Improved Regulatory T Cell Activity in Patients with Chronic Immune Thrombocytopenia Purpura Treated with Thrombopoietic Agents.
Authors: Bao Weili , Heck Susanne , Karpoff Marissa , Bussel James B. , Yazdanbakhsh Karina 
Published: October 2019

Abstract
Abstract 684
Immune thrombocytopenic purpura (ITP) is an autoimmune, autoantibody-mediated disease with accelerated platelet destruction and impaired platelet production resulting in. thrombocytopenia and bleeding due to breakdown of immune tolerance. We and others have reported impaired regulatory CD4+CD25hi T cells (Treg) suppressive function in ITP patients. Clinical trials using thrombopoietic agents to stimulate platelet production have shown favorable outcomes in ITP patients with increased platelet counts and reduced bleeding. Two such agents, romiplostim (Nplate) and eltrombopag (Promacta), were recently licensed in the USA and elsewhere. Information on the immune responses of patients treated with these drugs pertaining to prognosis and response to treatment are lacking. We therefore studied the immunological profile of ITP patients with an average platelet counts of 21×109/L before treatment (n=6) and those who had been more than 3 months on treatment (total n=12, average platelet count 133 ×109/L) with either Nplate (n=5), eltrombopag (n=2) or an investigational thrombopoietic agent AKR-501 (n=5). We found no statistically significant differences in the Treg frequencies (Foxp3+CD25hi in the CD4+ population) of patients pre- and on-treatment (2.4± 0.6% versus 2.9± 0.5, p=0.5). However, the Treg activity as measured by suppression of proliferation of autologous CD4+CD25 cells at 1:1 and 1:4 ratios of Tregs : CD4+CD25 cells was significantly improved in patients on treatment compared to the pre-treatment group (at 1:1 ratio, 60% on-treatment versus 41% pre-treatment, p=0.001 and at 1:4 ratio, 44% versus 24%, p=0.04) and comparable to the overall Treg activity of controls (p=0.9).
Improved Treg function correlated with reduction in IL-2-producing Th cells (Pearsons r=0.6, p=0.01), and patients on treatment had a 30% reduced frequency single positive type 1 IFN-g-producing Th cells (p=0.04). Circulatory levels of pro-inflammatory sCD40L in platelet poor plasma (PPP) were decreased in patients on treatment versus pre-treatment (0.54 ± 0.05 ng/ml versus 0.89 ± 0.13ng/ml, p=0.02) and comparable to controls (p=0.9). TGF-b levels in PPP strongly correlated with improved platelet counts (Pearsons r=0.8, p<0.0001) and were increased in patients following treatment (2177 ± 258 ng/ml versus 1083 ± 127 ng/ml p=0.004). In summary, our data indicates that treatment with thrombopoietic agents results in improved immune regulation by Tregs which may be responsible for the observed dampening of the pro-inflammatory Th1-associated responses. The concomitant increase in TGF-b levels and decrease in sCD40L levels, both of which are platelet-derived, are consistent with a rise in platelet counts in patients on treatment. This raises the interesting possibility that platelets in patients on treatment may play a role in improving Treg function either directly through cell-cell interactions or indirectly by release of TGF-b. In conclusion, our findings suggest that thrombopoietic agents are not immunologically inert but rather have profound effects to restore immune tolerance.

Disclosures:
Bussel: Genzyme: Research Funding; GlaxoSmithKline: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding; Cangene: Research Funding; Amgen: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding; Immunomedics: Research Funding; Ligand: Membership on an entity's Board of Directors or advisory committees, Research Funding; Eisai, Inc: Research Funding; Sysmex: Research Funding; Scienta: Speakers Bureau; Shionogi: Membership on an entity's Board of Directors or advisory committees.