T Helper Type 1-Bias in a Regulatory T Cell-Deficient Mouse Model for Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is a T cell-mediated autoimmune disorder, in which IgG autoantibodies to platelet surface glycoproteins promote platelet clearance in the reticuloendthelial system. CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs) are known to play a crucial role in the maintenance of immune homeostasis to self-antigens. Several lines of recent evidence have shown that Tregs are decreased in number and are functionally impaired in patients with ITP. Recently, we have found that approximately one third of Treg-deficient mice spontaneously develop thrombocytopenia with increased platelet-associated IgG and proportion of reticulated platelets. Platelets eluates and culture supernatants of splenocytes prepared from thrombocytopenic mice contain IgG antibodies capable of binding to intact platelets, which are not detected in non-thrombocytopenic mice. The main target of anti-platelet autoantibodies is GPIb, and some mice also produce anti-GPIIIa antibodies. However, detailed mechanisms that elicit ITP during immune reconstitution through homeostatic proliferation in the absence of Tregs remain uncertain.

To evaluate T-helper (Th) cell balance that promotes anti-platelet autoantibody response in a Treg-deficient mouse model for ITP.

Treg-deficient mice were prepared by inoculation of Treg-depleted CD4⁺ T cells obtained from BALB/c mice into syngeneic T cell-deficient nude mice. Platelet count was determined using flow cytometry 4 weeks after inoculation, and Treg-deficient mice with platelet count < 0.33 × 10⁶/ul were regarded as ITP mice. Treg-deficient mice without thrombocytopenia were also used as a control. To evaluate cytokine profiles of Th cells, proportions of Th subsets in the freshly prepared splenic CD4⁺ T cells were evaluated by intracellular staining for IFN-γ, IL-4, and IL-17 followed by flow cytometry. Th1, Th2, and Th0 cells were defined as IFN-γ⁺IL-4⁻, IFN-γ⁻IL-4⁺, IFN-γ⁺IL-4⁺ cells, respectively, and Th17 and Th1/17 cells were defined as IFN-γ⁻IL-17⁺ and IFN-γ⁺IL-17⁺, respectively. In addition, CD4⁺ T cells were isolated from splenocytes using magnetic activated cell sorting, and were stimulated with phorbol 1,2-myristate 1,3-acetate and ionomycin for 4 days. The culture supernatants were subjected to a cytokine bead array to measure levels of interleukin (IL)-2, IL-4, IL-6, IL-10, IL-17, interferon (IFN)-γ, and tumor necrosis factor (TNF). Finally, to determine IgG subclasses of anti-platelet autoantibodies, splenocyte culture supernatants were incubated with platelets derived from BALB/c mice, followed by incubation with fluorescence-conjugated antibodies to IgG₁, IgG_2a, IgG_2b, or IgG₃. Then, the antibodies bound to platelets was detected by flow cytometry.

Fourteen ITP mice and 8 control mice were used at 6–8 weeks after inoculation. The proportions of Th1, Th2, and Th0 cells did not differ significantly between ITP and control mice, while the Th1/Th2 ratio was significantly increased in ITP mice than in control mice (8.3 versus 3.2, p < 0.01). The proportions of Th17 and Th1/17 cells were comparable between ITP and control mice. There was no difference in the in vitro production levels of cytokines except IL-4, which was lower in ITP mice compared to control mice (140 versus 600 pg/ml, p = 0.02). Increase in the IFN-γ/IL-4 ratio was noted in the culture supernatants from ITP mice, compared to those from control mice (15.6 versus 9.2, p = 0.04). The Th1/Th2 ratio detected by flow cytometric measurement and the IFN-γ/IL-4 ratio in in vitro cultures were correlated with each other (r = 0.85, p < 0.01). IgG subclasses of anti-platelet autoantibodies were heterogeneous among individual ITP mice, but IgG_2a was the predominant subclass in the majority of ITP mice. Interestingly, a high Th1/Th2 ratio was associated with production of IgG_2b anti-platelet antibodies, while the mice with a low Th1/Th2 ratio produced IgG₁ anti-platelet antibodies.

These findings suggest that induction of IgG anti-platelet autoantibody response in Treg-deficient mice is associated with Th1 bias, which is analogous to the Th balance in patients with primary ITP. The Th1/Th2 balance may modulate the autoimmune responses during expansion of CD4⁺ T cells in the absence of Tregs.

No relevant conflicts of interest to declare.