ABSTRACT. A pig interleukin-21 (IL-21) cDNA was successfully cloned and sequenced from porcine peripheral blood lymphocytes (PBL) stimulated with 10 碌g/ml concanavalin A (ConA), 10 碌g/ml phytohemagglutinin P (PHA), 50 ng/ml phorbol 12-myristate 13-acetate (PMA), and 0.5 碌g/ml anti-porcine CD3 antibody for 48 hr. The open reading frame of the porcine IL-21 cDNA is 459 base pairs in length and encodes 152 amino acids. The predicted amino acid sequence of the porcine IL-21 shows 86.2%, 77.7%, and 58.4% identi ty to the bovine, human, and murine IL-21, respectively. The porcine IL-21 gene was mapped to porcine chromosome 8 (8q22鈫抭23) by means of fluorescence in situ hybridization and radiation hybrid mapping, where the porcine IL-2 gene had been mapped nearby. The recombinant porcine mature IL-21 expressed by E. coli induced dose-dependent proliferation and IFN-纬 production from a human NK cell line, NK0. The porcine IL-21 identified in this study will be helpful for the enhancement of innate immune responses of pigs. KEY WORDS: cloning, innate immunity, interleukin-21, NK cell, porcine.J. Vet. Med. Sci. 66(3): 269-275, 2004 Interleukin-21 (IL-21) is a novel cytokine that regulates the proliferation of T and B cells, and involves the maturation and expansion of NK cells from bone marrow progenitor cells [20]. IL-21 is produced by activated T cells [20,21], and is a four-helix-bundle type I cytokine with structural similarity to IL-2, 21]. IL-21 receptor also has significant amino acid similarities with IL-2, , and shares the common 纬 chain for the subunit of the IL-21 receptor complex [2]. The common 纬 chain-containing receptor binded by its ligand such as IL-2 and IL-21 causes the activation of the Janus tyrosine kinase family 3 (JAK3) [6,17], and transduce its signal to the downstream molecules. Recent studies have indicated that IL-21 has various biological activities that affect T and NK cell functions [20,21] . Currently, IL-21 is thought to be a mediator cytokine, which promotes the transition from innate to adaptive immunity [9,21].One of the most interesting characteristics of IL-21 is its effect on NK cell maturation and expansion [20], because pigs have high NK cell populations in the peripheral blood [31], and many piglets suffer from diarrhea and respiratory diseases in their neonatal periods during which adaptive immunity is immature. We previously reported the cloning and expression of porcine , and the IFN-纬 induction by IL-18 from neonatal piglets [13]. Recently, Strengell et al. reported that IL-21 also enhances IFN-纬 production from human NK and T cells in synergy with . Taken, together, we would like to isolate porcine IL-21, a recent cytokine that promotes the transition from innate to adaptive immunity [9,21], in order to utilize this cytokine for the enhancement of neonatal immunity of pigs. We also reported the cloning, and the expression of bovine IL-21 using a baculovirus expression system [15].In the present study, cDNA encoding porcine IL-21 was cloned and characterized, and the chromos...