Caribou to the best of my knowledge are thriving, and not included in bans. The only issue I can see, would be in the sale, purchase of these antlers, internationally. Even within ones home country, there are legal ramifications in the sale/purchase of "animal parts" If you harvested these antlers, or bought them, as long as you have the tags/permit for the animal, your fine.
As a knife handle, be aware, unstabilized antler, will move more than say buffalo horn.
There are a couple ways to use it, depending if you want the natural outside rough texture, or if you intend to shape it like buffalo horn. I would think you intend to shape it and blend it smoothly with the rest of the handle. If the animal fed well, the outer dense clear bone, will be thicker, if it fought through a hard winter/spring, it might be fairly thin. Cut a portion slightly larger than your finished handle, remove as much of the inner hollow material as you can, fill the voids with epoxy, or use CA to help "stabilize it" You should be ok. If the outer bone is really thin, you may have issues.
Can I throw in a question about stag while we are at it? I have a few nice rounds that seem very dense, hardly any inner hollows. Do they behave about the same, should they be treated with anyhing? Intuitively, I would say they seem very dense and stable, but what do I know...
Stag is one of the best to use, most good quality rounds have a nice thick, dense, outerlayer, that has very little movement. It is most sought for the "popcorn" finish some higher quality pieces have for bowies, and fighters, as well as some hunters. You should have no, or very little issues.