Discussion
In this study, we sought to identify potential biomarkers to differentiate acute FPIES from mimicking diseases also associated with vomiting, such as enterocolitis or IgE-mediated anaphylaxis. Our study showed that the serum levels of IL-2 and IL-10 were both prominently elevated in patients with acute FPIES compared with those with enterocolitis or IgE-mediated anaphylaxis. Serum IL-2 and IL-10 increases were only detected in symptomatic FPIES patients. These increases were also confirmed in the symptomatic phase by the OFC test. The serum levels of these two cytokines were previously reported to be elevated in OFC-positive FPIES patients,4, 5 which is in line with our results.
Several studies on oral immunotherapy have demonstrated that IL-10 levels increase in the course of tolerance acquisition, suggesting a role of this cytokine in the induction of oral tolerance.5,9 Caubet et al.5reported increases in the serum levels of IL-10 in OFC-positive cases, whereas this increase was not observable in the supernatants of casein-stimulated peripheral blood mononuclear cells skewing toward T cells obtained from FPIES patients positive for milk OFC. This suggests that, in patients with active FPIES, since T cells have a reduced capacity to secrete IL-10, other cell populations such as monocytes produce compensatory IL-10 in response to antigen exposure. It has been reported that the activation of innate immune cells is involved in the pathogenesis of acute FPIES.10 Thus, the increase in serum IL-10 levels that we observed in symptomatic patients could be related to the activation of innate immune cells such as monocytes.
IL-2 is a representative helper T (Th) 1 cytokine, produced by naive and Th cells, that activates T cells, B cells, and monocytes/macrophages.11 Our finding that serum IL-2 was elevated in patients with acute FPIES was consistent with the results of Kimura et al.4, who reported that IL-2 elevation contributes to the activation of Th1 cells, which in turn plays a significant role in FPIES pathophysiology. Several lines of evidence suggest a key role of T cells in the secretion of pro-inflammatory cytokines that have an impact on intestinal permeability.12-16 On the other hand, the role of T cells has been questioned by some studies.5,10,17,18It has been reported that IL-2 signaling might be required for maintaining the homeostasis of regulatory T cells.19Zhou et al.20 recently reported that innate lymphoid cell (ILC) 3-derived, but not T cell-derived, IL-2 is essential for maintaining regulatory T cell immunological homeostasis throughout the gastrointestinal tract. This suggests that increased IL-2 serum levels observed in the very early phase of onset in acute FPIES patients could reflect increased IL-2 production promoted by other mechanisms such as ILC3 mediation in the gastrointestinal tract, although the pathophysiological role of IL-2 remains unknown at the mechanistic level.
Some studies have reported elevated serum IL-8 levels in symptomatic acute FPIES patients.4,5 The serum level of IL-8 was significantly elevated in the acute FPIES group compared with the enterocolitis group. However, there were no significant differences between patients with acute FPIES and IgE-mediated anaphylaxis, suggesting that IL-8 was not a useful biomarker to differentiate acute FPIES from mimicking diseases.
It has been reported that serum IL-2 and IL-10 levels are elevated in severe anaphylactic reactions and correlate with the severity.21 Furthermore, Jiang et al.22 and Newman et al.23 reported cytokine profiles in children with rotavirus infection and patients with norovirus and observed a significant increase in the IL-2 and IL-10 levels compared to that in the corresponding healthy control group or individuals before illness onset. The above-reported increases in IL-2 and IL-10 levels were not as high as those in our acute FPIES patients. Because the half-life of serum IL-2 is very short (6.9 min)24, the timing of sample collection is very important to detect higher levels of serum IL-2.
Lee et al.7 also sought features distinctive of acute FPIES by comparing patients who presented to an emergency department with FPIES symptoms and subjects with enterocolitis or sepsis. In their study, the laboratory data characteristic of acute FPIES included less pronounced CRP elevation, leukocytosis, lymphocytosis, and thrombocytosis; and elevated albumin/globulin ratio. Thus, we compared CRP as well as white blood cell, lymphocyte, and platelet counts in patients with acute FPIES, enterocolitis, and IgE-mediated anaphylaxis, and found no significant differences in these parameters among the three groups (data not shown). This discrepancy might reflect differences in the study design and timing of sample collection. Notably, in the study by Lee et al.7, acute FPIES was determined retrospectively based on the medical record database of an emergency department, and the schedule of sample collection was not specified. In contrast, in the present study, the diagnoses were made in suspected cases by well-trained pediatric allergists based on recent international consensus guidelines3, and the blood samples were all obtained within 3 h after the onset of vomiting.
In this study, we suggest that serum IL-2 and IL-10 may be suitable biomarkers for the diagnosis of acute FPIES. Acute FPIES can cause lethal hypovolemic shock, and thus, the elimination of offensive foods is crucial.25 However, causative foods may be reintroduced to patients unless acute FPIES is diagnosed by reliable biomarkers. We propose serum IL-2 and IL-10 as a convenient biomarker to differentiate acute FPIES from other mimicking diseases, thus avoiding the risk of lethal shock due to future accidental ingestion of the offending foods by the patients.
There are some limitations in this study. First, it is a pilot study conducted with a small sample size in a single center. Multicenter studies from different regions are warranted to confirm the diagnostic accuracy of serum IL-2 and IL-10 measurements for discrimination between acute FPIES and mimicking diseases that are also associated with vomiting. Second, we only considered infectious enterocolitis and IgE-mediated anaphylaxis as non-FPIES conditions. Since there are many other acute FPIES-mimicking diseases2, further analyses need to include other conditions such as necrotizing enterocolitis, sepsis, and lactose intolerance. Third, as we did not use age-matched normal subjects, we have not adjusted the cytokine levels by age.
To our knowledge, this is the first report to identify potential biomarkers capable of discriminating between acute FPIES and other diseases and, therefore, suitable for differential diagnosis. The measurement of serum IL-2 and IL-10 levels may substantially improve the diagnosis of acute FPIES, which now mainly relies on clinical manifestations.