CD4 and systemic lupus erythematosus: This observation might be due to the completely different genetic backgrounds of the mice or to the different time of lupus development in these two models, both of which could affect NKG2D+ cell numbers; however, the exact mechanisms underlying the distinctive increase of NKG2D+CD4+ T cells in B6.MRL/lpr mice but not in NZBW/F1 mice remain unclear and need to be clarified in future studies.