E coli exposure triggers apoptosis and destruction of CD161⁺⁺/MAIT cells in vitro. (A-B) Sections of colon from HIV-infected patients (n = 12) or controls (n = 12) were stained for lipopolysaccharide (LPS). (A) Representative images from control (left) and HIV-infected tissue (right) are shown. Scale bars, 50 μm. (B) Significantly more LPS+ cells were seen in the lamina propria in HIV infection. The number of LPS+ cells in the lamina propria was determined by manual counting as outlined in “Methods.” (C-G) PBMCs from healthy subjects (n = 12) were activated with PFA-fixed E coli at a bacteria per cell (BpC) ratio of 1, 10, or 100 or mock-treated and were analyzed after 20 hours incubation. (C) E coli–exposed CD161⁺⁺CD8⁺ cells (BpC of 10) had higher frequencies of activated caspase-3–positive cells versus mock-treated cells or E coli–exposed CD161⁻ and CD161⁺ cell populations. (D) Representative FACS plots, gated on CD8⁺ T cells, comparing activated caspase-3 expression in mock-treated and E coli–exposed PBMC cultures. (E) At all BpC of E coli, there was a reduction in survival of the CD161⁺⁺CD8⁺ T-cell population. No significant differences were noted in the CD161⁺ or the CD161⁻CD8⁺ T-cell populations. Cell survival was determined by normalizing the frequency of the cell population of interest in the E coli–exposed culture to that in the mock-treated culture. A value of 1 is equivalent to 100% survival. (F) Anti-MR1–blocking antibody reduced the frequency of activated caspase-3–positive CD161⁺⁺CD8⁺ T cells compared with the isotype control. Blocking of E coli–induced apoptosis was determined by normalizing the frequency of activated caspase 3⁺ CD161⁺⁺C8⁺ T cells treated as indicated to that of the culture exposed to E coli alone. (G) Anti-MR1–blocking antibody increased the survival of CD161⁺⁺CD8⁺ T cells in the E coli–exposed culture. Cell survival was determined by normalizing the frequency of CD161⁺⁺CD8⁺ T cells in the antibody treated, E coli–exposed culture to that of the mock-treated culture.