In this pilot study, our results indicate that mucosal dysregulation demonstrated by elevated antibody responses to CD-related enteric microbial antigens is present in a cohort of AS patients. Using a quartile analysis method that allows for a more sensitive evaluation of levels of these CD-related microbial antigens when compared with disease control cohorts, we detected an increased individual antibody response to I2 and ASCA, as well as a higher response to the combination of antibodies in patients compared with controls.
The quartile analysis methodology has yielded significant results in IBD studies utilizing these serologies as indicators of mucosal dysregulation. Demonstrating the strength of this analysis to determine differences in non-IBD but related individuals was previously demonstrated by Devlin and colleagues [17
]. Using quartile sums, the level of antibody responses in CD patients was related to the number of NOD2 variants (0-2) in a given individual. The same analysis was then repeated using unaffected family members of CD patients. Although some family members have elevated levels as defined by cut offs used to differentiated CD from normal controls, overall the levels were much lower than CD patients. However, when the quartile sums were recalculated using the levels of antibodies within entire unaffected family members as the cohort and not CD, the same relation of numbers of NOD2 variants to quartile sums was seen as that in the CD cohort. An important concept from these analyses is that the current laboratory methods are tailored to have optimal sensitivity and specificity for patients with CD. However, there are no established reference ranges or methods for detecting these antibody values in AS subjects. In the future, it may be possible to develop a method to study the AS population at large using appropriate controls.
In addition, we observed that the sum of antibody activity is higher in AS patients compared with controls if ANCA is excluded. In our study, ANCA levels in AS patients were not significantly elevated when compared with controls. In IBD studies where these issues are addressed, pANCA (perinuclear ANCA) positivity is most typically associated with UC as opposed to CD [18
]. This finding from our study is consistent with clinical observations that intestinal inflammation associated with AS more closely resembles the phenotype of CD rather than UC [4
]. Thus, our pattern of serological detection is consistent with what is hypothesized about the relation between AS and CD.
In IBD, detecting the loss of tolerance to certain antigens has diagnostic and prognostic significance. For example, determining the pANCA and ASCA status of IBD patients helps determine if the patient's phenotype will be more consistent with CD or UC with a high specificity [18
]. Furthermore, a pANCA antibody response in a CD patient signifies a predilection towards left-sided large intestine involvement [19
]. Outcome studies have revealed that anti-I2 activity is associated with increased likelihood of fibrostenosing CD, that anti-OmpC activity is associated with increased likelihood of perforation, and patients with loss of tolerance to multiple antigens are more likely to undergo small bowel surgery [15
]. Additionally, anti-CBir1 activity is associated with penetrating, fibrostenosing disease [16
]. Further detection of these serologies have also aided in revealing that mucosal dysregulation exists in unaffected relatives who carry genetic markers for IBD in comparison with controls who do not [17
With these findings reported herein, it is our hope that we can make similar pathologic and clinical associations with the presence of mucosal dysregulation and AS. Although the cause of AS is still unknown, associating mucosal dysregulation with disease onset of AS would bring us closer to potentially discovering the trigger that initiates the disease phenotype called AS. Further, as in IBD, these serologies may help provide clinical information in AS that could indicate which AS patients are more likely to have intestinal inflammation or even develop overt IBD. They may help determine patients who are more likely to have aggressive disease, different phenotypic patterns of ankylosis, or response to biologics.
There are obvious limitations to interpreting the results from this pilot study. First, our sample size is small. A larger study is required to more precisely determine the true prevalence as well as the significance of these serologies in AS. Furthermore, these patients are a random sampling from an AS cohort who are only characterized by the absence of IBD. Further studies will serve to place patients in a clinical context providing disease associations with disease onset and duration, clinical characteristics, disease severity, and treatment response.