The innate immune response to bacterial infections requires the interaction of neutrophils and platelets. Here, we show that a multistep reciprocal crosstalk exists between these two cell types, ultimately facilitating neutrophil influx into the lung to eliminate infections. Activated platelets adhere to intravascular neutrophils through P-selectin/P-selectin glycoprotein ligand-1 (PSGL-1)-mediated binding, a primary interaction that allows platelets glycoprotein Ibα (GPIbα)-induced generation of neutrophil-derived extracellular vesicles (EV). EV production is directed by exocytosis and allows shuttling of arachidonic acid into platelets. EVs are then specifically internalized into platelets in a Mac1-dependent fashion, and relocated into intracellular compartments enriched in cyclooxygenase1 (Cox1), an enzyme processing arachidonic acid to synthesize thromboxane A2 (TxA2). Finally, platelet-derived-TxA2 elicits a full neutrophil response by inducing the endothelial expression of ICAM-1, intravascular crawling, and extravasation. We conclude that critical substrate–enzyme pairs are compartmentalized in neutrophils and platelets during steady state limiting non-specific inflammation, but bacterial infection triggers regulated EV shuttling resulting in robust inflammation and pathogen clearance.

Beyond hemostasis, platelets actively participate in immune cell recruitment and host defense, yet their potential in the resolution of inflammatory processes remains unknown. Here, we demonstrate that platelets are recruited into the lung together with neutrophils during the onset of inflammation and alongside regulatory T (T reg) cells during the resolution phase. This partnering dichotomy is regulated by differential adhesion molecule expression during resolution. Mechanistically, intravascular platelets form aggregates with T reg cells, a prerequisite for their recruitment into the lung. This interaction relies on platelet activation by sCD40L and platelet P-selectin binding to PSGL-1 on T reg cells. Physical platelet–T reg cell interactions are necessary to modulate the transcriptome and instruct T reg cells to release the anti-inflammatory mediators IL-10 and TGFβ. Notably, the presence of platelet–T reg cell aggregates in the lung was also required for macrophage transcriptional reprogramming, polarization toward an anti-inflammatory phenotype, and effective resolution of pulmonary inflammation. Thus, platelets partner with successive immune cell subsets to orchestrate both the initiation and resolution of inflammation.

Pulmonary infection is a frequent pathology associated with excessive neutrophil infiltration. Ly49Q, an ITIM domain-bearing receptor expressed on different leukocytes, has been recently reported to impact neutrophil migration and polarization. Utilizing a murine model of induced pulmonary infection in combination with additional in vivo and in vitro assays, we show that Ly49Q is critically involved in different steps of the leukocyte adhesion cascade. Ly49Q deficiency is associated with a reduced rolling velocity, impaired crawling capacity, and diminished transmigration. We show that overactivation of the neutrophil β integrins Mac-1 and LFA-1 is responsible for increased adhesion and reduced neutrophil transmigration, resulting in a strongly impaired immune defense against pulmonary infection. Structure function analysis in vitro and in vivo demonstrated that different domains of Ly49Q are important for its function. In summary, Ly49Q regulates integrin activation and neutrophil recruitment and is required for an adequate immune response in pulmonary infection.

Acute kidney injury increases morbidity and mortality and previous studies have shown that remote ischemic preconditioning (RIPC) reduces the risk of acute kidney injury after cardiac surgery. RIPC increases urinary HMGB1 (high mobility group box protein-1) levels in patients which correlates with kidney protection. Here, we show that RIPC reduces renal ischemia-reperfusion injury and improves kidney function in mice. Mechanistically, RIPC increases HMGB1 levels in the plasma and urine and HMGB1 binds to Toll-like receptor 4 (TLR4) on renal tubular epithelial cells, inducing transcriptomic modulation of renal tubular epithelial cells and providing renal protection, whereas TLR4 activation on non-renal cells was shown to contribute to renal injury. This protection is mediated by activation of induction of AMPK⍺ and NF-kB, which induces the upregulation of Sema5b that triggers a transient, protective G1 cell-cycle arrest. In cardiac surgery patients at high risk for postoperative acute kidney injury, increased HMGB1 and Sema5b levels after RIPC were associated with renal protection after surgery. The results may help to develop future clinical treatment options for acute kidney injury.

Former prospective studies showed that the occurrence of relapse in Major Depressive Disorder (MDD) is associated with volume loss in the insula, hippocampus and dorsolateral prefrontal cortex (DLPFC). However, these studies were confounded by the patient’s lifetime disease history, as the number of previous episodes predict future recurrence. In order to analyze neural correlates of recurrence irrespective of prior disease course, this study prospectively examined changes in brain structure in patients with first-episode depression (FED) over 2 years. N = 63 FED patients and n = 63 healthy controls (HC) underwent structural magnetic resonance imaging at baseline and after 2 years. According to their disease course during the follow-up interval, patients were grouped into n = 21 FED patients with recurrence (FEDrec) during follow-up and n = 42 FED patients with stable remission (FEDrem). Gray matter volume changes were analysed using group by time interaction analyses of covariance for the DLPFC, hippocampus and insula. Significant group by time interactions in the DLPFC and insula emerged. Pairwise comparisons showed that FEDrec had greater volume decline in the DLPFC and insula from baseline to follow-up compared with FEDrem and HC. No group by time interactions in the hippocampus were found. Cross-sectional analyses at baseline and follow-up revealed no differences between groups. This longitudinal study provides evidence for neural alterations in the DLPFC and insula related to a detrimental course in MDD. These effects of recurrence are already detectable at initial stages of MDD and seem to occur without any prior disease history, emphasizing the importance of early interventions preventing depressive recurrence.