The inhibitory co-receptor programmed death 1 （PD-1, encoded by pdcdl） and its two ligands PD-L1 and PD-L2 comprise an important immune inhibitory signaling pathway for defense against microbes and for self-tolerance. Unlike other members of the B7-CD28 family, expression of PD-L1 and PD-L2 is not limited to the immune system. In this study, we determined that a polyclonal antibody （pAb） （R＆D Systems） against extracellular domains of mouse PD-L2 （mPD-L2） could recognize anti- gen（s） in diverse mouse tissues, including the anterior and intermediate pituitary gland, olfactory bulbs and olfactory epitheli- um, tongue epithelium, keratinized epithelial ceils and skin and whisker hair follicles. These findings differed from previous reports of mPD-L2 localization. Reverse transcription PCR and Western blot analyses, however, were unable to detect any mPD-L2 transcripts or proteins of the 25-kD predicted molecular weight in RNA and protein extracts, respectively, from the above tissues, suggesting that the anti-mPD-L2 pAb cross-reacts with certain novel antigen（s）. Developmental studies revealed that the earliest expression of mPD-L2-1ike antigen was in the olfactory epithelium at embryonic day 12.5 （E12.5）. At E14.5, mPD-L2-1ike antigen was present in the skin, tongue and follicles of the skin and whiskers. The distribution patterns of mPD-L2-1ike antigen remained similar from E18.5 to adulthood. The results of bioinformatic analysis and other experiments suggested neural cell adhesion molecule and hemicentin-1 as candidate proteins with cross-reactivity to the anti-mPD-L2 pAb. These results demonstrate that care is required in interpreting staining patterns generated when anti-PD-L2 pAb is used to lo-cate PD-L2-expressing cells in the central nervous system and epithelial tissues, such as the olfactory epithelium. In addition, this anti-PD-L2 pAb may be used as an alternative antibody for labeling the olfactory epithelium during embryonic develop-ment in mice.
