I just read a really interesting article about this. Basically, it said that while CD-57 can be helpful. it is not always meaningful. The doctor said that on average, her lyme patients had lower CD-57's, but that some of the sickest ones had really high levels.
She also said that the levels naturally vary by a large amount, and so it can be hard to tell what the "actual" level is.
So, a decreasing CD-57 could be a sign of increasing illness, or it might be random.
I'd love to give you a reference, or link to the article, but I'm having trouble finding it right now. If I find it, I'll post it.