Background/Purpose:

Programmed death-1 (PD-1) has been regarded as a negative regulatory signal in T cells. Our laboratory has shown that PD-1 is important in T cell regulation of autoimmunity, as treatment with neutralizing anti-PD1 Ab increased regulatory T cell function and dramatically delayed SLE onset in young BWF₁ females. We hypothesized that tight regulation of PD-1 signaling is required for maintenance of functional CD4⁺T_reg and subsequent control of autoimmunity in BWF₁ mice, and that regulatory capacity of the cells is sustained at least in part by resistance to apoptosis, resulting in CD4⁺T_reg survival, when PD-1 is expressed at a certain level – neither absent nor high.

Methods:

A neutralizating Ab against PD-1 or control isotype-matched IgG were injected i.p. into BWF₁ mice. Foxp3 and PD-1 expression were assessed by flow cytometry. Splenocytes from these animals were isolated for in vitro culture, and anti-PD-1 Ab at various concentrations were added in vitro to test for B cell apoptosis by Annexin V and 7-AAD, and CD4⁺CD25^– helper T cell (T_h) proliferation by CFSE. AutoAb production of anti-dsDNA and total IgG was tested by ELISA in the supernatant.

Results:

Blocking the full expression of PD-1 in vivo resulted in induction of CD4⁺CD25⁺Foxp3⁺T_reg with reduced, but not absent, PD-1 expression. These PD-1^loCD4⁺T_reg, compared to CD4⁺T_reg with PD-1^hi expression, had increased ability to induce B cell apoptosis and to suppress T_h proliferation. The PD-1^loCD4⁺T_reg suppressed syngeneic B cells production of anti-dsDNA and IgG. Apoptosis was significantly lower in the PD-1^loCD4⁺T_reg. In vitro PD-1 blockade confirmed that tight regulation of PD-1 expression is important in maintaining CD4⁺T_reg suppressivity. By adding anti-PD-1 Ab >25 hg/ml in vitro, PD-1^loCD4⁺T_reg from mice with in vivo PD-1 blockade lost their ability to induce B cell apoptosis and suppress T_h proliferation.  On the other hand, PD-1^hiCD4⁺T_reg from the isotype controls became more suppressive with the presence of anti-PD-1 Ab in vitro, until its concentration was > 75hg/ml. The concentration of anti-PD-1 Ab did not affect the survival of CD4⁺T_reg.

Conclusion:

PD-1 expression is central in the ability of CD4⁺T_reg to suppress autoimmunity; their PD-1 expression has to be finely tuned to permit CD4⁺T_reg to survive and retain suppressive capacities. One mechanism by which PD-1 sustains CD4⁺T_reg is by reducing their susceptibility to apoptosis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/pd-1-signaling-promotes-suppressive-function-of-cd4-regulatory-t-cells-in-new-zealand-black-x-new-zealand-white-f1-lupus-prone-mice-in-a-dose-dependent-manner/