Analysis of NK cell clones obtained using interleukin-2 and gene-modified K562 cells revealed the ability of “senescent” NK cells to lose CD57 expression and start expressing NKG2A

In this work, we analyzed the phenotype and growth of human NK cell clones obtained by the stimulation of individual NK cells with IL-2 and gene-modified K562 feeder cells expressing membrane-bound IL-21 (K562-mbIL21). We generated clones from NK cells at distinct differentiation and activation stages, determined by CD56, CD57 and HLA-DR expression levels. Less differentiated CD56(bright) NK cell subsets showed higher cloning efficiency compared with more differentiated CD56(dim) subsets, especially with the CD57(bright) subset. However, clones from the CD56(dim)CD57(–) subset lived longer on average than other subsets. Moreover, several clones with the highest cell numbers were derived from CD56(dim)CD57(–)HLA-DR(−)cells. Most of the clones including those derived from more differentiated CD56(dim)CD57(+/–)NKG2A(–) NK cells showed a less-differentiated NKG2A(+) phenotype. Also, CD57(–) cells were frequently observed in clones derived from CD57(+) NK cells suggesting the loss of CD57 during the cloning process. On the other hand, KIR surface expression once detected for a clone never disappeared entirely, confirming irreversibility of the KIR expression. In summary, we have demonstrated that in specific conditions terminally differentiated CD57(+) human NK cells are able to acquire the CD57(–) phenotype that was previously not observed and, thus, was considered impossible.