Low-affinity FcγR interactions can decide the fate of novel human IgG-sensitised red blood cells and platelets.
Bottom Line: 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.Considering therapeutic blocking Abs, G1Δnab offers lower FcγR binding and a greater bias towards inhibition than IgG2 and IgG4 constant regions.
Affiliation: Department of Pathology, University of Cambridge, Cambridge, UK.Show MeSH
Related in: MedlinePlus
Mentions: Each volunteer in the platelet survival study received two samples of autologous, HPA-1a1b platelets that had been left unsensitised or sensitised at saturating concentrations of B2 Ab (0.13 mg/mL) and then labelled with different radionuclides 10. The previous report focussed on the survival curves generated from the radioactivity in the cellular fractions of the blood samples 10. Platelet destruction can be inferred from radioactivity appearing in the plasma but only for platelets radiolabelled with 111In since 51Cr elutes too rapidly. This limits the number of data sets available for each type of platelet: unsensitised (n = 3), P-G1 (n = 5), P-G1Δnab (n = 4) or P-G1/G1Δnab (10% B2 G1/90% B2 G1Δnab, n = 2). Figure1 compares the plasma-associated 111In radioactivity levels measured for the four types of platelets and shows the corresponding platelet survival curves when data are restricted to these platelet samples. The graphs are limited to the first 24 h after injection because B2 Abs redistribute to the whole platelet population by this time point 10. Large error bars result from donor variation and the small group sizes mean statistics cannot be applied but there was a higher level of plasma 111In activity associated with P-G1 than for the other types of platelets. The result is particularly striking for P-G1/G1Δnab, given that the survival curves for these platelets and P-G1 are similar. In fact, one of the volunteers receiving the 111In-labelled P-G1/G1Δnab had significantly higher HPA-1a levels on their platelets than all other volunteers (UPN 18; see table 1 of 10). These P-G1/G1Δnab were cleared more quickly than all other samples of P-G1/G1Δnab but this was not accompanied by increased levels of 111In in the plasma.
Affiliation: Department of Pathology, University of Cambridge, Cambridge, UK.