In our research, 10 adolescent patients with CSU were treated with three sessions of omalizumab at 4-week intervals. We found that omalizumab helped to relieve urticarial activity and was effective for treating adolescent patients with CSU who are resistant to antihistamines. As shown in Table 1, ESR and CRP levels decreased significantly after omalizumab treatment. Additionally, the specific IgE of the two male patients decreased to normal range after treatment. Moreover, allergic symptoms and signs of CSU in our study were dramatically alleviated after omalizumab treatment. Several patients in our study claimed that there were no wheals or pruritus on their skin a few days before the last follow-up. As shown in Fig. 2, UAS7 at 4, 8, and 12 weeks after the first omalizumab treatment decreased dramatically compared to the baseline level.
The richness, evenness, and composition of the gut microbiota of the 10 patients with CSU before and after omalizumab treatment were compared based on 16S rRNA gene profiling. In our research, we found no significant differences on alpha diversity in patients with CSU before and after omalizumab treatment (Fig. 4), which suggests that omalizumab did not change the richness and evenness of the gut flora. Zhang et al. detected that there was no significant difference between the alpha diversity of patients with CSU and healthy individuals . Therefore, it could be inferred that the alpha diversity of the gut microbiota was not correlated with improvement in outcomes or complete remission of CSU after treatment. We discovered that the top four taxa in the relative abundance at the phylum level in the adolescent patients with CSU before omalizumab treatment were Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, which were highly in accordance with the results presented by Eckburg et al. and Wang et al. [15, 21]. Our results showed that the top four gut flora detected in patients with CSU before omalizumab treatment was also detected after treatment (Fig. 5a).
Alpha diversity demonstrated the distribution of gut flora within a sample whereas beta diversity is often used for the comparison of the diversity and similarity of gut flora between two different samples. We found that beta diversity was significantly different in adolescent patients with CSU before and after omalizumab treatment. The two groups clustered independently in the PCoA diagram (Fig. 6). We then analyzed the specific differences in the microbial communities at different levels and found that the relative abundances of Alphaproteobacteria and Betaproteobacteria at the class level which belong to the Proteobacteria at the phylum level decreased after omalizumab treatment. Although Proteobacteria are commonly found in the human intestine, changes in their relative abundance can cause microbiota dysbiosis, even in relation to diseases. It was reported that the abundance of Proteobacteria was increased in patients with asthma and allergic diseases [22, 23]. Lu et al. found that Proteobacteria increased in patients with chronic urticaria . Zhang et al. found that the relative abundance of Proteobacteria, including Alphaproteobacteria and Betaproteobacteria increased significantly in patients with CSU compared to healthy controls . Research findings suggested that the abundance of Proteobacteria was associated with the development of inflammatory skin diseases. When the abundance of Proteobacteria increased, the permeability of the gut mucus was enhanced, thereby allowing the bacteria to enter the gut mucus to cause damage of the gut mucosal barrier and leading to inflammatory skin diseases. In our study, we speculated that a higher abundance of Alphaproteobacteria and Betaproteobacteria belonging to Proteobacteria may have an important role in CSU etiology, and the reduction of both taxa indicated an improvement in urticarial severity. Notably, it has been reported that the abundance of Proteobacteria can be altered by drug action. For example, highly active antiretroviral therapy appeared to reduce these bacteria in HIV-positive patients, and atorvastatin had relatively increased the abundance of Proteobacteria in high-fat-diet rats and patients with atherosclerosis . As a kind of gut flora regulated by drugs, Proteobacteria is an excellent therapeutic target. We speculate that Alpha- and Betaproteobacteria may be potential therapeutic targets for omalizumab. The reduction in these bacteria may be a marker of CSU improvement in omalizumab treatment. More mechanistic studies are needed to confirm that omalizumab can change the abundance of Proteobacteria which is related to the occurrence and improvement of chronic urticaria. Moreover, Burkholderiales with significantly higher abundance were found in the low IgE asthma group than in the high IgE asthma group at the order level. As a result, the experts inferred that a potential correlation between the relative abundance of Burkholderiales order and total serum IgE from allergic diseases may exist and needed further investigation . Tsai et al. showed that compared with healthy controls, there was more abundant of the genus Rhodococcus which belongs to the order of Actinomycetales and the abundance of Rhodococcus was positively associated with total serum IgE levels in children with allergic conditions . Another study examined the changes at the genus level reported a higher abundance of Sphingomonas which belongs to the Sphingomonadales order, in 45 food-sensitized infants . According to these results, there seems to be an association among Burkholderiales, Rhodococcus, Sphingomonas and allergic diseases. We found that the abundance of the genera Burkholderia, Rhodococcus, and Sphingomonas decreased in patients with CSU, accompanied by a reduction in specific IgE in two male patients after omalizumab treatment. We hypothesized that omalizumab may reduce the abundance of Burkholderia, Rhodococcus, and Sphingomonas and alleviate the signs and symptoms of CSU.
In current clinical research, the intake of probiotics and/or prebiotics to modify the gut microbiota has been reported . Several researches have proved that Bifidobacterium and Lactobacillus are favorable in the improvement of chronic urticaria. The relative abundance of Lactobacillus were significantly higher in fecal samples from healthy controls than in those from patients with chronic urticaria . It has been revealed that the Th1/Th2 balance could be modulated by the probiotic bacteria, as a result, the development of inflammatory diseases including the allergies could be prevented . Experts have reported the administration of Bifidobacterium and Lactobacillus as probiotics to alleviate inflammatory responses by ameliorating dysbiosis [32, 33]. However, some studies have reported that the therapeutic efficacy of probiotic treatment differs among patients with chronic urticaria. Nettis et al. found that the probiotic therapy with Lactobacillus and Bifidobacterium was not advantageous for most patients with CSU by evaluating the efficacy of the therapy. According to our study, no differences in Bifidobacterium and Lactobacillus genera were in adolescent patients with CSU before and after omalizumab treatment, which demonstrated that the administration of omalizumab did not affect these bacteria in the gut. Based on the evidence presented by the experts above, a further study with large samples to investigate the efficacy of combined therapy with omalizumab and probiotics containing Bifidobacterium and Lactobacillus is warranted.
In the present study, our functional prediction results showed that the differences in patients before and after omalizumab treatment were mainly in the dioxin and xylene degradation pathways. Dioxins are found worldwide in the environment and accumulate in the food chain, mainly in the fatty tissues of animals. Xylene is an aromatic hydrocarbon which is a potential occupational hazard  and has been reported to possibly cause urticaria [36, 37]. However, no reports of urticaria associated with dioxins have been reported. We assumed that dioxin and xylene, which are not routinely tested as allergens, could be potential triggers for CSU. With the improvement of the disease treated with omalizumab, the abundance of dioxin and xylene degradation pathways decreased. A review of the contact history and allergen test results for dioxin and xylene is necessary before a diagnosis of CSU is made.
One limitation of this study was the small sample size and limited age group (i.e., teenagers) of participants. Larger sample sizes, including participants of all ages, will produce more reliable results. Additionally, more advanced techniques, such as metagenomics, can further extend and verify the 16S rRNA sequencing analysis results, and metabolomics analysis can be used to find more detailed metabolic and functional profiles associated with gut microbiota after omalizumab treatment.