Converting an Anti-Mouse CD4 Monoclonal Antibody into an scFv Positron Emission Tomography Imaging Agent for Longitudinal Monitoring of CD4+ T Cells.

Immuno-positron emission tomography (PET), a noninvasive imaging modality, can provide a dynamic approach for longitudinal assessment of cell populations of interest. Transformation of mAbs into single-chain variable fragment (scFv)-based PET imaging agents would allow noninvasive tracking in vivo of a wide range of possible targets. We used sortase-mediated enzymatic labeling in combination with PEGylation to develop an anti-mouse CD4 scFv-based PET imaging agent constructed from an anti-mouse CD4 mAb. This anti-CD4 scFv can monitor the in vivo distribution of CD4+ T cells by immuno-PET. We tracked CD4+ and CD8+ T cells in wild-type mice, in immunodeficient recipients reconstituted with monoclonal populations of OT-II and OT-I T cells, and in a B16 melanoma model. Anti-CD4 and -CD8 immuno-PET showed that the persistence of both CD4+ and CD8+ T cells transferred into immunodeficient mice improved when recipients were immunized with OVA in CFA. In tumor-bearing animals, infiltration of both CD4+ and CD8+ T cells increased as the tumor grew. The approach described in this study should be readily applicable to convert clinically useful Abs into the corresponding scFv PET imaging agents.