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

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⁺ T_SCM and to characterize their functional ontology.

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 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⁺ 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.

M. tb-specific T_SCM were not detected in QFT-negative persons. After QFT conversion frequencies of T_SCM increased to measurable levels and remained detectable thereafter, suggesting that primary M. tb infection induces T_SCM cells. Gene expression (GE) profiling of tetramer⁺ T_SCM showed that these cells were distinct from bulk CD4⁺ naïve T cells (T_N) and shared features of bulk T_SCM and M. tb-specific tetramer⁺ T_CM and T_EFF cells. These T_SCM were predominantly CD95⁺ and CXCR3⁺, markers typical of CD8⁺ T_SCM. Tetramer⁺ T_SCM expressed significantly higher protein levels of CCR5, CCR6, CXCR3, granzyme A, granzyme K, and granulysin than bulk T_N and T_SCM cells. M. tb-specific T_SCM 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.

Human infection with M. tb induced distinct, antigen-specific CD4⁺ T_SCM 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⁺ T_SCM should be considered for vaccination approaches that aim to generate long-lived memory T cells against M. tb.