To fulfil its orchestration of immune cell trafficking, a network of chemokines and receptors developed that capitalizes on specificity, redundancy, and functional selectivity. The discovery of heteromeric interactions in the chemokine interactome has expanded the complexity within this network. Moreover, some inflammatory mediators, not structurally linked to classical chemokines, bind to chemokine receptors and behave as atypical chemokines (ACKs). We identified macrophage migration inhibitory factor (MIF) as an ACK that binds to chemokine receptors CXCR2 and CXCR4 to promote atherogenic leukocyte recruitment. Here, we hypothesized that chemokine–chemokine interactions extend to ACKs and that MIF forms heterocomplexes with classical chemokines. We tested this hypothesis by using an unbiased chemokine protein array. Platelet chemokine CXCL4L1 (but not its variant CXCL4 or the CXCR2/CXCR4 ligands CXCL8 or CXCL12) was identified as a candidate interactor. MIF/CXCL4L1 complexation was verified by co-immunoprecipitation, surface plasmon-resonance analysis, and microscale thermophoresis, also establishing high-affinity binding. We next determined whether heterocomplex formation modulates inflammatory/atherogenic activities of MIF. Complex formation was observed to inhibit MIF-elicited T-cell chemotaxis as assessed by transwell migration assay and in a 3D-matrix-based live cell-imaging set-up. Heterocomplexation also blocked MIF-triggered migration of microglia in cortical cultures in situ, as well as MIF-mediated monocyte adhesion on aortic endothelial cell monolayers under flow stress conditions. Of note, CXCL4L1 blocked binding of Alexa-MIF to a soluble surrogate of CXCR4 and co-incubation with CXCL4L1 attenuated MIF responses in HEK293-CXCR4 transfectants, indicating that complex formation interferes with MIF/CXCR4 pathways. Because MIF and CXCL4L1 are platelet-derived products, we finally tested their role in platelet activation. Multi-photon microscopy, FLIM-FRET, and proximity-ligation assay visualized heterocomplexes in platelet aggregates and in clinical human thrombus sections obtained from peripheral artery disease (PAD) in patients undergoing thrombectomy. Moreover, heterocomplexes inhibited MIF-stimulated thrombus formation under flow and skewed the lamellipodia phenotype of adhering platelets. Our study establishes a novel molecular interaction that adds to the complexity of the chemokine interactome and chemokine/receptor-network. MIF/CXCL4L1, or more generally, ACK/CXC-motif chemokine heterocomplexes may be target structures that can be exploited to modulate inflammation and thrombosis.

Recent research has uncovered Arabidopsis thaliana proteins that are similar to the human inflammatory cytokine MIF. Plant MIF/D‐dopachrome tautomerase (D‐DT)‐like proteins (MDLs) can interact with human MIF, yet the significance of these findings in living organisms has not been investigated. Given MIF's key role in acute respiratory distress syndrome promoting pulmonary inflammation, pathology, and leukocyte infiltration, here we set out to investigate the interplay between MIF and MDL1, one of three A. thaliana MIF orthologs, in an in vivo mouse model of MIF‐induced acute lung injury (ALI). Human MIF and MDL1 were administered to C57BL/6 mice via inhalation, individually or in combination. Inhalation of MIF promoted various parameters of lung injury as evaluated by flow cytometry, immunofluorescence microscopy, RT‐qPCR, and ELISA, while MDL1 inhalation alone had no effect. Intriguingly, combined treatment with MIF and MDL1 synergistically enhanced pulmonary infiltration of neutrophils and monocytic cells, accompanied by an upregulation of pro‐inflammatory cytokine genes. Thus, the plant‐derived MIF ortholog MDL1 potentiates MIF‐induced inflammation in ALI. These data support the growing evidence of interactions between plant‐derived compounds and human inflammatory mediators and illustrate how they may impact human health.