Anti-PF4 immunothrombosis without proximate heparin or adenovirus vector vaccine exposure

Abstract

Platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies and anti-PF4 antibodies cause heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombocytopenia and thrombosis (VITT), respectively. Diagnostic and treatment considerations differ somewhat between HIT and VITT. We identified patients with thrombocytopenia and thrombosis without proximate heparin exposure or adenovirus-based vaccination who tested strongly positive by PF4/polyanion EIA and negative/weakly-positive by heparin-induced platelet activation (HIPA) test but strongly positive by PF4-induced platelet activation (PIPA) test (i.e., VITT-like profile). We tested these patients by a standard chemiluminescence assay that detects anti-PF4/heparin antibodies found in HIT (HemosIL® AcuStar HIT-IgG(PF4-H)) as well as a novel chemiluminescence assay for anti-PF4 antibodies found in VITT. Representative control sera included an exploratory anti-PF4 antibody-positive but HIPA-negative/weak cohort obtained prior to 2020 (n=188). We identified 9 patients with a clinical-pathological profile of a VITT-like disorder in the absence of proximate heparin or vaccination, with a high frequency of stroke (arterial, n=3; cerebral venous sinus thrombosis, n=4), thrombocytopenia (median platelet count nadir, 49×109/L), and hypercoagulability (greatly elevated D-dimer levels). VITT-like serological features included strong reactivity by PIPA (aggregation <10min in 9/9 sera) and positive testing in the novel anti-PF4 chemiluminescence assay (3/9 also tested positive in the anti-PF4/heparin chemiluminescence assay). Our exploratory cohort identified 13 additional patient sera obtained before 2020 with VITT-like anti-PF4 antibodies. Platelet-activating VITT-like anti-PF4 antibodies should be considered in patients with thrombocytopenia, thrombosis and very high D-dimer levels, even without a proximate exposure to heparin or adenovirus vector vaccines.

Vaccine-Induced Immune Thrombotic Thrombocytopenia: Clinicopathologic Features and New Perspectives on Anti-PF4 Antibody-Mediated Disorders

“…A similar finding was also reported by Schönborn et al [72], who identified nine patients with VITT-like clinical and humoral profiles from a retrospective analysis of repository serum samples dated before 2020 [72]. These individuals experienced acute thrombocytopenia, significantly elevated D-dimer, and severe thrombotic events, including arterial strokes and CVST, without recent exposure to heparin or adenoviral vector-based vaccines [72]. Thus, the association of pro-thrombotic anti-PF4 antibodies with disorders showing clinical and/or pathological similarities to VITT may be indicative of an unknown trigger for pathogenic antibody production beyond our current scope of understanding.…”

“…Serum samples from these two patients were positive for anti-PF4 antibodies in both solid and fluid phases assays, and in PF4-enhanced platelet-activation assays, and demonstrated binding to the heparin-binding region on PF4 [64]. A similar finding was also reported by Schönborn et al [72], who identified nine patients with VITT-like clinical and humoral profiles from a retrospective analysis of repository serum samples dated before 2020 [72]. These individuals experienced acute thrombocytopenia, significantly elevated D-dimer, and severe thrombotic events, including arterial strokes and CVST, without recent exposure to heparin or adenoviral vector-based vaccines [72].…”

Autoimmune Heparin-Induced Thrombocytopenia

“…A promising new approach is the recent development of a rapid chemiluminescence immunoassay (CLIA) that detects anti-PF4 (not anti-PF4/heparin) antibodies [188]; such an assay could be complementary to the existing, commercially-available, PF4/heparin-CLIA [189,190]. Approximately 30% of HIT sera yielded a positive result in the novel PF4-CLIA-presumably reflecting detection of aHIT antibodies-in addition to the expected positive result in the standard PF4/heparin-CLIA.…”

The Biodistribution of the Spike Protein after Ad26.COV2.S Vaccination Is Unlikely to Play a Role in Vaccine-Induced Immune Thrombotic Thrombocytopenia

“…It was originally reported that the binding of the adenoviral particle to PF4 could play a role in the development of VITT [21], but recent studies showed no binding between PF4 and Ad26.COV.2.S [22,23]. There is also conflicting literature associating human adenovirus infections with a prothrombotic disorder resembling VITT [24,25]. However, the high prevalence of natural human adenovirus infections without a high frequency of associated prothrombotic disorders, and the fact that TTS-like disease has also been described in patients with COVID-19 [26,27], suggests a multifactorial mechanism for VITT.…”