| Summary: | Risk of celiac disease (CD) is increased in relatives of CD patients due to genetic and possible environmental factors. We recently reported increased seropositivity to anti-<i>Saccharomyces cerevisiae</i> (ASCA), <i>Pseudomonas fluorescens</i>-associated sequence (anti-I2) and <i>Bacteroides caccae</i> TonB-linked outer membrane protein (anti-OmpW) antibodies in CD. We hypothesized these markers also to be overrepresented in relatives. Seropositivity and levels of ASCA, anti-I2 and anti-OmpW were compared between 463 first-degree relatives, 58 untreated and 55 treated CD patients, and 80 controls. CD-associated human leukocyte antigen (HLA)-haplotypes and transglutaminase (tTGab) and endomysium (EmA) antibodies were determined. One or more of the microbial antibodies was present in 75% of relatives, 97% of untreated and 87% of treated CD patients and 44% of the controls. The relatives had higher median ASCA IgA (9.13 vs. 4.50 U/mL, <i>p</i> < 0.001), ASCA IgG (8.91 vs. 5.75 U/mL, <i>p</i> < 0.001) and anti-I2 (absorbance 0.74 vs. 0.32, <i>p</i> < 0.001) levels than controls. There was a weak, positive correlation between tTGab and ASCA (r = 0.31, <i>p</i> < 0.001). Seropositivity was not significantly associated with HLA. To conclude, seropositivity to microbial markers was more common and ASCA and anti-I2 levels higher in relatives of CD patients than controls. These findings were not associated with HLA, suggesting the role of other genetic and environmental factors.
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