Rick Kapur

Nouvelle cuisine: platelets served with inflammation.

Platelets are small cellular fragments with the primary physiological role of maintaining hemostasis. In addition to this well-described classical function, it is becoming increasingly clear that platelets have an intimate connection with infection and inflammation. This stems from several platelet characteristics, including their ability to bind infectious agents and secrete many immunomodulatory cytokines and chemokines, as well as their expression of receptors for various immune effector and regulatory functions, such as TLRs, which allow them to sense pathogen-associated molecular patterns. Furthermore, platelets contain RNA that can be nascently translated under different environmental stresses, and they are able to release membrane microparticles that can transport inflammatory cargo to inflammatory cells. Interestingly, acute infections can also result in platelet breakdown and thrombocytopenia. This report highlights these relatively new aspects of platelets and, thus, their nonhemostatic nature in an inflammatory setting.

Pathogenesis and Therapeutic Mechanisms in Immune Thrombocytopenia (ITP)

Immune thrombocytopenia (ITP) is a complex autoimmune disease characterized by low platelet counts. The pathogenesis of ITP remains unclear although both antibody-mediated and/or T cell-mediated platelet destruction are key processes. In addition, impairment of T cells, cytokine imbalances, and the contribution of the bone marrow niche have now been recognized to be important. Treatment strategies are aimed at the restoration of platelet counts compatible with adequate hemostasis rather than achieving physiological platelet counts. The first line treatments focus on the inhibition of autoantibody production and platelet degradation, whereas second-line treatments include immunosuppressive drugs, such as Rituximab, and splenectomy. Finally, third-line treatments aim to stimulate platelet production by megakaryocytes. This review discusses the pathophysiology of ITP and how the different treatment modalities affect the pathogenic mechanisms.

CD20+ B-cell depletion therapy suppresses murine CD8+ T-cell-mediated immune thrombocytopenia.

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder with a complex pathogenesis, which includes both antibody- and T-cell-mediated effector mechanisms. Rituximab (an anti-human CD20 monoclonal antibody [mAb]) is one of the treatments for ITP and is known to deplete B cells but may also work by affecting the T-cell compartments. Here, we investigated the outcome of B-cell depletion (Bdep) therapy on CD8(+) T-cell-mediated ITP using a murine model. CD61 knockout (KO) mice were immunized with CD61(+) platelets, and T-cell-mediated ITP was initiated by transfer of their splenocytes into severe combined immunodeficiency (SCID) mice. The CD61 KO mice were administrated an anti-mouse CD20 mAb either before or after CD61(+) platelet immunization. This resulted in efficient Bdep in vivo, accompanied by significant increases in splenic and lymph node CD4(+) and CD8(+) T cells and proportional increases of FOXP3(+) in CD4(+)and CD8(+) T cells. Moreover, Bdep therapy resulted in significantly decreased splenic CD8(+) T-cell proliferation in vitro that could be rescued by interleukin-2. This correlated with normalization of in vivo platelet counts in the transferred SCID mice suggesting that anti-CD20 therapy significantly reduces the ability of CD8(+) T cells to activate and mediate ITP.

C-reactive protein enhances murine antibody-mediated transfusion-related acute lung injury

Transfusion-related acute lung injury (TRALI) is a syndrome of respiratory distress triggered by blood transfusions and is the leading cause of transfusion-related mortality. TRALI has primarily been attributed to passive infusion of HLA and/or human neutrophil antigen antibodies present in transfused blood products, and predisposing factors such as inflammation are known to be important for TRALI initiation. Because the acute-phase protein C-reactive protein (CRP) is highly upregulated during infections and inflammation and can also enhance antibody-mediated responses such as in vitro phagocytosis, respiratory burst, and in vivo thrombocytopenia, we investigated whether CRP affects murine antibody-mediated TRALI induced by the anti-major histocompatibility complex antibody 34-1-2s. We found that BALB/c mice injected with 34-1-2s or CRP alone were resistant to TRALI, however mice injected with 34-1-2s together with CRP had significantly enhanced lung damage and pulmonary edema. Mechanistically, 34-1-2s injection with CRP resulted in a significant synergistic increase in plasma levels of the neutrophil chemoattractant macrophage inflammatory protein-2 (MIP-2) and pulmonary neutrophil accumulation. Importantly, murine MIP-2 is the functional homolog of human interleukin-8, a known risk factor for human TRALI. These results suggest that elevated in vivo CRP levels, like those observed during infections, may significantly predispose recipients to antibody-mediated TRALI reactions and support the notion that modulating CRP levels is an effective therapeutic strategy to reduce TRALI severity.

T regulatory cells and dendritic cells protect against transfusion-related acute lung injury via IL-10

Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-related fatalities and is characterized by acute respiratory distress following blood transfusion. Donor antibodies are frequently involved; however, the pathogenesis and protective mechanisms in the recipient are poorly understood, and specific therapies are lacking. Using newly developed murine TRALI models based on injection of anti-major histocompatibility complex class I antibodies, we found CD4+CD25+FoxP3+ T regulatory cells (Tregs) and CD11c+ dendritic cells (DCs) to be critical effectors that protect against TRALI. Treg or DC depletion in vivo resulted in aggravated antibody-mediated acute lung injury within 90 minutes with 60% mortality upon DC depletion. In addition, resistance to antibody-mediated TRALI was associated with increased interleukin-10 (IL-10) levels, and IL-10 levels were found to be decreased in mice suffering from TRALI. Importantly, IL-10 injection completely prevented and rescued the development of TRALI in mice and may prove to be a promising new therapeutic approach for alleviating lung injury in this serious complication of transfusion.

Platelets as immune-sensing cells

Publishers Note: This article has a companion Point by Brass et al. Publishers Note: Join in the discussion of these articles at Blood Advances Community Conversations.

Thymic-derived tolerizing dendritic cells are upregulated in the spleen upon treatment with intravenous immunoglobulin in a murine model of immune thrombocytopenia

Abstract Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by low platelet counts. First-line treatment includes intravenous immunoglobulin (IVIg), however, its working mechanism remains incompletely understood. We investigated splenic and thymic dendritic cell (DC) subsets upon IVIg treatment in a well-characterized active murine model of ITP. During active disease, there was a significant peripheral deficiency of splenic tolerizing SIRPα+ DCs which could be rescued by IVIg therapy, increasing platelet counts. These splenic tolerizing DC changes were associated with an abrogation of the thymic-retention of tolerizing DCs, suggesting that IVIg may raise platelet counts in ITP by modulating peripheral numbers of tolerizing DCs.

Low levels of interleukin-10 in patients with transfusion-related acute lung injury

Transfusion related acute lung injury (TRALI) is the leading cause of transfusion-related fatalities (FDA Report 2016) (1) and is characterized by the acute onset of respiratory distress within 6 hours following blood transfusion (2,3). The clinical diagnosis is confirmed in case of newly developing acute respiratory distress: PaO 2 /FiO 2 ratio (ratio of arterial oxygen partial pressure to fractional inspired oxygen) <300 mmHg or arterial oxygen saturation <90% at room air; newly developed or worsened bilateral pulmonary infiltrates indicative of pulmonary edema on chest X-ray; emergence of all symptoms within 6 hours upon blood transfusion and exclusion of cardiac ischemia and transfusion associated circulatory overload (TACO). Apart from supportive measures, such as oxygen or ventilation, no specific therapies are available. The pathogenesis is incompletely understood, however, a two-hit model is usually assumed to underlie the disease pathology. The first hit consists of a pre-disposing factor present in the recipient such as inflammation while the second hit is conveyed by factors present in the transfused blood product such as anti-leukocyte antibodies (3). For example, the acute phase protein C-reactive protein (CRP) which rapidly increases during infection and inflammation was shown to enhance antibody-mediated TRALI in mice (4) and in line with that data, CRP was found to be elevated in human TRALI patients (5) confirming its role as a first hit risk factor in human TRALI.

Elevation of C-reactive protein levels in patients with transfusion-related acute lung injury

Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-related fatalities and is characterized by the onset of acute respiratory distress within six hours following blood transfusion. In most cases, donor antibodies are suggested to be involved, however, the pathogenesis is poorly understood. A two-hit model is generally assumed to underlie TRALI pathogenesis where the first hit consists of a patient predisposing factor such as inflammation and the second hit is due to donor antibodies present in the transfused blood. We recently demonstrated that the acute phase protein C-reactive protein (CRP) could enhance murine anti-major histocompatibility complex (MHC) class I-mediated TRALI. Whether CRP is increased in human TRALI patients which would support its role as a risk factor for human TRALI, is currently unknown. For that purpose, we measured CRP levels in the plasma of human TRALI patients and found CRP levels to be significantly elevated compared to transfused control patients. These data support the notion that CRP may be a novel first hit risk factor in human TRALI and that modulation of CRP levels could be an effective therapeutic strategy for this serious adverse event of transfusion.

Targeting Transfusion-Related Acute Lung Injury: The Journey From Basic Science to Novel Therapies

Objectives: Transfusion-related acute lung injury is characterized by the onset of respiratory distress and acute lung injury following blood transfusion, but its pathogenesis remains poorly understood. Generally, a two-hit model is presumed to underlie transfusion-related acute lung injury with the first hit being risk factors present in the transfused patient (such as inflammation), whereas the second hit is conveyed by factors in the transfused donor blood (such as antileukocyte antibodies). At least 80% of transfusion-related acute lung injury cases are related to the presence of donor antibodies such as antihuman leukocyte or antihuman neutrophil antibodies. The remaining cases may be related to nonantibody-mediated factors such as biolipids or components related to storage and ageing of the transfused blood cells. At present, transfusion-related acute lung injury is the leading cause of transfusion-related fatalities and no specific therapy is clinically available. In this article, we critically appraise and discuss recent preclinical (bench) insights related to transfusion-related acute lung injury pathogenesis and their therapeutic potential for future use at the patients’ bedside in order to combat this devastating and possibly fatal complication of transfusion. Data Sources: We searched the PubMed database (until August 22, 2017). Study Selection: Using terms: “Transfusion-related acute lung injury,” “TRALI,” “TRALI and therapy,” “TRALI pathogenesis.” Data Extraction: English-written articles focusing on transfusion-related acute lung injury pathogenesis, with potential therapeutic implications, were extracted. Data Synthesis: We have identified potential therapeutic approaches based on the literature. Conclusions: We propose that the most promising therapeutic strategies to explore are interleukin-10 therapy, down-modulating C-reactive protein levels, targeting reactive oxygen species, or blocking the interleukin-8 receptors; all focused on the transfused recipient. In the long-run, it may perhaps also be advantageous to explore other strategies aimed at the transfused recipient or aimed toward the blood product, but these will require more validation and confirmation first.