Key Points
Antibodies causing FNAIT have decreased Fc fucosylation, unlike in refractory thrombocytopenia. Decreased Fc fucose increases affinity to FcγRIIIa/b, enhances platelet phagocytosis, and correlates with increased disease severity.
Fetal and neonatal alloimmune thromboctyopenia due to maternal human platelet antigen (HPA)-1a antibodies affects primigravidas. Immunization must occur early in pregnancy before fetal platelets enter maternal blood via fetomaternal hemorrhage. The HPA-1a antigen is located on platelet glycoprotein (GP)IIIa (CD61, beta3 integrin), which is also present on the placental syncytiotrophoblast (ST) and in direct contact with maternal blood. Since ST debris is shed into maternal blood during pregnancy, this material might be immunogenic in vivo. For experimental purposes, we prepared and characterized ST microparticles (STMPs) in vitro from term placentas. Phenotype analysis by flow cytometry and Western blotting showed that STMP expressed more placental alkaline phosphatase (PLAP) than GPIIIa. Quantitative real-time PCR demonstrated expression of human placental lactogen (HPL), human chorionic gonadotrophin (HCG), and GPIIIa by STMP, in the order HPL > HCG > GPIIIa. PLAP, HPL, and HCG are trophoblast-specific proteins. These STMPs may be a useful model for studying the natural ST debris in plasma of pregnant women.
Flow cytometry may enable the crude estimation of the percentage of small volumes (<5 mL) of transfused D+ red cells, but in this study it was found that this method was not sufficiently accurate to determine the initial clearance rate, red cell half-life, or mean cell lifespan. If the proportion of transfused cells in the recipient is about 0.2 percent or less, the use of radioisotopes for labeling cells for quantitative in vivo red cell clearance or survival data should remain the method of choice.
SummaryFetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs most frequently when human platelet antigen (HPA)-1a-positive fetal platelets are destroyed by maternal HPA-1a immunoglobulin (Ig)G antibodies. Pregnancies at risk are treated by administration of high-dose intravenous Ig (IVIG) to women, but this is expensive and often not well tolerated. Peptide immunotherapy may be effective for ameliorating some allergic and autoimmune diseases. The HPA-1a/1b polymorphism is Leu/Pro33 on β3 integrin (CD61), and the anti-HPA-1a response is restricted to HPA-1b1b and HLA-DRB3*0101-positive pregnant women with an HPA-1a-positive fetus. We investigated whether or not HPA-1a antigen-specific peptides that formed the T cell epitope could reduce IgG anti-HPA-1a responses, using a mouse model we had developed previously. Peripheral blood mononuclear cells (PBMC) in blood donations from HPA-1a-immunized women were injected intraperitoneally (i.p.) into severe combined immunodeficient (SCID) mice with peptides and HPA-1a-positive platelets. Human anti-HPA-1a in murine plasma was quantitated at intervals up to 15 weeks. HPA-1a-specific T cells in PBMC were identified by proliferation assays. Using PBMC of three donors who had little T cell reactivity to HPA-1a peptides in vitro, stimulation of anti-HPA-1a responses by these peptides occurred in vivo. However, with a second donation from one of these women which, uniquely, had high HPA1a-specific T cell proliferation in vitro, marked suppression of the anti-HPA-1a response by HPA-1a peptides occurred in vivo. HPA-1a peptide immunotherapy in this model depended upon reactivation of HPA-1a T cell responses in the donor. For FNAIT, we suggest that administration of antigen-specific peptides to pregnant women might cause either enhancement or reduction of pathogenic antibodies.
The observation that different rPrP production protocols influence QuIC performance indicates that caution should be exercised when comparing inter-laboratory QuIC results.
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