TLR2/4 are novel activating receptors for SARS-CoV-2 spike protein on NK cells, 2024, Landolina et al.

[SNT Gatchaman]

TLR2/4 are novel activating receptors for SARS-CoV-2 spike protein on NK cells
Landolina, Nadine; Ricci, Biancamaria; Veneziani, Irene; Alicata, Claudia; Mariotti, Francesca Romana; Pelosi, Andrea; Quatrini, Linda; Mortari, Eva Piano; Carsetti, Rita; Vacca, Paola; Tumino, Nicola; Azzarone, Bruno; Moretta, Lorenzo; Maggi, Enrico

In early infected or severe coronavirus disease 2019 (COVID-19) patients, circulating NK cells are consistently reduced, despite being highly activated or exhausted. The aim of this paper was to establish whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike glycoprotein (SP) may directly trigger NK cells and through which receptor(s).

SP-stimulated human NK cells have been evaluated for the expression of activation markers, cytokine release, and cytotoxic activity, as well as for gene expression profiles and NF-kB phosphorylation, and they have been silenced with specific small interfering RNAs.

SPs from the Wuhan strain and other variants of concern (VOCs) directly bind and stimulate purified NK cells by increasing activation marker expression, cytokine release, and cytolytic activity, prevalently in the CD56brightNK cell subset. VOC-SPs differ in their ability to activate NK cells, G614, and Delta-Plus strains providing the strongest activity in the majority of donors. While VOC-SPs do not trigger ACE2, which is not expressed on NK cells, or other activating receptors, they directly and variably bind to both Toll-like receptor 2 (TLR2) and TLR4. Moreover, SP-driven NK cell functions are inhibited upon masking such receptors or silencing the relative genes. Lastly, VOC-SPs upregulate CD56dimNK cell functions in COVID-19 recovered, but not in non-infected, individuals.

TLR2 and TLR4 are novel activating receptors for SP in NK cells, suggesting a new role of these cells in orchestrating the pathophysiology of SARS-CoV-2 infection. The pathogenic relevance of this finding is highlighted by the fact that free SP providing NK cell activation is frequently detected in a SARS-CoV-2 inflamed environment and in plasma of infected and long-COVID‐19 subjects.


Link | PDF (Frontiers in Immunology) [Open Access]

 

[SNT Gatchaman]

Some quotes from discussion —

This study provides evidence that SP, a crucial component of SARS-CoV-2, binds to NK cells through novel receptors, namely, TLR2 and TLR4. Their engagement triggers cell activation leading to induction of cytotoxicity and cytokine production.

The novel message of this study is that VOC-rSPs can directly bind and activate in vitro resting NK cells from HD, enrolled irrespective of previous infection or vaccination. SP subunit S1 induced higher NK cell stimulation than S2, whereas among SARSCoV-2 gps, E, but not M and N gps, slightly activated the CD56bright NK cell subset. In addition to the expression of activation markers (CD25 and CD69), different VOC-rSPs increased both cytokine release and cytolytic activity of freshly isolated NK cells. Although not shown, rSP-stimulated NK cells neither modulate LAG3, TIGIT, and PD1 genes nor determine any increase of CD57 or Annexin V surface expression. By contrast, NKG2C expression was higher in SP-stimulated than in untreated NK cells, likely mimicking the non-specific expansion of the “adaptive” NK cell subset and suggesting the induction of an activated rather than an exhausted profile.

Further experiments revealed that rSP induced the activation of IRAK1, IRF3, and IKBKG genes and of the NF-kB pathway in NK cells, suggesting the involvement of one or more TLRs. This notion is not new since it is known that NK cells express functional TLRs involved in the recognition of pathogen-associated molecular patterns from Herpes-viridiae and bind the majority of gps from such virus family through TLR2 or, partially, TLR4.

Taken together, these data suggest that the interaction of rSP with TLR2 and TLR4 on NK cells may have a strong impact on the immunopathology of COVID-19 and provides a clue that such receptors and/or their intracellular pathways could represent potential therapeutic targets.

The finding that recently recovered subjects show an increased proportion of NK cells that can be activated in vitro by rSP (high CD25/CD69 expression on CD56 dim subset and high IFN-g release) could explain why NK cells (mainly CD56 dim subset) are highly activated in vivo during infection and recovery.

In conclusion, this report provides evidence that SARS-CoV-2 rSP, as the majority of gps of many viruses, directly engages both TLR2 and TLR4 molecules on NK cells and triggers intracellular pathways inducing their activation and function. This mechanism may allow to dampen viral infection at early stages, while contributing to excessive inflammation at late stages. Our present data strongly suggest an important role of NK cells in orchestrating the pathophysiology of different phases of COVID-19 and of some post-vaccination side effects.