Platelet, not endothelial, P-selectin expression contributes to generation of immunity in cutaneous contact hypersensitivity

From Classical to Unconventional: The Immune Receptors Facilitating Platelet Responses to Infection and Inflammation

Platelets have long been recognized for their role in maintaining the balance between hemostasis and thrombosis. While their contributions to blood clotting have been well established, it has been increasingly evident that their roles extend to both innate and adaptive immune functions during infection and inflammation. In this comprehensive review, we describe the various ways in which platelets interact with different microbes and elicit immune responses either directly, or through modulation of leukocyte behaviors.

Revisiting Platelets and Toll-Like Receptors (TLRs): At the Interface of Vascular Immunity and Thrombosis

While platelet function has traditionally been described in the context of maintaining vascular integrity, recent evidence suggests that platelets can modulate inflammation in a much more sophisticated and nuanced manner than previously thought. Some aspects of this expanded repertoire of platelet function are mediated via expression of Toll-like receptors (TLRs). TLRs are a family of pattern recognition receptors that recognize pathogen-associated and damage-associated molecular patterns. Activation of these receptors is crucial for orchestrating and sustaining the inflammatory response to both types of danger signals. The TLR family consists of 10 known receptors, and there is at least some evidence that each of these are expressed on or within human platelets. This review presents the literature on TLR-mediated platelet activation for each of these receptors, and the existing understanding of platelet-TLR immune modulation. This review also highlights unresolved methodological issues that potentially contribute to some of the discrepancies within the literature, and we also suggest several recommendations to overcome these issues. Current understanding of TLR-mediated platelet responses in influenza, sepsis, transfusion-related injury and cardiovascular disease are discussed, and key outstanding research questions are highlighted. In summary, we provide a resource—a “researcher’s toolkit”—for undertaking further research in the field of platelet-TLR biology.

Platelets are important for the development of immune tolerance: Possible involvement of TGF-β in the mechanism

Platelets have diverse roles in immune processes in addition to their key functions in haemostasis and thrombosis. Some studies imply that platelets may be possibly related to the immune tolerance induction. However, the role of platelets in the development of immune tolerance is not fully understood. The purpose of this study was to investigate the role of platelets in the development of regulatory mechanisms responsible for cutaneous inflammation using a mouse model of low zone tolerance (LZT). Mice were treated with 2,4,6-trinitro-1-chlorobenzene (TNCB) 8 times every other day for tolerance induction with administration of anti-platelet antibody or control antibody during the tolerance induction phase every 3 days. After the treatment for the tolerance induction, mice were sensitized and then challenged with TNCB. The contact hypersensitivity (CHS) was significantly decreased at 24 hours after challenge in the mice with LZT than in those without LZT. Platelet depletion via administration of anti-platelet antibody reversed the inhibition of CHS and reduced the frequency of Foxp3⁺ Tregs in the inflamed skin and draining lymph nodes in mice with LZT. In addition, repeated low-dose skin exposure resulted in elevated plasma levels of transforming growth factor (TGF)-β1. Interestingly, platelet depletion reduced plasma TGF-β1 levels of mice with LZT. Furthermore, the CHS response was reduced by administration of recombinant TGF-β1 during platelet depletion in mice with LZT. Administration of anti-TGF-β antibody reversed the inhibition of the CHS responses. These results suggest that platelets are involved in the induction of immune tolerance via the release of TGF-β1.

A cell-extrinsic ligand acquired by activated T cells in lymph node can bridge L-selectin and P-selectin

P-selectin expressed on activated endothelia and platelets supports recruitment of leukocytes expressing P-selectin ligand to sites of inflammation. While monitoring P-selectin ligand expression on activated CD8+ T cells in murine adoptive transfer models, we observed two distinct ligands on responding donor cells, the canonical cell-intrinsic P-selectin ligand PSGL-1 and a second undocumented P-selectin ligand we provisionally named PSL2. PSL2 is unusual among selectin ligands in that it is cell-extrinsic, loaded onto L-selectin expressed by activated T cells but not L-selectin on resting naïve CD8+ T cells. PSL2 display is highest on activated T cells responding in peripheral lymph nodes and low on T cells responding in spleen suggesting that the original source of PSL2 is high endothelial venules, cells known to produce L-selectin ligands. PSL2 is a ligand for both P-selectin and L-selectin and can physically bridge the two selectins. The L-selectin/PSL2 complex can mediate P-selectin-dependent adherence of activated T cells to immobilized P-selectin or to activated platelets, either independently or cooperatively with PSGL-1. PSL2's capacity to bridge between L-selectin on activated T cells and P-selectin reveals an undocumented and unanticipated activity of cell-extrinsic selectin ligands in mediating selectin-selectin connectivity. The timing and circumstances of PSL2 detection on T cells, together with its capacity to support adherence to P-selectin-bearing substrates, are consistent with P-selectin engagement of both PSGL1 and the L-selectin/PSL2 complex during T cell recruitment. Engagement of PSGL-1 and L-selectin/PSL2 would likely deliver distinct signals known to be relevant in this process.

Isolation of T cells from the skin

T cells can be found in skin under steady-state conditions as well as in inflammatory processes. T cells in skin play an important role in immune homeostasis as well as control of infectious, inflammatory diseases or tumors. In addition, several important and frequent skin diseases such as psoriasis, atopic dermatitis, autoimmune disease, and contact allergy are initiated by T cells. In skin diseases, the majority of antigen-specific T cells can be found in the tissue, not the peripheral blood. Here, we present a protocol suitable for isolation of skin-resident (inflammatory) T cells that can be used for an in-depth characterization of their frequency, function, and role for the respective inflammatory condition.

The role of platelets in the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is an inflammatory disease of unknown etiology characterized by widespread organ dysfunction due to fibrosis and ischemia. Its nebulous pathogenic background and the consequent absence of an etiological therapy prevent the adoption of satisfying treatment strategies, able to improve patients’ quality of life and survival and stimulate researchers to identify a unifying pathogenic target. Platelets show a unique biological behavior, lying at the crossroads between vascular function, innate and adaptive immunity, and regulation of cell proliferation. Consequently they are also emerging players in the pathogenesis of many inflammatory diseases, including SSc. In the setting of SSc platelets are detectable in a persistent activated state, which is intimately linked to the concomitant presence of an injured endothelium and to the widespread activation of the innate and adaptive immune system. As a consistent circulating source of bioactive compounds platelets contribute to the development of many characteristic phenomena of SSc, such as fibrosis and impaired vascular tone.

Neutralization of macrophage migration inhibitory factor (MIF) by fully human antibodies correlates with their specificity for the β-sheet structure of MIF

The macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine that recently emerged as an attractive therapeutic target for a variety of diseases. A diverse panel of fully human anti-MIF antibodies was generated by selection from a phage display library and extensively analyzed in vitro. Epitope mapping studies identified antibodies specific for linear as well as structural epitopes. Experimental animal studies revealed that only those antibodies binding epitopes within amino acids 50-68 or 86-102 of the MIF molecule exerted protective effects in models of sepsis or contact hypersensitivity. Within the MIF protein, these two binding regions form a β-sheet structure that includes the MIF oxidoreductase motif. We therefore conclude that this β-sheet structure is a crucial region for MIF activity and a promising target for anti-MIF antibody therapy.

Platelet P-selectin reflects a state of cutaneous inflammation: possible application to monitor treatment efficacy in psoriasis

Haemostasis-maintaining platelets are also recognized as important modulators in the regulation of immune response. Activated platelets expressing P-selectin (CD62P) are involved in the extravasation of leucocytes. This study evaluated platelet P-selectin expression as a biomarker for cutaneous inflammation. P-selectin expression was assessed by flow cytometry in 147 successive patients suffering from an inflammatory or infectious skin condition at the day of admission for in-patient treatment as well as a day prior to demission. Forty-one patients admitted for allergy testing served as controls. A commercially available ELISA was used in 17 patients to determine soluble P-selectin in the plasma. In patients with psoriasis, the Psoriasis Area and Severity Index (PASI) was documented as a measure for disease severity. We observed a significant increase in platelet P-selectin expression in patients with inflammatory or infectious disorders, when compared to the control group (3,01% vs. 1,46%; P < 0.000001). Successful treatment resulted in a significant decrease in P-selectin expression to the level of the control group. In the case of psoriasis (n = 47), we found highly significant correlation between P-selectin and PASI (r = 0.51; P < 0.000001), as well as between the change in the PASI and the change in P-selectin expression (r = 0.4; P = 0.006). Platelet P-selectin expression as determined by flow cytometry correlated well with the results of soluble P-selectin, determined by ELISA (r = 0.63; P < 0.01). Thus, platelet P-selectin expression may be used as an efficacy biomarker to monitor treatment success in psoriasis. As platelet P-selectin correlates with soluble P-selectin in patient plasma, which can be measured by ELISA, the latter is feasible also for routine use.