To determine whether monocytes in rheumatoid arthritis (RA) are activated to produce proinflammatory cytokines in the peripheral circulation before entering the synovium and whether the pattern of cytokines that is expressed correlates with disease activity.
Cytokine messenger RNA (mRNA) levels were assessed in peripheral blood mononuclear cells (PBMC) from 14 RA patients and 14 healthy controls by semiquantitative reverse transcription-polymerase chain reaction technology. The method employed was sufficiently sensitive to assess cytokine mRNA levels in freshly isolated cells without the necessity of in vitro stimulation. Thus, an estimate of the in vivo state of activation could be obtained.
Interleukin-8 (IL-8) mRNA levels were elevated in all 14 RA patients compared with normal controls, whereas 7 of 14 RA patients had elevated levels of mRNA for IL-6 or IL-10. IL-1beta mRNA levels were below the normal range in 3 of 14 patients, within normal limits in 4 of 14, and elevated in 7 of 14. Tumor necrosis factor alpha mRNA levels were within the normal range in 9 of 14 patients and below normal in 5 of 14. There was a statistically significant difference between the mean IL-10 (P < 0.05) and IL-8 (P < 0.001) mRNA levels in RA patients and normal controls. Of note, the 7 patients with elevated IL-1beta mRNA levels also expressed the highest levels of IL-8 mRNA. Whereas a strong correlation between the expression of IL-1beta and IL-8 mRNA (P < 0.001) was found, expression of all other mRNA occurred independently of each other. Levels of cyclooxygenase 2 (COX-2) mRNA were also determined to evaluate the status of myeloid cell activation more completely. COX-2 mRNA levels were within the normal range in 4 of 11 patients and below normal in 7 of 11, but did not correlate with the expression of any of the cytokine mRNA.
Elevated levels of mRNA for selected cytokines that are predominantly produced by monocytes can be found in the PBMC of many RA patients. The data indicate that myeloid precursor cells become activated to produce cytokines before they enter the synovium, a finding which emphasizes the systemic nature of RA.