Isolation and characterization of a cDNA encoding a putative cytokine which is induced by stimulation via the CD2 structure on human T lymphocytes

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Abstract
To define activation‐specific sequences in human T lymphocytes, a cDNA library was constructed by subtractive hybridization using resting and stimulated peripheral blood lymphocyte pairs. Stimulation of peripheral blood lymphocytes was achieved by triggering with mitogenic anti‐CD2 (T11) monoclonal antibodies. Differential library screening with cDNA probes derived from stimulated vs. resting peripheral blood lymphocytes led to identification of a novel sequence, termed HC21. The predicted primary and secondary structure of HC21 deduced from the translated nucleotide sequence suggest that the gene encodes a secreted protein of 92 amino acids including a 23‐residue leader sequence. Northern blot analysis demonstrates that HC21 mRNA is induced in peripheral blood T lymphocytes and interleukin 2‐dependent T cell clones within 10 min following stimulation via CD2, while steady‐state RNA expression is maximal by 2 h. Although the kinetics of HC21 expression are similar after stimulation via the antigen/major histocompatibility complex receptor (CD3‐Ti), CD3‐Ti triggering leads to less accumulation of HC21 RNA than CD2 triggering. In contrast, recombinant interleukin 2 does not induce detectable HC21 RNA expression in T cell clones. Transient expression of the HC21 cDNA in COS cells results in a readily detectable protein band in sodium dodecyl sulfate‐polyacrylamide gel electrophoresis analysis of radiolabeled cell supernatants consistent with HC21 encoding a secreted product. Protein sequence analysis reveals a striking homology (up to 76% identity at amino acid level) with other members of a recently described lymphokine family whose functions are yet to be defined.

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