Alpaca (Vicugna pacos), the first nonprimate species with a phosphoantigen-reactive Vγ9Vδ2 T cell subset.

Vγ9Vδ2 T cells are a major γδ T cell population in the human blood expressing a characteristic Vγ9JP rearrangement paired with Vδ2. This cell subset is activated in a TCR-dependent and MHC-unrestricted fashion by so-called phosphoantigens (PAgs). PAgs can be microbial [(E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate, HMBPP] or endogenous (isopentenyl pyrophosphate, IPP) and PAg sensing depends on the expression of B7-like butyrophilin (BTN3A, CD277) molecules. IPP increases in some transformed or aminobisphosphonate-treated cells, rendering those cells a target for Vγ9Vδ2 T cells in immunotherapy. Yet, functional Vγ9Vδ2 T cells have only been described in humans and higher primates. Using a genome-based study, we showed in silico translatable genes encoding Vγ9, Vδ2, and BTN3 in a few nonprimate mammalian species. Here, with the help of new monoclonal antibodies, we directly identified a T cell population in the alpaca (Vicugna pacos), which responds to PAgs in a BTN3-dependent fashion and shows typical TRGV9- and TRDV2-like rearrangements. T cell receptor (TCR) transductants and BTN3-deficient human 293T cells reconstituted with alpaca or human BTN3 or alpaca/human BTN3 chimeras showed that alpaca Vγ9Vδ2 TCRs recognize PAg in the context of human and alpaca BTN3. Furthermore, alpaca BTN3 mediates PAg recognition much better than human BTN3A1 alone and this improved functionality mapped to the transmembrane/cytoplasmic part of alpaca BTN3. In summary, we found remarkable similarities but also instructive differences of PAg-recognition by human and alpaca, which help in better understanding the molecular mechanisms controlling the activation of this prominent population of γδ T cells.