ITP three R's: regulation, routing, rituximab

Tregs create a controlling milieu that promotes immune suppression and autoimmune tolerance. Their versatility and adaptability makes them the “multitalented masters of immune regulation.”¹  In this issue of Blood, 3 studies have shown this concept to be true in patients with chronic ITP. Collectively, the studies show that functional deficiencies of Tregs are responsible for the immune etiology of ITP and that targeting them may alleviate the disease.

Our immune system has elegantly evolved to arm us with many mechanisms that destroy invading microorganisms or stop the spread of tumors. Our system also has extensive built-in mechanisms for preventing attack on healthy self tissues.¹  This “self-tolerance” mechanism involves the elimination of self-reactive T and B lymphocytes during selection in the thymus and bone marrow, respectively. However, since these central mechanisms are not absolute, we have evolved peripheral mechanisms to deal with immune cells that “escape” central tolerance. Over the past 50 years, immunologists have postulated the existence of suppressor T cells that police the peripheral immune system to stop unwanted self immune responses. But this postulated entity was cast into doubt because of hard-to-reproduce model systems, each with complexities and idiosyncrasies.¹  With an explosion of biochemical and molecular advances, however, the field of immune suppression has been resurrected Phoenixlike, and the notion of T regulatory cells (Tregs), marked by specific cell surface molecules (eg, CD4⁺ CD25⁺ Foxp3⁺), arose.¹  Tregs appear to be a relatively rare CD4⁺ T-cell subset that comprise many subpopulations, including IL-10–producing “Tr1′ cells10, TGF-β–producing T helper type 3 cells, CD8⁺ T suppressor cells, natural killer T cells, CD4-CD8–T cells and γδ T cells.”^2-5  Some of these cells originate in the thymus during ontogeny and are referred to as “natural” Tregs. Some Tregs can also be induced from naive T cells in the periphery.⁶  These Tregs appear to be the natural immune “magic bullets” that keep all of our normal and abnormal immune responses in check. They are critical to our survival, and absence of them can lead to either autoimmunity or inflammatory disorders, with fatal consequences in both mice and humans.^1-5  Although defects of these cell types have previously been described in immune thrombocytopenic purpura (ITP),^7-9  3 papers in this issue collectively suggest that a deficient Treg compartment allows enhanced T-cell and B-cell autoreactivity in ITP, and that therapies like rituximab actually correct the deficiency and reverse autoimmune platelet reactivity, thus leading to increased platelet counts.

The first study in this series was based on the authors' previous observations that rituximab (Rituxan) is an efficacious therapy for patients with ITP and that this is due to normalizing abnormal autoreactive T-cell responses in ITP.^10,11  Stasi and colleagues have extended these intriguing results to show that rituximab primarily reverses the Treg deficiency in patients with ITP. The authors studied 26 adult patients with chronic ITP (a different cohort from their last paper) who were treated with rituximab; they examined Tregs by flow cytometry and assessed their regulatory function by cell proliferation assays. Compared with control individuals, pretreatment patients with ITP had a significantly reduced number and defective suppressive capacity of Tregs. In addition, the Tregs in the patients with ITP showed a polyclonal spectratype. In contrast, upon treatment with ri-tuximab, patients, particularly responders, showed restored numbers of Tregs as well as restored regulatory functions. The authors suggested that patients with active ITP have a defective Treg compartment, which can be significantly modulated by a B-cell targeted therapy.

In the second paper, Yu and colleagues studied 17 patients with chronic ITP and tested the frequency and functional capabilities of CD4⁺ CD25⁺ Foxp3⁺ Tregs in peripheral blood. Although they found a similar frequency of Tregs in controls and patients, the ability to functionally suppress in vitro T proliferation was significantly reduced in ITP patients. These data further support the notion that functional defects in Tregs probably contribute to a breakdown in self-tolerance in chronic ITP.

In the third paper, Olsson and colleagues studied the bone marrow and peripheral blood of 26 patients with chronic ITP and found, particularly in the bone marrow, increased numbers of infiltrating activated CD3⁺ T cells with elevated surface expression of VLA-4 and CX3CR1. Compared with controls, the increased T-cell number in the bone marrow was also associated with significantly lower numbers of bone marrow Tregs. They suggest that chronic ITP is a disease of increased activated T cell due to a Treg defect within the bone marrow and that this may contribute to suppressed megakaryocyte production in ITP.¹² 

The 3 studies collectively shed light on how Tregs may initiate and/or mediate the autoimmunity of ITP. It appears that a central deficiency of Tregs breaks tolerance and allows unchecked activation of autoreactive Th1 cells and B cells. These autoreactive cells, in turn, react against platelet autoantigenic targets, leading to the production of antiplatelet antibodies and cytotoxic T cells. Like autoantibodies, the activated T cells can route to the bone marrow and additionally promote suppression or killing of megakaryocytes. Therapies that either directly or indirectly affect the Treg compartment, as in the case of rituximab, can rescue tolerance and inhibit the abnormal platelet autoreactivity, thereby raising platelet counts. These papers are important not only because they confirm that defective Tregs are at the heart of the autoimmune dysregulation in ITP but because they suggest that development of therapies targeted at Tregs may be the best way to significantly and permanently control the disease.