Rap1 and integrin inside-out signaling

NK-like CD8+ γδ T cells are expanded in persistent Mycobacterium tuberculosis infection

The response of gamma delta (γδ) T cells in the acute versus chronic phases of the same infection is unclear. How γδ T cells function in acute Mycobacterium tuberculosis (Mtb) infection is well characterized, but their response during persistent Mtb infection is not well understood, even though most infections with Mtb manifest as a chronic, clinically asymptomatic state. Here, we analyze peripheral blood γδ T cells from a South African adolescent cohort and show that a unique CD8+ γδ T cell subset with features of "memory inflation" expands in chronic Mtb infection. These cells are hyporesponsive to T cell receptor (TCR)-mediated signaling but, like NK cells, can mount robust CD16-mediated cytotoxic responses. These CD8+ γδ T cells comprise a highly focused TCR repertoire, with clonotypes that are Mycobacterium specific but not phosphoantigen reactive. Using multiparametric single-cell pseudo-time trajectory analysis, we identified the differentiation paths that these CD8+ γδ T cells follow to develop into effectors in this infection state. Last, we found that circulating CD8+ γδ T cells also expand in other chronic inflammatory conditions, including cardiovascular disease and cancer, suggesting that persistent antigenic exposure may drive similar γδ T cell effector programs and differentiation fates.

Toll-like Receptor-Mediated Immunomodulation of Th1-Type Response Stimulated by Recombinant Antigen of Type 2 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2)

PRRSV infects CD163-positive macrophages and skews their polarization toward an M2 phenotype, followed by T-cell inactivation. In our previous study, we found that recombinant protein A1 antigen derived from PRRSV-2 was a potential vaccine or adjuvant for immunization against PRRSV-2 infection due to its ability to repolarize macrophages into M1 subtype, thereby reducing CD163 expression for viral entry and promoting immunomodulation for Th1-type responses, except for stimulating Toll-like receptor (TLR) activation. The aim of our current study was to evaluate the effects of another two recombinant antigens, A3 (ORF6L5) and A4 (NLNsp10L11), for their ability to trigger innate immune responses including TLR activation. We isolated pulmonary alveolar macrophages (PAMs) from 8- to 12-week-old specific pathogen free (SPF) piglets and stimulated them with PRRSV (0.01 MOI and 0.05 MOI) or antigens. We also investigated the T-cell differentiation by immunological synapse activation of PAMs and CD4+ T-cells in the cocultured system. To confirm the infection of PRRSV in PAMs, we checked the expression of TLR3, 7, 8, and 9. Our results showed that the expression of TLR3, 7, and 9 were significantly upregulated in PAMs by A3 antigen induction, similar to the extent of PRRSV infection. Gene profile results showed that A3 repolarizes macrophages into the M1 subtype potently, in parallel with A1, as indicated by significant upregulation of proinflammatory genes (TNF-α, IL-6, IL-1β and IL-12). Upon immunological synapse activation, A3 potentially differentiated CD4 T cells into Th1 cells, determined by the expression of IL-12 and IFN-γ secretion. On the contrary, antigen A4 promoted regulatory T cell (T-reg) differentiation by significant upregulation of IL-10 expression. Finally, we concluded that the PRRSV-2 recombinant protein A3 provided better protection against PRRSV infection, suggested by its capability to reeducate immunosuppressive M2 macrophages into proinflammatory M1 cells. As M1 macrophages are prone to be functional antigen-presenting cells (APCs), they can call for TLR activation and Th1-type immune response within the immunological synapse.

Thromboinflammation in Myeloproliferative Neoplasms (MPN)-A Puzzle Still to Be Solved

Myeloproliferative neoplasms (MPNs), a group of malignant hematological disorders, occur as a consequence of somatic mutations in the hematopoietic stem cell compartment and show excessive accumulation of mature myeloid cells in the blood. A major cause of morbidity and mortality in these patients is the marked prothrombotic state leading to venous and arterial thrombosis, including myocardial infarction (MI), deep vein thrombosis (DVT), and strokes. Additionally, many MPN patients suffer from inflammation-mediated constitutional symptoms, such as fever, night sweats, fatigue, and cachexia. The chronic inflammatory syndrome in MPNs is associated with the up-regulation of various inflammatory cytokines in patients and is involved in the formation of the so-called MPN thromboinflammation. JAK2-V617F, the most prevalent mutation in MPNs, has been shown to activate a number of integrins on mature myeloid cells, including granulocytes and erythrocytes, which increase adhesion and drive venous thrombosis in murine knock-in/out models. This review aims to shed light on the current understanding of thromboinflammation, involvement of neutrophils in the prothrombotic state, plausible molecular mechanisms triggering the process of thrombosis, and potential novel therapeutic targets for developing effective strategies to reduce the MPN disease burden.

Thromboinflammation in Myeloproliferative Neoplasms (MPN)-A Puzzle Still to Be Solved

Myeloproliferative neoplasms (MPNs), a group of malignant hematological disorders, occur as a consequence of somatic mutations in the hematopoietic stem cell compartment and show excessive accumulation of mature myeloid cells in the blood. A major cause of morbidity and mortality in these patients is the marked prothrombotic state leading to venous and arterial thrombosis, including myocardial infarction (MI), deep vein thrombosis (DVT), and strokes. Additionally, many MPN patients suffer from inflammation-mediated constitutional symptoms, such as fever, night sweats, fatigue, and cachexia. The chronic inflammatory syndrome in MPNs is associated with the up-regulation of various inflammatory cytokines in patients and is involved in the formation of the so-called MPN thromboinflammation. JAK2-V617F, the most prevalent mutation in MPNs, has been shown to activate a number of integrins on mature myeloid cells, including granulocytes and erythrocytes, which increase adhesion and drive venous thrombosis in murine knock-in/out models. This review aims to shed light on the current understanding of thromboinflammation, involvement of neutrophils in the prothrombotic state, plausible molecular mechanisms triggering the process of thrombosis, and potential novel therapeutic targets for developing effective strategies to reduce the MPN disease burden.

The small GTPase Rap1b negatively regulates neutrophil chemotaxis and transcellular diapedesis by inhibiting Akt activation

Mice lacking the small GTPase Rap1b exhibit enhanced neutrophil recruitment to inflamed lungs and susceptibility to endotoxin shock via enhance PI3K-Akt activation.

Neutrophils are the first line of cellular defense in response to infections and inflammatory injuries. However, neutrophil activation and accumulation into tissues trigger tissue damage due to release of a plethora of toxic oxidants and proteases, a cause of acute lung injury (ALI). Despite its clinical importance, the molecular regulation of neutrophil migration is poorly understood. The small GTPase Rap1b is generally viewed as a positive regulator of immune cell functions by controlling bidirectional integrin signaling. However, we found that Rap1b-deficient mice exhibited enhanced neutrophil recruitment to inflamed lungs and enhanced susceptibility to endotoxin shock. Unexpectedly, Rap1b deficiency promoted the transcellular route of diapedesis through endothelial cell. Increased transcellular migration of Rap1b-deficient neutrophils in vitro was selectively mediated by enhanced PI3K-Akt activation and invadopodia-like protrusions. Akt inhibition in vivo suppressed excessive Rap1b-deficient neutrophil migration and associated endotoxin shock. The inhibitory action of Rap1b on PI3K signaling may be mediated by activation of phosphatase SHP-1. Thus, this study reveals an unexpected role for Rap1b as a key suppressor of neutrophil migration and lung inflammation.

ICAM-1-mediated leukocyte adhesion is critical for the activation of endothelial LSP1

Leukocyte-endothelial interaction triggers signaling events in endothelial cells prior to transendothelial migration of leukocytes. Leukocyte-specific protein 1 (LSP1), expressed in endothelial cells, plays a pivotal role in regulating subsequent recruitment steps following leukocyte adhesion. In neutrophils, LSP1 is activated by phosphorylation of its serine residues by molecules downstream of p38 MAPK and PKC. Whether leukocyte adhesion to endothelial cells is required for endothelial LSP1 activation remains elusive. In addition, discrepancies in the functions of endothelial and leukocyte LSP1 in leukocyte adhesion prevail. We demonstrate that adhesion of wild-type (Lsp1(+/+)) neutrophils to LSP1-deficient (Lsp1(-/-)) endothelial cells was significantly reduced compared with adhesion to Lsp1(+/+) endothelial cells. Immunoblotting revealed increased phosphorylated endothelial LSP1 in the presence of adherent Lsp1(-/-) neutrophils [stimulated by macrophage inflammatory protein-2 (CXCL2), TNF-α, or thapsigargin], but not cytokine or chemokine alone. Pharmacological inhibition of p38 MAPK by SB-203580 (10 μM) significantly blunted the phosphorylation of endothelial LSP1. Functionally blocking endothelial ICAM-1 or neutrophil β2-integrins diminished neutrophil adhesion and phosphorylation of endothelial LSP1. The engagement of endothelial ICAM-1 cross-linking, which mimics leukocyte adhesion, resulted in phosphorylation of endothelial LSP1. In neutrophil-depleted Lsp1(+/+) mice, administration of ICAM-1 cross-linking antibody resulted in increased phosphorylation of LSP1 and p38 MAPK in TNF-α-stimulated cremaster muscle. In conclusion, endothelial LSP1 participates in leukocyte adhesion in vitro, and leukocyte adhesion through ICAM-1 fosters the activation of endothelial LSP1, an effect at least partially mediated by the activation of p38 MAPK. Endothelial LSP1, in contrast to neutrophil LSP1, is not phosphorylated by cytokine or chemokine stimulation alone.

Integrin bi-directional signaling across the plasma membrane

Integrins are heterodimeric cell adhesion molecules that are important in many biological functions, such as cell migration, proliferation, differentiation, and survival. They can transmit bi-directional signals across the plasma membrane. Inside-out activating signal from some cell surface receptors bound with soluble agonists triggers integrins conformational change leading to high affinity for extracellular ligands. Then binding of ligands to integrins results in outside-in signaling, leading to formation of focal adhesion complex at the integrin cytoplasmic tail and activation of downstream signal pathways. This bi-directional signaling is essential for rapid response of cell to surrounding environmental changes. During this process, the conformational change of integrin extracellular and transmembrane/cytoplasmic domains is particularly important. In this review, we will summarize recent progress in both inside-out and outside-in signaling with specific focus on the mechanism how integrins transmit bi-directional signals through transmembrane/cytoplasmic domains.