Objective: To examine the correlation between C-reactive protein (CRP) and CRP-inducing cytokines (IL-1 beta, IL-6, TNF-alpha) and IL-1 receptor antagonist (IL-1ra), as well as to study their relationship with systemic lupus erythematosus disease activity (SLEDAI) in newly diagnosed, untreated lupus patients.
Methods: Sera from newly diagnosed untreated lupus and rheumatoid arthritis (RA) patients were examined for CRP and cytokines. Data were compared among patient groups and correlated individually among the lupus group. Lupus monocytes and neutrophils were cultured in vitro to produce IL-1ra and experimental results were related to CRP levels and SLEDAI.
Results: Within lupus, serum CRP, IL-6, IL-1 beta and TNF-alpha levels were significantly lower than those of RA (all p values were < 0.005) and generally higher than those in the controls (p = 0.002, < 0.001, > 0.2, and < 0.001, respectively). Except IL-1ra, which was correlated with CRP (p = 0.045), no substantial correlation was discovered between CRP and IL-6, IL-1 beta or TNF-alpha individually. Moreover, excluding IL-1ra (p = 0.024), there was no association between cytokines and SLEDAI. In vitro IL-1ra as secreted by monocytes correlated with serum CRP and SLEDAI.
Conclusion: In lupus patients, serum IL-1 beta, IL-6 or TNF-alpha levels failed to correlate with low CRP levels. This indicates a complicated CRP production process, which can not be explained solely by single cytokines as reported previously. Both serum and in vitro produced IL-1ra may be applied clinically as a surrogate CRP marker in untreated lupus patients as they are both correlated with serum CRP.