Despite being the first genetic disease described, sickle cell disease (SCD) continues to afflict patients with immense pain, significant comorbidities and premature death. SCD has only recently benefited from new interventions with L-glutamine (2017), voxelotor (2019) and crizanlizumab (2019) representing the first Food and Drug Administration approved medications for SCD since hydroxyurea in 1997. These interventions have demonstrated some ability to reduce vaso-occlusive pain crisis episodes, improve hemoglobin (HGB), or reduce markers of hemolysis and have largely been used as preventative care measures. While these and additional approaches, such as hematopoietic stem cell transplant and gene therapy, can improve SCD care, many patients with SCD continue to suffer from severe acute SCD complications that can result in organ damage and early death.1,2 Unfortunately, in these situations, supportive care remains the primary approach to alleviate complications. The lack of more targeted approaches in part reflects an incomplete understanding of the pathophysiology and accompanying pharmacological targets that could specifically mitigate acute disease complications. We present a summary of three cases of children with SCD who developed significant acute complications that demonstrate underlying complement-mediated thrombotic microangiopathy (CM-TMA). These cases include a delayed hemolytic transfusion reaction (DHTR), vasoocclusive crisis (VOC) and drug-induced immune hemolytic anemia (DIIHA).
Patient #1 is a 14-year-old male with a history of two episodes of DHTR. At the age of 12 years, he received a transfusion pre-operatively for hip core-decompression. He presented 7 days later with severe diffuse body pain, hemoglobinuria, fever, and total HGB of 9.4 g/dL with hemoglobin A (HbA) at 17% (Figure 1A). Further testing revealed a previously detected anti-U alloantibody, negative direct antiglobulin test (DAT) and evidence for intravascular hemolysis. On the night of admission, he became hypertensive with headache and sluggish mentation. A brain magnetic resonance imaging scan was normal. HGB dropped to 5.6 g/dL within 30 hours of admission, and the patient rapidly deteriorated to multiorgan failure (MOF) with thrombocytopenia (Figure 1A). Due to strong suspicion for DHTR with hyperhemolysis and CM-TMA, he was treated with eculizumab 600 mg intravenously (IV) and erythropoietin 150 IU/kg to augment erythropoiesis. Over the next 24 hours, he developed a new consolidation in the left lung consistent with acute chest syndrome (ACS). He received one unit of crossmatch compatible, U-negative red blood cells (RBC) after a dose of 1 g/kg intravenous immunoglobulin (IVIG) on day 9. Over the next few days, he made a gradual improvement in clinical and laboratory status. Eculizumab 600 mg was continued weekly for a total of four doses. Subsequent analysis revealed significant alternative complement pathway (ACP) activation at the peak of hemolysis, as evidenced by increased complement component fragment Bb (Bb) levels, anaphylatoxins (C3a and C5a) and terminal complement complex (C5b-9) (Table 1). Testing for complement regulatory genes (CFH, CFI, MCP (CD46), CFB, CFHR5, C3, THBD, DGKE, PLG, ADAMTS13 and MMACHC) revealed a homozygous deletion of complement factor H-related protein (CFHR) 3 and CFHR1, but criteria for DEAP-HUS (deficiency of CFHR plasma proteins and autoantibody positive form of HUS) were not met due to the absence of factor H autoantibodies.
Patient #2 is a 15-year-old female with a history of VOC episodes and DHTR, who was transferred to our care at the age of 9 years. Since transfer, she suffered from multiple episodes of VOC, each accompanied with a drop in HGB. Five of these episodes are shown in the dotted lines (Figure 1B, top). During episode #3, with no recent RBC transfusion history, the patient presented with an HGB of 6.7 g/dL which decreased to 3.8 g/dL over 5 days, accompanied by lethargy, hypoxia and respiratory distress. Given her past history of DHTR and rapid decompensation, one dose of eculizumab 900 mg IV was administered along with erythropoietin 210 IU/kg daily and IVIG at 1 g/kg. The patient’s symptoms resolved with a rise in HGB and she was discharged home within 4 days. Six months later, she presented again (#4) with VOC, worsening hypoxia, and HGB 7.9 g/dL (Figure 1B, bottom). On admission, she received erythropoietin and IVIG at the doses outlined above. The HGB dropped to a nadir of 3.6 g/dL over 3 days, associated with severe headaches. Due to inadequate response to the above measures, eculizumab 900 mg was administered with rapid improvement in 48 hours. Complement analyses (Table 1) indicated clear evidence of ACP activation during episodes #1 and #5, as shown by elevation of Bb levels. During episodes #3 and #4, eculizumab was administered in anticipation of worsening organ function, which could explain why the complement activation markers were not significantly elevated (unlike in other episodes) and why her rapid HGB response and prompt reversal of organ dysfunction were observed. Complement gene evaluation revealed a heterozygous deletion of CFHR3/CFHR1.
Patient #3 is a 3-year-old male with a history of splenectomy at the age of 2 years for recurrent acute splenic sequestration. He presented with fever, tachypnea, HGB of 7 g/dL, with a negative chest x-ray (CXR). He developed increased work of breathing within 3 hours of receiving ceftriaxone. Repeat CXR revealed bilateral infiltrates requiring emergent intubation. HGB dropped to 4 g/dL, and he received extended phenotypematched and crossmatch-compatible RBC transfusions. Additional laboratory work-up revealed intravascular hemolysis and MOF (Figure 1C). The DAT was positive for complement component 3 (C3) only. The patient required escalation of respiratory support to high frequency oscillatory ventilation and nitric oxide. Oliguric acute kidney injury and hypertension required the use of continuous renal replacement therapy and multiple antihypertensives. Multiple common and rare causes for his rapid multi-organ failure were entertained including sepsis (negative blood/respiratory cultures), cold agglutinin syndrome (negative testing for mycoplasma serology and Donath-Landsteiner antibody), hemophagocytic lymphohistiocytosis (normal soluble interleukin-2 receptor, CD107a, perforin/granzyme B, Epstein Barr and cytomegaloviral load) and ceftriaxone-induced hemolysis. As his clinical course was consistent with CM-TMA, complement inhibition with eculizumab 600 mg IV was initiated on day 5. Following eculizumab, he demonstrated a rapid response with weaning of his ventilator support and dialysis, along with the reduced need for blood products. Follow-up testing was notable for strongly positive ceftriaxone-dependent antibodies consistent with DIIHA. Complement analyses confirmed significant activation of ACP (factor Bb elevation, see Table 1). Proximal and terminal complement pathway activation were likewise observed as indicated by increased C3 and C5 activation and C5b-9, respectively. Additionally, hypocomplementemia with reduced C3, C4, and CH50, seen in this patient, suggests worse disease. These markers improved following complement inhibition, which correlated with improved clinical status within 11 days of initial presentation. Eight months after this episode, this patient’s renal function is gradually improving. He remains on eculizumab 300 mg every 2 weeks pending full renal recovery. Complement genetic analysis was negative.
All patients and/or their guardians of cases described in this report provided written consent for the off-label use of eculizumab. These patients received meningococcal and pneumococcal vaccinations as part of routine SCD standard-of-care or given right before eculizumab and continued on a prophylactic antibiotic regimen while on treatment.
CM-TMA from an underlying mutation involving the complement regulatory genes is traditionally called atypical hemolytic uremic syndrome or ‘atypical HUS’ (aHUS). CM-TMA occurring secondary to complement amplifying disorders such as hematopoietic stem cell transplantation, malignancy, infections, or autoimmune diseases are termed ‘secondary HUS’.3,4 The above clinical vignettes suggest that some complications of SCD may also reflect complement activation-induced secondary HUS (Figure 2). As SCD is a chronic hemolytic condition and plasma free HGB and heme can activate complement, 5,6 additional complement activation during episodes of disease exacerbation may lead to increased hemolysis and a sustained positive feedback loop that leads to life-threatening anemia. As sickle erythrocytes are uniquely susceptible to complement-mediated damage, 7 this may also result in further exacerbation of the disease. Heme-dependent endothelial dysfunction seen in SCD can also be modulated by complement activation.8 In addition, heme can regulate the coagulation system, which likely reflects another important feature of SCD pathophysiology that contributes to inflammation and thrombosis.9 Increased C5a production has also been shown to cause acute lung injury and vasoocclusion in animal models.10,11 Prior reports suggested the role of ACP in the pathophysiology of SCD, and a protective effect of eculizumab in some settings.12-15 However, the present data suggest that some complications in SCD may not only reflect exuberant ACP activation, but actually represent an underlying CM-TMA. In these situations, chronic hemolysis and endothelial dysfunction may saturate scavenging and detoxifying mechanisms, reducing the capacity of patients with SCD to manage elevated plasma HGB and heme during periods of crises. In two of our patients, we detected variants in the complement genes. The CFH gene encodes soluble plasma factor H, which is a principal inhibitor of ACP. Further genetic studies focusing on complotype are needed to help understand its role in SCD. Figure 2 compares salient clinical features seen in aHUS and CM-TMA in SCD. In this way, the underlying pathophysiology of SCD may prime individuals for secondary HUS through ACP activation, with a subset of these patients reaching an inflection point during periods of crises (second-hit) that lead to additional hyperhemolysis and MOF. Case #3 is the first report of successful use of eculizumab to treat life-threatening DIIHA. Ceftriaxone-induced hemolysis often occurs secondary to immunglobulin M (IgM) anticeftriaxone antibodies, which typically engage the classical complement pathway and induce hemolysis. However, the impact of eculizumab did not appear to reflect inhibition of IgM-induced hemolysis as marked activation of APC was evidenced by increased levels of factor Bb, which likewise responded to eculizumab, consistent with a positive feedback loop that drives additional APC activation in these patients. While rare, this event in particular holds public health and preventative importance, as ceftriaxone is a widely used medication in SCD, and there is a high prevalence of ceftriaxone-induced RBC antibodies in these patients.16
This collection of cases therefore emphasizes a previously underappreciated role of CM-TMA and complement across a broad range of SCD presentations, and may reflect novel insights into the pathophysiology of acute exacerbations of SCD that may be sensitive to complement inhibition to avoid severe hemolytic complications in SCD.
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