Lymphatic drainage from peripheral tissues to the lymph nodes carries tissue-specific information in the form of passively transported products and active transport by dendritic cells. In turn, this shapes the quantity and quality of the immune response in a tissue-specific manner. In addition, Ataide et al. now describe a role for lymphatic migration of unconventional T cells (UTCs) — γδ T cells, mucosal-associated invariant T (MAIT) cells and natural killer T (NKT) cells — in this process.

When single-cell suspensions from skin-draining, mediastinal and mesenteric lymph nodes were globally activated, the production of signature cytokines differed between lymph node sites. For example, skin-draining lymph nodes (sdLNs) produced high levels of IL-17 and IFNγ, whereas mediastinal lymph nodes (medLNs) produced high levels of IL-17 and IL-4. UTCs were identified as the source of these cytokines.

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Consistent with these in vitro data, vaccinia virus infection of mice by intranasal or subcutaneous routes resulted in higher levels of IL-4 production by NKT cells in medLNs compared with sdLNs, which was required for the generation of IgG1-producing B cells after pulmonary vaccinia virus infection. In a model of Staphylococcus aureus infection of skin or lungs, the γδ T cell population was expanded in sdLNs but this was not observed in medLNs even after repeat infection or simultaneous skin infection. This suggests that these UTCs do not recirculate between lymph nodes, which contrasts with the previous distinction between tissue-resident UTCs and circulating UTCs in blood. Indeed, ~30% of γδ T cells in sdLNs and medLNs did not express CD62L, which is required for recirculation, and the expansion of γδ T cells in sdLNs continued after CD62L blockade. Thus, unique populations of UTCs in lymph nodes at distinct sites generate distinct humoral and cellular immune responses.

Single-cell RNA-sequencing (scRNA-seq) of UTCs from different lymph nodes showed that CD62L+ recirculating UTCs were similar between sites, whereas CD62L non-recirculating UTCs differed in terms of lineage composition and transcriptional landscape. For example, CD62L TH17-like UTCs had transcriptional differences between sdLNs and medLNs and were largely absent from mesenteric lymph nodes (mesLNs). Each of the scRNA-seq transcriptional clusters consisted of all three UTC lineages — γδ T cells, MAIT cells and NKT cells — which suggests that functional categorization of UTCs might be as important as their categorization based on T cell receptor sequence and MHC restriction.

Having shown that the composition of UTCs in the tissue (lungs, gut or skin) mirrors that in the draining lymph nodes, the authors proposed that UTCs seed the lymph nodes from peripheral tissues via the lymph. This was confirmed by lymph node transplantation, as well as using photoconvertible mice in which photoswitched CD62L UTCs could be found in draining lymph nodes 24 hours after skin exposure.

In mice lacking a particular UTC lineage, the same number and transcriptional clusters of UTCs were found in different lymph nodes. Regardless of lineage, all TH17-like UTCs were shown to have similar intranodal migration and localization after infection with S. aureus; all three lineages contributed to IL-17 production; and the total number of IL-17-producing UTCs remained the same in mice lacking one of the lineages. Similar results were shown for IFNγ-producing UTCs.

“lymph node UTCs of different lineage operate as connected functional units, sharing transcriptional identity and cytokine output according to the draining tissue”

Thus, lymph node UTCs of different lineage operate as connected functional units, sharing transcriptional identity and cytokine output according to the draining tissue and having numerical and functional compensation between lineages. The effect of these tissue-specific UTCs on infection control was shown by comparing subcutaneous and intraperitoneal infection with S. aureus; IL-17-dependent recruitment of neutrophils to sdLNs was not observed in mesLNs, which lack TH17-like UTCs. In line with the idea of functional units, bacterial loads were unaltered in sdLNs from mice lacking a single UTC lineage.