Monocytes Represent One Source of Bacterial Shielding from Antibiotics following Influenza Virus Infection

IL-10 Counteracts IFN-γ to Alleviate Acute Lung Injury in a Viral-Bacterial Superinfection Model

Immune activation is essential for lung control of viral and bacterial infection, but an overwhelming inflammatory response often leads to the onset of acute respiratory distress syndrome. IL-10 plays a crucial role in regulating the balance between antimicrobial immunity and immunopathology. In the present study, we investigated the role of IL-10 in acute lung injury induced by influenza A virus and methicillin-resistant Staphylococcus aureus coinfection. This unique coinfection model resembles patients with acute pneumonia undergoing appropriate antibiotic therapies. Using global IL-10 and IL-10 receptor gene-deficient mice, as well as in vivo neutralizing antibodies, we show that IL-10 deficiency promotes IFN-γ-dominant cytokine responses and triggers acute animal death. Interestingly, this extreme susceptibility is fully preventable by IFN-γ neutralization during coinfection. Further studies using mice with Il10ra deletion in selective myeloid subsets reveal that IL-10 primarily acts on mononuclear phagocytes to prevent IFN-γ/TNF-α hyperproduction and acute mortality. Importantly, this antiinflammatory IL-10 signaling is independent of its inhibitory effect on antiviral and antibacterial defense. Collectively, our results demonstrate a key mechanism of IL-10 in preventing hypercytokinemia and acute respiratory distress syndrome pathogenesis by counteracting the IFN-γ response.

Microbial and Immune Regulation of the Gut-Lung Axis during Viral-Bacterial Coinfection

Viral-bacterial coinfections of the respiratory tract have long been associated with worsened disease outcomes. Clinical and basic research studies demonstrate that these infections are driven via complex interactions between the infecting pathogens, microbiome, and host immune response, although how these interactions contribute to disease progression is still not fully understood. Research over the last decade shows that the gut has a significant role in mediating respiratory outcomes, in a phenomenon known as the "gut-lung axis." Emerging literature demonstrates that acute respiratory viruses can modulate the gut-lung axis, suggesting that dysregulation of gut-lung cross talk may be a contributing factor during respiratory coinfection. This review will summarize the current literature regarding modulation of the gut-lung axis during acute respiratory infection, with a focus on the role of the microbiome, secondary infections, and the host immune response.

IFN-γ Drives TNF-α Hyperproduction and Lethal Lung Inflammation during Antibiotic Treatment of Postinfluenza Staphylococcus aureus Pneumonia

Inflammatory cytokine storm is a known cause for acute respiratory distress syndrome. In this study, we have investigated the role of IFN-γ in lethal lung inflammation using a mouse model of postinfluenza methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. To mimic the clinical scenario, animals were treated with antibiotics for effective bacterial control following MRSA superinfection. However, antibiotic therapy alone is not sufficient to improve survival of wild-type animals in this lethal acute respiratory distress syndrome model. In contrast, antibiotics induce effective protection in mice deficient in IFN-γ response. Mechanistically, we show that rather than inhibiting bacterial clearance, IFN-γ promotes proinflammatory cytokine response to cause lethal lung damage. Neutralization of IFN-γ after influenza prevents hyperproduction of TNF-α, and thereby protects against inflammatory lung damage and animal mortality. Taken together, the current study demonstrates that influenza-induced IFN-γ drives a stepwise propagation of inflammatory cytokine response, which ultimately results in fatal lung damage during secondary MRSA pneumonia, despite of antibiotic therapy.

Zinc and SARS‑CoV‑2: A molecular modeling study of Zn interactions with RNA‑dependent RNA‑polymerase and 3C‑like proteinase enzymes

RNA‑dependent RNA‑polymerase (RdRp) and 3C‑like proteinase (3CLpro) are two main enzymes that play a key role in the replication of SARS‑CoV‑2. Zinc (Zn) has strong immunogenic properties and is known to bind to a number of proteins, modulating their activities. Zn also has a history of use in viral infection control. Thus, the present study models potential Zn binding to RdRp and the 3CLpro. Through molecular modeling, the Zn binding sites in the aforementioned two important enzymes of viral replication were found to be conserved between severe acute respiratory syndrome (SARS)‑coronavirus (CoV) and SARS‑CoV‑2. The location of these sites may influence the enzymatic activity of 3CLpro and RdRp in coronavirus disease 2019 (COVID‑19). Since Zn has established immune health benefits, is readily available, non‑expensive and a safe food supplement, with the comparisons presented here between SARS‑CoV and COVID‑19, the present study proposes that Zn could help ameliorate the disease process of COVID‑19 infection.

Influenza sequelae: from immune modulation to persistent alveolitis

Acute influenza virus infections are a global public health concern accounting for millions of illnesses worldwide ranging from mild to severe with, at time, severe complications. Once an individual is infected, the immune system is triggered in response to the pathogen. This immune response can be beneficial ultimately leading to the clearance of the viral infection and establishment of immune memory mechanisms. However, it can be detrimental by increasing susceptibility to secondary bacterial infections and resulting in permanent changes to the lung architecture, in the form of fibrotic sequelae. Here, we review influenza associated bacterial super-infection, the formation of T-cell memory, and persistent lung injury resulting from influenza infection.

NF-κB/TWIST1 Mediates Migration and Phagocytosis of Macrophages in the Mice Model of Implant-Associated Staphylococcus aureus Osteomyelitis

Staphylococcus aureus (S. aureus) infection-induced osteomyelitis is a great challenge in clinic treatment. Identification of the essential genes and biological processes that are specifically changed in mononuclear cells at an early stage of S. aureus osteomyelitis is of great clinical significance. Based on transcriptional dataset GSE16129 available publicly, a bioinformatic analysis was performed to identify the differentially expressed genes of osteomyelitis caused by S. aureus infection. ERBB2, TWIST1, and NANOG were screened out as the most valuable osteomyelitis-related genes (OMRGs). A mice model of implant-associated S. aureus osteomyelitis was used to verify the above genes. We found significantly up-regulated expression of TWIST1 in macrophages and accumulation of macrophages around the infected implant. Meanwhile, S. aureus infection increased the expression of TWIST1, MMP9, and MMP13, and stimulated the migration and phagocytosis function of Raw 264.7 cells. Additionally, knock-down of the expression of TWIST1 by siRNA could significantly down-regulate MMP9 and MMP13 and suppress the migration and phagocytosis ability of macrophages in response to S. aureus infection. Furthermore, we found that NF-κB signaling was activated in Raw 264.7 cells by S. aureus and that inhibition of NF-κB signaling by Bay11-7082 blocked the expression of TWIST1, MMP9, and MMP13 as well as cell migration and phagocytosis evoked by S. aureus. Our findings demonstrate that NF-κB/TWIST1 is necessary for migration and phagocytosis of macrophages in response to S. aureus infection. Our study highlights the essential role of NF-κB/TWIST1 in early innate immune response to S. aureus infection in bone.

Linezolid Attenuates Lethal Lung Damage during Postinfluenza Methicillin-Resistant Staphylococcus aureus Pneumonia

Postinfluenza methicillin-resistant Staphylococcus aureus (MRSA) infection can quickly develop into severe, necrotizing pneumonia, causing over 50% mortality despite antibiotic treatments. In this study, we investigated the efficacy of antibiotic therapies and the impact of S. aureus alpha-toxin in a model of lethal influenza virus and MRSA coinfection. We demonstrate that antibiotics primarily attenuate alpha-toxin-induced acute lethality, even though both alpha-toxin-dependent and -independent mechanisms significantly contribute to animal mortality after coinfection. Furthermore, we found that the protein synthesis-suppressing antibiotic linezolid has an advantageous therapeutic effect on alpha-toxin-induced lung damage, as measured by protein leak and lactate dehydrogenase (LDH) activity. Importantly, using a Panton-Valentine leucocidin (PVL)-negative MRSA isolate from patient sputum, we show that linezolid therapy significantly improves animal survival from postinfluenza MRSA pneumonia compared with vancomycin treatment. Rather than improved viral or bacterial control, this advantageous therapeutic effect is associated with a significantly attenuated proinflammatory cytokine response and acute lung damage in linezolid-treated mice. Together, our findings not only establish a critical role of alpha-toxin in the extreme mortality of secondary MRSA pneumonia after influenza but also provide support for the possibility that linezolid could be a more effective treatment than vancomycin to improve disease outcomes.