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Case Report
35%) and remained low when the patient was getting better (day 7, 36%). In contrast, platelet activation returned to a high level at the time of clinical deteriora- tion, day 15 (79%). Time related changes of total IgG plasma levels mirrored those of PF4-SRA (Figure 1). While the total IgG level was rather low at diagnosis, it rose above 30 g/L after IVIg administration and then rap- idly decreased, as observed on day 15, with a return to normal values for a healthy adult. These data support the hypothesis that the rebound of platelet activation observed in PF4-SRA at the time of clinical deterioration could be due to a rapid elimination of the infused immunoglobulins and the loss of their competing effect with platelet activating anti-PF4 antibodies on platelet FcɣRIIa.
Additional experiments were performed with PF4-SRA to consolidate our hypothesis that the levels of IgG can explain the exacerbation The same samples were diluted either in normal plasma (containing normal IgG level) or in modified Tyrode’s buffer at 1/10, 1/20, 1/40 dilution ratios. When samples were diluted in normal plasma, platelet activation decreased proportionally with the dilution, and was completely abolished at 1/40, except for the day 1 sample, which still slightly activate platelets under these conditions (Figure 2). In contrast, when the samples were diluted in Tyrode’s buffer, platelet activation remained high (near 100%) for the samples collected on day 1 and day 15 but increased for those obtained at days 5 and 7, approaching 100% (Figure 2). These results strongly support that platelet activation by VITT antibodies was inhibited by normal IgG, and that lowering the concentrations of normal IgG led to the reappearance of platelet activation by loss of competition between the IVIg and anti-PF4 IgG.
This case supports the concept that proper monitoring using an appropriate functional assay could help in the clinical decision making since PF4-SRA mirrored with the clinical evolution of the patient. Such an observation has also been made in a recent study, which demonstrat- ed that platelet activation by VITT antibodies was inhib- ited in patients treated with IVIg.11 Nevertheless, this needs to be confirmed. Interestingly, the inhibitory effect of normal polyclonal IgG on the platelet activation induced by PF4-specific antibodies could also vary from one patient to another, as previously demonstrated in HIT patients.12 It is also important to note that adminis- tration of IVIg reduces the activation of platelets as assessed by the PF4-SRA. This has major consequences when collecting samples for confirmation of VITT diag- nosis and dilution in appropriate buffer, as we did in our experiments, could be recommended to assess the com- petitive interaction between anti-PF4 IgG and IVIg. However, this test lacks worldwide availability, and can- not easily be used for emergency patient monitoring. Therefore, as total IgG concentration measured in the patient inversely correlated with platelet activation in PF4-SRA, quantitatively assaying anti-PF4 IgG antibod- ies levels and total IgG concentration in the patient’s plasma could help to identify situations where the com- petition between normal polyclonal IgG and anti-PF4 IgG on FcgRIIa may switch in favor of the platelet acti- vating antibodies.13
Even though data on IVIg clearance parameters and target concentrations are lacking for such a very peculiar condition, a rapid decrease in total IgG concentrations within the normal range (i.e., 7-16 g/L)14 could alert to possible therapeutic escape, and the need for re-admin- istration of IVIg, especially in a situation where anti-PF4 IgG remains high. In the patient, total IgG concentration
was reduced by half within 8 days, which is substantial- ly faster than the median half-life of 30 days generally reported in the literature for IVIg.15 Although further studies are needed to understand the accelerated clear- ance and to assess the clinical relevance of total IgG measurement to monitor the efficacy of IVIg, it appears a very affordable tool in medical practice in combination with anti-PF4 IgG antibodies testing.
Jonathan Douxfils,1,2* Caroline Vayne,3* Claire Pouplard,3 Thomas Lecompte,4 Julien Favresse,1,5 Florence Potier,6
Emy Gasser,7 Valérie Mathieux,8 Jean-Michel Dogné,1
Yves Gruel,3 Jérôme Rollin3# and François Mullier9#
1University of Namur, Department of Pharmacy, Namur Research for Life Sciences, Namur Thrombosis and Hemostasis Center, Namur, Belgium; 2QUALIblood s.a., Namur, Belgium; 3University of Tours, EA7501 GICC, CHRU de Tours, Department of Hemostasis, Tours, France; 4Département de Médecine, Hôpitaux Universitaires de Genève, Service d’Angiologie et d’Hémostase et Faculté de Médecine, Geneva Platelet Group (GpG), Université de Genève, Geneva, Switzerland; 5Clinique Saint-Luc Bouge, Department of Laboratory Medicine, Bouge, Belgium; 6Service de Gériatrie, CHU UCL Namur site Sainte-Elisabeth, Namur, Belgium; 7Université Catholique de Louvain, Service de Gériatrie, CHU UCL Namur site Sainte- Elisabeth, Namur, Belgium; 8CHU UCL Namur |site Sainte- Elizabeth, Université Catholique de Louvain, Department of Hematology, Namur Research Institute for Life Sciences, Namur Thrombosis and Hemostasis Center, Yvoir, Belgium and
9Université Catholique de Louvain, CHU UCL Namur, Namur Thrombosis and Hemostasis Center, Hematology Laboratory, Namur Research Institute for Life Sciences, Yvoir, Belgium.
*JD and CV contributed equally as co-first authors #JR and FM contributed equally as co-senior authors Correspondence: JONATHAN DOUXFILS - jonathan.douxfils@unamur.be doi:10.3324/haematol.2021.279509
Received: June 28, 2021. Accepted: August 3, 2021. Pre-published: August 12, 2021.
Disclosures: JD is the CEO and founder of QUALIblood S.A.,
a contract research organization manufacturing the DP-Filter, is a co-inventor of the DP-Filter (patent application number: PCT/ET2019/ 052903) and reports personal fees from Daiichi-Sankyo, DOASense Gmbh, Gedeon Richter, Mithra Pharmaceuticals, Norgine, Portola, Stago, Roche and Roche Diagnostics outside the submitted work;
TL reports non-personal fees from IRIS and Stago; FM reports institutional fees from Stago, Werfen, Nodia, Roche Sysmex and Bayer as well as speaker fees from Boehringer Ingelheim, Bayer Healthcare, Bristol- Myers Squibb-Pfizer, Stago, Sysmex and Aspen all outside the submitted work. The other authors have no conflicts of interest
to disclose.
Contributions: JD and CV analyzed the results, wrote the first draft of the manuscript and designed the figures; JR, YG, CP, CV provided and analyzed the results and revised the manuscript; TL analyzed the results and thoroughly revised the manuscript; JF provided and analyzed the results and revised the manuscript; FP, EG and VM managed the patient and thoroughly revised the manuscript; J-MD analyzed the results; FM designed and supervised the experiments, provided and analyzed the data, and interpreted the results.
Acknowledgments: the authors would like to thank the technical staff of the CHU UCL Namur, Mrs Justine Baudar, Mrs Maïté Guldenpfennig and the technical staff of the CHRU Tours,
Mrs Séverine Augereau.
Informed consent and ethical committee approval: as the patient died, an independent Review Board of the Clinique Sainte-Elisabeth
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