Osama A. Hamad

Are Therapeutic Human Mesenchymal Stromal Cells Compatible with Human Blood

Multipotent mesenchymal stromal cells (MSCs) are tested in numerous clinical trials. Questions have been raised concerning fate and function of these therapeutic cells after systemic infusion. We therefore asked whether culture‐expanded human MSCs elicit an innate immune attack, termed instant blood‐mediated inflammatory reaction (IBMIR), which has previously been shown to compromise the survival and function of systemically infused islet cells and hepatocytes. We found that MSCs expressed hemostatic regulators similar to those produced by endothelial cells but displayed higher amounts of prothrombotic tissue/stromal factors on their surface, which triggered the IBMIR after blood exposure, as characterized by formation of blood activation markers. This process was dependent on the cell dose, the choice of MSC donor, and particularly the cell‐passage number. Short‐term expanded MSCs triggered only weak blood responses in vitro, whereas extended culture and coculture with activated lymphocytes increased their prothrombotic properties. After systemic infusion to patients, we found increased formation of blood activation markers, but no formation of hyperfibrinolysis marker D‐dimer or acute‐phase reactants with the currently applied dose of 1.0–3.0 × 106 cells per kilogram. Culture‐expanded MSCs trigger the IBMIR in vitro and in vivo. Induction of IBMIR is dose‐dependent and increases after prolonged ex vivo expansion. Currently applied doses of low‐passage clinical‐grade MSCs elicit only minor systemic effects, but higher cell doses and particularly higher passage cells should be handled with care. This deleterious reaction can compromise the survival, engraftment, and function of these therapeutic cells. Stem Cells2012;30:1565–1574

Do cryopreserved mesenchymal stromal cells display impaired immunomodulatory and therapeutic properties

We have recently reported that therapeutic mesenchymal stromal cells (MSCs) have low engraftment and trigger the instant blood mediated inflammatory reaction (IBMIR) after systemic delivery to patients, resulting in compromised cell function. In order to optimize the product, we compared the immunomodulatory, blood regulatory, and therapeutic properties of freeze‐thawed and freshly harvested cells. We found that freeze‐thawed MSCs, as opposed to cells harvested from continuous cultures, have impaired immunomodulatory and blood regulatory properties. Freeze‐thawed MSCs demonstrated reduced responsiveness to proinflammatory stimuli, an impaired production of anti‐inflammatory mediators, increased triggering of the IBMIR, and a strong activation of the complement cascade compared to fresh cells. This resulted in twice the efficiency in lysis of thawed MSCs after 1 hour of serum exposure. We found a 50% and 80% reduction in viable cells with freshly detached as opposed to thawed in vitro cells, indicating a small benefit for fresh cells. In evaluation of clinical response, we report a trend that fresh cells, and cells of low passage, demonstrate improved clinical outcome. Patients treated with freshly harvested cells in low passage had a 100% response rate, twice the response rate of 50% observed in a comparable group of patients treated with freeze‐thawed cells at higher passage. We conclude that cryobanked MSCs have reduced immunomodulatory and blood regulatory properties directly after thawing, resulting in faster complement‐mediated elimination after blood exposure. These changes seem to be paired by differences in therapeutic efficacy in treatment of immune ailments after hematopoietic stem cell transplantation. Stem Cells 2014;32:2430–2442

Complement activation triggered by chondroitin sulfate released by thrombin receptor-activated platelets

Summary.  Background: Chondroitin sulfate (CS) is a glycosaminoglycan released by activated platelets. Objective: Here we test the hypothesis that CS released by activated platelets can trigger complement activation in the fluid phase. Methods and results: Thrombin receptor‐activating peptide (TRAP)‐6 was used to activate platelets in platelet‐rich plasma and blood, anticoagulated with the thrombin inhibitor lepirudin. TRAP activation induced fluid‐phase complement activation, as reflected by the generation of C3a and sC5b‐9, which could be attenuated by the C3 inhibitor compstatin. Chondroitinase ABC treatment of supernatants from activated platelets totally inhibited the activation, indicating that platelet‐derived CS had initiated the complement activation. Furthermore, addition of purified CS to plasma strongly triggered complement activation. C1q was identified as the recognition molecule, as it bound directly to CS, and CS‐triggered complement activation could be restored in C1q‐depleted serum by adding purified C1q. TRAP activation of whole blood increased the expression of CD11b on leukocytes and generation of leukocyte–platelet complexes. It was demonstrated that these leukocyte functions were dependent on C3 activation and signaling via C5a, as this expression could be inhibited by compstatin and by a C5aR antagonist. Conclusions: We conclude that platelets trigger complement activation in the fluid phase by releasing CS, which leads to inflammatory signals mediated by C5a.

Platelets, Complement, and Contact Activation : Partners in inflammation and thrombosis

Platelet activation during thrombotic events is closely associated with complement and contact system activation, which in turn leads to inflammation. Here we review the interactions between activated platelets and the complement and contact activation systems in clotting blood. Chondroitin sulfate A (CS-A), released from alpha granules during platelet activation, is a potent mediator of crosstalk between platelets and the complement system. CS-A activates complement in the fluid phase, generating anaphylatoxins that mediate leukocyte activation. No complement activation seems to occur on the activated platelet surface, but C3 in the form of C3(H(2)O) is bound to the surfaces of activated platelets . This finding is consistent with the strong expression of membrane-bound complement regulators present at the platelet surface. CS-A exposed on the activated platelets is to a certain amount responsible for recruiting soluble regulators to the surface. Platelet-bound C3(H(2)O) acts as a ligand for leukocyte CR1 (CD35), potentially enabling platelet-leukocyte interactions. In addition, platelet activation leads to the activation of contact system enzymes, which are specifically inhibited by antithrombin, rather than by C1INH, as is the case when contact activation is induced by material surfaces. Thus, in addition to their traditional role as initiators of secondary hemostasis, platelets also act as mediators and regulators of inflammation in thrombotic events.

The role of complement factor C3 in lipid metabolism

Abundant reports have shown that there is a strong relationship between C3 and C3a-desArg levels, adipose tissue, and risk factors for cardiovascular disease, metabolic syndrome and diabetes. The data indicate that complement components, particularly C3, are involved in lipid metabolism. The C3 fragment, C3a-desArg, functions as a hormone that has insulin-like effects and facilitates triglyceride metabolism. Adipose tissue produces and regulates the levels of complement components, which promotes generation of inflammatory initiators such as the anaphylatoxins C3a and C5a. The anaphylatoxins trigger a cyto/chemokine response in proportion to the amount of adipose tissue present, and induce inflammation and mediate metabolic effects such as insulin resistance. These observations support the concept that complement is an important participant in lipid metabolism and in obesity, contributing to the metabolic syndrome and to the low-grade inflammation associated with obesity.

The lectin complement pathway serine proteases (MASPs) represent a possible crossroad between the coagulation and complement systems in thromboinflammation.

Essentials The lectin pathways MASP‐1/2 activates coagulation factors but the trigger of the activation is unknown. MASP‐1/2 activation was assessed by quantifying complexes between MASPs and antithrombin/C1‐inhibitor. Activated platelets and fibrin were demonstrated to activate MASP‐1 and MASP‐2 both in vitro and in vivo. These findings may represent a crossroad between the complement and the coagulation systems.

Activated Platelets Provide a Functional Microenvironment for the Antiangiogenic Fragment of Histidine-Rich Glycoprotein

The angiogenesis inhibitor histidine-rich glycoprotein (HRG) constitutes one of several examples of molecules regulating both angiogenesis and hemostasis. The antiangiogenic properties of HRG are mediated via its proteolytically released histidine- and proline-rich (His/Pro-rich) domain. Using a combination of immunohistochemistry and mass spectrometry, we here provide biochemical evidence for the presence of a proteolytic peptide, corresponding to the antiangiogenic domain of HRG, in vivo in human tissue. This finding supports a role for HRG as an endogenous regulator of angiogenesis. Interestingly, the His/Pro-rich peptide bound to the vessel wall in tissue from cancer patients but not to the vasculature in tissue from healthy persons. Moreover, the His/Pro-rich peptide was found in close association with platelets. Relesate from in vitro–activated platelets promoted binding of the His/Pro-rich domain of HRG to endothelial cells, an effect mediated by Zn2+. Previous studies have shown that zinc-dependent binding of the His/Pro-rich domain of HRG to heparan sulfate on endothelial cells is required for inhibition of angiogenesis. We describe a novel mechanism to increase the local concentration and activity of an angiogenesis inhibitor, which may reflect a host response to counteract angiogenesis during pathologic conditions. Our finding that tumor angiogenesis is elevated in HRG-deficient mice supports this conclusion. (Mol Cancer Res 2009;7(11):1792–802)

Evaluation of the blood compatibility of materials, cells, and tissues: basic concepts, test models, and practical guidelines.

Medicine today uses a wide range of biomaterials, most of which make contact with blood permanently or transiently upon implantation. Contact between blood and nonbiological materials or cells or tissue of nonhematologic origin initiates activation of the cascade systems (complement, contact activation/coagulation) of the blood, which induces platelet and leukocyte activation. Although substantial progress regarding biocompatibility has been made, many materials and medical treatment procedures are still associated with severe side effects. Therefore, there is a great need for adequate models and guidelines for evaluating the blood compatibility of biomaterials. Due to the substantial amount of cross talk between the different cascade systems and cell populations in the blood, it is advisable to use an intact system for evaluation. Here, we describe three such in vitro models for the evaluation of the biocompatibility of materials and therapeutic cells and tissues. The use of different anticoagulants and specific inhibitors in order to be able to dissect interactions between the different cascade systems and cells of the blood is discussed. In addition, we describe two clinically relevant medical treatment modalities, the integration of titanium implants and transplantation of islets of Langerhans to patients with type 1 diabetes, whose mechanisms of action we have addressed using these in vitro models.

Do ABO Blood Group Antigens Hamper the Therapeutic Efficacy of Mesenchymal Stromal Cells

Investigation into predictors for treatment outcome is essential to improve the clinical efficacy of therapeutic multipotent mesenchymal stromal cells (MSCs). We therefore studied the possible harmful impact of immunogenic ABO blood groups antigens – genetically governed antigenic determinants – at all given steps of MSC-therapy, from cell isolation and preparation for clinical use, to final recipient outcome. We found that clinical MSCs do not inherently express or upregulate ABO blood group antigens after inflammatory challenge or in vitro differentiation. Although antigen adsorption from standard culture supplements was minimal, MSCs adsorbed small quantities of ABO antigen from fresh human AB plasma (ABP), dependent on antigen concentration and adsorption time. Compared to cells washed in non-immunogenic human serum albumin (HSA), MSCs washed with ABP elicited stronger blood responses after exposure to blood from healthy O donors in vitro, containing high titers of ABO antibodies. Clinical evaluation of hematopoietic stem cell transplant (HSCT) recipients found only very low titers of anti-A/B agglutination in these strongly immunocompromised patients at the time of MSC treatment. Patient analysis revealed a trend for lower clinical response in blood group O recipients treated with ABP-exposed MSC products, but not with HSA-exposed products. We conclude, that clinical grade MSCs are ABO-neutral, but the ABP used for washing and infusion of MSCs can contaminate the cells with immunogenic ABO substance and should therefore be substituted by non-immunogenic HSA, particularly when cells are given to immunocompentent individuals.

Prediction of inflammatory responses induced by biomaterials in contact with human blood using protein fingerprint from plasma.

Inappropriate complement activation is often responsible for incompatibility reactions that occur when biomaterials are used. Complement activation is therefore a criterion included in legislation regarding biomaterials testing. However, no consensus is yet available regarding appropriate complement-activation-related test parameters. We examined protein adsorption in plasma and complement activation/cytokine release in whole blood incubated with well-characterized polymers. Strong correlations were found between the ratio of C4 to its inhibitor C4BP and generation of 10 (mainly pro-inflammatory) cytokines, including IL-17, IFN-γ, and IL-6. The levels of complement activation products correlated weakly (C3a) or not at all (C5a, sC5b-9), confirming their poor predictive values. We have demonstrated a direct correlation between downstream biological effects and the proteins initially adhering to an artificial surface after contact with blood. Consequently, we propose the C4/C4BP ratio as a robust, predictor of biocompatibility with superior specificity and sensitivity over the current gold standard.