The role of P-selectin in the immune destruction of platelets

Association between P-Selectin Autoantibody Positive and Response to Steroid Treatment in Newly Diagnosed Immune Thrombocytopenia Patients

OBJECTIVE

This study aimed to detect the association between P-selectin autoantibody positive and response to steroid treatment in newly diagnosed immune thrombocytopenia (ITP) patients.

METHODS

The data from 105 newly diagnosed adult ITP patients administered with first-line of steroid treatment from October 2016 to May 2021 were retrospectively analyzed. Treatment responses were evaluated within 3 months after the onset of treatment.

RESULTS

Among the 105 patients, 80.00% (84/105) of patients presented with platelet glycoprotein-specific antibody positive; 44.76% (47/105) patients were anti-P-selectin positive, while 35.24% (37/105) were anti-P-selectin negative. No significant difference in overall response was observed between patients who were anti-P-selectin positive and those who were anti-P-selectin negative (74.47% vs. 89.19, χ2 = 2.910, p = 0.088). But patients who were anti-P-selectin negative had significantly higher complete response rate, compared to those who were anti-P-selectin positive (72.97% vs. 48.94%, χ2 = 4,965, p = 0.026). Logistic regression analysis revealed that anti-GP IIb/IIIa positive (OR = 3.114, p = 0.010, 95% CI: 1.313-7.388) and anti-P-selectin positive (OR = 0.309, p = 0.036, 95% CI: 0.127-0.753) were two factors that could affect patients' response.

CONCLUSIONS

Our study found that ITP patients with anti-GP IIb/IIIa may have a higher response to steroid treatment, but anti-P-selectin-mediated-ITP might be less responsive to steroid treatment. In adults with ITP, the presence of anti-P-selectin autoantibodies is a predictive factor for poor response to steroid treatment.

Adding to the complexity of fetal and neonatal alloimmune thrombocytopenia: Reduced fibrinogen binding in the presence of anti-HPA-1a antibody and hypo-responsive neonatal platelets

BACKGROUND

In fetal and neonatal alloimmune thrombocytopenia (FNAIT), maternal alloantibodies directed against paternally-derived platelet antigens are transported across the placenta to the fetus, where they may cause thrombocytopenia. The most serious complication of FNAIT is an intracranial hemorrhage (ICH), which may cause death or life-long disability of the child. Apart from alloantibody-mediated platelet destruction, the clinical outcome in FNAIT may be affected by properties of neonatal platelets and possible functional effects on platelets caused by maternal alloantibodies.

METHODS AND RESULTS

The function of umbilical cord blood platelets was compared with adult platelets in two assays, impedance aggregometry (Multiplate) and rotational thromboelastometry (Rotem). Both revealed a decreased in vitro neonatal platelet function compared to adult platelets. Consistent with this finding, activation using TRAP revealed less pronounced changes in the expression of CD62P, PAC-1, CD41 and CD42a in umbilical cord blood platelets compared to adult platelets. Furthermore, a monoclonal anti-HPA-1a antibody, derived from an immunized mother of two children with FNAIT, blocked fibrinogen binding to resting and activated umbilical cord blood and adult HPA-1aa and HPA-1ab platelets, interfered with platelet activation by TRAP, and impaired the function of umbilical cord blood HPA-1aa platelets in rotational thromboelastometry.

DISCUSSION AND CONCLUSIONS

Reduced fibrinogen binding in the presence of anti-HPA-1a antibodies may disturb the neonatal hemostatic balance, characterized by poorly responsive platelets. This effect may operate in parallel to platelet destruction and contribute to the clinical outcome in FNAIT.

Low-affinity FcγR interactions can decide the fate of novel human IgG-sensitised red blood cells and platelets

G1Δnab is a mutant human IgG1 constant region with a lower ability to interact with FcγR than the natural IgG constant regions. Radiolabelled RBCs and platelets sensitised with specific G1Δnab Abs were cleared more slowly from human circulation than IgG1-sensitised counterparts. However, non-destructive splenic retention of G1Δnab-coated RBCs required investigation and plasma radioactivities now suggest this also occurred for platelets sensitised with an IgG1/G1Δnab mixture. In vitro assays with human cells showed that G1Δnab-sensitised RBCs did not cause FcγRI-mediated monocyte activation, FcγRIIIa-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) or macrophage phagocytosis although they did adhere to macrophages. Thus, FcγRII was implicated in the adhesion despite the Δnab mutation reducing the already low-affinity binding to this receptor class. Additional contacts via P-selectin enhance the interaction of sensitised platelets with monocytes and this system provided evidence of FcγRII-dependent activation by G1Δnab.

These results emphasise the physiological relevance of low-affinity interactions: It appears that FcγRII interactions of G1Δnab allowed splenic retention of G1Δnab-coated RBCs with inhibitory FcγRIIb binding preventing RBC destruction and that FcγRIIb engagement by G1Δnab on IgG1/G1Δnab-sensitised platelets overcame activation by IgG1. Considering therapeutic blocking Abs, G1Δnab offers lower FcγR binding and a greater bias towards inhibition than IgG2 and IgG4 constant regions.

Developing recombinant HPA-1a-specific antibodies with abrogated Fcgamma receptor binding for the treatment of fetomaternal alloimmune thrombocytopenia

Fetomaternal alloimmune thrombocytopenia (FMAIT) is caused by maternal generation of antibodies specific for paternal platelet antigens and can lead to fetal intracranial hemorrhage. A SNP in the gene encoding integrin beta3 causes a clinically important maternal-paternal antigenic difference; Leu33 generates the human platelet antigen 1a (HPA-1a), whereas Pro33 generates HPA-1b. As a potential treatment to prevent fetal intracranial hemorrhage in HPA-1a alloimmunized pregnancies, we generated an antibody that blocks the binding of maternal HPA-1a-specific antibodies to fetal HPA-1a1b platelets by combining a high-affinity human HPA-1a-specific scFv (B2) with an IgG1 constant region modified to minimize Fcgamma receptor-dependent platelet destruction (G1Deltanab). B2G1Deltanab saturated HPA-1a+ platelets and substantially inhibited binding of clinical HPA-1a-specific sera to HPA-1a+ platelets. The response of monocytes to B2G1Deltanab-sensitized platelets was substantially less than their response to unmodified B2G1, as measured by chemiluminescence. In addition, B2G1Deltanab inhibited chemiluminescence induced by B2G1 and HPA-1a-specific sera. In a chimeric mouse model, B2G1 and polyclonal Ig preparations from clinical HPA-1a-specific sera reduced circulating HPA-1a+ platelets, concomitant with transient thrombocytopenia. As the Deltanab constant region is uninformative in mice, F(ab')2 B2G1 was used as a proof of principle blocking antibody and prevented the in vivo platelet destruction seen with B2G1 and polyclonal HPA-1a-specific antibodies. These results provide rationale for human clinical studies.

Agonist-inducible platelet activation in chronic idiopathic autoimmune thrombocytopenia

OBJECTIVE

There are only few studies on agonist-inducible platelet activation in chronic idiopathic autoimmune thrombocytopenia (cAITP).

MATERIALS AND METHODS

We compared agonist (TRAP-6, ADP, Arachidonic acid, Epinephrine, and Ristocetin) -inducible P-selectin expression and PAC-1 binding in 40 patients with cAITP (f/m ratio 23/17) with those in 20 healthy controls. Results were correlated with platelet counts, detectable platelet antibodies, and reticulated platelets.

RESULTS

The in vivo activation of platelets determined the in vitro inducible response to agonists. The stronger the in vivo activation the less the number of platelets responding to agonists, as illustrated by the inverse correlation of P-selectin expression ex vivo and the relative increase after the exogenous addition of agonists. The agonist-inducible platelet activation was not associated with the presence of detectable platelet antibodies to GPIb/IX or GPIIb/IIIa. Agonist-inducible platelet activation was also not correlated with counts of reticulated platelets.

CONCLUSION

Agonist-inducible activation of platelets in cAITP is affected mainly by their in vivo activation.

HPA-1a antibody potency and bioactivity do not predict severity of fetomaternal alloimmune thrombocytopenia

BACKGROUND

The antenatal management of fetomaternal alloimmune thrombocytopenia (FMAIT) due to HPA-1a antibodies remains controversial, and a test identifying pregnancies that do not require therapy would be of clinical value.

STUDY DESIGN AND METHODS

The statistical correlation was analyzed between clinical outcome and 1) anti-HPA-1a potency in maternal serum samples determined by a monoclonal antibody immobilization of platelet (PLT) antigen assay with an international anti-HPA-1a potency standard and 2) anti-HPA-1a biological activity measured by a monocyte chemiluminescence (CL) assay.

RESULTS

A total of 133 pregnancies with FMAIT due to anti-HPA-1a were analyzed. In 97 newly diagnosed cases, there was no difference in antibody potency or CL signal between cases with intracranial hemorrhage (ICH; n = 15), those with no ICH but a PLT count of less than 20 x 10(9) per L (n = 52), and those with a PLT count of at least 20 x 10(9) per L (n = 30). In 22 previously known pregnancies, the positive predictive value of maternal anti-HPA-1a of greater than 30 IU per mL for a PLT count of less than 20 x 10(9) per L was 90 percent, but the negative predictive value was only 66 percent. Antibody potency tended to stay stable throughout pregnancy (n = 16) and from one pregnancy to the next (n = 16).

CONCLUSION

Neither severe thrombocytopenia nor ICH in HPA-1a-alloimmunized pregnancies can be predicted with sufficient sensitivity and specificity for clinical application from maternal anti-HPA-1a potency or bioactivity.