CD94/NKG2A Inhibits NK Cell Activation by Disrupting the Actin Network at the Immunological Synapse

Abstract

An adequate immune response is the result of the fine balance between activation and inhibitory signals. The exact means by which inhibitory signals obviate activation signals in immune cells are not totally elucidated. Human CD94/NKG2A is an ITIM-containing inhibitory receptor expressed by NK cells and some CD8+ T cells that recognize HLA-E. We show that the engagement of this receptor prevents NK cell activation by disruption of the actin network and exclusion of lipid rafts at the point of contact with its ligand (inhibitory NK cell immunological synapse, iNKIS). CD94/NKG2A engagement leads to recruitment and activation of src homology 2 domain-bearing tyrosine phosphatase 1. This likely explains the observed dephosphorylation of guanine nucleotide exchange factor and regulator of actin, Vav1, as well as ezrin-radixin-moesin proteins that connect actin filaments to membrane structures. In contrast, NK cell activation by NKG2D induced Vav1 and ezrin-radixin-moesin phosphorylation. Thus, CD94/NKG2A prevents actin-dependent recruitment of raft-associated activation receptors complexes to the activating synapse. This was further substantiated by showing that inhibition of actin polymerization abolished lipid rafts exclusion at the iNKIS, whereas cholesterol depletion had no effect on actin disruption at the iNKIS. These data indicate that the lipid rafts exclusion at the iNKIS is an active process which requires an intact cytoskeleton to maintain lipid rafts outside the inhibitory synapse. The net effect is to maintain an inhibitory state in the proximity of the iNKIS, while allowing the formation of activation synapse at distal points within the same NK cell.

Wiskott‐Aldrich syndrome protein (WASp) and N‐WASp are involved in the regulation of NK‐cell migration upon NKG2D activation

“…This effect may be disrupted when inhibitory signals dominate. While CD94/NKG2A signaling disrupts synapse formation [29], NKG2D engagement is known to promote a stop signal [20]. Our results are consistent with these studies, as we analyzed how this stop signal affects NK-cell motility after stimulating NKG2D, while activation of an inhibitory receptor such as NKG2A reverses this effect.…”

Line of attack: NK cell specificity and integration of signals

“…Engagement of inhibitory receptors prevents actin cytoskeleton dynamics [47*,48], thereby preventing actin-dependent processes, such as coalescence of lipid rafts [49], recruitment and phosphorylation of co-activation receptors 2B4 and NKG2D to lipid rafts [50,51], and dephosphorylation of ezrin-radixin-moesin proteins, which connect actin filaments to membrane structures [48]. A direct substrate of SHP-1 during inhibition is Vav1, which is an essential regulator of actin dynamics [52].…”

Cell Death Mechanisms Induced by Cytotoxic Lymphocytes

“…The activating or inhibitory function of the heterodimer complex depends on the cytoplasm region of NKG2 molecules. Accordingly, NKG2A and B have immunoreceptor tyrosine-based inhibition motif (ITIM) groups that act as inhibitors (14)(15)(16), whereas NKG2C, NKG2D, and NKG2E lack ITIMs, instead they transmit activating signals (17)(18)(19). NKG2C is associated with a 12 kDa DNA-activating protein (DAP-12), a factor containing an immunoreceptor tyrosine-based activating motif (ITAM) that provides cellular activating signals (17).…”