A full-length cDNA encoding chicken tumor necrosis factor superfamily 15 (TNFSF15) was isolated and its functional role was investigated. TNFSF15 transcripts were primarily expressed in spleen, liver, intestinal intraepithelial lymphocytes (IEL), peripheral blood lymphocytes and bursa. In vitro infection of HTC macrophages with three species of Eimeria sporozoites induced TNFSF15 gene expression. In vivo experiments revealed that TNFSF15 gene was highly increased following primary infections with Eimeria acervulina or Eimeria maxima. In contrast, no consistent changes in transcript levels were seen following primary infection with Eimeria tenella, or following secondary infection with any of the three Eimeria species. Following infection with E. acervulina and E. maxima, TNFSF15 transcripts were primarily expressed in intestinal CD4(+) and TCR2(+) IEL, respectively. A dose-dependent cytotoxic effect of recombinant TNFSF15 protein was observed on HTC and LSCC-RP9 tumor cells. These results indicate that TNFSF15 plays an important role in local inflammatory response to Eimeria.