MR1-dependent activation of CD161⁺⁺CD8⁺ T cells by E coli. Healthy isolated PBMCs were exposed to E coli overnight and analyzed for CD69 expression and IFNγ and IL17A production by CD8⁺ T cells as described in “Methods.” (A) There was a significant increase in the cell surface expression of CD69 on the CD161⁺⁺ population in response to E coli exposure compared with the mock-infected control and to the CD161⁺ and CD161⁻E coli–exposed populations (n = 12). (B-C) A greater proportion of the E coli–exposed CD161⁺⁺ population produced IFNγ or IL17A than the CD161⁻ population (n = 12). Note data points lie on the x-axis (values of 0% were arbitrarily ascribed a value of 0.01% or 0.001% so as to appear on the log scale). (D) Increasing concentrations of anti-MR1 blocking antibody reduced the level of activation of the CD161⁺⁺CD8⁺ T cells compared with the isotype antibody (n = 6). “Activation” on the y-axis is measured as a ratio of the geometric mean fluorescent intensity of CD69 on the antibody-treated E coli–exposed CD161⁺⁺CD8⁺ T cells compared with that of the E coli–exposed CD161⁺⁺CD8⁺ T cells. Plots are gated on CD8⁺ T cells.