Chemokine

Chemokines are 8 to 12 kDa proteins, around fifty of them in humans, signalling through roughly twenty G protein-coupled receptors. Their names come from the arrangement of conserved cysteines near the amino terminus, giving the CXC, CC, CX3C and XC families. Receptor engagement triggers integrin activation, converting a leukocyte rolling along the endothelium into one firmly arrested and able to transmigrate.

Some axes are genuinely druggable. CXCL12 acting on CXCR4 retains haematopoietic stem cells in the marrow, so blocking that receptor mobilises them into blood for collection; plerixafor, a small molecule, was approved for this in 2008, and motixafortide, a peptide CXCR4 antagonist, was approved in 2023 for stem cell mobilisation in multiple myeloma. Maraviroc, approved in 2007, antagonises CCR5, the coreceptor most HIV strains use to enter cells. These successes share a single axis doing a job nothing else does.

Elsewhere redundancy dominates. Most receptors bind several ligands and most ligands bind several receptors, so removing one chemokine rarely stops recruitment, which is the standard explanation for the long run of failed anti-chemokine programmes in rheumatoid arthritis. Judging a chemokine as a target means asking whether its axis is non-redundant in that tissue, not whether it is elevated there.

Serum chemokine panels are sold as inflammation profiling, and they are the weakest possible readout of a system that operates over micrometres. A plasma concentration is spillover from local gradients whose shape is unmeasured, and the relationship between the two is unestablished for most ligands. Absolute values also differ substantially between multiplex bead assays and single-analyte ELISAs, so comparing a result against a reference range from another platform is not a valid comparison.