Retinoic acid signaling during priming licenses intestinal CD103+ CD8 TRM cell differentiation

CD8 tissue-resident memory T (TRM) cells provide frontline protection at barrier tissues; however, mechanisms regulating TRM cell development are not completely understood. Priming dictates the migration of effector T cells to the tissue, while factors in the tissue induce in situ TRM cell differentiation. Whether priming also regulates in situ TRM cell differentiation uncoupled from migration is unclear. Here, we demonstrate that T cell priming in the mesenteric lymph nodes (MLN) regulates CD103+ TRM cell differentiation in the intestine. In contrast, T cells primed in the spleen were impaired in the ability to differentiate into CD103+ TRM cells after entry into the intestine. MLN priming initiated a CD103+ TRM cell gene signature and licensed rapid CD103+ TRM cell differentiation in response to factors in the intestine. Licensing was regulated by retinoic acid signaling and primarily driven by factors other than CCR9 expression and CCR9-mediated gut homing. Thus, the MLN is specialized to promote intestinal CD103+ CD8 TRM cell development by licensing in situ differentiation.

The accumulation of Vγ4 T cells with aging is associated with an increased adaptive Vγ4 T cell response after foodborne Listeria monocytogenes infection of mice

BACKGROUND

It is generally accepted that aging has detrimental effects on conventional T cell responses to systemic infections. However, most pathogens naturally invade the body through mucosal barriers. Although mucosal sites are highly enriched in unconventional immune sentinels like γδ T cells, little is currently known about the impact of aging on unconventional mucosal T cell responses. We previously established that foodborne infection with a mouse-adapted internalin A mutant Listeria monocytogenes (Lm) generates an adaptive intestinal memory CD44^hi CD27^neg Vγ4 T cells capable of co-producing IL-17A and IFNγ. Therefore, we used this model to evaluate the impact of aging on adaptive Vγ4 T cell responses elicited by foodborne infection.

RESULTS

Foodborne Lm infection of female Balb/c and C57BL/6 mice led to an increased adaptive CD44^hi CD27^neg Vγ4 T cell response associated with aging. Moreover, Lm-elicited CD44^hi CD27^neg Vγ4 T cells maintained diverse functional subsets despite some alterations favoring IL-17A production as mice aged. In contrast to the documented susceptibility of aged mice to intravenous Lm infection, mice contained bacteria after foodborne Lm infection suggesting that elevated bacterial burden was not a major factor driving the increased adaptive CD44^hi CD27^neg Vγ4 T cell response associated with mouse age. However, CD44^hi CD27^neg Vγ4 T cells accumulated in naïve mice as they aged suggesting that an increased precursor frequency contributes to the robust Lm-elicited mucosal response observed. Body mass did not appear to have a strong positive association with CD44^hi CD27^neg Vγ4 T cells within age groups. Although an increased adaptive CD44^hi CD27^neg Vγ4 T cell response may contribute to foodborne Lm resistance of C57BL/6 mice aged 19 or more months, neither anti-TCRδ or anti-IL-17A treatment impacted Lm colonization after primary infection. These results suggest that γδTCR signaling and IL-17A are dispensable for protection after primary foodborne Lm infection consistent with the role of conventional T cells during the early innate immune response to Lm.

CONCLUSIONS

Lm-elicited adaptive Vγ4 T cells appear resistant to immunosenescence and memory Vγ4 T cells could be utilized to provide protective immune functions during enteric infection of aged hosts. As such, oral immunization might offer an efficient therapeutic approach to generate unconventional memory T cells in the elderly.

A blend of broadly-reactive and pathogen-selected Vγ4 Vδ1 T cell receptors confer broad bacterial reactivity of resident memory γδ T cells

Although murine γδ T cells are largely considered innate immune cells, they have recently been reported to form long-lived memory populations. Much remains unknown about the biology and specificity of memory γδ T cells. Here, we interrogated intestinal memory Vγ4 Vδ1 T cells generated after foodborne Listeria monocytogenes (Lm) infection to uncover an unanticipated complexity in the specificity of these cells. Deep TCR sequencing revealed that a subset of non-canonical Vδ1 clones are selected by Lm infection, consistent with antigen-specific clonal expansion. Ex vivo stimulations and in vivo heterologous challenge infections with diverse pathogenic bacteria revealed that Lm-elicited memory Vγ4 Vδ1 T cells are broadly reactive. The Vγ4 Vδ1 T cell recall response to Lm, Salmonella enterica serovar Typhimurium (STm) and Citrobacter rodentium was largely mediated by the γδTCR as internalizing the γδTCR prevented T cell expansion. Both broadly-reactive canonical and pathogen-selected non-canonical Vδ1 clones contributed to memory responses to Lm and STm. Interestingly, some non-canonical γδ T cell clones selected by Lm infection also responded after STm infection, suggesting some level of cross-reactivity. These findings underscore the promiscuous nature of memory γδ T cells and suggest that pathogen-elicited memory γδ T cells are potential targets for broad-spectrum anti-infective vaccines.

A blend of broadly-reactive and pathogen-selected Vγ4Vγδ1 T cell receptors confer broad bacterial reactivity of resident memory γδ T cells

γδ T cells are pleiotropic sentinels involved in immune defense at barrier tissues. While most γδ T cell subsets are associated with innate-like responses, foodborne Listeria monocytogenes (Lm) infection elicits a substantial resident memory Vγ4Vδ1 T cell response that appears specific to the priming pathogen despite limited T cell receptor (TCR) diversity. As our understanding of memory γδ T cells remains elusive, memory γδ T cells were examined using heterologous challenge infections coupled with TCRδ sequencing to reveal an unanticipated complexity in this memory compartment. CDR3δ sequence analysis revealed that a subset of non-canonical Vδ1 clones expanded during primary and secondary Lm infection but contracted at memory, consistent with antigen-specific clonal expansion. Additionally, canonical clones comprised most of the response, suggesting that some cells may not be Lm-restricted. Indeed, ex vivo stimulation of Lm-elicited memory γδ T cells with Salmonella enterica serovar typhimurium (STm), Yersinia pseudotuberculosis, and Citrobacter rodentium (Cr) induced a robust multifunctional response. Furthermore, heterologous STm-induced colitis infection and intraperitoneal Cr infection of Lm-immune mice led to the TCR-dependent recall of memory γδ T cells, demonstrating their broad reactivity in vivo. Interestingly, most of the non-canonical Vγ4Vδ1 T cell clones that expanded in an antigen-specific manner after Lm infection did not respond after STm infection, suggesting some level of specificity. These findings underscore the complex and promiscuous nature of memory γδ T cells and suggest that pathogen-elicited memory γδ T cells are potential targets for broad-spectrum anti-infective vaccines.

Mesenteric lymph node priming licenses intestinal CD103+ CD8 tissue-resident memory T cell development

CD8 tissue-resident memory T (TRM) cells provide front-line protective immunity at barrier tissues; however, mechanisms regulating their development are not completely understood. Priming dictates the migration of effector CD8 T cells to the tissue, while factors in the tissue induce in situ TRM cell differentiation. Whether priming also regulates in situ differentiation of CD8 TRM cells has not been addressed. Here, we demonstrate that T cell priming in the mesenteric lymph nodes (MLN) was the principal determinant of CD103+ CD8 TRM cell differentiation in the intestine. In contrast, CD8 T cells primed in the spleen were inefficient at differentiating into intestinal CD103+ TRM cells. Moreover, both CD103− and CD103+ naïve T cells were highly efficient in differentiating into CD103+ TRM cells in the intestine after priming in the MLN, suggesting preconditioning of naïve T cells by TGF-b during homeostasis had little impact on intestinal CD103+ TRM cell differentiation. We further demonstrate that MLN priming initiated CD103+ CD8 TRM cell signature gene expression and licensed rapid CD103+ CD8 TRM precursor cell differentiation in response to factors in the tissue by providing retinoic acid signaling to CD8 T cells. Thus, MLN priming is specialized to license intestinal CD103+ CD8 TRM cell development.

Distinct memory γδ T cell subsets are fine-tuned by TCR engagement and soluble signals to shape IFNγ and IL-17A responses

Unconventional T cells are essential for tissue homeostasis and protection against invading pathogens at barrier sites. γδ T cells are enriched at these interfaces where they can form memory populations that can be either beneficial or detrimental depending on the functional response elicited. However, our understanding of the factors regulating memory γδ T cell generation, activation and function remains limited. Foodborne Listeria monocytogenes (Lm) infection of Batf3−/− mice revealed a critical role for Batf3-dependent dendritic cells in the generation and function of adaptive γδ T cells. In line with this observation, IFNγ and IL-17A production and IFNγ/IL-17A co-production by memory γδ T cells were induced by heat-killed Lm after 24 hours in culture, and these responses were inhibited after MHC-II or CD11c depletion. The induction of cytokine production was tightly associated with Zap70/Syk phosphorylation, indicative of TCR activation. Although IL-12 is usually associated with type 1 responses, neutralization of IL-12/23p40 dramatically impaired IL-17A production whereas IFNγ response was only marginally affected. Surprisingly, classical TH17 polarizing cytokines had little impact on IL-17A production. Despite this conventional-like response, memory γδ T cells remained able to quickly produce IL-17A in response to supernatant or live Lm independently of TCR signaling. Thus, adaptive γδ T cells are a functionally diverse population that encompasses innate- and adaptive-like responses that are fine-tuned by the γδTCR and soluble signals. These insights may facilitate the development of new strategies eliciting specific functional subsets of memory γδ T cells tailored to various intestinal disease settings.