Sialyl lewis x antigen‐expressing human CD4 + T and CD8 + T cells as initial immune responders in memory phenotype subsets

Cytokine production by memory T cells in secondary immune responses has a critical role in host defenses. Previously, we had demonstrated that a unique antigen composed of sialyl lewis x (sLe x ) was expressed on CD45RO + memory‐phenotype subsets of human T cells. Here, we found that the sLe x antigen was up‐regulated on CD45RA + naïve human CD4 + T and CD8 + T cells by TCR stimulation. In addition, sLe x antigen‐expressing CD4 + T and CD8 + T cells in human PBMCs were activated immediately by cytokine stimulations composed of IL‐2 plus IL‐12 or IL‐15 in an antigen‐independent manner. Moreover, the sLe x ‐positive human CD8 + T cells significantly enhanced reverse antibody‐dependent cellular cytotoxicity compared with a sLe x ‐negative population. These findings clearly indicate that sLe x antigen‐expressing memory phenotype CD4 + T and CD8 + T cells contribute to early‐stage immunity by providing a source of IFN‐γ and cytotoxicity, suggesting that they would be a key immunomodulator in host defenses.