Adherent cell inhibition of human leukemic in vitro agar clonal growth by short-term cell-to-cell contact

The effects of peripheral blood adherent cells from normal donors on human myeloid leukemic cluster growth in agar were studied. A prior co- incubation of nonadherent leukemic cells with adherent cell monolayers from 9 out of 10 donors in liquid cultures over a 4-hr period was sufficient to reduce subsequent leukemic growth in semisolid agar cultures. Inhibition was seen with adherent to leukemic cell ratios of as low as 0.5:1. Conversely, identical numbers of adherent cells in agar cultures but separated from the leukemic cells enhanced growth more than the cultures containing human placental conditioned media alone. Because leukemic cell exposure to adherent cells was brief, a cytotoxic mechanism appeared likely; however, this could not be detected by 51Cr release. Human peripheral blood adherent cells not activated by any in vitro mechanism suppress clonal growth of human myeloid leukemic cells by a mechanism requiring cell to cell contact. Examination of the inhibition of clonal growth appears to be more sensitive than 51Cr release as an indicator of adherent cell effects on myeloid leukemia.