In what sense are angels elect?

In 1 Timothy 5:21 we read: >I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favoritism. *1 Timothy 5:21 (NIV)* My understanding is that a Reformed Protestant would use the word "elect" to describe Christians predestined by God to be saved from their sin, and brought into His kingdom . I'm not aware of any biblical evidence that angels have gone through that process, so it seems odd for Paul to use that word here. **How would a Reformed Protestant understand Paul describing angels as elect?**