Both live attenuated influenza vaccines (LAIV) and inactivated influenza vaccines (IIV) induce protective immunity against influenza. There is evidence that LAIV induces superior protection in children, whereas IIV may induce superior protection in adults. The immune mechanisms responsible for these differences have not been identified. We previously compared LAIV and IIV in young children of 6 to 36 months of age, and we demonstrated that while both induced similar hemagglutination inhibition (HAI) antibody responses, only LAIV induced significant increases in T cell responses. In the present study, 37 healthy adult subjects of 18 to 49 years of age were randomized to receive seasonal influenza vaccination with LAIV or IIV. Influenza virus-specific HAI, T cell, and secretory IgA (sIgA) responses were studied pre- and postvaccination. In contrast to the responses seen in young children, LAIV induced only minimal increases in serum HAI responses in adults, which were significantly lower than the responses induced by IIV. Both LAIV and IIV similarly induced only transient T cell responses to replication-competent whole virus in adults. In contrast, influenza virus-specific sIgA responses were induced more strongly by LAIV than by IIV. Our previous studies suggest that LAIV may be more protective than IIV in young children not previously exposed to influenza virus or influenza vaccines due to increased vaccine-induced T cell and/or sIgA responses. Our current work suggests that in adults with extensive and partially cross-reactive preexisting influenza immunity, LAIV boosting of sIgA responses to hemagglutinin (HA) and non-HA antigenic targets expressed by circulating influenza virus strains may be an important additional mechanism of vaccine-induced immunity.

Mycobacterium tuberculosis infection (Mtb) is the leading cause of death due to a single infectious agent and is among the top ten causes of all human deaths worldwide. CD4 T cells are essential for resistance to Mtb infection, and for decades it has been thought that IFNγ production is the primary mechanism of CD4 T-cell-mediated protection. However, IFNγ responses do not correlate with host protection, and several reports demonstrate that additional anti-tuberculosis CD4 T-cell effector functions remain unaccounted for. Here we show that the tumour-necrosis factor (TNF) superfamily molecule CD153 (encoded by the gene Tnfsf8) is required for control of pulmonary Mtb infection by CD4 T cells. In Mtb-infected mice, CD153 expression is highest on Mtb-specific T helper 1 (T1) cells in the lung tissue parenchyma, but its induction does not require T1 cell polarization. CD153-deficient mice develop high pulmonary bacterial loads and succumb early to Mtb infection. Reconstitution of T-cell-deficient hosts with either Tnfsf8 or Ifng CD4 T cells alone fails to rescue mice from early mortality, but reconstitution with a mixture of Tnfsf8 and Ifng CD4 T cells provides similar protection as wild-type T cells. In Mtb-infected non-human primates, CD153 expression is much higher on Ag-specific CD4 T cells in the airways compared to blood, and the frequency of Mtb-specific CD153-expressing CD4 T cells inversely correlates with bacterial loads in granulomas. In Mtb-infected humans, CD153 defines a subset of highly polyfunctional Mtb-specific CD4 T cells that are much more abundant in individuals with controlled latent Mtb infection compared to those with active tuberculosis. In all three species, Mtb-specific CD8 T cells did not upregulate CD153 following peptide stimulation. Thus, CD153 is a major immune mediator of host protection against pulmonary Mtb infection and CD4 T cells are one important source of this molecule.

Mucosal-associated invariant T cells (MAITs) are positioned in airways and may be important in the pulmonary cellular immune response against infection, particularly prior to priming of peptide-specific T cells. Accordingly, there is interest in the possibility that boosting MAITs through tuberculosis (TB) vaccination may enhance protection, but MAIT responses in the lungs during tuberculosis are poorly understood. In this study, we compared pulmonary MAIT and peptide-specific CD4 T cell responses in-infected rhesus macaques using 5-OP-RU-loaded MR-1 tetramers and intracellular cytokine staining of CD4 T cells following restimulation with an -derived epitope megapool (MTB300), respectively. Two of four animals showed a detectable increase in the number of MAIT cells in airways at later time points following infection, but by ∼3 weeks postexposure, MTB300-specific CD4 T cells arrived in the airways and greatly outnumbered MAITs thereafter. In granulomas, MTB300-specific CD4 T cells were ∼20-fold more abundant than MAITs. CD69 expression on MAITs correlated with tissue residency rather than bacterial loads, and the few MAITs found in granulomas poorly expressed granzyme B and Ki67. Thus, MAIT accumulation in the airways is variable and late, and MAITs display little evidence of activation in granulomas during tuberculosis in rhesus macaques.