New aspects in the pathogenesis and treatment of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Thrombotic microangiopathies: an update

Thrombotic microangiopathies (TMA) are microvascular occlusive disorders characterized by hemolytic anemia caused by fragmentation of erythrocytes and thrombocytopenia due to increased platelet aggregation and thrombus formation, eventually leading to disturbed microcirculation with reduced organ perfusion. Although several disease states may manifest as TMA, the two most relevant conditions associated with TMA are thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), characterized by prominent brain or renal lesions, respectively. However, occasionally the clinical distinction between these two conditions can be difficult. In this review, we focus on the epidemiologic and diagnostic criteria as well as on the most recent insights into the pathophysiology and treatment of these two conditions.

Disseminated histoplasmosis presenting as thrombotic microangiopathy

BACKGROUND

Thrombotic microangiopathies (TMA) are systemic vasoocclusive disorders associated with significant morbidity and mortality. Rapid and reliable diagnosis of TMA is critical. The diagnosis is complicated by a lack of objective and sensitive laboratory testing as well as multiple concurrent diseases, including infectious processes.

CASE STUDY

We report two cases of disseminated histoplasmosis associated with TMA manifestations in renal transplant recipients, including one patient with histologically documented renal microthrombi; both patients were referred for plasma exchange. After the diagnosis of histoplasmosis, the treatment plan was changed to antifungal medications, reduced immuno-suppression, and supportive care, with progressive resolution of TMA manifestations.

CONCLUSION

TMA occurs in transplant populations in association with infections, medications, and other factors. Appropriate management includes recognition and treatment of possible etiological factors. Disseminated histoplasmosis should be considered in transplant patients presenting with TMA.

The pathogenicity of von Willebrand factor in thrombotic thrombocytopenic purpura: reconsideration of treatment with cryopoor plasma

New developments in the understanding of thrombotic thrombocytopenic purpura (TTP) provide opportunities for improved patient care. A widely held historical model of TTP microvascular thrombosis implicated circulating ultra large von Willebrand factor (ULVWF) in causing spontaneous platelet (PLT) aggregation. From this pathogenic model, concerns about ULVWF in fresh-frozen plasma (FFP) used to treat patients led to widespread use of cryopoor plasma (CPP) as an alternative. There is scant evidence, however, that circulating ULVWF contributes to microvascular thrombosis in TTP. New evidence suggests that the formation of PLT aggregates in TTP may be mediated by VWF in the process of being released from endothelium. Moreover, clinical studies do not demonstrate superior efficacy of CPP compared to FFP in the treatment of TTP. Because CPP may have reduced concentrations of factors important in the treatment of TTP, including ADAMTS13 metalloprotease, a reappraisal of the use of CPP in the treatment of TTP is warranted.

Advances in the pathogenesis, diagnosis and treatment of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

The thrombotic microangiopathies are microvascular occlusive disorders characterized by hemolytic anemia caused by fragmentation of erythrocytes and thrombocytopenia due to increased platelet aggregation and thrombus formation, eventually leading to disturbed microcirculation with reduced organ perfusion. Depending on whether brain or renal lesions prevail, two different entities have been described: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). However, not rarely the clinical distinctions between these two conditions remain questionable. Recent studies have contributed greatly to our current understanding of the molecular mechanisms leading to TTP and HUS. In this review, we briefly focus on the most important advances in the pathophysiology, diagnosis and treatment of these two thrombotic microangiopathies.

Characterization of a human monoclonal antibody against Shiga toxin 2 expressed in Chinese hamster ovary cells

Shiga toxin-producing Escherichia coli infections can often lead to the development of hemolytic-uremic syndrome (HUS) in a small percentage of infected humans. Patients with HUS receive only supportive treatment as the benefit of antibiotic therapy remains uncertain. We have previously reported the generation and preclinical evaluation of neutralizing human monoclonal antibodies (HuMAbs) against the Shiga toxins (Stx). In this paper, we describe the expression in Chinese hamster ovary (CHO) cells of 5C12 HuMAb, which is directed against the A subunit of Stx2. The cDNAs of the light and heavy chain immunoglobulin (Ig) variable regions of 5C12 HuMAb were isolated and cloned into an expression vector containing human IgG1 constant regions. The vector was transfected into CHO cells, and transfectants secreting Stx2-specific antibody were screened by an Stx2-specific enzyme-linked immunosorbent assay. The CHO-produced recombinant 5C12 (r5C12) showed similar specificity and binding affinity to Stx2 as the parent hybridoma-produced 5C12. More significantly, the r5C12 displayed the same neutralizing activity as the parent 5C12 in vitro and in vivo. In the mouse toxicity model, both antibodies significantly and equally prolonged survival at a dose of 0.312 microg/mouse. The data showed that since r5C12, produced in CHO cells, was equally effective as the parent 5C12, it is our choice candidate as a potential prophylactic or therapeutic agent against hemolytic-uremic syndrome.

Antibody therapy in the management of shiga toxin-induced hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is a disease that can lead to acute renal failure and often to other serious sequelae, including death. The majority of cases are attributed to infections with Escherichia coli, serotype O157:H7 strains in particular, which cause bloody diarrhea and liberate one or two toxins known as Shiga toxins 1 and 2. These toxins are thought to directly be responsible for the manifestations of HUS. Currently, supportive nonspecific treatment is the only available option for the management of individuals presenting with HUS. The benefit of antimicrobial therapy remains uncertain because of several reports which claim that such intervention can in fact exacerbate the syndrome. There have been only a few specific therapies directed against neutralizing the activities of these toxins, but none so far has been shown to be effective. This article reviews the literature on the mechanism of action of these toxins and the clinical manifestations and current management and treatment of HUS. The major focus of the article, however, is the development and rationale for using neutralizing human antibodies to combat this toxin-induced disease. Several groups are currently pursuing this approach with either humanized, chimeric, or human antitoxin antibodies produced in transgenic mice. They are at different phases of development, ranging from preclinical evaluation to human clinical trials. The information available from preclinical studies indicates that neutralizing specific antibodies directed against the A subunit of the toxin can be highly protective. Such antibodies, even when administered well after exposure to bacterial infection and onset of diarrhea, can prevent the occurrence of systemic complications.