Quantification of human interleukin 18 mRNA expression by competitive reverse transcriptase polymerase chain reaction

Interleukin 18 (IL-18) is a recently identified cytokine, originally called interferon gamma inducing factor, due to its capacity to induce interferon gamma production in Thr type cells. IL-18 is expressed by a wide variety o f cell types including mononuclear phagocytes, osteoblasts, keratinocytes a nd adrenal cortex cells. To quantify human IL-18 mRNA expression in small-s cale cell samples the authors developed a competitive reverse transcriptase polymerase chain reaction using a competitive template as an internal stan dard. This assay was demonstrated as a valid, sensitive and precise tool to quantify human IL-18 mRNA expression. IL-18 mRNA expression of primary per ipheral blood monocytes, CD4(+) T cells, CD8(+) T cells, B cells and NK cel ls was assessed by competitive RT-PCR. Basal IL-18 expression could be dete cted in all types of peripheral blood mononuclear cells (PBMC). The kinetic s of IL-18 mRNA expression in PBMC from healthy donors was defined in vitro after monocyte-specific (lipopolysaccharide LPS), T-cell-specific (anti-CD 3) and polyclonal-unspecific stimulation (phytohaemagglutinin PHA), Only LP S led to a strong increase of IL-18 mRNA expression peaking after 2 h, Thes e results indicate that IL-18 is expressed constitutionally by all major PB MC subtypes. However, only monocyte specific stimulation resulted in a sign ificant induction of IL-18 mRNA expression suggesting activated monocytes e .g. in inflammation as the main source of IL-18 expression. (C) 1999 Academ ic Press.