There's a developer beta of the latest version of Mac OS X 10.11 El Capitan (Did they really call it that?)
Before you read any further please note WE DO NOT ADVISE YOU INSTALLING ANY BETA SOFTWARE ON A WORKING PRO TOOLS SYSTEM
Anyway, some brave soul has indeed installed it and tested it with Pro Tools 11.2.1 and it seems to be running without any issue.
Pro Tools opens and records and plays back as expected.
Remember Avid only support qualified systems so of course always wait for Avid to qualify an OS before updating.