In vivo application of mAb directed against the γδ TCR does not deplete but generates invisible γδ T cells

mAb targeting the gamma delta TCR have been used for gamma delta T-cell depletion with varying success. Although the depletion-capacity of the anti-gamma delta TCR mAb clone GL3 has been disputed repeatedly, many groups continue to use gamma delta T-cell depletion protocols involving the mAb clone UC7-13D5 and find significant biological effects. We show here that treatment with both GL3 and UC7-13D5 antibodies does not deplete gamma delta T cells in vivo, but rather leads to TCR internalization and thereby generates invisible gamma delta T cells. We addressed this issue using anti-gamma delta TCR mAb injections into WT mice as well as into reporter TCR delta locus-histone 2B enhanced GFP knock-in mice, in which gamma delta T cells can be detected based on an intrinsic green fluorescence. Importantly, the use of TCR delta locus-histone 2B enhanced GFP mice provided here for the first time direct evidence that the depleted gamma delta T cells were actually still present. Our results show further that GL3 and UC7-13D5 mAb are in part cross-competing for the same epitope. Assessed by activation markers, we observed in vitro and in vivo activation of gamma delta T cells through mAb. We conclude that gamma delta T-cell depletion experiments must be evaluated with caution and discuss the implications for future studies on the physiological functions of gamma delta T cells.