SARS-CoV-2 infection impairs NK cell functions via activation of the LLT1-CD161 axis

Synergistic effects of Candida albicans and Porphyromonas gingivalis biofilms on epithelial barrier function in a 3D aspiration pneumonia model

Introduction

Polymicrobial infections involving Candida albicans and Porphyromonas gingivalis represent a significant challenge in maintaining epithelial barrier integrity. This study explores their synergistic effects on epithelial cells using an air-liquid interface (ALI) model.

Methods

Mixed-species biofilms were developed and analyzed for their impact on epithelial permeability and tight junction proteins. The effects of biofilm supernatants on IL-8 secretion and oxidative stress markers were also evaluated. The role of P. gingivalis proteases was assessed using a gingipain-null mutant (ΔKΔRAB) compared to the wild-type strain (W83). Survival experiments were conducted using Galleria mellonella larvae to examine the pathogenicity of dual-species biofilms.

Results

Mixed-species biofilms significantly increased epithelial permeability and disrupted tight junction proteins, as evidenced by reduced levels of ZO-1 and E-cadherin. These changes were accompanied by oxidative stress, characterized by decreased HO-1 expression and enhanced Bax/Bcl-xL ratios, indicating increased pro-apoptotic activity. Supernatants from dual-species biofilms demonstrated a pronounced effect on epithelial cells, modulating IL-8 secretion and exacerbating oxidative damage. C. albicans was identified as the dominant driver of pro-inflammatory responses, while P. gingivalis contributed through immune modulation and enzymatic activity, primarily via gingipains. The ΔKΔRAB mutant biofilms caused less epithelial disruption and oxidative stress compared to the wild-type, highlighting the critical role of gingipains in pathogenesis.

Discussion

Survival experiments using Galleria mellonella larvae supported these findings, highlighting the reduced survival associated with dual-species biofilms and the potential for high-dose antimicrobial therapies to mitigate this effect. These results emphasize the cooperative mechanisms of C. albicans and P. gingivalis in compromising epithelial barriers and underline the importance of combination therapies targeting both fungal and bacterial components in polymicrobial infections.

KLRB1 expression in nasopharyngeal mucosa as a prognostic biomarker in COVID-19 patients

The resurgence of COVID-19 and the rise in severe outcomes emphasize the need for reliable prognostic markers to guide patient care and optimize ICU and hospital resources. This study investigates the potential of nasopharyngeal swabs to identify biomarkers that predict ICU admission or death in hospitalized COVID-19 patients. We analyzed nasopharyngeal exudates from 95 hospitalized patients in 2020 using high-plex RNA quantification on the NanoString® nCounter platform. Comparative analysis identified four genes, with KLRB1 (Killer cell lectin like receptor B1) (Odds Ratio OR 0.5, 95% CI: 0.27-0.96), along with age (OR 3.3, 95% CI: 1.25-8.93) emerging as independent prognostic markers in multivariate analysis. These findings were validated using qRT-PCR in an independent cohort of 168 patients hospitalized in 2022. While univariate analysis identified a significant association between KLRB1 expression and vaccination status (p < 0.05), only low KLRB1 expression (OR 1.135, 95% CI: 1.0-1.280), and age (OR 1.033, 95% CI: 1.006-1.061) were confirmed as independent risk factors for ICU admission or death, regardless of other studied variables such as comorbidities, vaccination status, or smoking habits. Our findings suggest that KLRB1 expression could improve prognostic tools by identifying patients at higher risk upon admission. Incorporating KLRB1 into multiplex diagnostic kits alongside SARS-CoV-2 detection could streamline prognostic assessment, providing a more comprehensive and efficient approach to patient management.

NK-cell receptor modulation in viral infections

Natural killer (NK) cells play a crucial role in controlling viral infections. The ability to kill infected cells without prior immunization, yet being tolerant to self, healthy cells, depends on the balance of germ-line encoded surface receptors. NK-cell receptors are divided into either activating, leading to activation of NK cell and its cytotoxic and pro-inflammatory activity, or inhibitory, providing tolerance for a target cell. The signals from inhibitory receptors dominate and NK-cell activation requires stimulation of activating receptors. In viral infections, NK-cell interaction with infected cells can result in activation, memory-like NK-cell differentiation, or NK-cell exhaustion, which constitutes one of the viral immune evasion mechanisms. All of these states are associated with the modulation of NK-cell receptor expression. In this review, we summarize the current knowledge of NK-cell receptors and their role in viral infection control, as well as the alterations of their expression observed in acute or chronic infections. We present recently discovered SARS-CoV-2-mediated modulation of NK-cell receptor expression and compare them with other human viral infections. Finally, since modulation of NK-cell receptor activation gives a promising addition to currently used antiviral therapies, we briefly discuss the clinical significance and future perspective of the application of agonists or antagonists of activating and inhibitory receptors, respectively. In sum, our review shows that although much is known about NK-cell receptor biology, a deeper understanding of NK-cell receptors role in viral infections is still needed.

Natural Killer Cells Do Not Attenuate a Mouse-Adapted SARS-CoV-2-Induced Disease in Rag2-/- Mice

This study investigates the roles of T, B, and Natural Killer (NK) cells in the pathogenesis of severe COVID-19, utilizing mouse-adapted SARS-CoV-2-MA30 (MA30). To evaluate this MA30 mouse model, we characterized MA30-infected C57BL/6 mice (B6) and compared them with SARS-CoV-2-WA1 (an original SARS-CoV-2 strain) infected K18-human ACE2 (K18-hACE2) mice. We found that the infected B6 mice developed severe peribronchial inflammation and rapid severe pulmonary edema, but less lung interstitial inflammation than the infected K18-hACE2 mice. These pathological findings recapitulate some pathological changes seen in severe COVID-19 patients. Using this MA30-infected mouse model, we further demonstrate that T and/or B cells are essential in mounting an effective immune response against SARS-CoV-2. This was evident as Rag2-/- showed heightened vulnerability to infection and inhibited viral clearance. Conversely, the depletion of NK cells did not significantly alter the disease course in Rag2-/- mice, underscoring the minimal role of NK cells in the acute phase of MA30-induced disease. Together, our results indicate that T and/or B cells, but not NK cells, mitigate MA30-induced disease in mice and the infected mouse model can be used for dissecting the pathogenesis and immunology of severe COVID-19.

Lectin-like Transcript-1 (LLT1) Expression in Oral Squamous Cell Carcinomas: Prognostic Significance and Relationship with the Tumor Immune Microenvironment

Lectin-like transcript-1 (LLT1) expression is detected in different cancer types and is involved in immune evasion. The present study investigates the clinical relevance of tumoral and stromal LLT1 expression in oral squamous cell carcinoma (OSCC), and relationships with the immune infiltrate into the tumor immune microenvironment (TIME). Immunohistochemical analysis of LLT1 expression was performed in 124 OSCC specimens, together with PD-L1 expression and the infiltration of CD20+, CD4+, and CD8+ lymphocytes and CD68+ and CD163+-macrophages. Associations with clinicopathological variables, prognosis, and immune cell densities were further assessed. A total of 41 (33%) OSCC samples showed positive LLT1 staining in tumor cells and 55 (44%) positive LLT1 in tumor-infiltrating lymphocytes (TILs). Patients harboring tumor-intrinsic LLT1 expression exhibited poorer survival, suggesting an immunosuppressive role. Conversely, positive LLT1 expression in TILs was significantly associated with better disease-specific survival, and also an immune-active tumor microenvironment highly infiltrated by CD8+ T cells and M1/M2 macrophages. Furthermore, the combination of tumoral and stromal LLT1 was found to distinguish three prognostic categories (favorable, intermediate, and adverse; p = 0.029, Log-rank test). Together, these data demonstrate the prognostic relevance of tumoral and stromal LLT1 expression in OSCC, and its potential application to improve prognosis prediction and patient stratification.

The proportion of CD161 on CD56+ NK cells in peripheral circulation associates with clinical features and disease activity of primary Sjögren's syndrome

OBJECTIVES

The purpose of this study was to examine the proportion of CD161 on CD56+ natural killer (NK) cells in peripheral blood of primary Sjögren's syndrome (pSS) and investigate its clinical relevance of pSS.

METHODS

The proportion of CD56+ NK cells and CD161 on CD56+ NK cells was detected by flow cytometry in 31 pSS patients and 29 healthy controls (HCs). The correlations between the proportion of CD161+CD56+ NK cells and clinical features and disease activity of pSS were further analyzed. Meanwhile, we drew the receiver operating characteristic curve to evaluate the diagnostic value of CD161+CD56+ NK cells in pSS. In addition, we evaluated the differences in the effects of CD161+ cells and CD161- cells in peripheral blood on the function of CD56+ NK cells in 5 pSS patients.

RESULTS

The proportion of CD56+ NK cells and CD161+CD56+ NK cells decreased markedly in pSS patients compared to HCs. The correlation analysis showed that the proportion of CD161+CD56+ NK cells negatively correlated with white blood cells, Immunoglobulin A (IgA), IgM, IgG, European League Against Rheumatism Sjogren's Syndrome Patient Reported Index and European League Against Rheumatism Sjogren's Syndrome Disease Activity Index, and positively correlated with complement C4. The proportion of CD161+CD56+ NK cells in pSS patients with decayed tooth, fatigue, arthralgia, skin involvement, primary biliary cirrhosis, interstitial lung disease, anti-SSA/Ro60 positive, anti-SSB positive and high IgG was lower than that in negative patients. Furthermore, compared with inactive patients, the proportion of CD161+CD56+ NK cells decreased obviously in active patients. The area under the curve was 0.7375 (p = .0016), the results indicated that CD161+CD56+ NK cells had certain diagnostic values for pSS. In addition, the proportion of CD86, HLA-DR, Ki67, FasL, TNF-α, and IFN-γ on CD161+CD56+ NK cells was lower than that on CD161-CD56+ NK cells in the peripheral blood of pSS patients.

CONCLUSION

This study suggested that the proportion of CD56+ NK cells and CD161+CD56+ NK cells decreased significantly in pSS patients, and the proportion of CD161+CD56+ NK cells negatively associated with the clinical features and disease activity of pSS patients. CD161 expression inhibited the function of CD56+ NK cells in peripheral blood of pSS patients. The CD161+CD56+ NK cells may present as a potential target for therapy and a biomarker of disease activity in pSS.

Cell type dependent stability and virulence of a recombinant SARS-CoV-2, and engineering of a propagation deficient RNA replicon to analyze virus RNA synthesis

Engineering of reverse genetics systems for newly emerged viruses allows viral genome manipulation, being an essential tool for the study of virus life cycle, virus-host interactions and pathogenesis, as well as for the development of effective antiviral strategies. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emergent human coronavirus that has caused the coronavirus disease (COVID-19) pandemic. The engineering of a full-length infectious cDNA clone and a fluorescent replicon of SARS-CoV-2 Wuhan-Hu-1, using a bacterial artificial chromosome, is reported. Viral growth and genetic stability in eleven cell lines were analyzed, showing that both VeroE6 cells overexpressing transmembrane serin protease 2 (TMPRSS2) and human lung derived cells resulted in the optimization of a cell system to preserve SARS-CoV-2 genetic stability. The recombinant SARS-CoV-2 virus and a point mutant expressing the D614G spike protein variant were virulent in a mouse model. The RNA replicon was propagation-defective, allowing its use in BSL-2 conditions to analyze viral RNA synthesis. The SARS-CoV-2 reverse genetics systems developed constitute a useful tool for studying the molecular biology of the virus, the development of genetically defined vaccines and to establish systems for antiviral compounds screening.

C-type lectin receptor 2d forms homodimers and heterodimers with TLR2 to negatively regulate IRF5-mediated antifungal immunity

Dimerization of C-type lectin receptors (CLRs) or Toll-like receptors (TLRs) can alter their ligand binding ability, thereby modulating immune responses. However, the possibilities and roles of dimerization between CLRs and TLRs remain unclear. Here we show that C-type lectin receptor-2d (CLEC2D) forms homodimers, as well as heterodimers with TLR2. Quantitative ligand binding assays reveal that both CLEC2D homodimers and CLEC2D/TLR2 heterodimers have a higher binding ability to fungi-derived β-glucans than TLR2 homodimers. Moreover, homo- or hetero-dimeric CLEC2D mediates β-glucan-induced ubiquitination and degradation of MyD88 to inhibit the activation of transcription factor IRF5 and subsequent IL-12 production. Clec2d-deficient female mice are resistant to infection with Candida albicans, a human fungal pathogen, owing to the increase of IL-12 production and subsequent generation of IFN-γ-producing NK cells. Together, these data indicate that CLEC2D forms homodimers or heterodimers with TLR2, which negatively regulate antifungal immunity through suppression of IRF5-mediated IL-12 production. These homo- and hetero-dimers of CLEC2D and TLR2 provide an example of receptor dimerization to regulate host innate immunity against microbial infections.

Early expansion of activated adaptive but also exhausted NK cells during acute severe SARS-CoV-2 infection

The analysis of immunological parameters during the course of a SARS-CoV-2 infection is of great importance, both to identify diagnostic markers for the risk of a severe course of COVID-19, and to better understand the role of the immune system during the infection. By using multicolor flow cytometry we compared the phenotype of Natural Killer (NK) cells from hospitalized COVID-19 patients during early SARS-CoV-2 infection with samples from recovered and SARS-CoV-2 naïve subjects. Unsupervised high-dimensional analysis of 28-color flow cytometric data revealed a strong enrichment of NKG2C expressing NK cells in response to the acute viral infection. In addition, we found an overrepresentation of highly activated NK cell subsets with an exhausted phenotype. Moreover, our data show long-lasting phenotypic changes within the NK cell compartment that did not completely reverse up to 2 months after recovery. This demonstrates that NK cells are involved in the early innate immune response against SARS-CoV-2.

Initial immune response after exposure to Mycobacterium tuberculosis or to SARS-COV-2: similarities and differences

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) and Coronavirus disease-2019 (COVID-19), whose etiologic agent is severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), are currently the two deadliest infectious diseases in humans, which together have caused about more than 11 million deaths worldwide in the past 3 years. TB and COVID-19 share several aspects including the droplet- and aerosol-borne transmissibility, the lungs as primary target, some symptoms, and diagnostic tools. However, these two infectious diseases differ in other aspects as their incubation period, immune cells involved, persistence and the immunopathological response. In this review, we highlight the similarities and differences between TB and COVID-19 focusing on the innate and adaptive immune response induced after the exposure to Mtb and SARS-CoV-2 and the pathological pathways linking the two infections. Moreover, we provide a brief overview of the immune response in case of TB-COVID-19 co-infection highlighting the similarities and differences of each individual infection. A comprehensive understanding of the immune response involved in TB and COVID-19 is of utmost importance for the design of effective therapeutic strategies and vaccines for both diseases.