High Calprotectin concentration in amniotic fluid predicts intra-amniotic infection

The study carried out by N. Abersek et al. found Calprotectin concentrations in amniotic fluid were significantly higher in the intra-amniotic infection group compared with the other groups. Moreover, the bacterial presence in the amnio-chorionic niche was higher in IAI group as published in European Journal of Obstetrics & Gynecology and Reproductive Biology.

A complex network of different overlapping pathophysiological pathways, including molecular and genetic factors, contributes to the occurrence of PTL. Intra-amniotic inflammation (IAI), defined as presence of intra-amniotic inflammatory mediators in amniotic fluid, is accompanied in most PTL cases. This inflammation may be triggered by microbial invasion of the amniotic cavity (MIAC), called intra-amniotic infection or other non-microbial immunologically mediated processes, defined as sterile IAI. Sterile IAI is determined by elevated concentrations of amniotic fluid interleukin (IL)-6 in the absence of MIAC. The mechanism of sterile IAI is still not completely understood but damage and infection of the fetal membranes and placenta are believed to be among the inducing factors. During infection and inflammation leukocytes invade amniotic cavity, neutrophils and macrophages being the most frequent. Their amount differs depending on the phenotype of inflammation and they are more abundant in infection compared to sterile IAI.

Endogenous molecules called alarmins are released from leukocytes, triggering inflammatory responses through pattern recognition receptors (PRR). Calprotectin is one of the proteins, secreted from cytosol of immunocompetent cells. It is widely used in the followup of inflammatory processes and has even previously been associated with both IAI and spontaneous PTD.

The aim of the study carried out by N. Abersek et al. was to evaluate amniotic fluid calprotectin concentrations with respect to the presence and absence of sterile IAI and intra-amniotic infection. The second aim was to assess the presence of microorganisms and/or their nucleic acids in the amnio-chorionic niche with respect to the presence and absence of sterile IAI and intra-amniotic infection.

Seventy-nine women with singleton pregnancies and preterm labor with intact membranes (PTL) were included in the study. Amniotic fluid was collected at the time of admission by amniocentesis and calprotectin levels were analyzed from frozen/thawed samples using ELISA. Interleukin (IL)-6 concentration was measured by point-of-care test. Samples from amniotic fluid and the amnio-chorionic niche (space between amniotic and chorionic membranes) were microbiologically analyzed. Microbial invasion of the amniotic cavity (MIAC) was diagnosed based on a positive PCR result for Ureaplasma species, Mycoplasma hominis, 16S rRNA or positive culture. Intra-amniotic inflammation (IAI) was defined as amniotic fluid point-of-care IL-6 concentration ≥ 745 pg/mL. The cohort of included women was divided into 4 subgroups based on the presence or absence of IAI/MIAC; i) intra-amniotic infection, ii) sterile IAI, iii) intra-amniotic colonization and iv) neither MIAC nor IAI.

Women with intra-amniotic infection had a significantly higher intra-amniotic calprotectin concentration (median; 101.6 µg/mL) compared with women with sterile IAI (median; 9.2 µg/mL), women with intra-amniotic colonization (median; 2.6 µg/mL) and women with neither MIAC nor IAI (median 4.6 µg/mL) (p = 0.001).

Moreover, significantly higher amniotic fluid calprotectin concentration was seen in women who delivered within 7 days (p = 0.003). A significant negative correlation was found between amniotic fluid calprotectin and gestational age at delivery (p = 0.003). Relatively more bacteria in the amnio-chorionic niche were found in the sterile IAI group compared with the other groups.

The main findings of this study were that;

• i)amniotic fluid calprotectin concentrations were elevated in the presence of both sterile IAI and intra-amniotic infection in pregnancies complicated with PTL,
• ii)amniotic fluid calprotectin concentrations were significantly higher in intra-amniotic infection compared to sterile IAI and
• iii)calprotectin concentrations were significantly higher in amniotic fluid of women delivered within 7 days from amniotic fluid sampling compared to delivery after 7 days.

It seems that MIAC itself does not significantly affect calprotectin concentrations. Moreover, this study has shown that amniotic fluid calprotectin concentrations were significantly higher in intra-amniotic infection compared to sterile IAI, indicating that the inflammatory events in amniotic fluid are more intense when caused by microbial agents and not only by alarmins. It has been hypothesized that sterile IAI could be caused by inflammatory mediators originating from the space between the fetal membranes. Bacteria that reside in the amnio-chorionic niche and the subsequent maternal inflammatory response, triggered by this microbial presence, could be the factor that cause inflammation without any bacteria found in the amniotic fluid.

Calprotectin concentrations in amniotic fluid were elevated in both sterile IAI and intra-amniotic infection, where concentrations were higher in cases of intra-amniotic infection. Relatively more bacteria in the amnio-chorionic niche were found in the sterile IAI group compared with the other groups.

Source: N. Abersek et al.; European Journal of Obstetrics & Gynecology and Reproductive Biology 272 (2022) 24–29

https://doi.org/10.1016/j.ejogrb.2022.03.006