The development of T cell memory from naive precursors is influenced by molecular cues received during T cell activation and differentiation. In this study, we describe a novel role for the chemokine receptors CCR5 and CXCR3 in regulating effector CD8+ T cell contraction and memory generation after influenza virus infection. We find that Ccr5−/− Cxcr3−/− cells show markedly decreased contraction after viral clearance, leading to the establishment of massive numbers of memory CD8+ T cells. Ccr5−/− Cxcr3−/− cells show reduced expression of CD69 in the lung during the peak of infection, which coincides with differential localization and the rapid appearance of memory precursor cells. Analysis of single chemokine receptor–deficient cells revealed that CXCR3 is primarily responsible for this phenotype, although there is also a role for CCR5 in the enhancement of T cell memory. The phenotype could be reversed by adding exogenous antigen, resulting in the activation and contraction of Ccr5−/− Cxcr3−/− cells. Similar results were observed during chronic Mycobacterium tuberculosis infection. Together, the data support a model of memory CD8+ T cell generation in which the chemokine-directed localization of T cells within infected tissues regulates antigen encounter and controls the extent of CD8+ T cell activation and differentiation, which ultimately regulates effector versus memory cell fate decisions.

Type 2 inflammation is a defining feature of infection with parasitic worms (helminths), as well as being responsible for widespread suffering in allergies. However, the precise mechanisms involved in T helper (Th) 2 polarization by dendritic cells (DCs) are currently unclear. We have identified a previously unrecognized role for type I IFN (IFN‐I) in enabling this process. An IFN‐I signature was evident in DCs responding to the helminth Schistosoma mansoni or the allergen house dust mite (HDM). Further, IFN‐I signaling was required for optimal DC phenotypic activation in response to helminth antigen (Ag), and efficient migration to, and localization with, T cells in the draining lymph node (dLN). Importantly, DCs generated from Ifnar1 ^−/− mice were incapable of initiating Th2 responses in vivo . These data demonstrate for the first time that the influence of IFN‐I is not limited to antiviral or bacterial settings but also has a central role to play in DC initiation of Th2 responses.

The dendritic cell signals required for the in vivo priming of IL-4–producing T cells are unknown. We used RNA sequencing to characterize DCs from skin LN of mice exposed to two different Th2 stimuli: the helminth parasite Nippostrongylus brasiliensis (Nb) and the contact sensitizer dibutyl phthalate (DBP)-FITC. Both Nb and DBP-FITC induced extensive transcriptional changes that involved multiple DC subsets. Surprisingly, these transcriptional changes were highly distinct in the two models, with only a small number of genes being similarly regulated in both conditions. Pathway analysis of expressed genes identified no shared pathways between Nb and DBP-FITC, but revealed a type-I IFN (IFN-I) signature unique to DCs from Nb-primed mice. Blocking the IFN-I receptor at the time of Nb treatment had little effect on DC migration and antigen transport to the LN, but inhibited the up-regulation of IFN-I–induced markers on DCs and effectively blunted Th2 development. In contrast, the response to DBP-FITC was not affected by IFN-I receptor blockade, a finding consistent with the known dependence of this response on the innate cytokine TSLP. Thus, the priming of Th2 responses is associated with distinct transcriptional signatures in DCs in vivo, reflecting the diverse environments in which Th2 immune responses are initiated.

The immune mechanisms that orchestrate protection against tuberculosis as a result of BCG vaccination are not fully understood. We used the immunomodulatory properties of fingolimod (FTY720) treatment to test whether the lung‐resident memory T lymphocytes generated by BCG vaccination were sufficient to maintain immunity against challenge infection with mycobacteria (BCG). Mice were given daily fingolimod treatment, starting either immediately before s.c. BCG vaccination or during subsequent BCG i.n. challenge, to prevent LN effector and memory lymphocytes from entering the periphery either during priming or challenge, respectively. Treatment with fingolimod during vaccination reduced vaccine‐mediated protection against subsequent infection. By contrast, BCG‐vaccinated mice were protected when fingolimod was given during the infectious challenge, suggesting that memory lymphocytes that migrate to the lung following vaccination are sufficient for protection. Notably, the antigen‐reactive IFN‐γ or multicytokine‐producing CD4⁺ T cells present in the lung when fingolimod was given during BCG challenge did not correlate with protection; however, expression of MHC class II on macrophages isolated from the lungs post BCG challenge was increased in the protected mice. We conclude that protection conferred by BCG vaccination is dependent on memory lymphocytes retained in the lung, although IFN‐γ production by this population is not correlated with vaccine‐mediated protection.

Protective immune responses against respiratory pathogens, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus, are initiated by the mucosal immune system. However, most licensed vaccines are administered parenterally and are largely ineffective at inducing mucosal immunity. The development of safe and effective mucosal vaccines has been hampered by the lack of a suitable mucosal adjuvant. In this study we explore a class of adjuvant that harnesses mucosal-associated invariant T (MAIT) cells. We show evidence that intranasal immunization of MAIT cell agonists co-administered with protein, including the spike receptor binding domain from SARS-CoV-2 virus and hemagglutinin from influenza virus, induce protective humoral immunity and immunoglobulin A production. MAIT cell adjuvant activity is mediated by CD40L-dependent activation of dendritic cells and subsequent priming of T follicular helper cells. In summary, we show that MAIT cells are promising vaccine targets that can be utilized as cellular adjuvants in mucosal vaccines.

Dendritic cells (DC) are critical for the initiation of immune responses; however, their role in priming IL-4–producing Th2 cells in vivo is not fully understood. We used a model of intradermal injection with fluorescent-labeled, nonviable larvae from the helminth parasite nonviable Nippostrongylus brasiliensis L3 larvae (Nb), a strong inducer of Th2 responses, together with IL-4–GFP reporter mice that enable a sensitive detection of IL-4 production to examine the contribution of DC to the priming of IL-4–producing CD4+ T cells in vivo. We found that parasite material is taken up by two distinct DC populations in draining lymph nodes: a mostly CD11cintMHC class II (MHCII)hiCD11b+Ly6C− dermal DC population and a CD11chiMHCIIintCD11b+Ly6C+ monocyte-derived DC population. After Nb treatment, these two DC populations appeared in the draining lymph nodes in comparable numbers and with similar kinetics; however, treatment with pertussis toxin blocked the migration of dermal DC and the priming of IL-4–producing T cells, but only partially affected monocyte-derived DC numbers. In line with this observation, transfer of OVA-loaded CD11cintMHCIIhi DC from Nb-treated mice into naive hosts could sensitize OVA-specific CD4+ T cells to IL-4 production, whereas transfer of CD11cintMHCIIhi DC from naive mice, or CD11chiMHCIIint DC from Nb-treated or naive mice, induced CD4+ T cell expansion but no IL-4 production. Phenotypic analysis of Nb-loaded CD11cintMHCIIhi DC revealed expression of programmed death ligand 2, CD301b, IFN regulatory factor 4, and moderate upregulation of OX40 ligand. However, thymic stromal lymphopoietin and OX40 ligand were not required for Th2 priming. Thus, our data suggest that appropriate stimuli can induce DC to express the unique signals sufficient to direct CD4+ T cells to Th2 differentiation.