IL-21–Induced MHC Class II+ NK Cells Promote the Expansion of Human Uncommitted CD4+ Central Memory T Cells in a Macrophage Migration Inhibitory Factor–Dependent Manner

Abstract

NK cells are critical for innate immunity–mediated protection. The main roles of NK cells rely on their cytotoxic functions or depend on the tuning of Th1 adaptive immunity by IFN-γ. However, the precise influence of inflammatory cytokines on NK cell and CD4 T lymphocyte interactions was never investigated. In this study, we provide evidence that IL-21, a cytokine produced during chronic inflammation or infectious diseases, promotes the differentiation of a specific subset of NK cells coexpressing CD86 and HLA-DR and lacking NKp44. More importantly, IL-21–propagated HLA-DR+ NK cells produce macrophage migration inhibitory factor and provide costimulatory signaling during naive CD4+ T cell priming inducing the differentiation of uncommitted central memory T cells. Central memory T cells expanded in the presence of HLA-DR+ NK cells are CXCR3+CCR6−CCR4−CXCR5− and produce IL-2, as well as low levels of TNF-α. Costimulation of CD4+ T cells by HLA-DR+ NK cells prevents the acquisition of effector memory phenotype induced by IL-2. Moreover, we identified this population of NK HLA-DR+ macrophage migration inhibitory factor+ cells in inflammatory human appendix. Collectively, these results demonstrate a novel function for IL-21 in tuning NK and CD4+ T cell interactions promoting a specific expansion of central memory lymphocytes.