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

Myeloid cells play a crucial role in controlling infection. Activation of these cells needs to be tightly regulated, because their potent effector functions can damage host tissue. Inhibitory receptors expressed by immune cells play an important role in restricting immune cell activation. In this study, we have characterized a hitherto unidentified ITIM-bearing receptor that is highly expressed on human neutrophils and monocytes: signal inhibitory receptor on leukocytes-1 (SIRL-1). The chromosomal location of SIRL-1 is adjacent to the human leukocyte receptor complex on chromosome 19q13.4 and contains two ITIMs in its cytoplasmic tail. As a classical ITIM-bearing receptor, SIRL-1 is capable of inhibiting FcepsilonRI-mediated signaling and can recruit the Src homology 2 domain-containing phosphatases Src homology region 2 domain-containing phosphatases 1 and 2. To investigate the specific involvement of the individual ITIMs in this study, mutational analysis was performed, which revealed that both ITIMs are crucial for SIRL-1 inhibitory function and phosphatase recruitment. When primary cells were stimulated in vitro, SIRL-1(high) monocytes produce less TNF-alpha than SIRL-1(low) monocytes. Thus, SIRL-1 is a novel inhibitory immune receptor belonging to the growing family of ITIM-bearing receptors that is implied in the regulation of phagocytes.