Using a metabolomics approach, this study, for the first time, identified 108 short-term metabolites and 122 long-term metabolites that were significantly associated with the infection of SFTSV. Pathway analysis revealed three noteworthy metabolic pathways, including GPL in short-term effect, and pentose phosphate pathway, and alanine, aspartate, and glutamate metabolism in long-term effects. Furthermore, cytokines were mainly correlated with metabolites on the pentose phosphate pathway and alanine, aspartate, and glutamate metabolism, with weak correlations observed with lipids in GPL.
Lipids constitute most of the cell membrane bilayers and modulate various biological responses such as inflammation, immunity, cell proliferation, and apoptosis(Zeng et al., 2017). Sixty-four lipids and lipid-like molecules were identified as short-term metabolites, suggesting substantial alternation of lipids metabolism after SFTSV infection. GPL was the sole significant metabolic pathway identified among the short-term metabolites, and predominantly consisted of PC and LysoPC. PC and LysoPC are crucial membrane components engaged in signal transduction and immune regulation(Papangelis & Ulven, 2022), where LysoPC is capable of binding to G protein-coupled signaling receptors and Toll-like receptors to play a pro-inflammatory role(Ren et al., 2022). However, we observed only a weak association between lipids in GPL and cytokines, suggesting that lipids alternation in GPL may not be directly related to inflammation. It has been shown that RNA viruses may disrupt PE to build membrane-bound viral replicase complexes for robust replication(Xu & Nagy, 2015). Many viruses also induce the remodeling of lipid metabolism in host cells, which is used to construct their cytosolic membranes and promote replication(Yan et al., 2022; Yuan et al., 2019). In this study, we observed decreased plasma levels of most lipids in GPL, suggesting that SFTSV may employ similar mechanisms to induce remodeling of host lipid metabolites for its own replication, resulting in reduced plasma lipids levels and abnormal alterations in GPL. However, further investigations are required to establish the precise mechanisms.
The pentose phosphate pathway and the alanine, aspartate and glutamate metabolism were found to be abnormal not only in the acute phase but also did not return to normal levels after SFTSV clearing. The pentose phosphate pathway is a key pathway in central carbon metabolism and is considered as a major regulator of redox homeostasis, biosynthesis and a major source of nicotinamide adenine dinucleotide phosphate (NADPH), which is required for scavenging reactive oxygen species (ROS)(Ge et al., 2020; Patra & Hay, 2014). Chemokines such as MIP-1α and MIP-1β, as well as VEGF, have been shown to promote ROS production and induce oxidative stress(Cheng et al., 2019; Tatara et al., 2009). We observed the increased concentrations of these cytokines during the convalescent period, suggesting a long-term presence of oxidative stress. Furthermore, metabolites within the pentose phosphate pathway exhibited significant upregulation in both acute and convalescent phases and were correlated with various cytokines, suggesting that the activation of the pentose phosphate pathway may be related to cytokines and oxidative stress(Hu et al., 2018; Park et al., 2021).
The alanine, aspartate and glutamate metabolism also exhibited long-term effects after SFTSV infection, with significant metabolites including α-ketoglutaric acid, succinic acid, L-aspartic acid, D-aspartic acid, and L-asparagine. α-Ketoglutarate is a rate-limiting intermediate in the tricarboxylic acid (TCA) cycle that plays a vital role in cellular energy metabolism and serves as a significant source of glutamate and glutamine(Wu et al., 2016). α-Ketoglutarate is generated with γ-aminobutyric acid (GABA) to succinate semialdehyde in the action of mitochondrial GABA transaminase (GABA-T), which is further dehydrogenated to succinic acid [38]. Succinic acid inhibits lipid peroxidation in mitochondria and has an elimination effect on peroxides but does not participate as a substrate in the regeneration of NADPH(Tretter et al., 2016). The elevated levels of succinic acid in the acute and convalescent phases of SFTS in the present study might be associated with the mitigation of oxidative stress, whereas α-ketoglutarate was used for the synthesis of succinic acid and appeared to be downregulated. Although α-ketoglutarate can be self-sufficient in the adult stage, this may not be the case in the human senescent stage(Chin et al., 2014). Given that the susceptible population of SFTS is mainly middle-aged and elderly individuals (Huang et al., 2021), the use of α-ketoglutarate as a dietary supplement for SFTS patients after hospital discharge may help minimize the long-term physical effects of SFTSV. However, further verification and evidence are necessary to support this claim.
Several limitations have to be considered. First, our sample size is relatively small from a statistical viewpoint. Second, we were only able to know that participants in IgG + HC had been asymptomatically infected by SFTSV, but the specific time of infection was not available, which may have influenced the results. Third, dietary composition affects plasma metabolite profiles, and although our sample was from the same region with similar dietary habits, we lacked this information to rule out bias.
In summary, our study aimed to investigate the short-term and long-term metabolic and cytokine responses after SFTSV infection. We observed a downregulation in the levels of several GPL lipids, especially PC and LysoPC, which may be attributed to SFTSV self-replication. The altered concentrations of certain cytokines, the activation of two metabolic pathways persisted during the convalescent period, and their correlations indicate the prolonged presence of oxidative stress in the body, which can have a lasting impact on the health of SFTS patients even after their recovery.