Rapid titration of retroviral vectors using a β-lactamase protein fragment complementation assay

Abstract

The availability of rapid and quantitative titration assays for retroviral vectors is important, especially in the context of clinical applications. In this report, we describe a novel assay to titrate lentiviral and gamma retroviral vectors. This rapid assay is based on protein fragment complementation involving the N-terminal (Bla1) and the C-terminal (Bla2) fragments of TEM-1 β-lactamase (BLAK). The Bla1 protein fragment is incorporated in the vector's envelope during vector production. Bla1-bearing vectors are titrated on Bla2-expressing cells. Upon transduction, Bla1 and Bla2 heterodimerize and restore BLAK's enzymatic function. The enzymatic activity of BLAK is quantified by flow cytometry using the green fluorescent CCF2/AM substrate, which is converted into a blue fluorescent product. The enzymatic conversion of the CCF2/AM substrate was found to be directly related to vector entry, as a neutralizing antibody completely blocked the conversion. The titers obtained using this rapid assay correlated well with the titers measured by functional transduction assays. The whole assay can be finished within 8 h. Thus, it is considerably less time consuming compared with other transduction-based titration assays for lentiviral and gamma retroviral vectors.