Impaired Periodontitis-Induced Cytokine Production by Peripheral Blood Monocytes and Myeloid Dendritic Cells in Patients with Rheumatoid Arthritis: A Case–Control Study

<b>Background</b>: Immune cells from rheumatoid arthritis (RA) patients display a reduced in vitro response to <i>Porphyromonas gingivalis</i> (<i>P. gingivalis</i>), which may have functional immune consequences. The aim of this study was to characterize, by flow cytometry, the frequency/activity of monocytes and naturally occurring myeloid dendritic cells (mDCs) in peripheral blood samples from patients with periodontitis and patients with periodontitis and RA. <b>Methods</b>: The relative frequency of monocytes and mDCs in the whole blood, the frequency of these cells producing TNFα or IL-6 and the protein expression levels for each cytokine, before and after stimulation with lipopolysaccharide (LPS) from <i>Escherichia coli</i> plus interferon-γ (IFN-γ), were assessed by flow cytometry, in peripheral blood samples from 10 healthy individuals (HEALTHY), 10 patients with periodontitis (PERIO) and 17 patients with periodontitis and RA (PERIO+RA). <b>Results</b>: The frequency of monocytes and mDCs producing IL-6 or TNF-α and the expression of IL-6 and TNF-α in the PERIO group were generally higher. Within the PERIO+RA group, <i>P. gingivalis</i> and related antibodies were negatively correlated with the monocyte and mDC expression of IL-6. A subgroup of the PERIO+RA patients that displayed statistically significantly lower frequencies of monocytes producing IL-6 after activation presented statistically significantly higher peptidylarginine deiminase (PAD)2/4 activity, anti-arg-gingipain (RgpB) IgG levels, mean probing depth (PD), periodontal inflamed surface area (PISA) and bleeding on probing (BoP). <b>Conclusions</b>: In the patients with PERIO+RA, innate immune cells seemed to produce lower amounts of pro-inflammatory cytokines, which are correlated with worse periodontitis-related clinical and microbiological parameters.