Posttranscriptional inhibition of interferon-gamma production by lead

Abstract

Lead (Pb) is known to preferentially suppress the activation and development of type-1 CD4+ helper T cell (Th1) responses, whereas it enhances the development of type-2 CD4+ helper T cell (Th2) responses. The inhibition of interferon-gamma (IFNgamma) production has been demonstrated in vitro with a Th1 clone and DO11.10 ovalbumin-transgenic (OVA-tg) CD4+ T cells, and in vivo with wild-type and OVA-tg BALB/c mice; however, the mechanisms responsible for the Pb-induced downregulation of IFNgamma have not been reported. Here, we assessed the modulation of IFNgamma production at the mRNA and protein levels. Pb did not significantly affect IFNgamma mRNA expression by a Th1 clone or activated splenocytes, as measured by reverse transcriptase-polymerase chain reaction (RT-PCR), ribonuclease protection, and real-time RT-PCR. However, Pb did significantly lower the amount of IFNgamma protein in supernatants and cell lysates of antigen-activated T cells in comparison to stimulated controls, suggesting that the lower amounts of IFNgamma released into culture supernatants were not due to a blockage of secretion that gave rise to a cytoplasmic accumulation of IFNgamma. Pb inhibition also was not prevented by addition of zinc or iron. Pb did not enhance protein degradation of IFNgamma, in that lactacystin, an effective blocker of proteosomal proteolysis, did not prevent loss of IFNgamma; additionally, Pb did not accelerate loss of IFNgamma after cycloheximide treatment. Pb did, however, significantly suppress IFNgamma biosynthesis, as investigated using 35S-incorporation in pulse/chase experiments, although it did not suppress total protein synthesis, indicating that Pb selectively inhibits IFNgamma biosynthesis. Thus, Pb appears to selectively interfere with the translation of certain proteins, such as IFNgamma. IL-12 blocked Pb's preferential promotion of Th2 cells, but absence of STAT6 did not prevent the Pb skewing. Thus, Pb may modulate unique regulatory pathways.