Molecular basis of fibrinogen recognition by the innate immune receptor LILRA2

Human leukocyte immunoglobulin-like receptors (LILRs) are cell surface receptors that are mainly expressed in immune cells. LILRs are involved in immune cell regulation. As a member of the LILR family, LILRA2 was reported to recognize the bacterially N-terminus truncated Ig (N-truncated Ig) for the induction of innate immune response. Fibrinogen, which is enzymatically converted to fibrin and forms fibrin-based blood clots, was recently shown to activate LILRA2-expressing immune cells. However, the molecular mechanisms of LILRA2-fibrinogen interaction remain unclear. In this study, we investigated the molecular recognition of fibrinogen by LILRA2, using biophysical methods. Surface plasmon resonance (SPR) analysis showed that LILRA2 specifically bound to fibrinogen with a relatively low dissociation constant (K_D) (∼10 μM), like N-truncated Ig. Furthermore, we found that high-molecular-weight fibrinogen exhibited a high-affinity interaction with immobilized LILRA2 owing to significant avidity effects. Domain-deletion and site-specific mutagenesis successfully identified the crucial amino acids of domains two and four of LILRA2 for fibrinogen binding. On the other hand, the D regions of fibrinogen are responsible for binding to LILRA2. These results enabled us to build a reasonable model of the LILRA2-fibrinogen complex, which provides insights into the molecular recognition and therapeutic potential of LILR-mediated immune responses.