In an uncommon turn, the commercial mortgage backed securities sector lead structured finance ratings upgrades in 2005, according to Fitch Ratings. However, the ratings agency warned that rising interest rates could harm commercial MBS this year and residential MBS are expected to fare better. Fitch’s U.S. structured finance ratings had an upgrade-to-downgrade ratio of 3.1 to 1 in 2005, compared to 2.6 to 1 in 2004, according to a recent report by the rating agency.…