Preferential Expansion of Foxp3+ T Regulatory Cells in CTLA-4-Deficient and CTLA-4-Haploinsufficient C57BL/6 Mice.

Foxp3⁺ cells and CTLA-4 have been ascribed major roles in downregulating immune responses. To address the relationship between CTLA-4 expression and Foxp3⁺ cells, we generated littermate CTLA-4-sufficient (Ctla4 ^+/+), CTLA-4-haploinsufficient (Ctla4 ^+/-), and CTLA-4-deficient (Ctla4 ^-/-) Foxp3-gfp knock-in C57BL/6 mice, permitting us to characterize the phenotype of Foxp3⁺ cells and to test their ex vivo T regulatory (Treg) suppressor activity. CD3⁺, CD4⁺, and CD8⁺ cells, but not CD19⁺ cells, were markedly expanded in Ctla4 ^-/- mice compared with Ctla4 ^+/+ or Ctla4 ^+/- mice. In Ctla4 ^-/- mice, the relative expansion of the Foxp3⁺ population was greater than that of the CD3⁺, CD4⁺, or CD8⁺ populations because of increased survival of Foxp3⁺ cells. Foxp3⁺ Treg cells from Ctla4 ^-/- mice and Foxp3⁺ Treg cells from Ctla4 ^+/+ mice exerted identical ex vivo suppressor function. This may be related to differential expression of GITR, CD73, and CD39 on Foxp3⁺ Treg cells from Ctla4 ^-/- mice versus that on corresponding cells from littermate Ctla4 ^+/+ or Ctla4 ^+/- mice, with GITR and CD39 being upregulated and CD73 being downregulated on Foxp3⁺ Treg cells from Ctla4 ^-/- mice. Moreover, CTLA-4 expression in Ctla4 ^+/+, Ctla4 ^+/-, and Ctla4 ^-/- mice correlated with their percentages of Foxp3⁺ cells, suggesting an important role for CTLA-4 expression in Treg cell homeostasis. This may have vital ramifications for the treatment of patients for whom augmentation of suppressor function would be beneficial (e.g., patients with autoimmune diseases) and for whom diminution of suppressor function would be beneficial (e.g., patients with cancer).