Here's an interesting recent paper from Lim, Vesely, and George on the use of rituximab in preventing relapse of TTP. Bear in mind this drug is not approved by the US FDA for TTP treatment (it's actually approved for use with non-Hodgkin lymphomas).

Anyway here's the link to the full paper at the American Society of Hematology:-
http://www.bloodjournal.org/content/125/10/1526.long

ASH journal paper