Infinity Ward has just released MW3 Collection 1 DLC drops 1-6 for the PC on Steam. MW3 Collection 1 contains 4 new multiplayer maps and 2 new Spec Ops missions.
Also, Infected and FFA Gunplay have been added in the Community playlists.
MP Maps: Liberation, Piazza, Overwatch, Black Box
Spec Ops Missions: Black Ice, Negotiator
Visit our DLC page for more details on these drops.
UPDATE: The patch that went live with support for Collection 1 for PC, brought prestige levels 16-20. These new levels will be added to the consoles soon. Prestige Emblems for 16-20:
SOURCE: Infinity Ward