E x t e n d e d r e p o r t Table 2 Association between HLA-SE and rheumatoid arthritis (RA) risk in the Nurses’ Health Studies with stratification by serological status Model General Dominant HLA-SE None Single SE Double SE None Any SE All RA, unadjusted* OR (95% CI) All RA, adjusted{ OR (95% CI) Seropositive RA, Seronegative RA, adjusted{ OR (95% CI) adjusted{ OR (95% CI) 1.0 (reference) 1.54 (1.13 to 2.10) 3.19 (1.88 to 5.43) 1.0 (reference) 1.80 (1.35 to 2.41) 1.0 (reference) 1.60 (1.16 to 2.22) 3.78 (2.13 to 6.71) 1.0 (reference) 1.90 (1.40 to 2.58) 1.0 (reference) 1.88 (1.34 to 2.65) 4.41 (2.53 to 7.68) 1.0 (reference) 2.25 (1.64 to 3.10) 1.0 (reference) 1.07 (0.72 to 1.59) 1.55 (0.75 to 3.21) 1.0 (reference) 1.14 (0.78 to 1.65) *Conditional logistic regression. {conditional logistic regression, adjusting for smoking (pack-years), parity, breast feeding and menstrual irregularity, and age at menarche, menopausal status and postmenopausal hormone use at diagnosis. {Unconditional logistic regression adjusting for year of birth, smoking (pack-years), parity, breast feeding and menstrual irregularity, and age at menarche, menopausal status and postmenopausal hormone use at blood draw and at diagnosis. HLA-SE, human leukocyte antigen shared epitope; OR, odds ratio. When stratifying pack-years into three categories, the highest odds for RA were in the 10–20 pack-year and HLA-SE group in all RA and seropositive RA analyses (table 5). Comparing each stratum to the referent demonstrated significant additive interactions for 10–20 pack-year stratum (p = 0.008) for all RA and for 10–20 (p = 0.003) and for .20 year strata (p = 0.002) for seropositive RA with a borderline multiplicative interaction (p = 0.09). When stratifying smoking status into never, past, or current (table 6), there was little evidence for interactions except a modest additive interaction between past smoking and HLA-SE for seropositive RA (p = 0.04). DISCUSSION In this nested case-control study of women, we demonstrate significant additive and multiplicative interaction between the HLA-DRB1 shared epitope and heavy cigarette smoking of at least 10 pack-years. The observed interaction between HLA-SE and smoking was strongest for seropositive RA with little evidence for association with seronegative RA. Evidence for interaction was less evident when smoking status was analysed as never/ever smoking or never/past/current, suggesting the importance of considering cumulative ‘‘dose’’ of smoking when testing for gene–environment interaction in RA. Interactions between HLA-SE alleles and smoking in RA risk have been demonstrated in several large epidemiological studies. In the Epidemiologic Investigations in Rheumatoid Arthritis (EIRA) study, a strong additive interaction between HLA-SE and smoking was demonstrated for RF-positive and anti-CCP-positive RA, but not for seronegative RA13 19; a 21-fold increased risk of CCP-positive RA was observed among smokers carrying a double Gene–environment interaction of human leukocyte antigen shared epitope (HLA-SE) and smoking in the Nurses’ Health Studies Table 3 HLA-SE None None Any Any Additive interaction: AP (95% CI) p For interaction RERI Synergy Index Multiplicative interaction: p For interaction None None Any Any Additive interaction: AP (95% CI) p For interaction RERI Synergy index Multiplicative interaction: p For interaction Smoking status Never Ever Never Ever (10 pack-years smoking .10 pack-years smoking (10 pack-years smoking .10 pack-years smoking All RA OR (95% CI)* 1.00 (reference) 0.99 (0.65 to 1.51) 1.65 (1.05 to 2.61) 2.14 (1.39 to 3.29) 0.23 (20.14 to 0.61) padd = 0.23 0.49 (20.35 to 1.33) 1.77 (0.50 to 6.25) pmulti = 0.38 OR (95% CI)* 1.00 (reference) 1.10 (0.73 to 1.67) 1.58 (1.08 to 2.33) 2.75 (1.75 to 4.31) 0.39 (0.08 to 0.69) padd = 0.01 1.06 (20.06 to 2.17) 2.54 (0.81 to 7.97) pmulti = 0.14 Seropositive RA OR (95% CI){{ 1.00 (reference) 1.05 (0.66 to 1.68) 1.78 (1.08 to 2.95) 2.94 (1.84 to 4.69) 0.38 (0.05 to 0.70) padd = 0.02 1.10 (20.02 to 2.22) 2.32 (0.72 to 7.47) pmulti = 0.18 OR (95% CI){{ 1.00 (reference) 1.08 (0.67 to 1.72) 1.72 (1.13 to 2.61) 3.61 (2.26 to 5.78) 0.50 (0.24 to 0.77) padd,0.001 1.82 (0.35 to 3.30) 3.30 (1.06 to 10.33) pmulti = 0.05 Seronegative RA OR (95% CI){1" 1.00 (reference) 1.04 (0.64 to 1.69) 1.18 (0.67 to 2.08) 1.23 (0.72 to 2.10) 0.004 (20.69 to 0.70) padd = 0.99 0.005 (20.85 to 0.86) 1.02 (0.02 to 47.34) pmulti = 1.00 OR (95% CI) 1.00 (reference) 1.16 (0.72 to 1.89) 1.09 (0.68 to 1.77) 1.45 (0.81 to 2.56) 0.13 (20.49 to 0.75) padd = 0.68 0.19 (20.75 to 1.13) 1.74 (0.07 to 42.22) pmulti = 0.74 padd is the p value for the attributable proportion due to interaction, pmulti is the p value for multiplicative interaction term between binary smoking variable and binary HLA-SE genotype with 1 degree of freedom. *Conditional logistic regression, adjusting for parity, breast feeding and menstrual irregularity, and age at menarche, menopausal status and postmenopausal hormone use. {Unconditional logistic regression adjusting for year of birth, parity, breast feeding and menstrual irregularity, and age at menarche, menopausal status and postmenopausal hormone use. {Seropositive RA cases and all controls; 1seronegative RA cases and all controls. AP, attributable proportion due to interaction, an index of additive interactions between binary smoking variable and binary HLA-SE genotype; RA, rheumatoid arthritis; RERI, relative excess risk due to interaction. Ann Rheum Dis 2010;69:54–60. doi:10.1136/ard.2008.102962 57