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.

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

Antibody-mediated platelet destruction is a poorly understood process, although several lines of evidence suggest that Fcgamma receptor (FcgammaR)-expressing splenic macrophages may be involved. In this study, chemiluminescence (CL) was used to measure the in vitro metabolic response of human monocytes to platelets sensitized with a human immunoglobulin (Ig)G1 recombinant antihuman platelet antigen-1a (anti-HPA-1a) antibody (B2G1; P-hrIgG1). CL responses were inhibited, but not abrogated, in the presence of 10 micro g/ml human IgG or murine IgG2a, suggesting that FcgammaRI was principally involved. Experiments to determine the effect of Fab fragments to FcgammaRII found that CL responses to P-hrIgG1 were significantly enhanced, indicating that crosslinking of monocyte FcgammaRII by platelet-bound hIgG may modulate concomitant activation by FcgammaRI. Several observations suggested that the CL responses to P-IgG were dependent on the activation of resting platelets during their co-culture with monocytes and their subsequent P-selectin-mediated adhesion. First, the magnitude of the CL response was related to the level of P-selectin expression following platelet activation with alpha-thrombin. Second, CL responses were inhibited in the presence of antibodies that block the binding of P-selectin to P-selectin glycoprotein ligand-1 but not when platelets were pretreated and then washed. Third, the addition of anti-HPA-1a to monocytes from HPA-1a-negative donors preincubated with HPA-1a-positive platelets resulted in rapid CL responses. Finally, PGI2 inhibited the CL response to resting P-hrIgG1. Thus, evidence is presented that the interaction of human monocytes with P-hrIgG1 is mediated by FcgammaRI, modulated via FcgammaRII, and enhanced by the presence of P-selectin on the platelet membrane.

The effect of methylene blue photoinactivation and methylene blue removal on the quality of fresh-frozen plasma.

BACKGROUND: T: he effects of using fresh or frozen-thawed plasma, WBC reduction of plasma before freezing, and the use of two different methylene blue (MB) removal filters on the quality of MB-treated plasma were compared. STUDY DESIGN AND METHODS: In a paired study (n = 11/arm) plasma was frozen within 8 hours of collection, thawed, MB photoinactivated, and then filtered using one of two MB removal filters. Fresh plasma (n = 16) and plasma WBC reduced before freezing (n = 19) were MB inactivated. RESULTS: Freeze-thawing resulted in loss of activity of FXII and VWF of 0.06 and 0.04 units per mL, respectively, but no significant loss of activity of factors II through XI or fibrinogen. Further loss of activity occurred after MB treatment: FII (0.07 IU/mL), FV (0.11 U/mL), FVII (0.08 IU/mL), FVIII (0.28 IU/mL), F IX (0.12 IU/mL), FX (0.16 IU/mL), FXI (0.28 U/mL), FXII (0.15 U/mL), VWF antigen (0.05 IU/mL), VWF activity (0.06 U/mL), and fibrinogen (0.79 g/L). Losses due to this step were significantly (5-10%) lower in fresh plasma compared to frozen-thawed plasma. Neither MB removal filter resulted in significant loss of activity of any factor studied. CONCLUSION: MB removal, by either of the available filters, has little impact on the coagulation factor content of plasma, but freezing of plasma before MB treatment results in a small additional loss.