This document discusses various thrombotic microangiopathies (TMAs) including their causes, pathophysiology, clinical findings, and treatments. It covers hemolytic uremic syndrome, thrombotic thrombocytopenic purpura, atypical hemolytic uremic syndrome, and antiphospholipid syndrome. The key pathological finding in all TMAs is endothelial injury and thrombus formation in small blood vessels. Recent research suggests dysregulation of the alternative complement pathway may be a unifying factor. Emerging treatments targeting complement, such as eculizumab, show promise in halting disease progression.

1. Thrombotic Microangiopathies
Richard McCrory

2. Objectives
• Thrombotic Microangiopathy: Pathology – 5 mins
• The Alternative Complement Pathway – 5 minutes
• STEC related HUS – 5 mins
• Atypical HUS – 5 mins
• TTP and ADAMTS13 – 5 minutes
• Treatment and Transplantation in TMA – 10 minutes
• Antiphospholipid Syndrome – 10 minutes

3. Thrombotic Microangiopathy
Describes a process comprising:
1) Consumptive Thrombocytopenia
2) Microangiopathic Haemolytic Anaemia
3) Microvascular Thrombosis

4. Clinical Spectrum of TMA
• Haemolytic uremic syndrome
• Thrombotic thrombocytopenic purpura
• Systemic sclerosis
• Malignant hypertension
• Preeclampsia – eclampsia
• Systemic lupus erythematosus
• Antiphospholipid antibody syndrome
• Renal transplant associated
• Drug induced
• Radiation therapy associated

5. Pathophysiology
Endothelial Cells key players in
pathogenesis of TMA
All TMA biopsies reflect endothelial injury
and thrombus formation

6. General Lab Findings in TMA
• Full blood count
– Severe thrombocytopenia and Anaemia
• Blood film
– Red cell fragmentation (‘schistocytosis’ >1%)
– Reticulocytosis
– Absent or Giant platelets
• Coombs test
– Negative
• Haemolysis screen
– Elevated LDH
– Serum haptoglobins low
• Liver enzymes and coagulation screen: Normal
• Creatinine: Elevated in renal involvement

7. Pathological Findings in TMA
TMA
Glomerular Vascular

8. Glomerular Lesions
Acute
Glomerular intracapillary thrombosis,
– Accumulation of fragmented erythrocytes
– Focally ischaemic or congested glomerular
tufts.
Chronic
• Membranoproliferative pattern with mesangial
interposition, double contours of the GBM, and
prominent mesangiolysis

10. Glomerular Tuft

11. Immunofluorescence

12. Bloodless glomeruli in TMA

13. Double Contours on Silver Stain

14. Vascular Changes
• Acute
• Thrombotic Occlusion of medium vessels

15. Vascular Changes
• Chronic
– Vessel-wall sclerosis, intraluminal organizing
thrombi
– Elastic lamina destruction with resultant
aneurysmal dilatation of the vessel wall

17. The Alternative Complement
Pathway

19. The Alternative Complement
Pathway

20. Regulation of C3 ‘Tickover’
MCP – Membrane Cofactor Protein;
found on all endothelial tisues

21. Haemolytic Uraemic
Syndrome

22. HUS: Epidemiology
Prevalence: 2 per 100,000 persons per year
– Peaks in children <5 yrs old
Shiga Toxin producing E. Coli (STEC)
contributes 90% of cases worldwide
– Can be sporadic or epidemic

23. Shiga Toxin

24. Pathogenesis of Shiga Toxin (1)
Localises to endothelium via
globotriaosylceramide (Gb3)
– Alpha subunit = N-glycosidase = Protein
Shutdown
Endothelial cell damage
– In the gut →Bloody Diarrhoea
– In the kidney → Intravascular thrombogenesis

25. Pathogenesis of Shiga Toxin (2)
Shiga toxin can de-regulate complement activation
Binds to Factor H
Orth et al. Journal of Immunology (2009)
Encourages expression of P-selectin on
endothelium, attracting C3a
Factor B deficient mice demonstrate less severe
disease with STEC organisms
Morigi et al. Journal of Immunology (2011)

26. E.Coli O157:H7 Outbreaks
Nov – Dec 1996 - Lanarkshire
21 elderly people die and another 500 were
made ill eating contaminated meat
2006 California
Linked to Spinach consumption from fields
contaminated with sewage water.
205 total cases, with 31 HUS cases, for an HUS
rate of about 15%.

27. Shiga-Toxin–Producing Escherichia
coli O104:H4 Outbreak in Germany
3816 lab confirmed cases
857 cases involved HUS
54 deaths
Frank et al. (NEJM 2011)

29. Question
Which of these causes of TMA can be
associated with a Coombs positive test?
Shiga Toxin Associated
Invasive Pneumococcal Infection
Calcineurin Inhibitor
Ticlopidine
Quinine

30. Pneumococcus associated HUS
• Streptococcal Neuramidase exposes a
novel antigen [Thomsen-Freidenreich (T)
Antigen] on RBCs/Platelets/ Glomeruli
• Antibodies lead to cell aggregation and
deposition in small vessels

31. ‘Atypical’ HUS
Non-STEC, Non-Pneumococcal
• Genetic and acquired factors leading to
dysregulation of the alternative
complement pathway.
• Much rarer (~1000 cases in registry data
from Europe/US)

32. Mutations in aHUS Registries
Factor H
MCP
Factor I
C3
Factor B
Hybrid Genes
Combined
Factor H antibody
The presence of a mutation
in an individual with aHUS
does not prove causation

33. Testing for Complement
Dysregulation
C3 /C4
Factor H, factor I, factor B
MCP (CD46) expression on PBMCs
Factor H autoantibodies
Mutations
• Direct exon sequencing of CFH, MCP,
CFI, CFB, C3
• Copy number variation across CFH-CFHR
locus

34. Thrombotic Thrombocytopenic
Purpura

35. TTP
• Prevalence 1 in 50,000 hospitalisations
• Mortality previously >90%, now around 20-30% with current therapy
The ‘classical’ pentad
• Neurological findings – 75% of patients
• Neurological Findings / Haemolytic Anaemia / Thrombocytopaenia –
74%
• Fever / Renal Impairment – 40%
• Elevated creatinine is a poor prognostic indicator

36. Pathophysiology
Reduced activity of the ADAMTS13 cleaving
protease with consequent disruption of
von Willebrand factor multimer processing
– Can be congenital or ‘functional’

37. TTP and Complement Activation
• Increased C3a activity detected in TTP
flares
• C3a activity declines with plasma
exchange treatment
Réti et al J Thromb Haemost.(2012)

38. Treatment and
Transplantation

39. Treatment
Supportive Management
• Avoid antibiotics in STEC infection -
increases HUS 17-fold
• Fluid balance, fluid resusciation,
electrolytes, early RRT if indicated
• Avoid platelet transfusion

40. Treatment
• ‘Urgent Empirical Plasma Exchange’
– Commonly when STEC/Pneumococcal
Negative
• May have a role in severe adult cases of STEC-
HUS but controversial
– Recommended in all other HUS/TTP settings

41. Transplantation
All forms of renal transplant available for:
– STEC-HUS
– MCP-aHUS
Other aHUS phenotypes
– Living-related donation ‘contraindicated’
– Unknown risk factors within family probands
– High risk recurrence in grafts with transplant
failure common.

42. Liver/Kidney Transplantation
May be considered as a solution to
deficiencies in complement proteins
Performed with limited success in paediatric
cases.

43. Eculizumab – The Great White Hope?
• Monoclonal Antibody to C5
• Prevents formation of membrane attack
complexes
• Multiple Case series in aHUS
– Also used in STEC-HUS during German
epidemic outbreak
– Use in transplantation to prevent graft
recurrence

44. Antiphospholipid Syndrome

45. Common autoimmune diseases
associated with APL antibodies
SLE -25-50%
Sjogren’s – 42%
Rheumatoid Disease - 33%
Autoimmune Thrombocytopenic Purpura - 30%
PA - 28%
Systemic Sclerosis - 25%
Mixed Connective Tissue Disease - 22%
Bechet - 20%
Polymyalgia Rheumatica - 20%

46. Pathogenesis of APL
Homeostatic regulation of blood coagulation is altered.
Exposure of membrane phospholipids to the binding of various plasma
proteins (b2gp1)
Production of antibodies against prothrombin, Protein C/ Protein S and
Annexins.
Activation of:
– platelets to enhance endothelial adherence.
– vascular endothelium—platelet and monocyte binding.
Antibodies against oxidized LDL—atherosclerosis.
Complement activation has been increasingly recognized as a possible
significant role in the pathogenesis of APS.
Blood. Jan 15 2007;109(2):422-30.
Nat Med. Nov 2004;10(11):1222-6.

47. The role of Complement in Miscarriage

49. The renal manifestations of APS
Systemic hypertension in association with livedo
reticularis
Renal Arterial Stenosis
Renal infarction
APS-associated nephropathy
Acutely – Thrombotic Microangiopathy
Chronic - Arteriosclerosis, Fibrous intimal hyperplasia,
focal cortical atrophy
Renal vein thrombosis and increased allograft
vascular thrombosis

50. Antiphospholipid syndrome (APS) -
Diagnosis
• At least one clinical and one laboratory criteria
• Clinical – vascular thrombosis or pregnancy
morbidity
– ≥ 1 venous, arterial or small vessel thrombosis with
unequivocal imaging or histology
– Unexplained fetal death ≥ 10wks gestation
– One or more premature births before 34wks gestation
because of eclampsia, pre-eclampsia or placental
insufficiency
– ≥ 3 pregnancy losses unexplained by chromosomal,
maternal or hormonal causes

51. APS - diagnosis
• Laboratory criteria
• The presence of APL ABs on ≥ 2 occasions at
least 12 wks apart
• No more than 5 yrs prior to clinical
manifestations
– Anticardiolipin Ab in moderate/high titre
– Β2-glycoprotein 1 Ab elevation
– Lupus anticoagulant (LA) activity ( APTT in vitro,
not corrected with normal plasma)

52. Catastrophic Antiphospholipid
Syndrome(CAPS)
• A syndrome of multisystem involvement (<1% of
patients)
• Acute onset
– 3 different organ systems within 2 weeks
– Acute microangiopathy is characteristic
Triggers
Infection (25-30%)
Surgery / Trauma (10%)
Anticoagulation Withdrawal (10%)

54. SLE with antiphospholipid Abs
 risk of thromboembolism & recurrent pregnancy loss
• 20-50% met criteria for APS at 10-20 yrs
• The higher the ACL Ab titre the greater risk of
thrombosis
• Aspirin if cardiovascular or genetic risk factors for
thromboembolic disease
• Address cardiovascular risk factors
• Avoid OCP - esp if high oestrogen content
• SLE pts should be regularly assessed for APL Abs
(ACL Ab, β2GP1 Ab, LAC)
• Aspirin and HCQ prophylaxis for those with persistent
LAC or ACL Ab at medium/high titre

55. TMA: Take Home Points
Pathological findings similar – causes are
multiple
Unifying pathological event appear to be
related to dysregulation of the alternative
complement pathway
Treatments aiming to stop the complement
cascade appear to promising targets for
therapy