Smoking is now well recognised not only as a risk factor for rheumatoid arthritis (RA), but also as a determinant of disease activity, severity, response to therapy, and possibly mortality. Recent studies have provided significant insights into the molecular and cellular mechanisms which underpin the pathogenesis of smokingrelated RA. These involve release of the enzymes, peptidylarginine deiminases (PADs) 2 and 4 from smoke-activated, resident and infiltrating pulmonary phagocytes. PADs, in turn, mediate the conversion of various endogenous proteins to putative citrullinated autoantigens. In genetically susceptible individuals, these autoantigens trigger the production of anti-citrullinated peptide/protein pathogenic autoantibodies (ACPA), an event which precedes the development of RA. This review is focused primarily on smoking-mediated harmful chronic inflammatory responses, both local and systemic, which promote the formation of ACPA, as well as the possible involvement of other types of outdoor and indoor pollution in the pathogenesis of RA. This is preceded by a brief overview of the evidence implicating smoking as a risk factor for development of ACPA-positive RA.Keywords: Anti-citrullinated peptide/protein antibodies; airway microbiota; atmospheric pollution; heavy metals; peptidylarginine deiminases; smoking cessation strategies. ImplicationsChronic inflammatory mechanisms operative in the lungs of smokers lead to the production of anti-citrullinated protein antibodies which, in turn, drive the development of rheumatoid arthritis. These mechanistic insights not only reinforce the association between smoking and risk for rheumatoid arthritis, but also the necessity to increase the level of awareness in those at highest risk.

Matrix metalloproteinase-3 (MMP-3) is involved in the immunopathogenesis of rheumatoid arthritis (RA), but little is known about its relationship to genetic susceptibility and biomarkers of disease activity, especially acute phase reactants in early RA. MMP-3 was measured by ELISA in serum samples of 128 disease-modifying, drug-naïve patients and analysed in relation to shared epitope genotype, a range of circulating chemokines/cytokines, acute phase reactants, autoantibodies, cartilage oligomeric protein (COMP), and the simplified disease activity index (SDAI). MMP-3 was elevated >1.86 ng/ml in 56.25% of patients (P < 0.0001), correlated with several biomarkers, notably IL-8, IL-6, IFN γ, VEGF and COMP (r values = 0.22–0.33, P < 0.014–0.0001) and with CRP and SAA levels (r = 0.40 and 0.41, resp., P < 0.0000) and SDAI (r = 0.29, P < 0.0001), but not with erosions or nodulosis. However, the correlations of CRP and SAA with SDAI were stronger (respective values of 0.63 and 0.54, P < 0.001 for both). COMP correlated with smoking, RF, and MMP-3. MMP-3 is significantly associated with disease activity, inflammatory mediators and cartilage breakdown, making it a potential biomarker of disease severity, but seemingly less useful than CRP and SAA as a biomarker of disease activity in early RA.

Our objective was to analyse the relationship between circulating cytokines, autoantibodies, acute phase reactants, and disease activity in DMARDs-naïve rheumatoid arthritis (RA) patients (n = 140). All cytokines were significantly higher in the RA cohort than in healthy controls. Moderate-to-strong positive intercorrelations were observed between Th1/Th2/macrophage/fibroblast-derived cytokines. RF correlated significantly with IL-1β, IL-2, IL-4, IL-10, IL-12, G-CSF, GM-CSF, IFN-γ, and TNF (P < .0001), and aCCP and aMCV with IL-1β, IL-2, IL-4, and IL-10 (P < .0002), while IL-6 correlated best with the acute phase reactants, CRP, and SAA (P < .0001). In patients with a DAS28 score of ≥5.1, IFN-γ, IL-1β, IL-1Ra, TNF, GM-CSF, and VEGF were significantly correlated (P < .04–.001) with high disease activity (HDA). Circulating cytokines in RA reflect a multifaceted increase in immune reactivity encompassing Th1 and Th2 cells, monocytes/macrophages, and synovial fibroblasts, underscored by strong correlations between these cytokines, as well as their relationships with RF, aCCP, and aMCV, with some cytokines showing promise as biomarkers of HDA.