Dear Yichao,
Before you embark into any big changes in your protocol, try cutting your
sections in warmer temperature. I find that kidney biopsies cut best at  -20
C or even warmer. Our protocol of collecting kidney biopsies is very close
to yours. How long is the delay from the obtaining the biopsy to freezing
it?
We collect the biopsy, put it onto saline moistened gauze, walk to the
cryostat (3-5 minutes), put the OCT onto cryostat chuck and orient the
biopsy and let freeze in the cryostat. If not cut same day the OCT with the
biopsy is detached from the chuck and stored at -80C until ready to cut and
do IF. I don't think you want to use a fixative or  you'd have validate your
IF staining protocol.
Only problem we have run into occasionally is the too long a delay before
biopsy is frozen, resulting in a dried up sections with non specific
staining. We do not have freezing artifact as long as you don't immerse the
biopsy into saline.
My two cents worth, I hope it helps... Katri

Katri Tuomala
Department of Pathology
St.Joseph's Health Care
Hamilton, Ontario, Canada