Page 53 - Haematologica March 2020

Seronegative antiphospholipid syndrome
Antibodies against phosphatidic acid, phosphatidylserine and phosphatidylinositol
In an effort to expand the panel of aPL to other negative- ly-charged phospholipids, antibodies against phospha- tidic acid (PA), phosphatidylserine (PS), and phos- phatidylinositol (PI), which fall under the category of anionic phospholipids, were proposed.43 Anti-PS antibod- ies inhibited the development and invasion of the tro- phoblast, decreased hCG levels, and retarded the forma- tion of syncytiotrophoblast in in vitro models.44 Few clinical studies have investigated this issue. In a first study on 866 women with recurrent pregnancy loss (RPL), the authors found that 87 of 866 women who were negative for aCL had a positivity for one of the other aPL.42 In a second study on 872 women with RPL, 49 (3.6%) were negative for both aCL and LA but positive for aPS.46 In this second study, the presence of aPS had a positive correlation with the number of consecutive pregnancy losses.46 This result was not confirmed when the same author analyzed a larg- er population of 1,020 woman with RPL.46 Moreover, Zhang et al. did not find any positivity for aPE, aPS or aPI in an evaluation study of 288 subjects (86 patients with APS, 30 patients with non-APS thrombosis, 32 patients with non-APS pregnancy-related morbidity, 42 patients with SLE, and 39 healthy controls).26
Based on the current evidence, testing for aPA, aPI and aPS is not recommended, as these antibodies appeared to overlap with the accepted diagnostic markers of APS. Nonetheless, the results obtained on RPL with a seroneg- ative profile suggest a potential role for aPA, aPI and aPS in defining SN-APS in this particular setting.
Anti-vimentin/cardiolipin complex
Vimentin is the most abundant type III intermediate fil- ament of the cytoskeletal system and it was recently local- ized on the surface of apoptotic neutrophils and T cells, activated macrophages, platelets, and vascular endothelial cells. After becoming antigenic with a still unexplained mechanism, Vimentin is exposed and could be bound by anti-vimentin antibodies (AVA).47,48 Vimentin could also electrically interact with cardiolipin on the surface of apoptotic cells generating the vimentin/cardiolipin com- plex. Antibodies against this complex (vimentin/cardi- olipin antibodies, AVA/CL) show a prothrombotic effect. Thus, Ortona et al. demonstrated an AVA/CL-mediated activation of the TLR4/IRAK/Nf-kB molecular pathway that leads to the release of pro-inflammatory and procoag- ulant factors by endothelial cells.47 Hence, AVA/CL could play a role in arterial thrombosis by inducing platelet and coagulation cascade activation.
The role of AVA/CL in SN-APS has been investigated only in a few clinical studies. Thus, Ortona et al.47 ana- lyzed serum IgG AVA/CL antibodies detected by ELISA in 29 SN-APS, 40 APS, 30 patients with SLE, 30 with rheumatoid arthritis, and 32 healthy controls. They found a positivity for AVA/CL in almost all APS patients (92%), and also in a large proportion of SN-APS (55%); interest- ingly, this positivity was persistent in almost all cases. Similarly, Conti et al. found AVA/CL positivity in 24 SN- APS patients.49 Moreover, in a retrospective analysis of 61 obstetric SN-APS, 76% resulted positive for AVA/CL.50 However, the overlapping presence of AVA/CL antibodies in SLE and APS weakens the specificity of such a diagnos-
tic marker. Hence, the observation by Ortona et al.47 needs to be confirmed by larger prospective clinical studies in order to better define the role of AVA/CL in SN-APS.
Anti-prothrombin and antiphosphatidylserine/prothrombin antibodies
Prothrombin is a plasma glycoprotein involved in the coagulation cascade converted to thrombin by extrinsic thromboplastin during the second stage of blood clotting.51 A large amount of data, obtained from various, mainly ret- rospective, studies gave contrasting evidence concerning the clinical significance of anti-prothrombin antibody (aPT). Thus, in a comparison between 106 subjects who experienced either a non-fatal myocardial infarction or cardiac death and 106 subjects without coronary disease, Vaarala et al. found that a high level of aPT (highest tertile of distribution) predicted a 2.5-fold increase in the risk of cardiovascular events.52 Conversely, Atsumi et al. did not find any correlation between clinical manifestation of aPT and APS in an evaluation of 265 APS patients.53 More recently, two prospective studies validated the role of aPT in predicting the first or recurrent risk of thrombosis in patients with APS.54,55 Considering a group of 142 LA pos- itive patients, Forastiero et al. found that a higher rate of thrombosis in patients with positive anti-PT compared with patients without anti-PT (8.6% vs. 3.5% per patient year). The highest incidence of thrombosis was detected in patients positive for both aβ2GPI and aPT (8.4% per patient year).54 Moreover, a 15-year longitudinal prospec- tive study by Bizzaro et al. identified IgG aPT antibody as the most useful thrombosis predictor in SLE patients.55
Another intriguing issue is represented by the different potential role of IgG/IgM antiphosphatidylserine/pro- thrombin (aPS/PT) compared to aPT. Indeed, a high corre- lation between APS classical antibody panel and aPS/PT IgG/IgM suggests that this marker may be useful in the evaluation of APS.56 The clinical significance of aPT and aPS-PT was evaluated by testing for the presence of these antibodies in 212 SLE patients and in 100 healthy individ- uals. Results show that aPT and aPS-PT were found in 47% of the patients (aPT in 31% and aPS-PT in 31%). Their presence did not correlate with that of aCL, aβ2GPI, LA and/or anti-protein S. IgG but not IgM aPT were more frequently found in patients with thrombosis than in those without. IgG and IgM aPS-PT were also more fre- quent in patients with thrombosis (venous and/or arterial) than in those without. Levels of IgG aPT and IgG and IgM aPS-PT were higher in patients with thrombosis than in those without. More significantly, 48% of the patients with aPL-related clinical features who were negative for standard tests had aPT.57 Recently, the clinical significance of aPS/PT antibodies was prospectively evaluated in a cohort of 191 aPL carriers:58 IgG aPS/PT antibodies were detected in 40 (20.9%) and IgM aPS/PT in 102 (53.4%) of the carriers. The cumulative incidence rate of thrombotic events was significantly higher in the IgG aPS/PT positive (P=0.035) but not in the IgM aPS/PT positive carriers. Similar results were obtained in a second study evaluating 152 patients with a previous thrombosis of whom 90 were SN-APS; 10% of SN-APS patients in this study were positive for aPS/PT.59 Of note, aPS/PT are associated with recurrent early or late abortions and with premature deliv- ery irrespective of other aPL.60
Based on the above studies, aPT and aPS/PT can be
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