Leukocyte‐associated Ig‐like receptor‐1 has SH2 domain‐containing phosphatase‐independent function and recruits C‐terminal Src kinase

Abstract

Most inhibitory receptors in the immune system contain one or several immunoreceptor tyrosine-based inhibitory motifs (ITIM) and recruit the SH2 domain-containing phosphatases SHP-1, SHP-2 and/or SHIP, which are generally believed to be essential for the inhibitory function. However, it has not been systematically investigated whether ITIM-bearing receptors exert their function through alternative interactions. Here we describe that leukocyte-associated Ig-like receptor (LAIR)-1 has inhibitory function in DT40 chicken B cells that lack both SHP-1 and SHP-2. In addition, we found that LAIR-1 did not recruit SHIP upon phosphorylation. Thus, LAIR-1 can function independently from SH2 domain-containing phosphatases and must recruit at least one other signaling molecule. Using a yeast-tri-hybrid system, we found that phosphorylated LAIR-1 bound the C-terminal Src kinase (Csk). The interaction required the SH2 domain of Csk and phosphorylation of the tyrosine in the N-terminal ITIM of LAIR-1. We propose that Csk is an additional player in the regulation of the immune system by ITIMbearing receptors. IntroductionAn appropriate response of the immune system depends on a balance between activating and inhibitory signals. Inhibitory receptors play an important role in the regulation of immune cells. This is illustrated by the fact that mutations in inhibitory receptors or in the molecules through which they signal, are associated with autoimmune disease [1]. Most inhibitory receptors in the immune system contain one or several immunoreceptor tyrosine-based inhibitory motifs (ITIM). This motif has the consensus sequence (I/V/L/S)-x-Y-x-x-(L/ V/I), where x represents any amino acid [2]. Upon engagement, ITIM-bearing receptors become phosphorylated and recruit the SH2 domain-containing inositol phosphatase SHIP and/or the SH2 domaincontaining tyrosine phosphatases SHP-1 and SHP-2. The phosphatases subsequently dephosphorylate and thereby inactivate key molecules involved in cellular activation [2,3]. It is widely believed that the recruitment of SH2 domain-containing phosphatases is required for the inhibitory function of ITIM-bearing receptors. However, several studies, including our previous work on leukocyte-associated Ig-like receptor (LAIR)-1, suggest that the phosphatases are not the sole down-stream effectors of ITIM-bearing receptors.LAIR-1 is an ITIM-bearing receptor that is broadly expressed in the immune system [4] and functions as an inhibitory receptor on NK cells, T cells and B cells [4][5][6][7][8][9]. In addition, LAIR-1 inhibits the differentiation of peripheral blood precursors towards dendritic cells [10] and induces apoptosis in myeloid leukemia cells [11,12] Here we show that LAIR-1 has an inhibitory capacity in SHP-1-and SHP-2-deficient DT40 chicken B cells and does not recruit SHIP. This indicates that, in contrast to the established dogma, LAIR-1 has an inhibitory function independent of SH2 domain-containing phosphatases. Therefore, we searched for other LAIR-1-interacting proteins. Using a yeast-tri...

The inhibitory collagen receptor LAIR-1 (CD305)

“…hLAIR-1 and mLAIR-1 recruit C-terminal Csk [33], a known negative regulator of immune cell function that inactivates Src family kinases [36]. Csk association is also reported for other ITIM-bearing receptors, including signal regulatory protein-␣, Ig-like transcript-2, and Fc␥RIIB [33,37,38] but not KIR3DL1 [39]. Thus, LAIR-1-mediated inhibition is not solely dependent on phosphatases, and Csk may be the key downstream effector of LAIR-1 in cells where phosphatase activity is limited [33].…”

Signal Inhibitory Receptor on Leukocytes-1 Is a Novel Functional Inhibitory Immune Receptor Expressed on Human Phagocytes

“…Their suppression of cell function is usually mediated via ITIMs in the intracellular tail with the consensus sequence V/L/I/SxYxxV/L/I, where x denotes any amino acid (5). ITIMs are phosphorylated upon receptor ligation, usually by Src-family kinases, and can consequently recruit the Src homology (SH)2-domain containing tyrosine phosphatases SH region 2 domain-containing phosphatase (SHP)-1, SHP-2, the inositol phosphatase SHIP, or C-terminal Src kinase (Csk) to mediate their inhibitory function (6,7).…”

LAIR and collagens in immune regulation

“…Upon cross-linking of the receptor with mAbs, the tyrosines in the cytoplasmic ITIMs of LAIR-1 become phosphorylated and the molecule recruits SHP-1 and SHP-2 and recruit C-terminal Src Kinase (Csk) [3]. Since LAIR-1 can still function in a cell line deficient for both phosphatases, Csk may be the key downstream effector of LAIR-1 in cells where phosphatase activity is limited.…”