Background/Purpose: In systemic lupus erythematosus (SLE), the dysregulated production of autoantibodies is a consequence of disrupted T cell homeostasis. Programmed death-1 (PD1), a negative regulatory signal in T cells, limits certain T-cell mediated immune responses. Increased PD1 expression on T cells inhibits cell activation and proliferation, and its blockade reinstates immune cell function. Our laboratory has shown that attenuated, but not absent PD1 signaling enables T_reg to survive, and suppress helper T cells (T_h) and B cells. These suppressors are designated PD1^loT_reg. Our gene array data have shown that attenuated PD1 expression in T_reg down-regulates several members of the TNF receptor family (TNRF). It has been reported that OX40, a member of the TNRF superfamily, is known to be up-regulated in immune cells from patients with autoimmune diseases. We therefore hypothesize that one mechanism by which PD1 sustains T_reg proliferation and suppressive function is by reducing OX40 expression on T_reg.

Methods: We treated lupus-prone BWF₁ mice with a neutralizing Ab against PD1 or control isotype-matched IgG intraperitoneally. T_reg from the spleens were isolated and cocultured with unmanipulated CD4⁺CD25^– T_h and B cells. Expression of OX40 and PD1 on T_reg was measured by flow cytometry. The ability of T_reg to regulate T_h was assessed by measuring the ratio of T_reg: T_h after coculture, with the phenotypes defined as: T_reg (Foxp3⁺CD4⁺CD25⁺CD127^–), T_h1 (CD4⁺CD25^–IFNγ⁺), T_h2 (CD4⁺CD25^–IL4⁺), T_h17 (CD4⁺CD25^–IL17a⁺). Cytokine production of T_reg and T_h was measured in the culture media by ELISA. Next, we treated T_reg with an agonistic OX40 Ab in vitro, and set up the colculture experiments and deterrmine cell proliferation and cytokine production as described above.

Results: OX40 expression was lower in PD1^loT_reg from anti-PD1-treated mice when compared to PD1^hiT_reg from controls. In particular, all Foxp3⁺PD1^loT_reg have either low or no OX40 expression. T_h was predominantly T_h2 over T_h1 and T_h17 in the coculture with PD1^loT_reg, with decreased expression of IFN-g and IL-17 in the culture media. When PD1^loT_reg were treated with agonistic OX40, their suppressive function was attenuated, but not completely abrogated. Although Foxp3 expression in PD1^loT_reg was not significantly diminished, there was decreased production of TGFβ . In the coculture, activation of OX40 was associated with increased proliferation of T_h1. However, treating PD1^–T_reg with antagonistic OX40 could not restore the suppressivity in T_reg.

Conclusion: Effective induction of T_reg is associated with low expression of PD1, which permits the cells to survive and perform a cell suppressive function. Attentuated PD-1 expression in T_reg reduced OX40 expression on T_reg, which helped restore the suppressive capacity and proliferation of T_reg. The suppressive function induced by low PD1 expression is influenced by, but not dependent on, low OX40 expression. PD1 and OX40 signaling most likely crosstalk to regulate the suppressive capacity and survival of T_reg to achieve peripheral tolerance in SLE but are not the only pathways involved.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/pd-1-signaling-interferes-with-ox40-to-alter-the-suppressive-function-and-proliferation-of-cd4-regulatory-t-cells-in-lupus-mice/