Regulation of 2B4 (CD244)‐mediated NK cell activation by ligand‐induced receptor modulation

Abstract

Natural killer (NK) cell activity can be stimulated by different surface receptors. 2B4 is a member of the signaling lymphocyte activation molecule (SLAM)-related receptor family and is important for stimulating human NK cell cytotoxicity and cytokine production. Here we show that stimulation of human NK cells by antibody-mediated 2B4 cross-linking or incubation with target cells expressing the 2B4 ligand CD48 results in a strong down-modulation of 2B4 surface expression. This down-modulation is observed in NK cell lines, purified human NK cells and NK cell clones, and is accompanied by an internalization of 2B4. The modulation of 2B4 is dependent on the activity of Src-family kinases, but independent of PI3 K activity or actin polymerization. Inhibitory receptors can interfere with 2B4-mediated signals and NK cell activation. However, co-engagement of inhibitory killer cell Ig-like receptors has no influence on the down-modulation of 2B4. This suggests that the modulation of 2B4 expression is independent of inhibitory receptors. The lower surface expression of 2B4 after ligandinduced down-modulation results in reduced 2B4-mediated NK cell activation and cytotoxicity. The modulation of activating surface receptors may therefore be another mechanism for the fine-tuning of NK cell activity and may lead to the adaptation of NK cell cytotoxicity in tissues with high ligand expression. IntroductionNatural killer (NK) cells have the ability to immediately kill infected or transformed cells and are important for the control of acute viral infections [1,2]. By modulating dendritic cell functions, NK cells also play an important role in the development of protective T cell responses against intracellular pathogens and tumors [3,4]. The activity of NK cells is regulated by a balance of positive and negative signals, mediated by different surface receptors [5,6]. Human NK cell inhibitory receptors include members of the killer cell Ig-like receptors (KIR), and the CD94/NKG2A heterodimer, which recognize MHC class I molecules on target cells [7]. NK cell activation can be mediated by a variety of different receptors, which include the natural cytotoxicity receptors (NCR) NKp30, NKp44, NKp46, the activating receptors NKG2D and DNAM-1, and members of the signaling lymphocyte activation molecule (SLAM)-related receptors (SRR) 2B4 (CD244), NTB-A and CRACC (CS1, CD319) [6,8,9].Under physiological conditions the positive and negative signals are balanced, ensuring that NK cells do not attack normal cells [10]. Loss of MHC class I expression on infected or transformed cells can reduce the negative signal mediated by inhibitory NK cell receptors, thereby tilting the balance toward activation, resulting in NK cell-mediated lysis. Alternatively, upregulation of ligands for the activating NKG2D receptor can enhance the positive signal, resulting in the lysis of target cells with normal MHC class I expression [11]. The activity of NK cells is not only dictated by the expression of stimulating or inhibiting ligands on target cells. The ...

Altered expression of signalling lymphocyte activation molecule (SLAM) family receptors CS1 (CD319) and 2B4 (CD244) in patients with systemic lupus erythematosus

“…In addition, the mean fluorescence intensity ratio (MFIR) of 2B4 was downregulated significantly by all 2B4-expressing cells, including NK cells (Table 2). A possible mechanism for this 2B4 downregulation can be suggested from previous studies, showing that 2B4-mediated stimulation of human NK cells leads to the down-regulation of surface 2B4 by reduced promoter activity as well as receptor internalization [57,58]. This implies that 2B4-CD48 interaction might be involved actively in SLE.…”

Functional role of human NK cell receptor 2B4 (CD244) isoforms

“…Also, in EBV infection, the infected B cells are known to up-regulate CD48 due to a viral protein acting on an enhancer element in the CD48 gene promoter that would result in increased NK cell stimulation and activation through 2B4 [39,40]. As reported earlier, triggering of 2B4 on human NK cells induces a strong down-modulation of 2B4 surface expression [41]. Upon engagement of the 2B4 receptor, its surface expression is down-modulated by receptor internalization as well as the expression of the 2B4 gene is reduced by inhibitory action at an Ets-element in its promoter [42].…”

“…Upon engagement of the 2B4 receptor, its surface expression is down-modulated by receptor internalization as well as the expression of the 2B4 gene is reduced by inhibitory action at an Ets-element in its promoter [42]. This leads to decreased 2B4-mediated NK cell activation and cytotoxicity and may therefore be another mechanism for the fine-tuning of NK cell activity [38,41,42]. Based on the expression of h2B4-A and h2B4-B mRNA levels, we hypothesize that there is a two-tier regulation of NK cell function.…”

Cryptococcus neoformansDirectly Stimulates Perforin Production and Rearms NK Cells for Enhanced Anticryptococcal Microbicidal Activity

“…However, whether the increased perforin expression was a result of cytokine release following activation of Toll-like receptor 4 by lipopolysaccharide is unknown. On the other hand, evidence of reduced NK cell activity following exposure to specific targets has been obtained, and this reduced activity is linked to loss of perforin, down-modulation of activating receptors, and inactivation of signaling pathways due to constant stimulation (3,51). Thus, NK cell binding to C. neoformans may induce a cytokine-independent signaling cascade resulting in perforin production, enhancing subsequent anticryptococcal activity, or it may result in NK cell exhaustion.…”