Engagement of the costimulatory molecule ICOS in tissues promotes establishment of CD8+ tissue-resident memory T cells

Elevated gene expression of the costimulatory receptor Icos is a hallmark of CD8⁺ tissue-resident memory (Trm) T cells. Here, we examined the contribution of ICOS in Trm cell differentiation. Upon transfer into WT mice, Icos^-/- CD8⁺ T cells exhibited defective Trm generation but produced recirculating memory populations normally. ICOS deficiency or ICOS-L blockade compromised establishment of CD8⁺ Trm cells but not their maintenance. ICOS ligation during CD8⁺ T cell priming did not determine Trm induction; rather, effector CD8⁺ T cells showed reduced Trm differentiation after seeding into Icosl^-/- mice. Icos^YF/YF CD8⁺ T cells were compromised in Trm generation, indicating a critical role for PI3K signaling. Modest transcriptional changes in the few Icos^-/- Trm cells suggest that ICOS-PI3K signaling primarily enhances the efficiency of CD8⁺ T cell tissue residency. Thus, local ICOS signaling promotes production of Trm cells, providing insight into the contribution of costimulatory signals in the generation of tissue-resident populations.

Arthritis flares mediated by tissue-resident memory T cells in the joint

Rheumatoid arthritis is a systemic autoimmune disease, but disease flares typically affect only a subset of joints, distributed in a distinctive pattern for each patient. Pursuing this intriguing pattern, we show that arthritis recurrence is mediated by long-lived synovial resident memory T cells (T_RM). In three murine models, CD8+ cells bearing T_RM markers remain in previously inflamed joints during remission. These cells are bona fide T_RM, exhibiting a failure to migrate between joints, preferential uptake of fatty acids, and long-term residency. Disease flares result from T_RM activation by antigen, leading to CCL5-mediated recruitment of circulating effector cells. Correspondingly, T_RM depletion ameliorates recurrence in a site-specific manner. Human rheumatoid arthritis joint tissues contain a comparable CD8+-predominant T_RM population, which is most evident in late-stage leukocyte-poor synovium, exhibiting limited T cell receptor diversity and a pro-inflammatory transcriptomic signature. Together, these findings establish synovial T_RM as a targetable mediator of disease chronicity in autoimmune arthritis.

Resident memory T cells in tumor-distant tissues fortify against metastasis formation

As a critical machinery for rapid pathogen removal, resident memory T cells (T_RMs) are locally generated after the initial encounter. However, their development accompanying tumorigenesis remains elusive. Using a murine breast cancer model, we show that T_RMs develop in the tumor, the contralateral mammary mucosa, and the pre-metastatic lung. Single-cell RNA sequencing of T_RMs reveals two phenotypically distinct populations representing their active versus quiescent phases. These T_RMs in different tissue compartments share the same TCR clonotypes and transcriptomes with a subset of intratumoral effector/effector memory T cells (T_Eff/EMs), indicating their developmental ontogeny. Furthermore, CXCL16 is highly produced by tumor cells and CXCR6^- T_Eff/EMs are the major subset preferentially egressing the tumor to form distant T_RMs. Functionally, releasing CXCR6 retention in the primary tumor amplifies tumor-derived T_RMs in the lung and leads to superior protection against metastases. This immunologic fortification suggests a potential strategy to prevent metastasis in clinical oncology.

Interstitial Migration of CD8αβ T Cells in the Small Intestine Is Dynamic and Is Dictated by Environmental Cues

The migratory capacity of adaptive CD8αβ T cells dictates their ability to locate target cells and exert cytotoxicity, which is the basis of immune surveillance for the containment of microbes and disease. The small intestine (SI) is the largest mucosal surface and is a primary site of pathogen entrance. Using two-photon laser scanning microscopy, we found that motility of antigen (Ag)-specific CD8αβ T cells in the SI is dynamic and varies with the environmental milieu. Pathogen-specific CD8αβ T cell movement differed throughout infection, becoming locally confined at memory. Motility was not dependent on CD103 but was influenced by micro-anatomical locations within the SI and by inflammation. CD8 T cells responding to self-protein were initially affected by the presence of self-Ag, but this was altered after complete tolerance induction. These studies identify multiple factors that affect CD8αβ T cell movement in the intestinal mucosa and show the adaptability of CD8αβ T cell motility.

T Cells in Nonlymphoid Tissues Give Rise to Lymph-Node-Resident Memory T Cells

Immunosurveillance of secondary lymphoid organs (SLO) is performed by central memory T cells that recirculate through blood. Resident memory T (Trm) cells remain parked in nonlymphoid tissues and often stably express CD69. We recently identified Trm cells within SLO, but the origin and phenotype of these cells remains unclear. Using parabiosis of "dirty" mice, we found that CD69 expression is insufficient to infer stable residence of SLO Trm cells. Restimulation of nonlymphoid memory CD8+ T cells within the skin or mucosa resulted in a substantial increase in bona fide Trm cells specifically within draining lymph nodes. SLO Trm cells derived from emigrants from nonlymphoid tissues and shared some transcriptional and phenotypic signatures associated with nonlymphoid Trm cells. These data indicate that nonlymphoid cells can give rise to SLO Trm cells and suggest vaccination strategies by which memory CD8+ T cell immunosurveillance can be regionalized to specific lymph nodes.

White Adipose Tissue Is a Reservoir for Memory T Cells and Promotes Protective Memory Responses to Infection

White adipose tissue bridges body organs and plays a fundamental role in host metabolism. To what extent adipose tissue also contributes to immune surveillance and long-term protective defense remains largely unknown. Here, we have shown that at steady state, white adipose tissue contained abundant memory lymphocyte populations. After infection, white adipose tissue accumulated large numbers of pathogen-specific memory T cells, including tissue-resident cells. Memory T cells in white adipose tissue expressed a distinct metabolic profile, and white adipose tissue from previously infected mice was sufficient to protect uninfected mice from lethal pathogen challenge. Induction of recall responses within white adipose tissue was associated with the collapse of lipid metabolism in favor of antimicrobial responses. Our results suggest that white adipose tissue represents a memory T cell reservoir that provides potent and rapid effector memory responses, positioning this compartment as a potential major contributor to immunological memory.

Sendai Virus Mucosal Vaccination Establishes Lung-Resident Memory CD8 T Cell Immunity and Boosts BCG-Primed Protection against TB in Mice

Accumulating evidence has shown the protective role of CD8⁺ T cells in vaccine-induced immunity against Mycobacterium tuberculosis (Mtb) despite controversy over their role in natural immunity. However, the current vaccine BCG is unable to induce sufficient CD8⁺ T cell responses, especially in the lung. Sendai virus, a respiratory RNA virus, is here engineered firstly as a novel recombinant anti-TB vaccine (SeV85AB) that encodes Mtb immuno-dominant antigens, Ag85A and Ag85B. A single mucosal vaccination elicited potent antigen-specific T cell responses and a degree of protection against Mtb challenge similar to the effect of BCG in mice. Depletion of CD8⁺ T cells abrogated the protective immunity afforded by SeV85AB vaccination. Interestingly, only SeV85AB vaccination induced high levels of lung-resident memory CD8⁺ T (T_RM) cells, and this led to a rapid and strong recall of antigen-specific CD8⁺ T cell responses against Mtb challenge infection. Furthermore, when used in a BCG prime-SeV85AB boost strategy, SeV85AB vaccine significantly enhanced protection above that seen after BCG vaccination alone. Our findings suggest that CD8⁺ T_RM cells that arise in lungs responding to this mucosal vaccination might help to protect against TB, and SeV85AB holds notable promise to improve BCG’s protective efficacy in a prime-boost immunization regimen.