A novel polypeptide secreted by activated human T lymphocytes.

We have identified two cDNA clones, I-309 and G-26, which define genes expressed abundantly in activated human PBMC, but at low or undetectable levels in resting PBMC. Based upon nucleotide sequence analysis, both clones are predicted to encode small, structurally related polypeptides, each containing a hydrophobic leader sequence characteristic of secreted proteins and a motif of four conserved cysteine residues. Further, I-309 and G-26 are structurally related to a growing family of genes that apparently encode small polypeptides whose secretion is induced upon cell activation. I-309 represents a previously undescribed human gene. We have generated an anti-peptide antiserum to the I-309 gene product which recognizes proteins in culture supernatants of an activated T cell clone and of COS cells transfected with the I-309 cDNA, supporting the idea that I-309 encodes a secreted protein. Because I-309 encodes a small protein secreted by activated T cells that displays structural features similar to other cytokines, we believe that it defines a novel cytokine with as yet unknown function.