INDUCTION OF CD14 ANTIGEN ON THE SURFACE OF U937 CELLS BY AN INTERLEUKIN-6 AUTOCRINE MECHANISM AFTER CULTURE WITH FORMALIN-KILLED GRAM-NEGATIVE BACTERIA

In order to better understand the regulation of CD14 antigen on the surface of the monocyte-like cell line U937 in response to bacteria, the expression and regulation of CD14 antigen on these cells when cultured with formalin-killed bacteria were determined using the monoclonal antibody MY-4 and analyzed by means of the indirect immunofluorescence method. CD14 expression was induced on the U937 cells after about 48 hours of culture with all of the formalin-killed Gram-negative bacteria used in this study but with none of the Gram-positive bacteria. Maximum expression was obtained after culture with formalin-killed Salmonella enteritidis strain 116-54. Various cytokines such as interleukin-1-beta, interleukin-2, interleukin-6, interferon-gamma and tumor-necrosis factor-alpha were assayed in the culture supernatant of U937 cells cultured with or without formalin-killed Salmonella enteritidis 116-54 using an enzyme-immunoassay or radioimmunoassay system. The U937 cells were found to produce a large amount of interleukin-6 in response to formalin-killed Salmonella enteritidis 116-54. On the other hand, culture supernatant (referred to as conditioned medium) obtained from the U937 cells after 72 h of culture with formalin-killed Salmonella enteritidis 116-54 also induced strong expression of CD14 antigen 48 to 72 h later, and this was blocked by the addition of anti-human interleukin-6 antibody. These findings suggest that the expression of CD14 antigen on the surface of U937 cells cultured with formalin-killed Gram-negative bacteria is induced by interleukin-6 and can be explained on the basis of the autocrine mechanism of interleukin-6.