Leukocyte immunoglobulin-like receptor A5 functions to induce ROS production on innate immune cells

Activating immune receptors provide mechanisms for phagocytes to elicit important effector functions that promote the killing of microbes. Leukocyte immunoglobulin-like receptor A5 (LILRA5) an orphan immune receptor expressed by human phagocytes and co-localising with FcRγ, remains poorly characterised. To address this, we developed a highly-specific anti-LILRA5 monoclonal antibody that has agonistic properties. Using a specific anti-LILRA5 antibody, we show LILRA5 expression on naïve monocytes and neutrophils, and that ligation of LILRA5 stimulates ROS production. While increased LILRA5 transcript copy numbers have been associated with sepsis, we also observed increased levels in patients with systemic infection but without sepsis complications. Ex vivo bacterial infection of whole blood did not alter surface LILRA5 expression, but LPS stimulation changed expression levels. This indicates that surface LILRA5 expression is dynamic and likely regulated posttranscriptionally, changing responses to different stimuli or over time. Soluble (s)LILRA5 was enhanced in sera from sepsis patients and in supernatants of monocytes that were LPS-stimulated, indicative that shedding of LILRA5 from cell surfaces or that expression of sLILRA5 isoforms provides a mechanism to regulate surface LILRA5 expression levels. Finally, we show that altered surface LILRA5 expression influences LILRA5-induced ROS production capacity. We propose that LILRA5 is a dynamically regulated activating receptor expressed on phagocytes that can stimulate ROS production.