Can I combine DefenAge and Retin-a?

You can layer. Apply the retin-a as usual and then apply the DefenAge serum or products. The serum "seals" in the other products, so they are last applied (except sunscreen or makeup)

It's a novel product development and thus far we've have consistent positive feedback.

The short answer is yes. But in truth you may not need to use both. 
A 2018 study compared the action of Defensin peptides which are the active ingredient in Defenage  against retinoic acid.  This study showed that Defenage rejuvenates facial skin similar to cosmetic retinoic acid, without the irritation.

Hello,
It is okay to combine DefenAge and Retin-A!  You can apply Retin-A a couple minutes after applying the BioSerum (try to wait for the serum to dry).
Happy Skin!
Dr. Berens