Promotion and inhibition of haemopoiesis by NK cells: a model for immune-mediated haemopoietic suppression.

Human NK cells, besides mediating cytotoxic functions, can exert a number of regulatory activities on the proliferation and maturation of hematopoietic stem cells through the release of soluble factors. A preliminary characterization of these factors demonstrates that NK cell-derived colony stimulating activities (CSA) are represented by GM-CSF and IL3, whereas the colony inhibiting activity (CIA) produced by NK cells is identifiable as TNF. CSA is mainly released by resting NK cells, while, following appropriate activation, the latter cells produce large amounts of CIA. NK cells also influence B cell activation by releasing a 50 kd BCGF and a 26 kd BCDF. The latter factor, which is active in vitro only in the presence of IL2, is identifiable as IL6. The production of regulatory cytokines by NK cells may have some bearing on the pathogenesis of a number of autoimmune disorders.