Peripheral blood mononuclear cells stimulated with C5a or lipopolysaccharide to synthesize equivalent levels of IL-1 beta mRNA show unequal IL-1 beta protein accumulation but similar polyribosome profiles.

Recent reports suggest that regulation of IL-1 beta gene expression in human monocytes may include translation level as well as transcription level control mechanisms. We studied IL-1 beta expression in PBMC stimulated with recombinant complement protein C5a or with LPS. IL-1 beta mRNA was expressed by C5a-treated PBMC, but very little or no IL-1 beta protein could be detected by ELISA or by Western blot analysis using a polyclonal Ab that reacts equally with pro-IL-1 beta and processed mature IL-1 beta. LPS-treated cells produced both IL-1 beta mRNA and protein. Velocity sedimentation analysis revealed, however, that IL-1 beta mRNA assembles into large polyribosomes in both C5a- and LPS-treated cells. Translation of the IL-1 beta mRNA in C5a-treated cells is not therefore blocked at the level of protein synthesis initiation. IL-1 beta mRNA was released from polyribosomes by treating the cells with puromycin, suggesting that the ribosomes are not frozen at the elongation phase of protein synthesis. One explanation for the data is that the rate of IL-1 beta polypeptide chain elongation or termination may be diminished in the C5a-stimulated PBMC.