IL-12 reverses CD16⁺ monocyte-mediated inhibition of Treg and CD4⁺IL-17⁺ proliferation. Sorted T cells were cocultured with CD14^hiCD16⁻ with or without CD16⁺ monocytes in the absence or presence of neutralizing anti–IFN-γ and IL-12 or isotype control antibodies. After 7 days of stimulation with anti-CD3, the frequencies of Foxp3^hi, IL-17, and IFN-γ–positive cells in divided CD4⁺ T cells was analyzed. Percent increase in proliferation of (A) Treg, (B) CD4⁺IL-17⁺, and (C) CD4⁺IFN-γ cells by CD16⁺ monocytes was calculated as 1 − (% CFSE^lo CD4⁺ (Foxp3^hi or IL-17⁺ or IFN-γ) in cocultures of T cells plus CD14^hiCD16⁻ plus CD16⁺ monocytes)/(% CFSE^lo CD4⁺(Foxp3^hi or IL-17⁺ or IFN-γ) in cocultures of T cells plus CD14^hiCD16⁻) in the presence of isotype control (“Control”) or the specific antibodies (anti–IFN-γ and anti–IL-12) are shown. Addition of anti–IL-12 decreases the inhibition of CD16⁺ monocytes on Treg and CD4⁺IL-17⁺ proliferative responses as well as its stimulatory effect on CD4⁺ IFN-γ⁺ expansion (P < .05, all paired t test). Anti–IFN-γ also has a significant inhibitory effect on proliferation of CD4⁺IL-17⁺ (P = .02).