Patients with ITP produce anti-platelet IgG antibodies (and more rarely IgM or IgA antibodies) [
28,
29,
30,
51,
52] which bind to platelets and mark them for phagocytic breakdown in the spleen and liver [
39]. These antibodies often bind to very abundant glycoproteins on the platelet surface, particularly GPαIIbβ3 (GPIIbIIIA) and GPIb-IX-V molecules [
31,
32]. However, in as many as 30% to 40% of the patients, no detectible antibodies can be found [
53,
54]. Whether the lack of antibodies in patients is due to the robustness of the antibody tests used or perhaps due to a purely T cell-mediated mechanism is still unknown. Of interest, in those patients positive for anti-platelet antibodies, other antibody specificities beside the classic surface glycoproteins have been found, including cytosolic proteins [
55], which may suggest that platelets undergo protein degradation by antigen presenting cells (APC) followed by antigen presentation to T cells [
56]. Moreover, other mechanisms have been proposed to be involved in antibody production in ITP including antigenic cross-reactivity (mimicry), somatic mutation [
16,
53], and defects in the elimination of autoreactive B-cell clones [
16]. In addition, oxidative stress, which favours the production of autoantibodies, may also be involved [
57]. The type of epitope targeted by autoantibodies may also be a marker of disease severity and, to some extent, of response to treatment, in mice at least [
58,
59]. Indeed, it has been hypothesised that certain antibody specificities are more prone to induce platelet clearance [
58] and apoptosis [
60,
61,
62] or to inhibit megakaryopoiesis [
35]. For example, anti-GPIb antibodies appear to induce a stronger platelet destruction by increasing the release of CD62P and phosphatidylserine and the clustering of GPIb receptors [
63], and, in mice, these antibodies tend to be more resistant to the effects of intravenous immunoglobulin (IVIg) treatment [
58]. Antibody-mediated platelet destruction has also been shown to be enhanced by the acute phase protein C-reactive protein (CRP) both in vitro and in vivo [
64]. Interestingly, increased levels of CRP at the diagnosis of childhood ITP predicted a slower platelet count recovery, but after IVIg treatment, the levels of CRP dropped, accompanied by a recovery in the platelet count and decreased bleeding severity [
64]. Furthermore, platelet opsonisation by autoreactive antibodies can affect platelet reactivity by modulating agonist stimulation and platelet secretory granule release [
65]. This observation may partially explain the variability of ITP bleeding severity as well as the differences in response to treatment observed in some patients with similar platelet counts. Furthermore, the presence of anti-platelet autoantibodies increases the risk of thrombotic events [
66,
67,
68], perhaps due to procoagulant microparticles released by activated platelets [
69,
70] or associated predispositions [
37,
63,
71].
Autoreactive antibodies are secreted by plasma cells, which have been reported to be present at higher levels in patients with ITP [
72], as well as the B cell regulator, and B cell-activating factor (BAFF, also called B cell stimulator (BlyS)), which is an important factor in B cell selection, survival, and proliferation. Indeed, BAFF promoter region polymorphisms as well as its up-regulation in the plasma have been strongly associated with ITP in humans and in a murine ITP model [
73,
74,
75,
76]. B cells were also shown to be increased in the red pulp of the spleens from patients with ITP [
77] and they appear to have higher proliferative rates in these splenic areas [
78]. Moreover CD19
+CD41
hiCD38
hi B-regulatory cells (Bregs), which promote peripheral tolerance, are also impaired in ITP [
79,
80]. They fail to reduce CD4
+ T cell activation and trigger the recruitment of CD4
+CD25
+FoxP3
+ T regulatory cells (Tregs), a subtype of CD4
+ T cells crucial for immune suppression and tolerance [
81] via IL-10 secretion [
82]. The CD19
+CD24
+ FOXP3
+ Breg subpopulation has also been recently shown to be significantly increased in the spleens of patients with ITP compared with control trauma patients [
83]. These studies suggest that, like Tregs, the peripheral deficiency of Bregs may be due to sequestration of these cells within lymphoid compartments. Nonetheless, there appears to be a central role for Bregs and IL-10 secretion in ITP and their modulating effects on Tregs in the pathogenesis of the disorder.
Taken together, these studies demonstrate that ITP patients present with impaired plasma cells, Bregs, and B cells, leading to the production of pathogenic antibodies. These antibodies, via platelet and MK opsonisation, trigger platelet destruction in the spleen and liver as well as defective megakaryopoiesis.