Age-associated decreases in primary CD8+ T cell responses occur, in part, due to direct effects on naive CD8+ T cells to reduce intrinsic functionality, but the precise nature of this defect remains undefined. Aging also causes accumulation of antigen-naive but semi-differentiated "virtual memory" (TVM) cells, but their contribution to age-related functional decline is unclear. Here, we show that TVM cells are poorly proliferative in aged mice and humans, despite being highly proliferative in young individuals, while conventional naive T cells (TN cells) retain proliferative capacity in both aged mice and humans. Adoptive transfer experiments in mice illustrated that naive CD8 T cells can acquire a proliferative defect imposed by the aged environment but age-related proliferative dysfunction could not be rescued by a young environment. Molecular analyses demonstrate that aged TVM cells exhibit a profile consistent with senescence, marking an observation of senescence in an antigenically naive T cell population.

Significance: Mucosal-associated invariant T (MAIT) cells are innate-like T lymphocytes with potent antibacterial reactivity. In this study, we investigated whether MAIT cells also contribute to immunity against influenza A viruses. Compared with those who succumbed, hospitalized patients who recovered from severe avian H7N9 influenza infection had higher numbers of MAIT cells. Subsequent in vitro analysis established that MAIT cells from healthy donors are indirectly activated by influenza infection via an IL-18–dependent (but not IL-12–dependent) mechanism requiring the involvement of CD14 ⁺ monocytes. Our findings highlight the potential for MAIT cells to promote protective immunity in human influenza.

The avian origin A/H7N9 influenza virus causes high admission rates (>99%) and mortality (>30%), with ultimately favourable outcomes ranging from rapid recovery to prolonged hospitalization. Using a multicolour assay for monitoring adaptive and innate immunity, here we dissect the kinetic emergence of different effector mechanisms across the spectrum of H7N9 disease and recovery. We find that a diversity of response mechanisms contribute to resolution and survival. Patients discharged within 2–3 weeks have early prominent H7N9-specific CD8⁺ T-cell responses, while individuals with prolonged hospital stays have late recruitment of CD8⁺/CD4⁺ T cells and antibodies simultaneously (recovery by week 4), augmented even later by prominent NK cell responses (recovery >30 days). In contrast, those who succumbed have minimal influenza-specific immunity and little evidence of T-cell activation. Our study illustrates the importance of robust CD8⁺ T-cell memory for protection against severe influenza disease caused by newly emerging influenza A viruses.

Immunization with the inactivated influenza vaccine (IIV) remains the most effective strategy to combat seasonal influenza infections. IIV activates B cells and T follicular helper (T_FH) cells and thus engenders antibody-secreting cells and serum antibody titers. However, the cellular events preceding generation of protective immunity in humans are inadequately understood. We undertook an in-depth analysis of B cell and T cell immune responses to IIV in 35 healthy adults. Using recombinant hemagglutinin (rHA) probes to dissect the quantity, phenotype, and isotype of influenza-specific B cells against A/California09-H1N1, A/Switzerland-H3N2, and B/Phuket, we showed that vaccination induced a three-pronged B cell response comprising a transient CXCR5⁻CXCR3⁺ antibody-secreting B cell population, CD21^hiCD27⁺ memory B cells, and CD21^loCD27⁺ B cells. Activation of circulating T_FH cells correlated with the development of both CD21^lo and CD21^hi memory B cells. However, preexisting antibodies could limit increases in serum antibody titers. IIV had no marked effect on CD8⁺, mucosal-associated invariant T, γδ T, and natural killer cell activation. In addition, vaccine-induced B cells were not maintained in peripheral blood at 1 year after vaccination. We provide a dissection of rHA-specific B cells across seven human tissue compartments, showing that influenza-specific memory (CD21^hiCD27⁺) B cells primarily reside within secondary lymphoid tissues and the lungs. Our study suggests that a rational design of universal vaccines needs to consider circulating T_FH cells, preexisting serological memory, and tissue compartmentalization for effective B cell immunity, as well as to improve targeting cellular T cell immunity.

Innate-like, evolutionarily conserved MR1-restricted mucosa-associated invariant T (MAIT) cells represent a large antimicrobial T-cell subset in humans. Here, we investigate the development of these cells in second trimester human fetal tissues. MAIT cells are rare and immature in the fetal thymus, spleen and mesenteric lymph nodes. In contrast, mature IL-18Rα(+) CD8αα MAIT cells are enriched in the fetal small intestine, liver and lung. Independently of localization, MAIT cells express CD127 and Ki67 in vivo and readily proliferate in response to Escherichia coli in vitro. Maturation is accompanied by the gradual post-thymic acquisition of the PLZF transcription factor and the ability to produce IFNγ and IL-22 in response to bacteria in mucosa. Thus, MAIT cells acquire innate-like antimicrobial responsiveness in mucosa before exposure to environmental microbes and the commensal microflora. Establishment of this arm of immunity before birth may help protect the newborn from a range of pathogenic microbes.