CD56 Is a Pathogen Recognition Receptor on Human Natural Killer Cells

Abstract

Aspergillus (A.) fumigatus is an opportunistic fungal mold inducing invasive aspergillosis (IA) in immunocompromised patients. Although antifungal activity of human natural killer (NK) cells was shown in previous studies, the underlying cellular mechanisms and pathogen recognition receptors (PRRs) are still unknown. Using flow cytometry we were able to show that the fluorescence positivity of the surface receptor CD56 significantly decreased upon fungal contact. To visualize the interaction site of NK cells and A. fumigatus we used SEM, CLSM and dSTORM techniques, which clearly demonstrated that NK cells directly interact with A. fumigatus via CD56 and that CD56 is re-organized and accumulated at this interaction site time-dependently. The inhibition of the cytoskeleton showed that the receptor re-organization was an active process dependent on actin re-arrangements. Furthermore, we could show that CD56 plays a role in the fungus mediated NK cell activation, since blocking of CD56 surface receptor reduced fungal mediated NK cell activation and reduced cytokine secretion. These results confirmed the direct interaction of NK cells and A. fumigatus, leading to the conclusion that CD56 is a pathogen recognition receptor. These findings give new insights into the functional role of CD56 in the pathogen recognition during the innate immune response.Invasive aspergillosis (IA), primarily caused by the mold Aspergillus fumigatus, is a devastating disease in immunocompromised patients suffering from hematological malignancies or undergoing allogeneic hematopoietic stem cell transplantation (HSCT) 1 . The mortality rate of HSCT patients diagnosed with IA ranges from 60-90% 2 and the prognosis for long-term survival is extremely poor 3 . Recently, it was shown that HSCT patients with probable/proven IA had a delayed reconstitution of natural killer (NK) cells for more than a year post HSCT 4 . In addition, patients with severe IA were found to have a lower NK cell count compared to patients with well-controlled IA, suggesting that NK cells play a critical role in immunity to IA.NK cells comprise 5-15% of the peripheral blood mononuclear cells (PBMCs) in healthy individuals and belong to the innate immune system 5 . Upon activation, NK cells release immune regulatory cytokines to stimulate other immune cells and display cytotoxicity directed against tumor or virus-infected cells by granule release 5 . NK cells are defined as CD56 positive and CD3 negative cells and can be distinguished into CD3 − CD56 dim CD16 + and CD3 − CD56 bright CD16 − cells. While CD56 dim cells are more cytotoxic, CD56 bright cells produce high levels of cytokines such as IFNγ and TNFα 6 . The function of NK cells is induced by the interplay of inhibitory and activating receptors 7 , leading to cytotoxicity directed against tumors and virus-infected cells. Besides the recognition of these cells, NK cells also recognize other infectious pathogens, become activated, and as a response induce either lysis of these pathogens or trigger activation of other...

Microbial killing by NK cells

“…Direct activation involves contact‐dependent recognition of the microbe leading to enhanced cell death, cytokine secretion, or expression of surface receptors that “license” other effector cells . In addition to recognition of infected host cells, NK cell activation may also occur following direct recognition of the invading pathogen . By contrast, indirect NK cell activation is primarily orchestrated by microbial products, cytokines or contact with activating ligands expressed by other immune cells responding to invading pathogens, which in turn leads to priming of NK cells .…”

“…Moreover, the CD56dim subset is activated by Paracoccidioides brasiliensis . By contrast, CD56 has been identified to bind directly to an unknown ligand on A. fumigatus , as Ab blocking of the receptor inhibits A. fumigatus ‐induced NK cell CD69 expression and cytokine secretion . Additionally, the fungal pathogen‐associated molecular pattern, β‐1,3 glucan, has recently been shown to activate NK cells for enhanced killing .…”

“…87 In addition to recognition of infected host cells, NK cell activation may also occur following direct recognition of the invading pathogen. 39,40,88 By contrast, indirect NK cell activation is primarily orchestrated by microbial products, cytokines or contact with activating ligands expressed by other immune cells responding to invading pathogens, which in turn leads to priming of NK cells. 89,90 Microbial products such as muramyl dipeptide, a pathogenassociated molecular pattern, has been demonstrated to synergize with IFN-and IL-12 to directly activate NK cells.…”

“…100 By contrast, CD56 has been identified to bind directly to an unknown ligand on A. fumigatus, as Ab blocking of the receptor inhibits A. fumigatus-induced NK cell CD69 expression and cytokine secretion. 88 Additionally, the fungal pathogenassociated molecular pattern, -1,3 glucan, has recently been shown to activate NK cells for enhanced killing. 51 These observations provide exciting evidence of direct NK cell activation by microbes; however, more studies are required to explore and characterize the underlying mechanisms.…”

Antifungal Innate Immunity: A Perspective from the Last 10 Years

“…The NKp30 receptor has been shown to be responsible for recognition of C. albicans and C. neoformans , and subsequent killing through perforin release [130], and the human NKp46 receptor and its mouse ortholog, NCR1, were reported to bind C. glabrata and mediate fungal killing [131]. More recently, CD56 was suggested to play a role in A. fumigatus -mediated NK cell activation and proinflammatory cytokine production [132]. Clinical studies have suggested that adoptively transferred NK cells could be an attractive strategy in the prophylaxis or treatment of invasive fungal infections in allogeneic hematopoietic stem cell transplant recipients, but we still know very little about the role of NK cells in human fungal diseases.…”