Caldwell, the leader of Toronto's defence, was made available to Orlando City SC and New York City FC along with fullback Mark Bloom, Brazilian midfielder Jackson and Ghanaian midfielder-forward Dominic Oduro.
MLS teams were allowed to protect 11 players, with Homegrown players also exempt.
Orlando and New York City will each select 10 players from the 18 other MLS clubs.
A club can lose no more than two players in the draft. If a team has a player claimed, it can protect an additional player.
Toronto is no doubt hoping that the age and salary of Caldwell will deter the two expansion teams. The 34-year-old Scot's salary was listed at US$364,166 last season.
But the protected lists do not always tell the whole story. Often deals are made with expansion teams to return talent.
Veteran Canadian international Dwayne De Rosario, whose 2015 option was not picked up, is also available on Toronto's unprotected list. Bloom, a bargain starter at $48,825, may get some attention Wednesday along with promising young TFC winger Daniel Lovitz.
Montreal didn't protect Canadian international midfielder Issey Nakajima-Farran, who criticized the team for being too predictable in post-season remarks, and defender Heath Pearce, who started 20 regular-season games in 2014.
The Impact have said they would not sign 35-year-old defender Matteo Ferrari and 33-year-old goalie Troy Perkins, who are both unprotected.
Montreal did protect 35-year-old Canadian midfielder Patrice Bernier, who is still in contract talks with the club.
The Whitecaps' unprotected list includes Mamadou Diouf, Sebastian Fernandez, Gabriel Mezquida, Carlyle Mitchell, Andy O'Brien, and Mauro Rosales.
The New York Red Bulls left unprotected designated players Tim Cahill and Thierry Henry, the French star who said recently he was leaving MLS. The 35-year-old Cahill made $3.65 million last season.
Portland made veteran goalkeeper Donovan Ricketts ($260,000) available and Seattle opted not to protect veteran striker Kenny Cooper ($265,625). New England has dangled 19-year-old Uruguayan forward Diego Fagundez.
D.C. United made available Canadian international defender Nana Attakora and midfielder Kyle Porter, as well as former Montreal Impact midfielder Davy Arnaud.
With files from Bill Beacon in Montreal
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