Hepatitis C virus E1 recruits high-density lipoprotein to support infectivity and evade antibody recognition.

Hepatitis C virus (HCV) is a member of the Flaviviridae family; however, unlike other family members, the HCV virion has an unusually high lipid content. HCV has two envelope glycoproteins, E1 and E2. E2 contributes to receptor binding, cell membrane attachment, and immune evasion. In contrast, the functions of E1 are poorly characterized due, in part, to challenges in producing the protein. This manuscript describes the expression and purification of a soluble E1 ectodomain (eE1) that is recognized by conformational, human monoclonal antibodies. eE1 forms a complex with apolipoproteins AI and AII, cholesterol, and phospholipids by recruiting high-density lipoprotein (HDL) from the extracellular media. We show that HDL binding is a function specific to eE1 and HDL hinders recognition of E1 by a neutralizing monoclonal antibody. Either low-density lipoprotein or HDL increases the production and infectivity of cell culture-produced HCV, but E1 preferentially selects HDL, influencing both viral life cycle and antibody evasion.IMPORTANCEHepatitis C virus (HCV) infection is a significant burden on human health, but vaccine candidates have yet to provide broad protection against this infection. We have developed a method to produce high quantities of soluble E1 or E2, the viral proteins located on the surface of HCV. HCV has an unusually high lipid content due to the recruitment of apolipoproteins. We found that E1 (and not E2) preferentially recruits host high-density lipoprotein (HDL) extracellularly. This recruitment of HDL by E1 prevents binding of E1 by a neutralizing antibody and furthermore prevents antibody-mediated neutralization of the virus. By comparison, low-density lipoprotein does not protect the virus from antibody-mediated neutralization. Our findings provide mechanistic insight into apolipoprotein recruitment, which may be critical for vaccine development.

Cutting Edge: Hyperinflammatory Monocytes Expressing CD56 Abound in Severe COVID-19 Patients.

Proinflammatory monocytes play a preponderant role in the development of a cytokine storm causing fatal consequences in coronavirus disease 2019 (COVID-19) patients, highlighting the importance of analyzing in more detail monocyte distribution in these patients. In this study, we identified an atypical monocyte subpopulation expressing CD56 molecules that showed a low level of HLA-DR and high level of l-selectin. They released higher amounts of TNF-α and IL-6 and expressed genes associated with an excessive inflammatory process. Remarkably, the frequency of CD56+ monocytes inversely correlated with that of CD16+ monocytes and a high CD56+/CD16+monocyte ratio was associated with both disease severity and mortality, as well as with serum concentration of type I IFN, a factor able to induce the appearance of CD56+ monocytes. In conclusion, severe COVID-19 is characterized by the abundance of hyperinflammatory CD56+ monocytes, which could represent a novel marker with prognostic significance and, possibly, a therapeutic target for controlling the inflammatory process occurring during COVID-19.

Circulating ILC precursors expressing CD62L exhibit a type 2 signature distinctly decreased in psoriatic patients.

Human CD117+ CRTH2neg innate lymphoid cells (ILC) comprise multipotent precursors (ILCp), which are able to differentiate into subtypes in response to different signals received in peripheral tissues. NKp46+ ILCp have been reported to associate with ILC3 whereas KLRG1+ ILCp with ILC2, although the latter can also generate other ILC subsets, thus, maintaining a substantial plasticity. We here showed that CD62L is expressed by ILCp exclusively within KLRG1+ population and its expression marks a loss of their broad differentiation potential. Analysis of cytokine production and relevant markers demonstrated that CD62L+ ILCp mainly differentiate into ILC2 whereas CD62Lneg counterpart can also differentiate into other ILC subsets depending on the signals they receive. Remarkably, in peripheral blood of psoriatic patients, where ILC3 are usually enriched, CD62L+ ILC were drastically reduced, whereas CD62Lneg ILC2 upregulated both RORγt and NKp46, thus, suggesting an ongoing conversion to ILC3. Therefore, CD62L now emerges as a potential marker to identify a skewing toward type 2 among ILCp.

Human Hepatitis B Virus Negatively Impacts the Protective Immune Crosstalk Between Natural Killer and Dendritic Cells.

BACKGROUND AND AIMS: Natural killer (NK) cells play a crucial role in the clearance of human viruses but their activity is significantly impaired in patients infected with chronic hepatitis B (CHB). Cooperation with dendritic cells (DCs) is pivotal for obtaining optimal NK cell antiviral function; thus, we investigated whether HBV might impact the ability of DCs to sustain NK cell functions. APPROACH AND RESULTS: Human DCs were poor stimulators of interferon-gamma (IFN-γ) production by NK cells when exposed to HBV, while maintaining the capability to trigger NK cell cytotoxicity. HBV prevented DC maturation but did not affect their expression of human leukocyte antigen class I, thus allowing DCs to evade NK cell lysis. Tolerogenic features of DCs exposed to HBV were further supported by their increased expression of IL-10 and the immunosuppressive enzyme indoleamine 2,3-dioxygenase, which contributed to the impairment of DC-mediated NK cell IFN-γ production and proliferation, respectively. HBV could also inhibit the expression of inducible immunoproteasome (iP) subunits on DCs. In fact, NK cells could induce iP subunit expression on DCs, but they failed in the presence of HBV. Remarkably, circulating blood DC antigen1 (BDCA1)+ DCs isolated from patients with CHB were functionally compromised, hence altering, in turn, NK cell responses. CONCLUSIONS: The abnormal NK-DC interplay caused by HBV may significantly impair the efficacy of antiviral immune response in patients with CHB.

Mechanical bacterial lysate enhances antimicrobial barrier mechanisms in human airway epithelial cells.

Polyvalent mechanical bacterial lysate is effective in the prevention of respiratory tract infections, although its mechanism of action is not entirely elucidated. Because epithelial cells constitute the frontline defense against infections, we investigated the molecular mechanisms of innate response exerted by bronchial epithelial cells in the presence of polyvalent mechanical bacterial lysate. By using primary human bronchial epithelial cells, we observed that polyvalent mechanical bacterial lysate was able to increase the expression of cellular adhesion molecules such as ICAM-1 and E-cadherin, as well as the expression of amphiregulin, a growth factor able to support human bronchial epithelial cell proliferation. Remarkably, polyvalent mechanical bacterial lysate promoted in human bronchial epithelial cells the de novo expression of human ß-defensin-2, a major antimicrobial peptide, conferring them a direct antimicrobial activity. Moreover, polyvalent mechanical bacterial lysate-stimulated human bronchial epithelial cells provided signals for increased IL-22 production by innate lymphoid cells via IL-23, which could further contribute to the release of antimicrobial peptides by epithelial cells. In agreement with these in vitro data, the concentration of both IL-23 and antimicrobial peptides (human ß-defensin-2 and LL-37) increased in the saliva of healthy volunteers after sublingual administration of polyvalent mechanical bacterial lysate. Altogether, these results indicate that polyvalent mechanical bacterial lysate administration might support mucosal barrier integrity and promote mechanisms of antimicrobial activity in airway epithelial cells.

Myeloma cells induce the accumulation of activated CD94low NK cells by cell-to-cell contacts involving CD56 molecules.

Natural killer (NK) cells represent innate effector cells potentially able to play a role during the immune response against multiple myeloma (MM). To better define the distribution and the specific properties of NK cell subsets during MM disease, we analyzed their features in the bone marrow and peripheral blood of newly diagnosed MM patients. Our findings revealed that, in both compartments, NK cells were more abundant than in healthy donors. Among total MM-NK cells, a significant increase of CD94lowCD56dim NK cell subset was observed, which already appears in clinical precursor conditions leading to MM, namely monoclonal gammopathy of undetermined significance and smoldering MM, and eventually accumulates with disease progression. Moreover, a consistent fraction of CD94lowCD56dim NK cells was in a proliferation phase. When analyzed for their killing abilities, they represented the main cytotoxic NK cell subset against autologous MM cells. In vitro, MM cells could rapidly induce the expansion of the CD94lowCD56dim NK cell subset, thus reminiscent of that observed in MM patients. Mechanistically, this accumulation relied on cell to cell contacts between MM and NK cells and required both activation via DNAM-1 and homophilic interaction with CD56 expressed on MM cells. Considering the growing variety of combination treatments aimed at enhancing NK cell-mediated cytotoxicity against MM, these results may also be informative for optimizing current immunotherapeutic approaches.