Monoclonal gammopathy of thrombotic significance (MGTS) is a recently described severe prothrombotic condition caused by monoclonal antibodies that recognize platelet factor 4 (PF4).1,2 Based on what is known about this entity, it presents with recurrent thrombosis and persistent thrombocytopenia. The presence of causative antibodies can be identified using tests traditionally employed for diagnosing heparin-induced thrombocytopenia (HIT), such as PF4–polyanion-based immunoassays — HIT enzyme-linked immunosorbent assays (ELISAs) — and the gold-standard serotonin-release assay (SRA).3,4 Recognition of this novel anti-PF4 antibody-mediated entity follows closely on the heels of another anti-PF4 antibody-mediated thrombotic thrombocytopenia identified after vaccine administration — vaccine-induced immune thrombotic thrombocytopenia (VITT).5-7
In a recent publication,8 we highlighted the diagnostic complexities encountered in a patient with recurrent thrombosis associated with only transient thrombocytopenia (coincident with thrombotic events). The 62-year-old male had a multiyear history of disease that included deep vein thrombosis, pulmonary embolism, splanchnic thrombosis (ultimately requiring splenectomy for hypersplenism), radial artery thrombosis, and ischemic cerebrovascular accident, with some thromboses occurring despite anticoagulation therapy. He was also found to have a monoclonal gammopathy of unknown significance (MGUS). The treating physician suspected possible spontaneous HIT. However, the patient tested negative in multiple HIT immunoassays approved by the U.S. Food and Drug Administration, as well as in an under-development VITT assay and the gold-standard SRA. Finally, the patient was evaluated using PF4-enhanced functional tests, including the PF4-SRA and the PF4-dependent P-selectin expression assay (PEA), which confirmed the presence of a PF4-dependent platelet-activating immunoglobulin G (IgG) antibody.
To evaluate if the patient’s MGUS was PF4-specific and thus causative of this syndrome, the MGUS antibody, an IgG2 kappa, was isolated, de novo sequenced, and recombinantly produced. Strikingly, the recombinant antibody replicated diagnostic testing results obtained using the patient’s serum (i.e., HIT ELISA- and SRA-negative, but PEA-positive), confirming the diagnosis of MGTS. Similar to HIT and VITT antibodies, high concentrations of heparin and blockade of the platelet IgG receptor FcγRIIa abolished platelet activation.
The nonclassical serological profile of this patient’s PF4-dependent, platelet-activating MGTS antibody underscores the potential complexities when diagnosing this new thrombophilia. Furthermore, from a clinical standpoint, making this diagnosis was not straightforward. The majority of platelet counts in this patient were in the normal range, and he only presented with transient thrombocytopenia coincident with thrombotic episodes. Thus, the possibility of normal platelet counts in this entity may need to be entertained (although asplenia, in this case, may have contributed to the normal counts). Timely and appropriate recognition of this highly prothrombotic syndrome may be critical, as new therapeutic modalities such as myeloma therapy are being used in MGTS with encouraging results.2
A new ‘flavor’ of pathogenic, platelet-activating anti-PF4 antibody
| . | Solid-phase ELISAs . | Automated immunoassays . | Heparin-based functional testing (e.g., SRA) . | PF4-based functional testing (e.g., PEA) . |
|---|---|---|---|---|
| HIT/HIT-like | + | + | + | + |
| VITT/VITT-like | + | – | –/+ | + |
| Patient | – | – | – | + |
| . | Solid-phase ELISAs . | Automated immunoassays . | Heparin-based functional testing (e.g., SRA) . | PF4-based functional testing (e.g., PEA) . |
|---|---|---|---|---|
| HIT/HIT-like | + | + | + | + |
| VITT/VITT-like | + | – | –/+ | + |
| Patient | – | – | – | + |
Abbreviations: ELISA, enzyme-linked immunoabsorbent assay; HIT, heparin-induced thrombocytopenia; PEA, P-selectin expression assay; PF4, platelet factor 4; SRA, serotonin-release assay; VITT, vaccine-induced immune thrombotic thrombocytopenia.
This table depicts the typical HIT/HIT-like and VITT/VITT-like antibody serology in antigen-based and functional anti-PF4 antibody assays. The patient’s antibody solely recognizes PF4 in the context of platelets.
In Brief
This case emphasizes the need to develop accurate and sensitive methodologies for diagnosing MGTS. The use of PF4-dependent functional testing and antibody evaluation by high-resolution mass spectrometry analysis, alongside conventional HIT assays such as ELISA and SRA, should aid in making a more accurate diagnosis, especially in patients with non-classical clinical/diagnostic profiles. A current challenge is the limited availability of some of these technologies in diagnostic laboratories. Recently, the use of mass spectrometry to detect antibodies and diagnose dysproteinemias has made rapid strides.9 Hence, it is expected that assays for MGTS will likely be available in the future. An intriguing question raised by this case is whether similar serological patterns can be observed in patients with HIT, i.e., ELISA- and SRA-negative patients with pathogenic platelet-activating antibodies. Future studies should address these important questions to enhance our understanding of anti-PF4 antibody-mediated thrombotic syndromes and help with timely and accurate diagnosis of these severe prothrombotic states.
Disclosure Statement
Drs. Kanack and Shah indicated no relevant conflicts of interest. Dr. Padmanabhan has pending and issued patents assigned to Mayo Clinic, Retham Technologies, and Versiti Blood Center of Wisconsin. He also reports equity ownership in and serving as an officer of Retham, as well as equity ownership in Veralox Therapeutics.
