In recent years, mRNA-miRNA integration analysis has been conducted for various chronic inflammatory diseases such as breast cancer, primary myelofibrosis, and idiopathic pulmonary fibrosis, and research on miRNAs that regulate pathological conditions is ongoing [15–17]. However, there are few reports for acute inflammatory diseases, and there are no reports on integrated mRNA-miRNA analysis in patients with CAP-associated sepsis to our knowledge. In this study, we performed the first ever integrated analysis of mRNA and miRNA in CAP-associated sepsis and elucidated that miRNA causes T-cell exhaustion through the RNA interference action of mRNA.
Pathogens are recognized by the host innate immune system via an array of PRRs. These PRRs recognize molecules released by both pathogens, i.e., PAMPs and injured cells from the human host, i.e., damage-associated molecular patterns. In most cases of infection, the burden of the pathogen is diminished and eventually eliminated by the innate immune system. However, pathogen persistence will lead to dysregulation of the host immune response, and when the pathogen is persistent, the infection becomes severe, and sepsis occurs due to the poor immune response [18].
Analysis of the expression variation of mRNAs and miRNAs specifically expressed in CAP-associated sepsis compared to the HCS revealed that mRNAs, miRNAs, and miRNA-targeted mRNAs showed differences in gene expression between the HCS and the CAP patient group (Fig. 2A, B). In addition, compared to the HCS, both mRNA and miRNA target mRNA-related signals, the inflammasome pathway, MSP-RON (macrophage-stimulating protein-recepteur d'origine Nantais) Signaling in Macrophages pathway, CREB Signaling in Neuron, and Factors Promoting Cardiogenesis in Vertebrates in the CAP patients. This suggests that systemic inflammatory responses are triggered by the immune response to infectious invasion and that a variety of inflammatory signals may be fluctuating in patients with CAP-associated sepsis.
PD-1 plays an important role in suppressing immune responses and promoting self-tolerance by regulating T-cell activity, activating antigen-specific T-cell apoptosis, and inhibiting regulatory T-cell apoptosis. PD-L1 is a transmembrane protein that is considered a co-inhibitor of immune responses and binds to PD-1 to suppress the proliferation of PD-1-positive cells, inhibit cytokine secretion, and induce apoptosis [19]. The PD-1/PD-L1 pathway also inhibits immune checkpoint [15]. In recent years, activation of anti-tumor immunity by inhibiting immune checkpoints in the PD-1/PD-L1 pathway has been considered a promising approach for cancer therapy, and research is underway [20].
In contrast, PD-1 on T cells and PD-L1 on monocytes have been found to be upregulated in patients with severe sepsis and septic shock. PD-L1 signaling may play an important role in sepsis-induced immunosuppression by acting on exhausted T cells and releasing immunosuppressive molecules [18, 21–23]. Mice in which PD1-PDL1 interaction was inhibited also showed improved survival after induction of sepsis. This suggests that T-cell exhaustion may play a detrimental role [23].
Patients with sepsis have suppressed CD4 + T helper 1 (TH1) cells, TH2 cells, and TH17 cells. Postmortem studies of patients who die of sepsis show both T-cell depletion and signs of T-cell exhaustion [23]. The anti-inflammatory environment of sepsis is also the result of altered immune cell positioning. Suppression of the inflammatory Th1 cell and monocyte responses further enhances the anti-inflammatory environment by reducing the expression of proinflammatory cytokines such as TNF, IL-1β, and IL-6 [18]. Whole blood RNA-Seq analysis in this study showed that PD-1 and PD-L1 cancer immunotherapy signaling was the most activated and Th1 signaling was the most inactivated in the CAP patients compared to the HCS (Fig. 3A, B).
Upstream regulator analysis in IPA showed mRNA expression of TNF, which is involved in PD-1 and PD-L1 cancer immunotherapy signaling, and IL-6, which is involved in Th1 signaling (Fig. 3I, J), suggesting that activation of these two signals is important in the pathogenesis of pneumonia-associated sepsis This suggests that activation of the two signals is important in the pathogenesis of sepsis derived from pneumonia and is consistent with reports that PD-1 activation is accompanied by suppression of cellular functions, especially that of T cells, and inactivation of the Th1 pathway, and that strong T-cell exhaustion occurs during the acute phase of pneumonia.
There are also reports of diverse associations between infections and human leukocyte antigens (HLA), and in sepsis, an association with HLA-DQB1 has been suggested [24]. T-bet (TBX21) and GATA-3, specific transcription factors for Th1 and Th2 cells, have been reported to be significantly lower in septic patients than in normal controls [25].
PD-L1 cancer immunotherapy signaling in mRNA was significantly suppressed in the septic patients compared with the HCS, and in the heat map of the Th1 pathway, the expression of T-bet (TBX21) and GATA-3 expression in patients with CAP-associated sepsis was significantly suppressed compared with the HCS, consistent with these reports (Fig. 2C, D). The same trend can also be observed in miRNA-targeted mRNA, suggesting the involvement of miRNAs in these mechanisms (Fig. 3G, H).
Among the upstream regulators inferred from the upstream analysis, miRNAs included in the PD-1 and PD-L1 cancer immunotherapy signals were miR-1275 and miR-3103-5p. Nine types of miRNAs were also included in the same signal: miR-148b-5p, miR-184, miR-199a-5p, miR-221-5p, miR-30a-3p, miR-374b-5p, miR-3909, miR-548ae-3p, and miR-6503-3p, and these miRNAs were associated with the severity of pneumonia (Fig. 4A). Among others, elevated miR-184 is speculated to play a role as a regulator of oral and lung cancers, and its expression in this signal is consistent with the results of previous reports [26, 27]. It is also considered to be a potential biomarker not only for neoplastic lesions but also for the diagnosis of CAP [28].
There were no miRNAs applicable for upstream analysis in the Th1 signal. Twelve miRNAs in the same signal, miR-148a-5p, miR-148b-5p, miR-184, miR-192-5p, miR-199a-5p, miR-221-5p, miR-30a-3p, miR-374b-5p, miR-3909, miR-4746-5p, miR-548ae-3 p, and miR-6503-3p, were associated with severity of pneumonia (Fig. 4B). Among them, elevated hsa-miR-199-5p was found to be involved in the progression of neoplastic lesions such as lung cancer and oral squamous cell carcinoma [29, 30]. Furthermore, in a mouse model of sepsis, mmu-miR-199-5p correlated with the inflammatory response process of macrophages in innate immunity and surfactant protein D and exacerbates intestinal barrier dysfunction by inhibiting the NF-κB pathway [31, 32]. These findings suggest that elevation of hsa-miR-199-5p may also be associated with inflammation in sepsis.
MiR-192-5p upregulation has also been noted in sepsis-induced myocardial injury and acute kidney injury and was found to be elevated in sepsis cases compared to those of systemic inflammatory response syndrome [33, 34]. This suggests that the elevation of miR-199-5p identified in this study may also be associated with the severity of CAP-associated sepsis.