Production of natural killer cell activity-augmenting factor (interleukin-6) by human epiphyseal chondrocytes

We sought to determine the capacity of human epiphyseal chondrocytes to modulate the cytotoxic activity of human natural killer (NK) cells by determining whether they release interleukin-6 (IL-6), a cytokine recently shown to stimulate NK cell activity.

Conditioned medium from human epiphyseal chondrocyte cultures (Ch-CM) was tested for IL-6 activity using the B9 cell hybridoma assay. Its NK cell-stimulating capacity in the presence of K562 (myelogenous leukemia) cells or human chondrocytes was evaluated in a 4-hour 51Cr-release assay. Ch-CM-derived IL-6/NK cell-augmenting factor activity was partially purified by high-performance liquid chromatography (HPLC) gel filtration and Western blot.

Ch-CM contained an NK cell-augmenting factor (NKAF) which was blocked by IL-2 or IL-6 antibodies. Ch-CM did not contain detectable IL-2 activity, but it stimulated IL-2 production by human peripheral blood lymphocytes (PBL). This IL-2-inducing capacity was inhibited by IL-6 antibodies, indicating that chondrocytes release an IL-6-like activity. Ch-CM significantly enhanced the proliferation of IL-6-dependent B9 hybridoma cells, and Western blot analysis of Ch-CM revealed specific bands corresponding to those of highly purified IL-6. Upon HPLC gel filtration, chondrocyte NKAF copurified with chondrocyte IL-6. Pure IL-6 and chondrocyte IL-6 were tested for their ability to stimulate the cytotoxic activity of human PBL against chondrocytes. Both mediators significantly enhanced chondrocyte killing. Lysis of chondrocytes by PBL was mediated by NK cells, since depletion of CD16+ cells resulted in inhibition of the activity.

Thus, upon stimulation, chondrocytes produce IL-6 which, through IL-2 induction, augments the activity of NK cells against K562 target cells as well as against chondrocytes.