The immunoglobulin (Ig) domain exon of human NKp30 contains internal splice sites. A unique intraexonic splicing mechanism produces a short Ig domain that lacks 25 residues but maintains the Ig-fold structure and is expressed on the cell surface. Unlike full-length NKp30, the short version (NKp30-S) does not bind the cancer cell ligand B7H6.
ABSTRACT
The activating receptor NKp30 is important in NK cell killing of cancer cells. Here, we demonstrated that a pair of splice signals in the Ig domain exon of human NKp30 is largely conserved among primates and placental mammals and produces an alternatively spliced NKp30 ectodomain (NKp30-S) with an in-frame, nonartefactual deletion of 25 amino acid residues. Transfection yielded NKp30-S bands in NKL cells but not in 293T cells, suggesting that the splicing mechanism is cell-specific. Molecular modeling indicated that the overall folding of NKp30-S is maintained compared with NKp30. NKp30-S was expressed at the cell surface but did not bind to the NKp30 ligand B7H6 in soluble fusion protein or reporter cell assays. Single-cell RNA sequencing showed that NKp30-S transcription was not restricted to major NK cell subsets but suggested that a small fraction of blood NK cells expressed only the NKp30-S splice variant. The ratio of NKp30-S to full-length transcript was increased after activation of blood NK cells with IL-2 or IL-15 or crosslinking with anti-CD16 antibody, suggesting that this unique splicing mechanism, not seen in other Ig superfamily proteins, is regulated and may play a role in modulating NK cell responsiveness toward B7H6+ cancer cells in vivo.