Unlocking weapon slots in Borderlands 2

I have two "locked" weapon slots. How can I unlock them?

Are the accessory slots locked too? Do they have some sort of level requirement?

Solution 1:

They're obtained in parts of the main story; one in the early Sanctuary missions and one is unlocked :

when you get back to Sactuary after it takes off. If I remember correctly.

The Relic/Class Mod/Grenade Mod/Shield slots are only locked during the tutorial, they're unlocked extremely early. You never have to do anything but complete story missions to unlock the slots.

Solution 2:

You get your first weapon slot upgrade upon finishing the mission where you get the message from Roland in Sanctuary.