on the shaft; a first end retainer attached to one end of the shaft and a second end retainer attached on an opposite end of the shaft, the first end retainer having a movable piston portion with an outer surface subject to a local hydrostatic pressure; a first rotary seal group having a first portion engaging the cutter ring assembly and a second portion engaging the first end retainer; and a second rotary seal group having a first portion engaging the cutter ring assembly and a second portion engaging the second end retainer; wherein a portion of the first rotary seal group is immersed in a lubricant; and wherein the movable piston portion is configured to increase the pressure of the lubricant in response to an increase in the local hydrostatic pressure, such that the pressure differential between the external hydrostatic pressure acting on the cutter assembly and the pressure of the lubricant acting on the portion of the seal group that is immersed in lubricant remains small; wherein the first end retainer comprises: a fixed retainer having at least one pressure port therethrough that is in fluid communication with the lubricant; and the movable piston portion of the first end retainer comprises a floating retainer that slidably and sealingly engages the fixed retainer such that a gap, defined between the fixed retainer and the floating retainer, is in fluid communication with the at least one pressure port, and wherein the gap is also filled with the lubricant; a fixed retainer having at least one pressure port therethrough that is in fluid communication with the lubricant; and the movable piston portion of the first end retainer comprises a floating retainer that slidably and sealingly engages the fixed retainer such that a gap, defined between the fixed retainer and the floating retainer, is in fluid communication with the at least one pressure port, and wherein the gap is also filled with the lubricant; characterized in that the floating retainer further comprises a through port to facilitate injecting lubricant into the gap and a plug that sealingly closes the through port.