this report is based on data from 2 case - control studies on environmental and genetic risk factors for ms . the first study is the epidemiological investigation of ms ( eims ) , with a study base comprising the swedish population aged 16 to 70 years . a case was defined as a person in the study base who had received a diagnosis of ms according to the mcdonald criteria . for each case , 2 controls were randomly selected from the national population register , frequency matched by age ( 5-year age groups ) , sex , and residential area . information regarding exposures and other circumstances was collected using a standardized questionnaire that was filled in at home . the study period for completed questionnaires were obtained from 1,798 cases and 3,907 controls , which is equivalent to 91% for the case group and 69% for the controls . since the impact of adolescent obesity was investigated in this study , cases with disease onset before age 20 , and their corresponding controls , were excluded ( 110 cases , 218 controls ) . the american case - control study used a study population of white non - hispanic people identified among members of kaiser permanente medical care plan , northern california region ( kpnc ) , using electronic medical records . kpnc is an integrated health services delivery system with a membership of 3.2 million that comprises about 25% to 30% of the population of a 22-county service area in northern california . cases of ms were required to have an ms diagnosis by a neurologist , self - identified white race / ethnicity , age 18 through 69 years , and kpnc membership at initial contact . controls were randomly selected from current kpnc members who did not have a ms diagnosis or related conditions , and were individually matched to cases on sex , birth date , race / ethnicity , and zip code of the case residence . in total , 1,087 cases and 687 controls were included in this report ; 128 cases with disease onset before age 20 were excluded , and 23 cases and 14 controls were excluded due to missing data on body mass index ( bmi ) . the kpnc study protocol was approved by the institutional review boards of kp division of research and the university of california , berkeley . in eims , blood samples were available from 1,510 cases ( 89% ) and 2,017 controls ( 55% ) . allelic dosage of hla - drb1 * 15 and hla - a*02 was obtained by 1 of 3 methods : ( 1 ) pcr amplification with sequence - specific primers ( olerup ssp low resolution typing kits , olerup ssp ab , stockholm , sweden ) ; ( 2 ) imputation using hla*imp:02 based on single nucleotide polymorphisms ( snps ) genotyped on the immunochip custom array in an overlapping swedish cohort ; or ( 3 ) taqman allelic discrimination of the snps rs9271366 ( assay i d : c__33416976_20 ) , which tags hla - drb1 * 15 or rs2975033 ( assay i d : c__15962692_20 ) and rs12206499 ( assay i d : c__31338281_10 ) , which combined tags for hla - a*02 . the distribution of method for classification of hla - drb1 * 15 status was as follows : 67% of cases/64% of controls were genotyped with the olerups ssp kit , 4% cases/1% controls had imputed genotypes , and 29% cases/35% controls were genotyped with taqman genotyping assays . the figures for distribution of method for classification of hla - a*02 status were the same with the exception of percentage of cases that were genotyped with olerups kit , which was 66% for hla - a*02 status . in the kpnc study , information on hla genotype was available for 937 ( 86% ) cases and 609 ( 87% ) controls . hla - drb1 genotypes were determined by use of a sequence - specific pcr method , as previously described . allelic dosage of hla - a*02 was obtained by taqman allelic discrimination of the snps rs2975033 ( assay i d : c__15962692_20 ) , which tags for hla - a*02 . the validity of imputing hla types from snps has been studied against classical methods of hla typing and congruence between methods is very good for most alleles , including hla - drb1 * 15 and hla - a*02 , studied herein . participants in both studies were classified according to the carriage of any hla - drb1 * 15 allele vs no carriage , and according to carriage of any hla - a*02 allele vs no carriage ( where absence of hla - a*02 is the risk factor ) . the questionnaire in eims contained questions regarding demographic factors , lifestyle factors , and socioeconomic circumstances . information was obtained regarding current body height and body weight at age 20 . using current height , we calculated bmi at age 20 by dividing weight in kilograms by height in meters squared . in the kpnc study , participants reported their current height at the time of the interview , as well as their highest and lowest ( nonpregnancy ) weight during their 20s . using current height , bmi during their 20s was calculated by dividing mean weight in kilograms by height in meters squared . in order to analyze the influence of bmi on the risk of developing ms , we categorized the subjects into the following groups based on bmi : < 18.5 , 18.5<21 , 21<23 , 23<25 , 25<27 , and 27 kg / m . however , in eims , fewer than 2% had an adolescent bmi exceeding 30 kg / m . the or among subjects with bmi 2730 and > 30 was fairly similar ( bmi 2730 : or 2.1 , 95% confidence interval [ ci ] 1.43.0 ; and bmi > 30 : or 2.5 , 95% ci 1.73.7 ) . therefore , we chose to combine these bmi groups into one group defined as obese . by means of unconditional logistic regression , the incidence of ms in subjects belonging to different bmi groups was compared with that of normal - weight subjects with bmi between 18.5 and 21 kg / m , by calculating ors with 95% cis . trend test for a dose - response relationship regarding adolescent bmi and risk of ms was performed by using a continuous variable for bmi ( kg / m ) in a logistic regression model . analyses of interaction between bmi and hla - drb1 - 15 as well as between bmi and hla - a*02 were performed , and the possible gene environment interaction was evaluated by estimating departure from additivity of effects using attributable proportion due to interaction ( ap ) as described . in eims however , only the results from the unmatched analyses are presented in this report since these were in close agreement with those from the matched analyses but had a higher degree of precision ( due to a higher number of controls ) . in the kpnc study , there were far more cases than controls , and in order to use information from the whole group we conducted unconditional logistic regression with adjustment for the matching factors . in eims , all analyses were adjusted for age , sex , and residential area as well as for ancestry and smoking . age was categorized into the following 8 strata : 1619 , 2024 , 2529 , 3034 , 3539 , 4045 , 4549 , and 5070 years of age . assessment of ancestry was based on whether the subject was born in sweden or not , and whether either of the subject 's parents had immigrated to sweden . a subject who was born in sweden , whose parents had not immigrated , was classified as swedish . smoking was considered prior to the index year in the cases ( i.e. , the year of disease onset ) and during the same period of time in the corresponding controls . adjustments were also made for heredity ( having a first- or second - degree relative with ms or not ) , educational level ( university degree or not ) , socioeconomic status ( according to an established socioeconomic classification ) , physical activity during ages 18 to 22 ( yes / no ) , oral contraceptive use ( ever / never ) , parity ( yes / no ) , ebna1immunoglobulin g ( high / low ) , vitamin d status ( more or less than 50 ng / ml ) , and ultraviolet radiation exposure habits ( high / low ) , but these factors had minor influence on the results of the study and were not retained in the final analyses . all analyses were conducted using sas version 9.2 . in the kpnc study , adjustments were made for age , sex , ancestry , and smoking . ancestry was categorized into the following stratas : northern europe , western europe , southern europe , eastern europe , other , and unknown . this report is based on data from 2 case - control studies on environmental and genetic risk factors for ms . the first study is the epidemiological investigation of ms ( eims ) , with a study base comprising the swedish population aged 16 to 70 years . a case was defined as a person in the study base who had received a diagnosis of ms according to the mcdonald criteria . for each case , 2 controls were randomly selected from the national population register , frequency matched by age ( 5-year age groups ) , sex , and residential area . information regarding exposures and other circumstances was collected using a standardized questionnaire that was filled in at home . the study period for completed questionnaires were obtained from 1,798 cases and 3,907 controls , which is equivalent to 91% for the case group and 69% for the controls . since the impact of adolescent obesity was investigated in this study , cases with disease onset before age 20 , and their corresponding controls , were excluded ( 110 cases , 218 controls ) . the american case - control study used a study population of white non - hispanic people identified among members of kaiser permanente medical care plan , northern california region ( kpnc ) , using electronic medical records . kpnc is an integrated health services delivery system with a membership of 3.2 million that comprises about 25% to 30% of the population of a 22-county service area in northern california . cases of ms were required to have an ms diagnosis by a neurologist , self - identified white race / ethnicity , age 18 through 69 years , and kpnc membership at initial contact . controls were randomly selected from current kpnc members who did not have a ms diagnosis or related conditions , and were individually matched to cases on sex , birth date , race / ethnicity , and zip code of the case residence . in total , 1,087 cases and 687 controls were included in this report ; 128 cases with disease onset before age 20 were excluded , and 23 cases and 14 controls were excluded due to missing data on body mass index ( bmi ) . the kpnc study protocol was approved by the institutional review boards of kp division of research and the university of california , berkeley . in eims , blood samples were available from 1,510 cases ( 89% ) and 2,017 controls ( 55% ) . allelic dosage of hla - drb1 * 15 and hla - a*02 was obtained by 1 of 3 methods : ( 1 ) pcr amplification with sequence - specific primers ( olerup ssp low resolution typing kits , olerup ssp ab , stockholm , sweden ) ; ( 2 ) imputation using hla*imp:02 based on single nucleotide polymorphisms ( snps ) genotyped on the immunochip custom array in an overlapping swedish cohort ; or ( 3 ) taqman allelic discrimination of the snps rs9271366 ( assay i d : c__33416976_20 ) , which tags hla - drb1 * 15 or rs2975033 ( assay i d : c__15962692_20 ) and rs12206499 ( assay i d : c__31338281_10 ) , which combined tags for hla - a*02 . the distribution of method for classification of hla - drb1 * 15 status was as follows : 67% of cases/64% of controls were genotyped with the olerups ssp kit , 4% cases/1% controls had imputed genotypes , and 29% cases/35% controls were genotyped with taqman genotyping assays . the figures for distribution of method for classification of hla - a*02 status were the same with the exception of percentage of cases that were genotyped with olerups kit , which was 66% for hla - a*02 status . in the kpnc study , information on hla genotype was available for 937 ( 86% ) cases and 609 ( 87% ) controls . hla - drb1 genotypes were determined by use of a sequence - specific pcr method , as previously described . allelic dosage of hla - a*02 was obtained by taqman allelic discrimination of the snps rs2975033 ( assay i d : c__15962692_20 ) , which tags for hla - a*02 . the validity of imputing hla types from snps has been studied against classical methods of hla typing and congruence between methods is very good for most alleles , including hla - drb1 * 15 and hla - a*02 , studied herein . participants in both studies were classified according to the carriage of any hla - drb1 * 15 allele vs no carriage , and according to carriage of any hla - a*02 allele vs no carriage ( where absence of hla - a*02 is the risk factor ) . the questionnaire in eims contained questions regarding demographic factors , lifestyle factors , and socioeconomic circumstances . information was obtained regarding current body height and body weight at age 20 . using current height , we calculated bmi at age 20 by dividing weight in kilograms by height in meters squared . in the kpnc study , participants reported their current height at the time of the interview , as well as their highest and lowest ( nonpregnancy ) weight during their 20s . using current height , bmi during their 20s was calculated by dividing mean weight in kilograms by height in meters squared . in order to analyze the influence of bmi on the risk of developing ms , we categorized the subjects into the following groups based on bmi : < 18.5 , 18.5<21 , 21<23 , 23<25 , 25<27 , and 27 kg / m . however , in eims , fewer than 2% had an adolescent bmi exceeding 30 kg / m . the or among subjects with bmi 2730 and > 30 was fairly similar ( bmi 2730 : or 2.1 , 95% confidence interval [ ci ] 1.43.0 ; and bmi > 30 : or 2.5 , 95% ci 1.73.7 ) . therefore by means of unconditional logistic regression , the incidence of ms in subjects belonging to different bmi groups was compared with that of normal - weight subjects with bmi between 18.5 and 21 kg / m , by calculating ors with 95% cis . trend test for a dose - response relationship regarding adolescent bmi and risk of ms was performed by using a continuous variable for bmi ( kg / m ) in a logistic regression model . analyses of interaction between bmi and hla - drb1 - 15 as well as between bmi and hla - a*02 were performed , and the possible gene environment interaction was evaluated by estimating departure from additivity of effects using attributable proportion due to interaction ( ap ) as described . in eims however , only the results from the unmatched analyses are presented in this report since these were in close agreement with those from the matched analyses but had a higher degree of precision ( due to a higher number of controls ) . in the kpnc study , there were far more cases than controls , and in order to use information from the whole group we conducted unconditional logistic regression with adjustment for the matching factors . in eims , all analyses were adjusted for age , sex , and residential area as well as for ancestry and smoking . age was categorized into the following 8 strata : 1619 , 2024 , 2529 , 3034 , 3539 , 4045 , 4549 , and 5070 years of age . assessment of ancestry was based on whether the subject was born in sweden or not , and whether either of the subject 's parents had immigrated to sweden . a subject who was born in sweden , whose parents had not immigrated , was classified as swedish . smoking was considered prior to the index year in the cases ( i.e. , the year of disease onset ) and during the same period of time in the corresponding controls . adjustments were also made for heredity ( having a first- or second - degree relative with ms or not ) , educational level ( university degree or not ) , socioeconomic status ( according to an established socioeconomic classification ) , physical activity during ages 18 to 22 ( yes / no ) , oral contraceptive use ( ever / never ) , parity ( yes / no ) , ebna1immunoglobulin g ( high / low ) , vitamin d status ( more or less than 50 ng / ml ) , and ultraviolet radiation exposure habits ( high / low ) , but these factors had minor influence on the results of the study and were not retained in the final analyses . all analyses were conducted using sas version 9.2 . in the kpnc study , adjustments were made for age , sex , ancestry , and smoking . ancestry was categorized into the following stratas : northern europe , western europe , southern europe , eastern europe , other , and unknown . selected characteristics of subjects by study and bmi at age 20 are presented in table e-1 on the neurology web site at neurology.org . in both studies , subjects with adolescent bmi 27 kg / m had an increased risk of developing ms compared to normal - weight subjects with bmi between 18.5 and 21 kg / m ( or 2.2 [ 95% ci 1.73.0 ] in eims and 1.8 [ 95% ci 1.22.6 ] in the kpnc study ) ( table 1 ) . ms risk in subjects with bmi between 25 and 27 kg / m was modestly increased ( or 1.4 [ 95% ci 1.051.9 ] in eims and 1.3 [ 95% ci 0.92.0 ] in the kpnc study ) . the observed trend of a higher bmi resulting in a higher risk of developing ms was statistically significant in both studies . the ors remained unchanged when the analyses were restricted to those who had been genotyped . odds ratios with 95% confidence intervals of developing multiple sclerosis for subjects in different categories according to body mass index at age 20 compared with normal - weight subjects a significant interaction was observed between the hla - drb1 * 15 allele and obesity with regard to risk for ms ( table 2 ) . the interaction was restricted to those with bmi 27 kg / m ( ap 0.6 , 95% ci 0.30.8 in eims and ap 0.5 , 95% ci 0.20.9 in the kpnc study ) . when the analysis of interaction between hla - drb1 * 15 and obesity ( in categories : drb1 * 15/bmi < 27 , drb1 * 15+/bmi < 27 , drb1 * 15/bmi 27 , and drb1 * 15+/bmi 27 ) was performed by hla - a*02 status in both materials combined , the ap was statistically significant in each strata ( p value 0.0003 among hla - a*02-positive subjects and < 0.0001 among hla - a*02-negative subjects ) . odds ratios with 95% confidence intervals of developing multiple sclerosis for subjects with different combinations of hla - drb1 * 15 status and bmi status similarly , a significant interaction was observed between absence of hla - a*02 and obesity ( ap 0.4 , 95% ci 0.10.7 in eims and ap 0.4 , 95% ci 0.010.8 in the kpnc study ) ( table 3 ) . when the analysis of interaction between absence of hla - a*02 and obesity ( in categories : a*02+/bmi < 27 , a*02/bmi < 27 , a*02+/bmi 27 , and a*02/bmi 27 ) was performed by hla - drb1 * 15 status in both materials combined , the ap was statistically significant in each strata ( p value 0.02 among hla - drb1 * 15-negative subjects and 0.01 among hla - drb1 * 15-positive subjects ) . odds ratios with 95% confidence intervals of developing multiple sclerosis for subjects with different combinations of hla - a*02 status and bmi status the magnitude of interaction between hla - drb1 * 15 and adolescent obesity remained the same regardless of which reference group was used ( table e-2 ) . finally , we calculated the ors associated with the most susceptible genotype ( carriage of drb1 * 15 but not hla - a*02 ) in subjects with bmi more or less than 27 kg / m , respectively , compared with nonobese subjects without these genetic risk factors . subjects with bmi less than 27 kg / m with the 2 risk genotypes displayed an or of 5.1 ( 95% ci 4.16.3 ) , whereas the same genotype for subjects with bmi 27 kg / m rendered an or of 16.2 ( 95% ci 7.535.2 ) . the corresponding ors in the kpnc study were 5.7 ( 4.08.0 ) and 13.8 ( 4.146.8 ) ( table 4 ) . odds ratios with 95% confidence intervals of developing multiple sclerosis for subjects with different combinations of bmi and the genetic risk factors carriage of the hla - drb1 * 15 allele and absence of the hla - a*02 allele , compared with nonobese subjects carrying none of the genetic risk factors we demonstrate striking interactions between adolescent obesity and 2 potent ms risk genes , properly replicated in an independent material , similar to the interaction between carriage of hla - drb1 * 15 , absence of hla - a*02 , and smoking in the development of ms . the biological explanations for these interactions are far from clear , but the data open up for mechanistically oriented studies . it should be emphasized that interaction measured this way does not necessarily imply evidence for direct protein protein interactions such as in a ligand  receptor pair , but rather indicates that the factors act synergistically in the same pathogenic pathway . in case of the bmi hla ms risk gene interaction , we primarily consider a low - grade chronic inflammation with activation of the innate immune system , promoted by the fat tissue , which may increase the options for hla - restricted activation of autoreactive t cells attacking the cns in ms . it is important to consider potential drawbacks , confounders , or biases in our study . both studies retrospectively gathered information regarding lifestyle factors and personal information such as weight and height . eims primarily included cases who had received the diagnosis within the past year in order to minimize recall bias . moreover , the questionnaire contained a wide range of questions regarding many potential environmental risk factors and no section in the questionnaire was given prime focus . thus , recall bias may have distorted the results . however , a validation study among women in the nurses ' health study found high correlations between recalled and measured past weight , and between reported current height and measured past height . participants slightly underreported weight at 18 years of age with a mean difference of 1.4 kg . there is no reason to believe that the results would be different in other populations . furthermore , body weight at age 30 was available in the american study , and no association was observed between bmi at age 30 and ms risk , which makes recall differences between cases and controls unlikely . any recall bias regarding weight would tend to bias toward the null , thus underestimating the or . the proportion of respondents with regard to participation in eims was 91% for cases and 69% for controls . a potential selection bias may result from the relatively high proportion of nonresponders among the controls . however , this bias is most likely to be modest because the prevalence of smoking among the controls , seen as an indicator of lifestyle , was in line with that of the general population at equivalent ages . furthermore , the increased risk for ms associated with overweight and obesity was similar among those who provided blood and those who did not . the molecular pathways responsible for the observed association between adolescent obesity and ms are still unknown , but there are different hypotheses attempting to explain the association . high levels of circulating 25-hydroxyvitamin d have been associated with reduced risk of ms in early adulthood . total body fat is inversely related to the levels of circulating 25-hydroxyvitamin d and consequently , obese people have lower levels of this metabolite than normal - weight people . the higher risk of ms among subjects who were obese during adolescence is consistent with a protective effect of vitamin d. however , our results based on eims remained unchanged when the analyses were adjusted for sun exposure habits and vitamin d status . furthermore , both sun exposure habits and vitamin d status seem to affect ms risk in adults independently of hla risk genes . we hypothesize that obesity - related inflammatory / immunologic mechanisms contribute to explain the association between adolescent obesity and increased ms risk . obesity induces a state of chronic , low - grade inflammation that arises from the production and secretion of inflammatory mediators driven by adipose tissue macrophages . the number of macrophages present in adipose tissue is directly correlated with adiposity . upon high fat feeding , adipose tissue macrophages undergo a phenotypic switch from an anti - inflammatory m2 polarization state to a proinflammatory m1 polarization . a recently published study demonstrated that the m1/m2 equilibrium in blood and cns favors mild experimental autoimmune encephalomyelitis ( eae ) , whereas imbalance toward m1 promotes relapsing eae . leptin is produced mainly by adipose tissue in proportion to body fat mass and has been considered a link among obesity , metabolic state , and autoimmunity . leptin promotes proliferation of effector t cells and constrains expansion of regulatory t cells , generally switching the phenotype toward a th1 response , promoting the onset and progression of autoimmune responses . in the early stages of ms , an inverse correlation has been demonstrated between serum leptin concentration and the number of regulatory t - cells , which are known to dampen autoreactive responses mediated by cd4 + cells . this protection is reversed by leptin administration and is associated with a switch from th2- to th1-type responses . obesity has been associated with increased susceptibility to inflammatory and autoimmune diseases such as psoriasis , autoimmune thyroiditis , diabetes , and ms . the increased incidence of both diabetes and ms has been parallel with the rise in childhood obesity . the finding of an interaction between obesity and hla genotype with regard to ms supports the hypothesis that the th1-promoting effects of obesity increase the risk of developing ms , in particular among subjects with a genetic susceptibility to the disease . an increased expression of hla class ii genes has been observed in adipose tissue macrophages specifically recruited to adipose tissue . the prime function of the molecules encoded by these genes is antigen presentation to t cells , and strong experimental data suggest that the association between hla genotype and ms risk depends on preferences in peptides allowing cns - directed autoimmunity . we thus hypothesize that by increasing the release of proinflammatory cytokines and promoting th1 responses , and decreasing the number of regulatory t cells , obesity may increase the risk of recruitment of autoaggressive cd4 + cells that target cns autoantigens . obesity in the context of hla risk genes may then further increase the risk of autoaggressive immunity that results in ms . supported by the following grants : the swedish research council for health , working life and welfare , the knut and alice wallenberg foundation , the afa foundation , the swedish brain foundation , the swedish association for persons with neurological disabilities , nih / national institute of neurological disorders and stroke 
 r01 ns049510 , r01 ns0495103 , and nih / niaid 
 r01 ai076544 . a. hedstrm , i. lima bomfim , l. barcellos , m. gianfrancesca , c. schaefer , and i. kockum report no disclosures . t. olsson served on scientific advisory boards for merck - serono , biogen idec , and sanofiaventis ; served as co - editor of current opinion in immunology ; received speaker honoraria from novartis and biogen ; and receives research support from bayer schering , sanofi - aventis , biogen idec , the swedish research council ( 07488 ) , eu fp6 neuropromise ( lshm - ct-2005 - 018637 ) , euratools ( lshg - ct-2005 - 019015 ) , the sderberg foundation , bibbi and nils jensens foundation , the montel williams foundation , and the swedish brain foundation . l. alfredsson receives research support from the swedish medical research council ( k2007 - 69x-14973 - 04 - 3 ) and swedish council for working life and social research ( dnr 2006 - 0655 ) .
objective : we investigated potential interactions between human leukocyte antigen ( hla ) genotype and body mass index ( bmi ) status in relation to the risk of developing multiple sclerosis ( ms).methods : we used 2 case - control studies , one with incident cases ( 1,510 cases , 2,017 controls ) and one with prevalent cases ( 937 cases , 609 controls ) . subjects with different genotypes and bmi were compared with regard to incidence of ms by calculating odds ratios ( ors ) with 95% confidence intervals ( cis ) employing logistic regression . 
 potential interactions between genotypes and bmi were evaluated by calculating the attributable proportion due to interaction.results:in both cohorts , a significant interaction was observed between hla - drb1 * 15 and obesity , regardless of hla - a*02 status . 
 similarly , there was a significant interaction between absence of a*02 and obesity , regardless of drb1 * 15 status . in the incident cohort , 
 obese subjects with the most susceptible genotype ( carriage of drb1 * 15 and absence of a*02 ) had an or of 16.2 ( 95% ci 7.535.2 ) compared to nonobese subjects without the genetic risk factors . 
 the corresponding or in the prevalent study was 13.8 ( 95% ci 4.146.8).conclusions : we observed striking interactions between bmi status and hla genotype with regard to ms risk . 
 hypothetically , a low - grade inflammatory response inherent to obesity synergizes with the adaptive , hla molecule  restricted arm of the immune system , causing ms . 
 prevention of adolescent obesity may thus lower the risk of developing ms , predominantly among people with a genetic susceptibility to the disease .