In this first-of-kind study, we prospectively evaluated the relationship between circulating anti-MUC1 antibody levels and risk of ovarian cancer. We observed a statistically significant interaction by age for the association between anti-MUC-1 antibodies and risk of ovarian cancer, with an inverse association among women <64 years at assessment (i.e., the controls had higher antibody levels on average than the cases) and a positive association in those ≥64 years (i.e., the controls had lower antibody levels on average than the cases). A decline of anti-MUC1 antibodies in healthy individuals with increasing age has been observed previously (4
). This may be due to a mechanism called immunosenescence, which reflects a declining immune response toward many common antigens with increasing age and time since antigen presentation, such as after immunization (17
). Many of the exposures that we have postulated could lead to protective antibodies occur during the reproductive years and their effect could wane with an increasing time interval from those events. This interpretation would be compatible with data from published epidemiologic studies that the protective effect of OC use (19
) and possibly with breast feeding (21
) and tubal ligation (22
), attenuates over time. In addition, the drop in antibody levels among controls matches age-specific increases in risk for ovarian cancer in the population; i.e. about a twofold change between women <64 compared to those ≥64, (27 vs. 56 cases per 100,000 in NHS data).
The events associated with antibody levels among controls found in our study include known risk factors for ovarian cancer. Notably, women who had a tubal ligation had about 20% higher antibody levels compared to those who had not; this is similar to a prior finding among 705 healthy women (9
). Explanations for the protective effect of tubal ligation on ovarian cancer risk have focused on blockage of potentially harmful contaminants from reaching the ovaries; however, our results suggest the injury associated with tubal ligation may trigger the release of MUC1 into the circulation and development of protective anti-MUC1 antibodies.
Increasing number of ovulatory cycles, which has been linked with higher risk for ovarian cancer, was associated with lower antibody levels in this study and in one prior study examining this association (23
). The ability of uninterrupted ovulations to lower anti-MUC1 antibody levels may be due to an absence of reproductive events, like OC use and breastfeeding that may have otherwise elicited antibody production. In addition, MUC1 has higher expression in secretory endometrium (24
) and circulating levels are highest during the luteal phase (25
). Thus, it is possible that repeated cyclic variation of MUC1 in blood over many years could lead to an immune tolerance that ultimately lowers anti-MUC1 antibody levels.
A prior study of 705 women without ovarian cancer noted associations between anti-MUC1 antibody levels and other factors, including OC use, hysterectomy, breastfeeding, bone fracture, and talc use (9
). Although we did not observe statistically significant associations for these exposures in this study (possibly due to the smaller number of controls), the direction of these relationships were similar to the prior study. For example, talc use was associated with lower anti-MUC1 antibody levels in both studies, fitting with its role as a factor that has been shown to increase risk for ovarian cancer (26
). Like ovulatory cycles, regular talc use may represent an exposure that can trigger a chronic inflammatory response, lead to MUC1 antigenemia, and result in MUC1-specific immune tolerance. Clearly, additional research is necessary to fully understand how “acute” events translate into protection and “chronic” events into increased risk and how these may relate to difference in the association by age. We believe this will require an understanding of the effects of the events and age on cellular immune reactions that may be most critical for cancer elimination or escape.
We suggested that the decline in anti-MUC1 antibodies in controls reflected immunosenescene. Clearly, the higher antibody levels observed among cases who were ≥age 64 at blood draw () requires a different interpretation. First, we point out that the level of antibodies in the oldest age group was about the same as the younger controls. This may indicate that there is a subset of women who do not have the expected decline in antibody levels as they age, perhaps due to chronic antigen challenge provided by an ongoing chronic inflammatory condition or a developing cancer. In addition, we note that cases who were ≥age 64 at blood draw had an age at diagnosis of 75 and overlapped with the group in who had the greatest lag between age at blood draw and cancer diagnosis and highest antibody levels. Unfortunately, our data cannot clarify whether the higher antibody levels in older cases represent a persistence of high antibodies that may delay onset of disease or new appearance of antibodies reflecting a chronic inflammatory response. Sorting the possibilities out will require larger studies with serial measurements of anti-MUC1 antibodies levels across the important mid-60’s age period. Besides anti-MUC1 antibodies, it may be necessary to measure free MUC1 antigen and immune complexes involving the antigen and antibodies. These have been described and could interfere with assays for either free antigen or free antibody (30
). A comprehensive study might also establish whether the appearance (or persistence) of high levels of anti-MUC1 antibodies in later life might represent an early detection signal many years in advance of ovarian cancer.
In the current study, we focused our attention on MUC1 because of the wealth of data that exists on its expression in normal tissues versus cancer, inflammation, and other pathological conditions (1
). We measured antibodies against the unglycosylated protein core of MUC1, commonly found in tumors. However, there are other aberrantly glycosylated MUC1 epitopes closely associated with tumor MUC1 that might be even more immunogenic eliciting additional protective immunity (32
). It also is possible that additional ovarian cancer antigens can be presented to the immune system through other non-malignant events and lead to the creation of antibodies and cellular immunity that would also be associated with a reduced risk for ovarian cancer. MUC1 thus represents one of many targets of immunosurveillance that collectively can elicit protective immunity and lower cancer risk. Development of new high throughput technologies may allow identification of antibodies that are present in circulation before ovarian cancer diagnosis and the exposures or events that are associated with increased antibody levels and define serologic signatures of cancer risk reduction. Further, understanding how antibodies against MUC1 or other antigens are formed during exposures or events that predate ovarian cancer development may suggest ways to duplicate or amplify these protective effects through vaccination.
A limitation of this study is that we were unable to measure MUC1-specific memory T cells due to the lack of archived viable white cells. However, T-cell activation is indirectly measured through the presence of various isotypes of anti-MUC1 antibodies, all of which require antigen-specific helper T cells for their generation. We also lacked information on certain exposures that previously have been associated with anti-MUC1 antibody levels, such as mastitis, mumps parotitis, and endometriosis. The relatively small number of cases in this study clearly limited power to examine the effect of antibodies by histologic type of ovarian cancer or by other potential effect modifiers; and the relatively high between-plate CVs, often seen for immune-based assays like that used here, also contributed to a lack of precision.
Although ours is the first prospective study investigating anti-MUC1 antibodies in relation to ovarian cancer, one study examined humoral immune responses to MUC1 among BRCA carrriers and related it to subsequent breast cancer, another MUC1-expressing cancer (34
). Women who carried a mutation had significantly lower anti-MUC1 Ig antibodies compared to controls; and carriers who went on to develop breast cancer had lower (although not significantly so) antibody levels than carriers not found to have cancer at prophylactic mastectomy. Thus, this study fits with our observations in suggesting that anti-MUC1 antibodies correlate with a known risk factor for the cancer and risk for developing disease.
In conclusion, our prospective study provides important evidence that circulating anti-MUC1 antibody, either as an effector mechanism or as a marker of a more comprehensive immune response, may be associated with a lower risk of ovarian cancer in women <64 years and offers an explanation for the mechanism of action of several factors known to alter risk of ovarian cancer. These data further suggest a new paradigm that immune surveillance may play an important role in ovarian carcinogenesis. Larger studies are necessary to clarify the role of MUC1 immunity in ovarian cancer, particularly in relation to the potential differential association by age at antibody assessment. Ultimately this avenue of research could lead to new methods for ovarian cancer prevention or early detection.