Heme oxygenase-1 is not required for mouse regulatory T cell development and function

CD4 regulatory T cells (Treg) ensure peripheral tolerance to self-antigens and limit the deleterious effects associated with inflammatory and immune responses by mechanisms that remain to be fully understood. The enzyme heme oxygenase-1 (HO-1), through its known anti-inflammatory activity, is a candidate for a functional role in Treg activity. We compared wild-type and heme oxygenase-1-deficient (hmox-1(-/-)) mice in order to assess the role of HO-1 in mouse Treg development and function under physiologic conditions. The frequency of CD25(+) and Foxp3(+) Treg was similar in hmox-1(-/-) and hmox-1(+/+) mice. More importantly, CD4(+)CD25(+) Treg purified from either hmox-1(-/-) or hmox-1(+/+) mice were equally efficient in controlling the proliferation in vitro and the expansion in vivo of CD4(+)CD25(-) T cells, whether or not these responder cells expressed HO-1. In addition, induction of expression of HO-1 in vivo did not affect Treg suppressor function. As shown before, expression of HO-1 was higher in Treg than in naive T cells; however, naturally activated Foxp3(-) T cells displayed equal amount of HO-1 mRNA as Treg. Finally, we conclude that under physiological conditions in mice, Treg development, maintenance and function are independent of HO-1 activity.