Phenotypical and Functional Analysis of Memory and Effector Human CD8 T Cells Specific for Mycobacterial Antigens

Mycobacterium tuberculosis infects one-third of the global population and claims two million lives every year. Because memory CD8 T cells exhibit a high heterogeneity in terms of phenotype and functional characteristic, we investigated the frequency, phenotype, and functional properties of Ag85A epitope-specific HLA-A*0201 CD8 T cells in children affected by tuberculosis (TB) before and 4 mo after chemotherapy and healthy contact children. Using Ag85A peptide/HLA-A*0201 pentamer, we found a low frequency of blood peptide-specific CD8 T cells in tuberculous children before therapy, which consistently increased after therapy to levels detected in healthy contacts. Ex vivo analysis of the expression of CD45RA and CCR7 surface markers indicated a skewed representation of Ag85A epitope-specific CD8 T cells during active TB, with a predominance of T central memory cells and a decrease of terminally differentiated T cells, which was reversed after therapy. Accordingly, pentamer-specific CD8 T cells from tuberculous patients produced low levels of IFN-gamma and had low expression of perforin, which recovered after therapy. The finding of an elevated frequency of pentamer-specific CD8 T cells with T effector memory and terminally differentiated phenotypes in the cerebrospinal fluid of a child with tuberculous meningitis strongly indicates compartmentalization of such CD8 effectors at the site of disease. Our study represents the first characterization of Ag-specific memory and effector CD8 T cells during TB and may help to understand the type of immune response that vaccine candidates should stimulate to achieve protection.