The results of the current study do not suggest that prenatal DES exposure has an appreciable influence autoimmune disease development. Considering only verified cases, there was no overall increase in autoimmune disease among women who were exposed to DES prenatally. The rates for SLE and ON appeared similar among DES-exposed and unexposed women. Among women younger than 45 years, a positive association between prenatal DES exposure and RA development was observed. In women 45 years and older, there was an inverse association between DES exposure and RA development. The rates of verified ITP cases appeared higher among prenatally DES-exposed women then among those unexposed. For the observed associations between DES and ITP rates as well as age-specific RA rates, the point effect estimates are based on small numbers and are imprecise.
In general, the results of the analyses considering all retained self-reported autoimmune disease cases were similar to those restricted to verified cases. As with the verified cases, small or no associations between DES exposure and ON or SLE were observed when considering retained self-reported cases of these two diseases. Also, as with the verified cases, the observed association between prenatal DES exposure and ITP was based on very few self-reported cases. The estimate of effect for DES on overall RA development when considering retained self-reported cases was similar to that when the analysis was restricted to verified cases. The directions of the associations observed in the age-specific analyses considering retained self-reported RA cases were also the same as those when only verified cases were considered. There was a positive association between prenatal DES exposure and RA development in younger women. This association is more pronounced in the analysis when only verified cases are considered than that where retained self-reported cases were included. Possibly the effect of DES on RA incidence in the latter analysis is underestimated due to over-reporting among the unexposed women in this age group. This may be suggested by the higher percentages of RA reports among younger unexposed women that were later ruled out by either the participants’ physicians or study investigators.
An inverse association between DES exposure and RA rates in older women was observed in both analyses. Again, the association was more pronounced in the analysis considering only verified cases than in the analysis in which retained self-reported cases were considered. The percentage of reports that were later retracted was higher in the older DES-exposed women than in the older unexposed women. A bias may still exist even after removing these cases from consideration, as was done in the current analysis, since the percentage of women complying with our request for records was higher in older unexposed women. Failure to provide these materials may reflect further over-reporting among the exposed women. Consequently, the inverse association may be stronger than the results currently indicate in this analysis. Alternatively, any difference in over-reporting may be a chance occurrence since the percentages of erroneous reporting or non-compliance are based on small numbers in this age group.
The current study has considerable notable strengths. The large cohort of women followed is unique in that the participants’ prenatal DES exposure status was verified at the onset of their participation, their cooperation in this prospective follow-up has been extensive, and the follow-up has continued for over twenty-five years. Furthermore, the strong participation rates are essentially the same among exposed and unexposed women. Also, the rates of RA and SLE among the verified unexposed women were similar to those observed in other studies. The study has limitations as well. The attempt to verify the reported diagnosis was only fairly successful. Historically, verification of autoimmune disease has been difficult. In one well-designed study of hormonally-related risk factors and RA, the disease was verified in only 8% of the participants who initially reported such a diagnosis. Approximately 22% of those who initially reported RA later retracted it in response to requests for verification materials10
. This may suggest that other forms of arthritis are initially mistaken for rheumatoid arthritis. In the current study, 29.3% of the initial RA reports were later retracted, and only 34.3% of the reporting participants complied with the request for verifying materials.
Other sources of study error exist as well. The low number of verified cases led to imprecise effect estimates. While controlling for known hormonal covariates of autoimmune disease was possible, there are risk factors for which no data were collected. Consequently, the influence of other suspected risk factors for autoimmune disease such as occupational exposure, family history, breastfeeding history, and nutrition on the effect estimates was not controlled in the current study 6, 7, 9–11
. There is no reason, however, to suspect that these factors are differentially distributed between the two exposure groups.
Diethylstilbestrol may have an effect on some autoimmune disease development that is mediated through parity. Specifically the incidence of RA is greater after childbirth8
. Furthermore, prenatal DES exposure has been associated with increased risk of pregnancy complications25
. It therefore follows that if women who are prenatally DES-exposed have a decreased probability of childbirth, an independent risk factor for RA development, they are at decreased risk for this disease. Adjusting for this difference in parity between DES-exposed and unexposed women in evaluating the association of DES and RA results in an estimate of the direct effect of prenatal DES exposure on RA incidence, while blocking any that is mediated by parity 27
The findings of this study are consistent with at least one other report of the effect of prenatal DES exposure on autoimmune disease where the rate of RA was elevated among DES-exposed women relative to that among unexposed women. In that study the average age the cohort participants was below 45 years where the positive association between DES and RA was observed in the current study13
. In the aforementioned study, there were, however, only 6 RA cases reported by exposed women; three of which are included in the current study as is the one case reported by an unexposed woman. Among the three women who were excluded in the current analysis, one did not complete any questionnaire after the cohorts were combined and the remaining two did not report autoimmune disease in any of the questionnaires in any of the follow-up cycles considered in the current study. In two other reports of prenatal DES exposure and overall autoimmune disease rates, no association was observed, which is also consistent with the current study14, 15
Other studies of autoimmune disease risk factors suggest that autoimmune disease development may have a hormonal component. Previous epidemiologic studies have positively and negatively associated RA and SLE risk with parity, lactation, oral contraceptive use and menopausal status6–12
. Animal studies suggest that estrogen exposure may play a role in autoimmune diseases, and that prenatal exposure to estrogenic insults may exacerbate adverse autoimmune effects given the immaturity of the prenatal immune system28
. Thymoctyes (T-cells) extracted from mice at 15 days of gestation and treated with DES underwent apotosis more readily than untreated cells3
. Thymocytes harvested from mice fetuses that were DES-exposed showed less differentiation and maturity than those that were not exposed1
. In mice prenatally exposed to DES and again exposed to DES as adults, apoptosis occurred more readily in highly differentiated T-cells compared to those in unexposed mice2
. In one study in humans, mononuclear cells from prenatally DES exposed women were observed to have an increased immune response to mitogen stimulation suggesting that prenatal DES exposure may play a role in the etiology of human autoimmune disease5
The three-fold increase in RA associated with DES exposure in the current study among women less than 45 years could be expected if the increase in RA immediately after childbirth observed in at least one study is more pronounced among DES-exposed women8
. The paucity of cases in the current study, however, precludes exploration of the modification of this parity effect by prenatal DES exposure. Furthermore, most of the cohort is no longer of reproductive age, and therefore this association will change very little with further follow-up. The inverse association between prenatal DES exposure and RA observed in women older than 45 years was unexpected. Possibly, DES-exposed women develop the disease earlier in their lifetime than unexposed women. This association could also be spurious given the small number of verified RA cases among older women. Nonetheless, as evidenced by the effect estimate based on the results for retained self-reported RA cases among older women, currently, prenatal DES exposure appears unlikely to be positively associated with RA development in older women. The cohort, however, is beginning to age into the period when RA is more commonly diagnosed24
and continued follow-up during this age period is warranted.
In conclusion, considering only verified cases, DES does not appear to influence overall autoimmune disease development. Possibly RA rates are higher in younger women who were prenatally DES-exposed than those unexposed. There are, however few verified cases of RA in this age group and the effect estimate is imprecise. There are similar associations between prenatal DES exposure and autoimmune diseases in the analyses considering self-reported autoimmune disease cases. Some of these, specifically those for prenatal DES exposure and overall RA development and overall autoimmune disease development, are more pronounced in the analyses considering self-reported cases. The possibility exists that this may be attributable to differential reporting. Further study of this cohort and more aggressive case verification is needed to clarify what the underlying reasons, if any, are for these associations.