Aim:To investigate the expression and regulation of PD-I ligand 1(PD-L1)inperipheral blood mononuclear cells(PBMC).Methods:The cDNA encoding hu-man PD-L1 precursor was cloned from the total RNA extracted from the restingand phorbol dibutyrate plus ionomycin-or phytohemagglutinin-activated PBMC,by reverse transcription polymerase chain reaction(RT-PCR),and independentclones were sequenced and analyzed.The expression and subcellular localizationwere examined in transiently transfected cells.The PD-L1 gene expression indifferent PBMC was also analyzed by RT-PCR.Results:A novel human PD-LIsplice variant was identified from the activated PBMC.It was generated by splic-ing out exon 2 encoding an immunoglobulin variable domain(Igv)-like domain butretaining all other exons without a frame-shift.Consequently,the putative trans-lated protein contained all other domains including the transmembrane regionexcept for the Igv-like domain.Furthermore,the conventional isoform was ex-pressed on the plasma surface whereas the novel isoform showed a pattern ofintracellular membrane distribution in transiently transfected K562 cells.In addition,the expression pattern of the PD-L1 splice variant was variable in different indi-viduals and in different cellular status.Conclusion:PD-L1 expression may beregulated at the posttranscriptional level through alternative splicing,and modu-lation of the PD-L1 isoform expression may influence the outcome of specificimmune responses in the peripheral tissues. Aim: To investigate the expression and regulation of PD-I ligand 1 (PD-L1) inperipheral blood mononuclear cells (PBMC). Methods: The cDNA encoding hu-man PD-L1 precursor was cloned from the total RNA extracted from the resting and phorbol dibutyrate plus ionomycin-or phytohemagglutinin-activated PBMC, by reverse transcription polymerase chain reaction (RT-PCR), and independent clones were sequenced and analyzed. The expression and subcellular localization were examined in transiently transfected cells. The PD-L1 gene expression indifferent PBMC was also analyzed by RT-PCR. Results: A novel human PD-LIsplice variant was identified from the activated PBMC. It was generated by splicing out exon 2 encoding an immunoglobulin variable domain (Igv) -like domain but not all other exons without a frame -shift.Consequently, the putative trans-lated protein contained all other domains including the transmembrane regionexcept for the Igv-like domain.Furthermore, the conventional isoform was ex-pressed on the plasm In addition, the expression pattern of the PD-L1 splice variant was variable in different indi-viduals and in different cellular status. Conlusion: PD-L1 expression may beregulated at the posttranscriptional level through alternative splicing, and modu-lation of the PD-L1 isoform expression may influence the outcome of specificimmune responses in the peripheral tissues.