Profiles and predictive value of cytokines in children with human metapneumovirus pneumonia

We grouped 59 children tested HMPV-positive by age and found that 75% were younger than 6 months and 92.4% were younger than 2 years. Therefore, the target of HMPV attack is mainly children under the age of two, which further narrows the scope of previous studies showing that children under the age of five are the main HMPV infections [25]. Children under the age of two are the major population for prevention and control of metapneumovirus pneumonia.

To confirm the immune status of children with HMPV infection and predictive ability of Th1/Th2 cytokines in children with HMPV pneumonia, 59 children with a single infection of HMPV were enrolled in this study. Compared with the HMPV pneumonia group and healthy control group, the former had a significantly lower level of IL-2 and higher levels of IL-4, IL-6, IL-10, and IFN-γ. In addition, IL-6 and TNF-α showed increased expression with severe group. Unlike previous studies, our results showed that the level of IL-2 was significantly lower in the HMPV-infected group, which may because IL-2 is not only very important for maintaining and generating regulatory T cells but also promotes the general proliferation of T lymphocytes, augmenting natural killer cells activity, and inhibiting the formation of granulocyte-macrophage colony. Hence, after infection, the level of IL-2 often decreases [21, 22]. IL-10 can deactivate the macrophage cells. The function and proliferation of T cells and natural killer cells can directly be inhibited by IL-10. Furthermore, the growth and differentiation of B cells, mast cells, granulocytes, dendritic cells, keratinocytes, and endothelial cells can be regulated by IL-10 [26, 27]. Alvarez et al. reported that up-regulation of IL-10 was found in mice with HMPV infection [28]. IL-6 is a major pro-inflammatory mediator that induces acute-phase responses and contributes to host defense against infection and tissue damage. Respiratory syncytial virus (RSV), which is closely related to HMPV, had demonstrated a positive correlation between high IL-6 level and the severity of RSV [29]. In addition, there was a research showed HMPV induced a more severe disease in mice than that of RSV [30]. These results were, to some extent, associated with higher level of IL-6. Consistent with our findings, IL-6 plays an important role in HMPV pathogenicity, which may account for the level of IL-6 was significantly increased in the severe group [31]. The relationship between TNF-α and HMPV has rarely been mentioned in previous studies, but some studies have shown that the expression of TNF-α is increased in RSV-infected patients [32]. In our research, the expression level of TNF-α was significantly increased in the severe group, the mechanism may be similar to RSV, but needs to be further explored. TNF-α may be helpful to distinguish MS and SS group of HMPV. The previous research showed that excessive expression of TNF-α can activate the signal transducer and activator of transcription 3 (STAT3) pathway through NF-κB-mediated IL-6. Unconstrained TNF-α production leads to the excessive activation of inflammatory cytokines, forming a cytokine storm [33]. This may be the reason for the high expression of TNF-α in the SS group than that of MS group. Consistent with previous studies, our results indicated that IL-4 levels were elevated in HMPV-infected group. IL-4 is a potent inflammatory response activator. Previous studies have shown that IL-6 may increase IL-4 and TNF-α production to induce inflammatory responses during the Th2 differentiation process [34]. However, IL-4 secreted by Th2 helper cells may be inhibited when pro-inflammatory cytokine levels are elevated. On the other hand, increased IL-4 level may induce suppression of pro-inflammatory cytokines during the late immune response phase [31, 32]. This may lead to various sensitivities among cytokines that may be helpful to differentiate multiple respiratory viruses. Interestingly, it is speculated that a balanced Th1 type immunity may lead to the clearance of HMPV through activation of IFN-γ secreted by T cells [35].

The hospitalization rate of HMPV infection was similar to that of influenza, using cytokines as biomarkers may be able to differentiate between HMPV and IV. In the early period of influenza-virus infection, NK cells produce IFN-γ. In the subsequent immune response, T cells are the major producer of IFN-γ. Influenza-specific effector CD8⁺ T cells produce a series of cytokines, including IFN-γ and TNF-α through a variety of antigen-dependent pathways [36]. Due to less studies on the differences of cytokine profiles between HMPV and IV, our findings suggested that the levels of IFN-γ and TNF-α are higher than those of influenza virus may be the two viruses have different inflammatory mechanisms, which require further research. Apart from of IL-6, IL-1β, IFN-γ, TNF-α and IL-8, few studies reported increased levels of IL-4 after influenza virus infection [36, 37]. This rule consists with our findings that HMPV-positive patients had a significantly higher level of IL-4. According to a study on influenza virus A, the levels of IL-6 in the site of initial virus infection were increased persistently for 6 days, this may be related the level of IL-6 in HMPV-positive patients was lower than that of IVA-positive patients [37].

Our study also has a little limitation. The sample size is not large enough and data collection time span is too long may affect the consistency of results. In further studies, we plan to increase the detection range and number of samples. In addition, the detection capacity of antigen assay we used in diagnosing HMPV is slightly lower than the gold standard PCR test.