For example, CXCL9 is mainly secreted by monocytes in response to IFN-γ stimulation (22), a process that leads to elevated IFN-γ production by T cells of IGRA-negative TB patients that may lead to T cell exhaustion, a phenomenon observed in patients suffering from several types of chronic viral infections (23, 24). Here, CXCL9 is linked to tuberculosis.