Gimme a brake: HPK1 regulates LFA-1 and neutrophil traction

Neutrophils are first responders when the innate immune system calls the inflammatory alarm. Produced in the bone marrow at a rapid rate of ∼10⁶ per second, neutrophils are expendable since they are cleared in the spleen and bone marrow within hours if not called into action from the blood stream. Neutrophils surveil the circulation for signs of infection and tissue insult and, at precisely the appropriate location, engage a highly efficient braking system to overcome the significant drag forces of blood flow. In response to acute inflammation, a multistep process has been identified that is initiated by tethering neutrophils via E- and P-selectin adhesion receptors upregulated on inflamed endothelium. Following capture, neutrophils roll a distance of only a few cell diameters (∼10 μm) during which they engage chemokines (eg, interleukin-8) that signal via G-protein coupled receptors that in turn activate integrins to bind intercellular adhesion molecules upregulated on inflamed endothelium.²  It is at this stage of the multistep process that the neutrophils, in cooperation with endothelium, make the decision to adhere tightly and proceed to immigrate to the site of tissue insult or be carried away in the circulation.

Perhaps The Clash summed up the situation best in their song lyrics: “…Should I stay or should I go now?/If I go there will be trouble/An’ if I stay it will be double…” The trouble is clinically manifest in patients with leukocyte adhesion deficiency (LAD) syndrome that presents early in life and manifests by infections without pus formation, despite a state of leukocytosis.³  In contrast to LAD-I and LAD-II syndromes, which lack expression of β₂-integrins and selectin ligands due to homozygous mutations in the genes that produce these adhesion molecules, a variant denoted LAD-III exhibits normal integrin and selectin ligand expression but has hematopoietic defects in receptor activation and adhesive functions that are attributed to the lack of expression of kindlin-3. Kindlin-3 forms a ternary complex with the cytoplasmic tail of β-integrin along with talin. The LAD-III defect has provided solid evidence that focal clusters of bound integrins effectively mechanotransduce outside-in signals that are critical to both stable adhesion and migration necessary to navigate the final steps of immigration.

In this issue of Blood, Jakob et al report that HPK1 participates during inside-out and outside-in signaling of neutrophil recruitment during acute inflammation.¹  Whereas HPK1 has previously been reported to participate in signaling lymphocyte functions, Jakob et al have provided the first data to reveal its function during chemokine induction of neutrophil arrest, adhesion strengthening, pseudopod formation, and cell migration via high-affinity lymphocyte function-associated antigen 1. Using HPK1-deficient mice, Jakob et al observed a significant defect in the neutrophils’ capacity to efficiently arrest and migrate along and across inflamed endothelium, analogous to the kindlin-3 defect in LAD-III. One caveat in this comparison is that HPK1 appears to specifically modulate LFA-1 and not Mac-1 function. A particularly exciting component of this study was the finding that HPK1 formed a complex with the adaptor mammalian actin-binding protein 1, previously shown by this same group to reinforce neutrophil adhesion-dependent functions.⁴  These observations begin to shed light on how tension transmitted through focal clusters of LFA-1 serve to reinforce its linkage to the cortical cytoskeleton via outside-in signaling. Still unknown is the relationship between HPK1, mammalian actin-binding protein 1, kindlin-3, and talin, all of which are enriched on the cytoplasmic domain of CD18 and function to stabilize high-affinity LFA-1. An intriguing possibility is that enrichment of these adaptors facilitates assembly of a cytoskeletal linkage critical to efficient neutrophil mechanotaxis, or directional sensing using shear stress. Such a process would involve signal transduction modulated by focal clusters of high-affinity LFA-1 bound at sites enriched in dimeric intercellular adhesion molecule 1 near endothelial junctions.⁵  Future studies should shed light on the precise nature of force-facilitated integrin activation and signaling. One proposed scenario is that LFA-1 functions as a traction sensor by converting tensile bond forces to conformational changes that in turn initiate dynamic recruitment of these linking molecules to the cytodomain of the integrin.⁶  HPK1 is now established as one such link that reinforces the integrin connection to the cortical actin cytoskeleton, perhaps functioning as a clutch that engages active mechanotactic crawling that guides a neutrophil efficiently to the site of inflammation.