Comic Book Movie brings us the following:
Sebastian Stan says if you freeze the rescue scene in "Captain America: The First Avenger" there's a set piece that gives a clue to how Bucky survives to wreak havoc upon Captain America in present-day America. Also, Anthony Mackie teases an epic beatdown Cap puts on Frank Grillo's Brock Rumlow.
Sebastian Stan says there are clues in the "set pieces" of "Captain America: The First Avenger" that are responsible for his return as the Winter Solider in the sequel. He says to "freeze the rescue scene" (when Captain America rescues Bucky who's in the middle of being experimented upon by Arnim Zola) but he warns it's "very subtle."Having not had a chance to verify this, I'm going to speculate that it's going to be Zola injecting Bucky with a version of the serum used on Schmidt/Red Skull. Since we know that it's the Super Soldier Serum that helped Cap survive being frozen at sea, it's only logical that something similar happened with Bucky when he froze in the river.
The film is scheduled to hit theaters on 04 April 2014.