Lactoferrin‐inducible monocyte cytotoxicity for K562 cells and decay of natural killer lymphocyte cytotoxicity

SUMMARY Monocyte‐enrichcd and lymphocyte‐enriched fractions of peripheral blood from three healthy volunteers were obtained by percoll density gradient centrifugation. The cytotoxie activity of each fraction against 51Cr‐labellcd K562 cells was quantified in a 2‐h assay using freshly isolated cells of each fraction and cells of each fraction which had been incubated with and without lactoferrin in complete medium for 18 h before performing the assay. We have thereby shown that cytotoxicity was not demonstrable in the lymphocyte fraction (containing 7.3 ± 2% large granular lymphocytes) after 18 h in medium, whereas the cytotoxicity of the monocyte fraction (containing 3.0 ± 4% large granular lymphocytes) was still significantly increased (P≤ 0.01) and that lactoferrin had no effect on lymphocyte fraction cytotoxicity while producing an 11‐fold increase in the cytotoxicity of the monocyte fraction. It is therefore possible to perform a relatively simple test of monocyte cytotoxicity using lactoferrin as a stimulant in a 2‐h 51Cr‐labelled K562 assay system by allowing 18 h to elapse for lymphocyte natural killer cytotoxicity to decay.