Targeting MAIT cells holds promise for the treatment of different diseases and infections . We previously showed that treatment of Mycobacterium tuberculosis infected mice with 5-OP-RU, a major antigen for MAIT cells, expands MAIT cells and enhances bacterial control . Here we treated M. tuberculosis infected rhesus macaques with 5-OP-RU intratracheally but found no clinical or microbiological benefit in M. tuberculosis infected macaques . In fact, after 5-OP-RU treatment MAIT cells did not expand, but rather upregulated PD-1 and lost the ability to produce multiple cytokines, a phenotype resembling T cell exhaustion . Furthermore, we show that vaccination of uninfected macaques with 5-OP-RU+CpG instillation into the lungs also drives MAIT cell dysfunction, and PD-1 blockade during vaccination partly prevents the loss of MAIT cell function without facilitating their expansion . Thus, in rhesus macaques MAIT cells are prone to the loss of effector functions rather than expansion after TCR stimulation in vivo, representing a significant barrier to therapeutically targeting these cells.