Extended report Matching factors Age, mean (SD) Postmenopausal (%) Current PMH use (%)* Other characteristics Ever cigarette smokers (%) Pack-years among smokers, mean (SD) Parous (%) Breastfed babies >12 months total (%){ RA features Mean age at diagnosis, (SD) Rheumatoid factor positive (%) Anti-CCP{ positive Seropositive (%) Rheumatoid nodules, (%) Radiographic changes, (%) Diagnosed by a member of ACR, (%) Table 1 Characteristics of rheumatoid arthritis (RA) cases and matched controls in the Nurses’ Health Study (NHS; 1976–2002) and the Nurses’ Health Study II (NHSII; 1989–2003) Nurses’ Health Study (390 matched pairs) Nurses’ Health Study II (49 matched pairs) RA cases 56.7 (9.4) 265 (68.0%) 114 (37.1%) 246 (63.1%) 23.9 (16.7) 355 (91.0%) 46 (13.7%) 56.7 (9.4) 224 (57.4%) 86 (47.8%) 238 (61.0%) 50 (12.8%) 118 (30.3%) 322 (84.1%) Controls 57.0 (9.3) 258 (66.2%) 106 (35.9%) 223 (57.2%) 24.1 (20.3) 368 (94.4%) 65 (18.4%) – – – – – – RA cases 43.1 (5.1) 14 (28.6%) 11 (68.8%) 19 (38.8%) 16.2 (9.1) 46 (93.9%) 18 (42.9%) 43.1 (5.1) 27 (55.1%) 19 (46.3%) 32 (65.3%) 6 (12.2%) 17 (34.7%) 47 (95.9%) Controls 43.1 (5.1) 14 (28.6%) 12 (75.0%) 22 (44.9%) 10.8 (5.5) 42 (85.7%) 18 (45.0%) – – – – – – *Percentage is calculated among postmenopausal women or parous women, with unknown/missing group excluded; for the rest of the variables, percentage was calculated with missing category included. {Among parous women. {Anti-cyclic citrullinated peptide (CCP) antibodies assayed in subset of cases (NHS = 180, NHSII = 41) with stored blood samples at collected at different points with respect to RA onset, up to 12 years prior to onset or after diagnosis. ACR, American College of Rheumatology; PMH, postmenopausal hormone. Table 1 shows the distribution of covariates for the RA cases and their matched controls at the time of RA diagnosis (or index date for the controls). A higher proportion of RA cases and controls were postmenopausal at RA diagnosis in NHS compared to NHSII cohorts. In NHSII a slightly higher percentage of women with RA were parous compared to their matched controls (93.9% and 85.7%), but not in the NHS cohort (91.0% of RA cases and 94.4% of controls). HLA-SE genotype distributions did not deviate from Hardy– Weinberg equilibrium. Overall, genotyping call rates were 98.5% for HLA-SE. The frequency of the HLA-SE was significantly higher among RA cases than controls (x2 with 1 degree of freedom, p,0.001 for pooled NHS/NHSII cohorts). In all, 49 (12.8%) NHS cases had 2 copies of the HLA SE allele as compared to 24 (6.3%) controls (p,0.001). Similar results were seen in NHSII participants with nine (18.4%) cases having two copies of the HLA-SE allele versus one (2.1%) in the controls (p = 0.03). The most common HLA-SE alleles in RA cases were 0401 (13.9%), 0404 (5.4%) and 0101 (9%). Table 2 includes the results of conditional logistic regression analyses for RA risk associated with HLA-SE for all RA and from unconditional logistic regression analyses stratified by seropo- sitivity. The adjusted model includes pack-years of cigarette smoking, age at menarche, regularity of menses, parity, breast feeding, menopausal status and postmenopausal hormone use. RA risk associated with a single copy of HLA-SE was elevated (odds ratio (OR) 1.60, 95% CI 1.16 to 2.22) and with a double copy was markedly elevated (OR 3.78, 95% CI 2.13 to 6.71). These effects of HLA-SE were limited to seropositive RA (double HLA-SE OR 4.41, 95% CI 2.53 to 7.68) with no significant association with seronegative RA. Interaction results Table 3 shows the results of analyses in which we tested for additive and multiplicative interactions between HLA-SE and smoking, categorised as ever/never smoking or dichotomised at (10 or .10 pack-years of smoking. There was a 2.14-fold 56 increased risk of RA (95% CI 1.39 to 3.29) for ever smokers who carried any HLA-SE compared to the referent group, never smokers with no HLA-SE, however, there was no evidence for gene–environment interaction. There was a modest additive but not multiplicative gene–environment interaction between ever smoking and the presence of the HLA-SE allele, with the proportion of risk due to additive interaction (AP) of 0.38 (95% CI 0.05 to 0.70, p = 0.02) for seropositive RA. In contrast, a 2.75-fold elevated risk of RA (95% CI 1.75 to 4.31) and a 3.6- fold elevated risk of seropositive RA (95% CI 2.26 to 5.78) were observed among heavy smokers (.10 pack-years) with any HLA-SE compared to the referent group ((10 pack-years without HLA-SE). We observed a significant additive, but not multiplicative, interaction between heavy cigarette smoking and the presence of the HLA-SE allele, with the proportion of risk due to additive interaction (AP) of 0.39 (95% CI 0.08 to 0.69, p = 0.01) for RA. A stronger additive interaction between heavy cigarette smoking and HLA-SE allele was observed for seropositive RA, with AP of 0.50 (95% CI 0.24 to 0.77, p,0.002), and a significant multiplicative interaction term (p = 0.05). When stratified by number of copies of HLA-SE there was strong evidence for increasing risk of RA with each copy of HLA- SE among heavy smokers, with a 6.6-fold increased risk of RA, (95% CI 2.49 to 17.46), for heavy smokers with 2 copies of HLA- SE as compared to the reference group, with evidence for additive interaction (table 4). The strongest evidence for additive interaction was for HLA-SE with heavy smoking in seropositive RA (OR 7.47, 95% CI 2.77 to 20.11), with a borderline multiplicative interaction (p = 0.07). Among HLA-SE subtypes, the only interaction was between the 0401 allele and heavy smoking, with evidence for additive interaction (p = 0.005) for all RA. The HLA-SE subtypes of 0401 and 0101 demonstrated significant additive interaction with heavy smoking (p = 0.003 and 0.01, respectively) for seropositive RA. There was no evidence for multiplicative interactions between HLA-SE subtypes and heavy smoking (data not shown). Ann Rheum Dis 2010;69:54–60. doi:10.1136/ard.2008.102962