SUMMARY A large and diverse array of chemoattractants control leukocyte trafficking, but how these apparently redundant signals collaborate in vivo is still largely unknown. We previously demonstrated an absolute requirement for the lipid chemoattractant leukotriene B4 (LTB4) and its receptor BLT1 for neutrophil recruitment into the joint in autoantibody-induced arthritis. We now demonstrate that BLT1 is required for neutrophils to deliver IL-1 into the joint to initiate arthritis. IL-1-expressing neutrophils amplify arthritis through the production of neutrophil-active chemokines from synovial tissue cells. CCR1 and CXCR2, two neutrophil chemokine receptors, operate non-redundantly to sequentially control the later phase of neutrophil recruitment into the joint and mediate all neutrophil chemokine activity in the model. Thus, we have uncovered a complex sequential relationship involving unique contributions from the lipid mediator LTB4, the cytokine IL-1, and CCR1 and CXCR2 chemokine ligands that are all absolutely required for effective neutrophil recruitment into the joint.

Loss-of-function mutations in the Arabidopsis (Arabidopsis thaliana) ENHANCED DISEASE RESISTANCE1 (EDR1) gene confer enhanced resistance to infection by powdery mildew (Golovinomyces cichoracearum). EDR1 encodes a protein kinase, but its substrates and the pathways regulated by EDR1 are unknown. To identify components of the EDR1 signal transduction pathway(s), we conducted a forward genetic screen for mutations that suppressed edr1-mediated disease resistance. Genetic mapping and cloning of one of these suppressor mutations revealed a recessive missense mutation in the KEEP ON GOING gene (KEG; At5g13530), which we designated keg-4. KEG encodes a multidomain protein that includes a RING E3 ligase domain, a kinase domain, ankyrin repeats, and HERC2-like repeats. The KEG protein has previously been shown to have ubiquitin ligase activity and to negatively regulate protein levels of the transcription factor ABCISIC ACID INSENSITIVE5. KEG mRNA levels were found to be 3-fold higher in edr1 mutant plants compared to wild type. Loss-of-function mutations in KEG are seedling lethal and are hypersensitive to glucose and abscisic acid (ABA). The keg-4 mutation, in contrast, conferred resistance to 6% glucose and suppressed edr1-mediated hypersensitivity to ABA, suggesting that the keg-4 mutation suppresses ABA signaling by altering KEG function. Several ABA-responsive genes were found to be further up-regulated in the edr1 mutant following ABA treatment, and this up-regulation was suppressed by the keg-4 mutation. We conclude that edr1-mediated resistance to powdery mildew is mediated, in part, by enhanced ABA signaling.

These findings provide novel evidence that corneal injury causes significant mobilization of endogenous MSCs into blood, and that MSCs home specifically to the injured cornea and promote regeneration, highlighting the therapeutic implications of MSC-mediated tissue repair in corneal injury.