Functional, Antigen-Specific Stem Cell Memory (TSCM) CD4+ T Cells Are Induced by Human Mycobacterium tuberculosis Infection

Background: Maintenance of long-lasting immunity is thought to depend on stem cell memory T cells (T-SCM), which have superior self-renewing capacity, longevity and proliferative potential compared with central memory (T-CM) or effector (T-EFF) T cells. Our knowledge of T-SCM derives primarily from studies of virus-specific CD8(+) T-SCM. We aimed to determine if infection with Mycobacterium tuberculosis (M. tb), the etiological agent of tuberculosis, generates antigen-specific CD4(+) Tscm and to characterize their functional ontology. Methods: We studied T cell responses to natural M. tb infection in a longitudinal adolescent cohort of recent QuantiFERON-TB Gold (QFT) converters and three cross-sectional OFT adult cohorts; and to bacillus Calmette Guerin (BCG) vaccination in infants. M. tb and/or BCG-specific CD4 T cells were detected by flow cytometry using major histocompatibility complex class II tetramers bearing Ag85, CFP-10, or ESAT-6 peptides, or by intracellular cytokine staining. Transcriptomic analyses of M. tb-specific tetramer(+) CD4+ T-SCM (CD45RA(+) CCR7(+) CD27(+)) were performed by microfluidic qRT-PCR, and functional and phenotypic characteristics were confirmed by measuring expression of chemokine receptors, cytotoxic molecules and cytokines using flow cytometry. Results: M. tb-specific Tscrd were not detected in OFT-negative persons. After OFT conversion frequencies of Isom increased to measurable levels and remained detectable thereafter, suggesting that primary M. tb infection induces T-SCM cells. Gene expression (GE) profiling of tetramer+ Isom showed that these cells were distinct from bulk CD4+ naive T cells (TN) and shared features of bulk Tscm and M. tb-specific tetramer+ T-OM and T-cells. These Clan were predominantly CD95(+) and CXCR3(+), markers typical of CD8+ T-SCM. Tetramer(+) Tsi;m expressed significantly higher protein levels of CCR5, CCR6, CXCR3, granzyme A, granzyme K, and granulysin than bulk TN and T-SCM cells. M. tb-specific TSCM were also functional, producing IL-2, IFN-gamma, and INF-alpha upon antigen stimulation, and their frequencies correlated positively with long-term BCG-specific CD4(+) T cell proliferative potential after infant vaccination. Conclusion: Human infection with M. tb induced distinct, antigen-specific CD4(+) Tscm cells endowed with effector functions, including expression of cytotoxic molecules and Th1 cytokines, and displayed chemokine receptor profiles consistent with memory Th1/17 cells. Induction of CD4(+) Tsuro should be considered for vaccination approaches that aim to generate long-lived memory T cells against M. tb.