Thrombotic microangiopathy after renal transplantation: Current insights in de novo and recurrent disease

Table 1 Morphological features in microangiopathy

Active lesions	Chronic lesions
Glomeruli: Thrombi - Endothelial swelling or denudation - Fragmented RBCs - Subendothelial flocculent material. EM: Mesangiolysis - Microaneurysms	Glomeruli: LM: Double contours of peripheral capillary walls, with variable mesangial interposition - EM: New subendothelial basement membrane - Widening of the subendothelial zone
Arterioles: Thrombi - Endothelial swelling or denudation-Intramural fibrin-Fragmented red blood cells-Intimal swelling-Myocyte necrosis	Arterioles: Hyaline deposits Arteries: Fibrous intimal thickening with concentric lamination (onion skin)
Arteries: Thrombi - Myxoid intimal swelling -Intramural fibrin- Fragmented red blood cells

Adapted from: Goodship et al[58]. EM: Electron microscopy; LM: Light microscopy.

Table 2 Risk of atypical hemolytic uremic syndrome recurrence according to the implicated genetic abnormality

Genemutation	Location	Functionalimpact	Mutation frequencyin aHUS (%)	Recurrence after transplantation (%)
CFH	Plasma	Loss	20-30	75-90
CFI	Plasma	Loss	2-12	45-80
CFB	Plasma	Gain	1-2	100
C3	Plasma	Gain	5-10	40-70
MCP	Membrane	Loss	10-15	15-20
THBD	Membrane	Loss	5	One case
HomozygousCFHR1 del (3%-8%)	Circulating	Undetermined	14-23 (> 90% with anti-CHF AB)	NA

Adapted from Salvadori et al[74]. NA: Not available; CFH: Complement factor H; CFI: Complement factor I; CFB: Complement factor B; C3: Complement component 3; MCP: Membrane cofactor protein; THBD: Thrombomodulin.

Table 3 Complement studies for atypical hemolytic uremic syndrome (aHUS)

Complement test	aHUS
Complement protein levels	C3, C4, FB1, C51
Complement regulatory protein levels	FH, FI, Properdin1, CD462
Complement split products	C3c1, C3d1, Bb1, sC5b-91
Complement functional assays	CH50, AH50, hemolytic assays, FH assays1
Autoantibodies	Anti-FH
Genetic screening	CFH, CFI, C3, CD46, CFB Genomic rearrangements across the FH-FHR locus (e.g., by MLPA) Sequencing of coding regions and assessment of CNV Non-complement genetic screening includes THBD and DGKE

¹Currently available only at specific laboratories; they are research and not clinically validated assays;

²CD46 is also known as MCP. Adapted from: Goodship et al[58]. AH50: Alternative pathway hemolytic assay; C3: Complement component 3; C4: Complement component 4; C5: Complement component 5; CFB: Complement factor B gene; CFH: Complement factor H gene; CFHR: Complement factor H related genes; CFI: Complement factor I gene; CH50: Classical pathway hemolytic assay; CNV: Copy number variation; DGKE gene: Diacylglycerol kinase epsilon gene; FB: Complement factor B; FH: Complement factor H; FI: Complement factor I; MLPA: Multiplex ligation-dependent probe amplification; sC5b-9: Soluble C5b-9; THBD: Thrombomodulin; aHUS: Atypical hemolytic uremic syndrome.

Table 4 Genotype-phenotype correlations in atypical hemolytic uremic syndrome (data refer to the period before introduction of eculizumab)

Gene	Risk of death or ESRD at onset or first yr	Risk ofrecurrence	Risk of death or ESRDafter 3-5 yr	Risk of recurrencein allograft
CFH or CFH-CFHR1/3 hybrid genes	50%-70%	50%	75%	75%-90%
CFI	50%	10%-30%	50%-60%	45%-80%
MCP single	0%-6%	70%-90%	6%-38%	< 20%
MCP combined1	30%-40%	50%	50%	50%-60%
C3	60%	50%	75%	40%-70%
CFB	50%	100%	75%	100%
THBD	50%	30%	54%	?
Anti-FH	30%-40%	40%-60%	35%-60%	Depends on antibody titers

¹Combined with CFH or CFI or C3 mutations. Adapted from: Goodship et al[58]. CFB: Complement factor B gene; CFH: Complement factor H gene; CFHR: Complement factor H-related genes; CFI: Complement factor I gene; FH: Factor H protein; THBD: Thrombomodulin gene.

Table 5 Eculizumab dosing in atypical hemolytic uremic syndrome based on dosing goal, one additional monitoring may be required during intercurrent events (e.g., infection, surgery, vaccination) to detect unblocked complement activity

Minimal dose	Desire to continue dosing with the minimal dose required to achieve a pre-identified level of complement blockade 1
	Dose reduction or interval extension
	Goal CH50 < 10% (recommended)
	Goal AH50 < 10% (recommended)
	Goal eculizumab trough > 100 μg/mL
Discontinuation	Desire to discontinue complement blockade: No consensus exists regarding tapering of dose

Adapted from: Goodship et al[58]. AH50: Alternative pathway hemolytic activity; CH50: Total complement activity.

Table 6 Monitoring eculizumab therapy

Description
CH50 (total complement activity)	Measures the combined activity of all of the complement pathways Tests the functional capability of serum complement components to lyse 50% of sheep erythrocytes in a reaction mixture Low in congenital complement deficiency (C1-8) or during complement blockade Normal range is assay dependent Recommended goal during therapeutic complement blockade: < 10% of normal
AH50 (alternative pathway hemolytic activity)	Measures combined activity of alternative and terminal complement pathways Tests the functional capability of alternate or terminal pathway complement components to lyse 50% of rabbit erythrocytes in a Mg2+-EGTA buffer Will be low in congenital C3, FI, FB, properdin, FH, and FD deficiencies or during terminal complement blockade Normal range is assay dependent Recommended goal during complement blockade: < 10% of normal
Eculizumab trough	May be a free or bound level ELISA: Using C5 coated plates, patient sera, and an anti-human IgG detection system Not affected by complement deficiencies Recommended trough level during complement blockade: 50-100 μg/mL
Alternative assays	The following assays are under investigation (or awaiting to be replicated in different laboratories)[83] as a means to monitor therapeutic complement blockade Free C5 In vitro human microvascular endothelial cell test sC5b -9 (also referred to as sMAC and TCC) may remain detectable in aHUS patients in remission and therefore is not recommended as a monitoring tool

Adapted from: Goodship et al[58]. aHUS: Atypical hemolytic uremic syndrome; C3: Complement component 3; C5: Complement component 5; EGTA: Ethyleneglycol tetraacetic acid; ELISA: Enzyme-linked immunosorbent assay; FB: Complement factor B; FD: Complement factor D; FH: Complement factor H; FI: Complement factor I; sC5b-9: Soluble C5b-9; sMAC: Soluble membrane attack complex; TCC: Terminal complement complex.