A simple procedure to determine the biological titer of recombinant retroviral vectors

Retroviral vectors are widely used to deliver genetic material to live cells both in experimental and clinical settings. The ability of these vectors to transduce target cells is an important aspect of their clinical applicability and one of the factors determining their transduction efficiency is vector functional titer. Current methods for titrating retroviral vectors involve measuring the number of target cells in culture transduced by a given volume of vector solution. In this report, we describe a new procedure which allows one to estimate the actual number of infectious particles capable of transducing a permissive cell type. Vector biological titer is calculated from the fractional decline in transduction efficiency observed when a given volume of vector solution is sequentially added to multiple dishes containing permissive cells. Values determined this way are greater than those obtained from a single transduction experiment, with the difference being inversely proportional to the degree of cell permissiveness for vector entry. The present procedure is simple, reliable and expeditious. It will be useful to standardize vector biological titers determined in different laboratories, and help implement strategies for efficient gene delivery protocols.