Avid Bioservices Inc (CDMO)

[freethemice]

It is late so this will be short. The literature on antiphospholipid syndrome (APS) and its associated antiphospholipid
antibodies (aPL), and on the target of these antibodies, beta2-glycoprotein I (beta2-GPI), is vast.
The following is from the introduction in last year's paper by Ramesh et al.

The antiphospholipid syndrome (APS) is an autoimmune disorder
characterized by the presence of circulating antiphospholipid antibodies
(aPL) and recurrent thrombosis (1). A link between APS and
greater risk of atherosclerosis in peripheral and coronary arteries
has also been established (2). aPL are directed not against phospholipids,
but rather against plasma proteins with affinity for anionic
cell surface phospholipids, and a pathogenetically important major
subset of aPL is directed against ß2-glycoprotein I (ß2GPI) (3–7).
Binding of aPL to phospholipid-bound ß2GPI causes its dimerization,
which further increases its affinity for negatively charged phospholipids
and cell surfaces (8). The endothelium is a primary target
of aPL, and pathogenic autoantibody binding to ß2GPI causes the
upregulation of adhesion molecule expression and a proinflammatory
and prothrombotic endothelial cell phenotype (9). How aPL
binding to ß2GPI on the endothelial cell surface induces a transmembrane
signal to modify endothelial cell behavior is unknown.
NO generated by the endothelial isoform of NOS (eNOS) is a key
determinant of vascular health that regulates several physiological
processes, including leukocyte adhesion, thrombosis, endothelial
cell migration and proliferation, vascular permeability, and vascular
smooth muscle cell growth and migration (10). The eNOS
enzyme, which generates NO upon the conversion of l-arginine to
l-citrulline, is activated by numerous extracellular stimuli and is
promoted primarily by increases in the phosphorylation of S1179
(in bovine eNOS; S1177 in human eNOS) by PI3 kinase/Akt kinase
and also by dephosphorylation of T497 (11–13). Whether aPL alter
eNOS function is unknown.
To better understand the molecular basis of APS, we designed
the present study to test the hypothesis that aPL-induced increases
in leukocyte–endothelial cell adhesion and thrombus formation
are caused by eNOS antagonism. In addition, we determined
whether aPL-induced eNOS inhibition involves ß2GPI, and if the
process also requires an LDL receptor (LDLR) family member, particularly
apoER2, which has the capacity to directly bind ß2GPI
(14, 15).

Here is figure 8 from the paper which summarizes some of the above, and what was found in this study.
This shows two molecules of beta2-GPI bound to PS on the surface of endothelial cells lining a blood vessel.
In APS this is not usually a tumor blood vessel, but the same will apply there too.
The two molecules of beta2-GPI are bound by the antiphospholipid antibody so that they are "dimerized",
which just means the two of them connected to each other. Note that the antibody is bound to
domain I of both of the beta2-GPI molecules. It has been found that these natural antibodies which bind
to domain I are highly associated with the thrombosis effects of APS. So what seems to happen is that
the domain V of the beta2-GPI molecule will interact with the end of the apoER2 receptor which is nearby.
This could be some sort of electrostatic effect because domain V has a positive charge. When this happens
a signal is propagated through the plasma membrane and the cytoplasmic tail of the apoER2 then interacts
with the enzyme PP2A. This causes PP2A to remove a phosphate group from the enzyme eNOS,
which is attached to the inside (cytoplasmic) side of the plasma membrane. The S1179 refers to the
amino acid Serine at position 1179 of the enzyme. PP2A is a phosphatase, which is an enzyme that
adds and removes phosphate groups from other molecules. The removal of this phosphate group
reduces the activity of eNOS, which means it produces less NO (nitric oxide). This then causes more
white blood cells to stick to the walls of the blood vessel which causes increased thrombosis.
It is complicated, but this chain of events is how a signal from outside the cell can get transmitted across
the cell membrane to the inside (cytoplasm) of the cell to turn on/off the production of molecules like NO.
In other cases signals can be carried into the nucleus so that genes can be turned on or off.
This process is called a signal transduction pathway.
The researchers found that by using a version of bavituximab, which binds to domain II of
beta2-GPI this pathway was broken because apoER2 would no longer bind to
domain V of the beta2-GPI bound to the plasma membrane. Without this happening a signal was
not sent down the pathway which means the PP2A did not remove a phosphate group from eNOS and
the production of NO was not reduced, so thrombosis was not increased. Therefore, bavi blocked the
chain of events and prevented the thrombotic effects of APS. This probably works because by
binding domain II, instead of domain I, there is a small change in the shape of the beta2-GPI,
maybe a twist in the structure, that results in the failure of the interaction between the domain V and
the end of the apoER2. That is the way it is many times, very small changes are all that is needed
because we are working on the atomic scale and the three-dimensional structure is very important.

The abstract from the AHA presentation also says that the aPL inhibited the normal repair of blood vessels
and that PGN635 allowed the repair to happen.
My thoughts on this are that PGN635 (fully human bavi) could be used when APS patients are having
acute episodes to prevent thrombosis. Of course, this would have to go through the whole clinical trial process,
but the large amount of safety data on bavi will help. I would guess that the reduction in thrombotic events could
be the trial endpoint, and that could mean a reduction in deaths too.

Here is a diagram showing bavi binding to domain II.