Can low doses of antideppressants aleviate the pain of interstitial nephritis?

It does't cause pain: Interstial nephritis is usually painless disease. The kidneys do not hhurt unless there are kidney stones, or any other disease that can distend the kidney capsule substantially as pyelonephritis, substantial hemorrhage into the kidney tissue, trauma of the kidney and few more rare.

possibly, yes: Although antidepressants have no direct effect on pain ( well, it can be argued - Cymbalta has indication for neuropathic pain, Effexor (venlafaxine) molecule looks like tramadol; Elavil is been used for decades) - they can decrease "perception of pain".