Background: The LA paradox
As its name implies, the LA acts
in vitro like an anticoagulant, prolonging the activated partial thromboplastin time (APTT) or other tests specifically designed to detect it. According to a 1961 review [
268], this effect was first reported in 1946 in a patient with ITP [
269]. Its frequent association with SLE was reported in a series of papers by Conley and colleagues from 1948 [
270] to 1952 [
2]. Both papers cited describe LA in the setting of hemorrhagic diatheses, as expected of an anticoagulant. The latter is usually cited as the origin of the LA test, although the name, LA, was not coined until 1972.
However, Bowie et al in 1963 described four cases of SLE with thrombosis despite presence of LA [
271]. This was the first clear statement of the paradox:
why should thrombosis occur in the presence of an anticoagulant? Efforts to answer this persist to the present.
Subsequently, a growing number of reports through the 1970s found association of LA with thrombosis at greater frequency than hemorrhage, culminating with recognition of the aPL syndrome (APS) in the mid-1980s, as earlier referenced [
3‐
5,
272].
Principles of LA testing
Although the LA is widely spoken of as an aPL, it must be stressed that LA is not defined by any specific antibody or other known agent, but only by its effect in an LA assay. In its simplest early form, the LA assay was the recalcification time (recal time) of platelet-poor plasma (PPP), together with the "platelet neutralization test" (PNT) for confirmation [
268]. Briefly, one adds sufficient calcium to overcome the citrate in the PPP and observes if the time to coagulation is abnormally long, indicating LA+, provided that in addition, the prolongation is reduced by the PNT (originally, addition of platelets or platelet membranes). This was interpreted to mean that some factor in the plasma, called the LA, was acting to block sites on the phospholipids (PL) essential for coagulation, suggesting that aPL is that factor. The rationality of the PNT is that an excess of PL overwhelms the ability of the LA to block enough sites to prolong the time. A variety of LA tests are now in use, beyond the scope of this review, but all are similar in principle.
LA associates best with thrombosis
Among the first persuasive reports of a strong association between LA and thrombosis was the retrospective analysis of Mueh at al in 1980 [
273]. Following recognition of the syndrome, APS, it became common to test LA in parallel with solid-phase ELISA of aPL, leading to several reports showing that LA has a stronger association with thrombosis than aPL measured by ELISA, notably by Derkens et al in 1988 [
274], later confirmed repeatedly [
6,
275‐
277], explicitly stated by Horbach et al in 1996 [
278] and by Arnout in 2001 [
11] and, if further confirmation was needed, in a meta-analysis by Galli in 2003 [
244]. Indeed, several studies found that thrombosis associated closely with LA but poorly or not at all with aPL by ELISA, e.g. [
276,
277].
Thus, it is firmly established that the association of LA+ assay with thrombophilic states is much stronger than ELISA of aPL such as aCL, anti-β2GPI, or other. Furthermore, this association is robust with respect to variant methods. However, if it is true that LA is an aPL, we are faced with the problem of explaining why LA correlates so much better with thrombosis than aPL measured by ELISA.
What is the LA? Why thrombogenic?
As explained by Triplett [
7], early results of Harris et al [
3] suggested to them that LA and aCL were one and the same entity. This was later shown to be incorrect since the two were clearly separable. Nevertheless, the assumption continued that LA is a manifestation of aPL, to the point where LA is spoken of almost as a synonym for aPL, or as a particular type of aPL [
279]. Indeed, Triplett has stated that LA is an aPL, by definition [
7].
As detailed in several reviews, such as by Arnout [
11], it has been shown that at least some LA are in fact expressions of anti-β
2GPI. Roubey et al showed that LA plasma added to normal plasma prolonged coagulation but not if the plasma was first depleted of β
2GPI [
279]. Around the same time, Arvieux showed that mouse antibodies against β
2GPI had LA-like activity [
280]. Arnout made a series of monoclonal antibodies (mAb) against β
2GPI and found that only 7 of 21 mAb had LA activity, variable in degree, one being very strong [
28,
281]. This suggested that the epitope targeted was critical for LA activity.
According to Arnout [
11], the now accepted explanation for the LA activity of anti-β
2GPI was developed by three groups, Willems at al in 1996 [
282]), Takeya et al in 1997 [
283], and Arnout et al in 1998 [
28]. The essence of it is that only those anti-β
2GPI which can bind two molecules of β
2GPI (divalently) in the soluble phase cause enhanced binding to PL and exhibit LA activity [
11]. However, not all LA depend on β
2GPI.
In the same timeframe, others had shown that aPT can also exert LA activity, notably Bevers et al and Galli et al in 1992 [
284], Oosting et al in 1993 [
72], and Permpikul et al in 1994 [
285], as referenced [
7,
11]. A method was devised to discriminate between LA that depended on PT from LA that depended on β
2GPI [
286]. The explanation given for why some but not all aPT exhibit LA activity is similar to that given for anti-β
2GPI. This explanation does not, however, directly account for thrombosis, except by way of the hypotheses listed above for aPL in general.
Field et al [
287,
288] in their introduction give references to about five different theories for LA but dismiss them all as unconvincing, and then supply evidence for a novel explanation of their own: IgG from LA plasma inhibits thrombin generation only under static conditions, whereas under the shearing conditions of natural blood flow, LA promotes thrombin generation. In that scenario, the LA effect is an
in vitro artifact.
Several workers have proposed anti-annexin V (aAnV) as the agent of LA, with direct bearing on thrombosis. Matsuda et al found that aAnV was common in SLE patients (positive in 12 of 47) and associated strongly with LA activity [
123]; for commentary, see [
124]. The LA activity of aAnV was further explored by Nakamura et al the next year [
90]. It is possible that aAnV may also bear on the antagonism between annexins and phospholipase A
2 (PLA2) [
126].
In a series of papers, Rand et al has proposed to resolve the LA paradox on the basis of competition between aPL and the natural anticoagulant function of AnV [
289‐
291]. They showed that aPL IgG inhibits the anticoagulant effect of exogenously added AnV. However, the strong action of added AnV is contrary to the hypothesis that AnV is naturally present in plasma in significant amounts. Although AnV exhibits an effect on coagulation indistinguishable from LA [
112], it has not been shown to play a significant role as a natural anticoagulant, except perhaps in pregnancy.
Most reviewers now accept the existence of multiple LA, not necessarily limited to anti-β
2GPI, aPT and/or aAnV. For example, anti-FVIII behaves like LA [
292]. A study of LA+ patients found that all of them reacted to at least one pure PL, and 95% were positive for aPE [
146]. The specific antigen(s) was not identified. We have speculated that TFPI (the inhibitor of tissue factor) could exhibit LA activity, since it prolongs coagulation, could be neutralized by excess PL, and in at least some circumstances, is elevated in thrombophilic state [
293]. Thus, quite different explanations are conceivable.