Platelet Allo-Antigens Flashcards
surface receptors and antigens on platelets
HPA (human platelet allo-antigens)
class I HLA
ABO
T or F. HP1b is more common than HP1a
F! “a” is more common/frequent
methods used to detect HPA antibodies
Luminex
- PakLX assay
among others
treatment of FNAIT
transfusion with maternal washed PLTS - preferably apheresis or compatible PLTs is antibody specificity unknown
unrelated HPA typed donors are often used
> randome PLT units may result in successful increments
percutaneous umbilical blood sampling (PUBS) can be performed to monitor PLT count in utero (and transfuse)
treatment of mom with IVIg during pregnancy
> IVIg therapy in baby has had mixed success
treatment of PTP
IVIg
corticosteroids
platelet transfusion
> transfusion of HPA compatible platelts when possible
plasmapheresis
in future, use of HPA matched units and IVIg pre-transfusion
GPIIb/IIIa
when activated, binds natural ligands and causes aggregation
= fibrinogen, Vitronectin, fibronectin, vWF
HPA alloantigens on both GPIIb/IIIa subunits that are screened for in Ab tests
- GPIIb = HPA-3
- GPIIIa = HPA-1,-4
this binds collagen and takes part in adhesion + activation
GPIa/IIa
HPA ALLOANIGENS ON GP1A ONLY
HPA-5
vWF receptor
GP1b/V/IX
how do antibodies to non-self platelet antigens develop?
after exposure to non-self platelet HPA following transfusion and pregnancy
what is FNAIT
fetal/neonatal allo-immune thrombocytopenia
- mom develops IgG Abs to baby’s plt allo-antigens
- passive thrombocytopenia (can occur in utero)
T or F. FNAIT occurs in first pregnancy
T
T or F. FNAIT is self-resolving
once baby is born, source of Abs (mom) is cut off
- thrombocytopenia 1-3 wks post-partum
lab detection of HPA Abs in FNAIT
- Ab screen in maternal serum
> during gestation if previous pregnancy was affected - HPA alloantigen typing of mom and dad
most common HPA antibody detected
anti HPA-1a
T or F. FNAIT lab detection is quantitative
F! Ab screen not quant but different sample dates can be tested in parallel for estimate of titre changes
methods for HPA Ab detection
platelet immunofluorescence (PIFT-FFC); read by flow
solid phase adherence assay (SPAA)
antigen capture ELISA (ACE)
monoclonal antibody-specific immobilization of platelet antigens (MAIPA)
Luminex (PakLX assay)
describe the Luminex screening/ID test
- uses phycoerythrin (PE) labelled anti-human IgG to detect antibodies bound to plt antigens on the beads
- amt of fluorescence detected (MFI) determines whether bead is + or -
- which Abs are present is determined by combination of beads that are reacting in the panel
what is the follow up done to antibody ID Testing?
HPA genotyping
> LinkSeq HPA typing kits (qPCR)
if HLA class I detected = Ab specificity and typing
describe HPA typing by qPCR
- specific DNa sequences amplified while also being quantified by spectrofluorimetric methods
=> Sybr Green = dsDNA binding fluorescent molecule - amplified product is then heated to identify melt curve characteristics that determine the presence/absence of HPA antigen
PTP
post transfusion purpura
- are event = acute episode of severe immune thrombocytopenia after ~1wk post-transfusion
- amnestic response to previous exposure to platelet allo-antigens
T or F. PTP more frequent in adults
T, more in females
T or F. PTP is self-resolving
T, but can be fatal
PLT counts in PTP
- pt’s own plts and transfused plts are destroyed so <15 plt count
- unknown mechanism
> immune complexes clearing all plts from circulation?
> auto Abs?
lab tests for PTP
- Luminex screening/ID test (same for HPA alloAbs)
- use serum from pt
- anti-HPA1a most common
- if required = HLA genotyping and HLA Ab specificity and typing
PLTRE
plt transfusion refractoriness = a repeated suboptimal response to platelet transfusions with lower-than-expected posttransfusion count increments
- most common = anti HLA Class I; some due to anti-HPA
often, it is recommended to have at least ____ episodes of unsatisfactory plt increment before refractoriness is investigates
two
correct count increment or CCI used for plt increment assessments
[post-transfusion - pre-transfusion plt count/uL x body SA (m2)] /
number of plts in component x 10^-11
altnernate cause of thrombocytopenia
fever
splenomegaly
DIC
drug-related immune destruction (quinidine, penicillin, etc.)
testing for PLT refractoriness
- class I HLA Ab screen
= if none … - HPA Ab
- HLA typing if required
- HPA genotyping if required
treatment of PTR
- use HLA matched units
- IVIg??? = studies vary
- continuous monitoring of PRA following future transfusions
this antigen is not well expressed on PLTs
C
guidelines for PLT transfusion
- matching for plt donor HLA-A/B antigens
- provide plts that lack Ag pt has antibody to
- substitute cross-reactive units if antigen matched not available
- matching for C not necessary
- mismatching for weakly expressed (ex: B44, B45)
- ABO expressed on plts BUT ABO matching usually not critical but should be considered if HLA matched are not effective
methods for HLA Ab testing in refractory pts
- serology: CDC/CDC-AHG
- solif phase assays:
