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

Abstract

BackgroundMaintenance of long-lasting immunity is thought to depend on stem cell memory T cells (TSCM), which have superior self-renewing capacity, longevity and proliferative potential compared with central memory (TCM) or effector (TEFF) T cells. Our knowledge of TSCM derives primarily from studies of virus-specific CD8+ TSCM. 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.MethodsWe 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 QFT+ 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+ TSCM (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.ResultsM. tb-specific TSCM were not detected in QFT-negative persons. After QFT conversion frequencies of TSCM increased to measurable levels and remained detectable thereafter, suggesting that primary M. tb infection induces TSCM cells. Gene expression (GE) profiling of tetramer+ TSCM showed that these cells were distinct from bulk CD4+ naïve T cells (TN) and shared features of bulk TSCM and M. tb-specific tetramer+ TCM and TEFF cells. These TSCM were predominantly CD95+ and CXCR3+, markers typical of CD8+ TSCM. Tetramer+ TSCM expressed significantly higher protein levels of CCR5, CCR6, CXCR3, granzyme A, granzyme K, and granulysin than bulk TN and TSCM cells. M. tb-specific TSCM were also functional, producing IL-2, IFN-γ, and TNF-α upon antigen stimulation, and their frequencies correlated positively with long-term BCG-specific CD4+ T cell proliferative potential after infant vaccination.ConclusionHuman 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+ TSCM should be considered for vaccination approaches that aim to generate long-lived memory T cells against M. tb.

The generation of T‐cell memory to protect against tuberculosis

“… More recently, a new subset termed stem cell memory T cells have been identified, that represent the earliest and longest lasting developmental stage of memory T cells and have a greater self‐renewing capacity and proliferative potential compared with T CM and effector memory T cells . In M. tuberculosis‐ infected individuals and BCG vaccines, antigen‐specific CD4 + stem cell memory T cells are detected and display effector expression including Th1 cytokine release and expression of cytotoxic molecules . Thus, a major challenge in TB vaccine development is to determine the relative importance of each of these memory T cells and define if effective vaccines should induce all or some of these subsets for maximal protection.…”

“…53 In M. tuberculosis-infected individuals and BCG vaccines, antigen-specific CD4 + stem cell memory T cells are detected and display effector expression including Th1 cytokine release and expression of cytotoxic molecules. 54 Thus, a major challenge in TB vaccine development is to determine the relative importance of each of these memory T cells and define if effective vaccines should induce all or some of these subsets for maximal protection.…”

Mycobacteria-specific CD4+IFN-γ+ cell expresses naïve-surface markers and confers superior protection against tuberculosis infection compared to central and effector memory CD4+ T cell subsets

“…Remarkably, these cells persisted at stable frequencies 25 years after yellow fever vaccination while T EM and T CM decreased over time [16]. Furthermore, CD45RA + CCR7 + CD27 + Mtb -Tetramer + positive cells were recently reported in human subjects with LTBI but not in healthy controls [24]. More than 50% of these tetramer positive cells were CD95 - .…”

“…Finally, the low frequencies of these cells detected in the peripheral blood make it challenging to induce large number of these cells by vaccination. However, long-term proliferative potential of CD4+ T cells after BCG vaccination was correlated with frequencies of BCG-specific sT EM cell like memory cells [24] suggesting that frequencies of these cells generated by a TB vaccine might be utilized as a marker of durable CD4+ T cell responses.…”

“…This is especially relevant as it was shown recently that in mouse models IFN- γ accounts for only ~30% of CD4 T cell-dependent cumulative bacterial control in the lungs and excess IFN-γ production may exacerbate lung pathology [31]. Additionally, we assessed the kinetics of T NLM specific for ESAT-6 and recent data seems to suggest that T SCM specific for Mtb antigen Ag85B secrete primarily IL-2, while BCG and CFP-10 specific cells produced IFN-γ, TNF-α and IL-2 [24]. Although we found ESAT-6-specific T NLM showed superior protection against Mtb compared to conventional memory subsets, it is not known what antigen-specificities of T NLM need to be generated by a TB vaccine to generate optimal, durable CD4+ memory response.…”