*4.1. Study Individuals and Lymph Node Needle Biopsy Sampling*

Individuals with arthralgia and elevated IgM-RF and/or ACPA levels, but without any evidence of arthritis upon examination were included (RA-risk individuals, phase c/d upon examination were included (RA-risk individuals, phase c/d; *n*= 23 [8]). Median follow up time of RA-risk individuals was 20.3 months (12.9–33.2 (IQR)) and none of the RA-risk individuals developed arthritis during this period. RA-risk individuals were not allowed to have systemic or intra-articular corticosteroid injection less than 28 days before enrolment. In addition, RA patients with established disease based on fulfillment of the American College of Rheumatology and European League Against Rheumatism (ACR/EULAR 2010 [60] criteria and as assessed by the rheumatologist were included (*n* = 24). Healthy individuals without any joint complaints and without elevated IgM-RF and/or ACPA level and without active viral infection or any history of autoimmunity or malignancy and no present or previous use of disease-modifying antirheumatic drugs (DMARDs), biologicals, or other experimental drugs served as the (voluntary) control group (*n* = 14). IgM-RF was measured using IgM-RF ELISA (Hycor Biomedical, Indianapolis, IN, USA) (ULN (upper limit of normal) 49 kU/mL)). ACPA was measured using anti-CCP2 ELISA CCPlus (Eurodiagnostica, Nijmegen, The Netherlands (ULN 25 kAU/L)). The study was performed according to the principles of the Declaration of Helsinki, approved by the institutional medical ethical review board of the Academic Medical Center (Ethical permission: NL20951.018.07, date: 25 February 2008 and NL52469.018.15, date: 17 July 2015), and all study individuals gave written informed consent. All study individuals underwent an ultrasound-guided inguinal LN needle core biopsy as previously described [61]. At the day of LN sampling none of the donors showed signs of an infection. Table 1 shows the demographics of the included individuals.



IgM-RF, IgM rheumatoid factor; ACPA, anti-citrullinated protein antibodies; ESR, erythrocyte sedimentation rate; CRP, C-reactive protein; TJC, tender joint count; NSAID, non-steroidal anti-inflammatory drug; DMARD, disease-modifying antirheumatic drugs. <sup>a</sup> levels missing from one individual, <sup>b</sup> levels missing from two individuals, <sup>c</sup> levels missing from six individuals, <sup>d</sup> levels missing from seven individuals, <sup>e</sup> levels missing from five individuals, <sup>f</sup> treatment unknown for five individuals. † Healthy individuals are significantly younger than RA-risk individuals and RA patients (*p* < 0.0050, tested by Kruskal–Wallis followed by a post Dunn's test).
