Progress towards a safe and effective vaccine for the prevention of tularemia has been hindered by a lack of knowledge regarding the correlates of protective adaptive immunity and a lack of tools to generate this knowledge. CD8+ T cells are essential for protective immunity against virulent strains of Francisella tularensis, but to-date, it has not been possible to study these cells in an antigen-specific manner. Here, we report the development of a tool for expression of the model antigen ovalbumin (OVA) in F. tularensis, which allows for the study of CD8+ T cell responses to the bacterium. We demonstrate that in response to intranasal infection with the F. tularensis Live Vaccine Strain, adoptively transferred OVA-specific CD8+ T cells expand after the first week and produce IFN-γ but not IL-17. Effector and central memory subsets develop with disparate kinetics in the lungs, draining lymph node and spleen. Notably, OVA-specific cells are poorly retained in the lungs after clearance of infection. We also show that intranasal vaccination leads to more antigen-specific CD8+ T cells in the lung-draining lymph node compared to scarification vaccination, but that an intranasal booster overcomes this difference. Together, our data show that this novel tool can be used to study multiple aspects of the CD8+ T cell response to F. tularensis. Use of this tool will enhance our understanding of immunity to this deadly pathogen.

Hydrogels generally have broad utilization in healthcare due to their tunable structures, high water content, and inherent biocompatibility. FDA-approved applications of hydrogels include spinal cord regeneration, skin fillers, and local therapeutic delivery. Drawbacks exist in the clinical hydrogel space, largely pertaining to inconsistent therapeutic exposure, short-lived release windows, and difficulties inserting the polymer into tissue. In this study, we engineered injectable, biocompatible hydrogels that function as a local protein therapeutic depot with a high degree of user-customizability. We showcase a PEG-based hydrogel functionalized with bioorthogonal strain-promoted azide-alkyne cycloaddition (SPAAC) handles for its polymerization and functionalization with a variety of payloads. Small-molecule and protein cargos, including chemokines and antibodies, were site-specifically modified with hydrolysable "azidoesters" of varying hydrophobicity via direct chemical conjugation or sortase-mediated transpeptidation. These hydrolysable esters afforded extended release of payloads linked to our hydrogels beyond diffusion; with timescales spanning days to months dependent on ester hydrophobicity. Injected hydrogels polymerize in situ and remain in tissue over extended periods of time. Hydrogel-delivered protein payloads elicit biological activity after being modified with SPAAC-compatible linkers, as demonstrated by the successful recruitment of murine T-cells to a mouse melanoma model by hydrolytically released murine CXCL10. These results highlight a highly versatile, customizable hydrogel-based delivery system for local delivery of protein therapeutics with payload release profiles appropriate for a variety of clinical needs.

MicroRNAs constitute a major post-transcriptional mechanism for controlling protein expression, and are emerging as key regulators during T cell development and function. Recent reports of augmented CD8 T cell activation and effector differentiation, and aberrant migratory properties upon ablation of Dicer/miRNAs in naïve cells have established a regulatory role of miRNAs during priming. Whether miRNAs continue to exert similar functions or are dispensable during later stages of CD8 T cell expansion and memory differentiation remains unclear. Here, we report a critical role of Dicer/miRNAs in regulating the balance of long-lived memory and short-lived terminal effector fates during the post-priming stages when CD8 T cells undergo clonal expansion to generate a large cytotoxic T lymphocyte (CTL) pool and subsequently differentiate into a quiescent memory state. Conditional ablation of Dicer/miRNAs in early effector CD8 T cells following optimal activation and expression of granzyme B, using unique dicerfl/fl gzmb-cre mice, led to a strikingly diminished peak effector size relative to wild-type antigen-specific cells in the same infectious milieu. Diminished expansion of Dicer-ablated CD8 T cells was associated with lack of sustained antigen-driven proliferation and reduced accumulation of short-lived effector cells. Additionally, Dicer-ablated CD8 T cells exhibited more pronounced contraction after pathogen clearance and comprised a significantly smaller proportion of the memory pool, despite significantly higher proportions of CD127Hi memory precursors at the effector peak. Combined with previous reports of dynamic changes in miRNA expression as CD8 T cells differentiate from naïve to effector and memory states, these findings support distinct stage-specific roles of miRNA-dependent gene regulation during CD8 T cell differentiation.

Chronic viral infections often result in T cell exhaustion. To determine the molecular signature of exhaustion, we compared the gene-expression profiles of dysfunctional lymphocytic choriomeningitis virus (LCMV)-specific CD8(+) T cells from chronic infection to functional LCMV-specific effector and memory CD8(+) T cells generated after acute infection. These data showed that exhausted CD8(+) T cells: (1) overexpressed several inhibitory receptors, including PD-1, (2) had major changes in T cell receptor and cytokine signaling pathways, (3) displayed altered expression of genes involved in chemotaxis, adhesion, and migration, (4) expressed a distinct set of transcription factors, and (5) had profound metabolic and bioenergetic deficiencies. T cell exhaustion was progressive, and gene-expression profiling indicated that T cell exhaustion and anergy were distinct processes. Thus, functional exhaustion is probably due to both active suppression and passive defects in signaling and metabolism. These results provide a framework for designing rational immunotherapies during chronic infections.

Early after priming, effector CD8 T cells are distinguished into memory precursor and short-lived effector cell subsets (MPECs and SLECs). Here, we delineated a distinct in vivo heterogeneity in killer cell lectin-like receptor G1 (KLRG-1) expression, which was strongly associated with diverse MPEC and SLEC fates. These in vivo MPECs and SLECs expressed equivalent levels of cytotoxic molecules and effector cytokines. Using a unique in vivo degranulation assay, we found that the MPECs and SLECs similarly encountered infected target cells and elaborated equivalent levels of cytotoxicity in vivo. These data provide direct in vivo evidence that memory-fated cells pass through a robust effector phase. Additionally, the preferential localization of the MPECs in the lymph nodes, where a lesser degree of cytotoxicity was elaborated, suggests that the MPECs may be protected from excessive stimulation and terminal differentiation by virtue of their differential tissue localization. These data provide novel mechanistic insights into the linear decreasing potential model of memory differentiation.

An important question in memory development is understanding the differences between effector CD8 T cells that die versus effector cells that survive and give rise to memory cells. In this study, we provide a comprehensive phenotypic, functional, and genomic profiling of terminal effectors and memory precursors. Using killer cell lectin-like receptor G1 as a marker to distinguish these effector subsets, we found that despite their diverse cell fates, both subsets possessed remarkably similar gene expression profiles and functioned as equally potent killer cells. However, only the memory precursors were capable of making interleukin (IL) 2, thus defining a novel effector cell that was cytotoxic, expressed granzyme B, and produced inflammatory cytokines in addition to IL-2. This effector population then differentiated into long-lived protective memory T cells capable of self-renewal and rapid recall responses. Experiments to understand the signals that regulate the generation of terminal effectors versus memory precursors showed that cells that continued to receive antigenic stimulation during the later stages of infection were more likely to become terminal effectors. Importantly, curtailing antigenic stimulation toward the tail end of the acute infection enhanced the generation of memory cells. These studies support the decreasing potential model of memory differentiation and show that the duration of antigenic stimulation is a critical regulator of memory formation.

Type-I interferon (IFN-I) signals exert a critical role in disease progression during viral infections. However, the immunomodulatory mechanisms by which IFN-I dictates disease outcomes remain to be fully defined. Here we report that IFN-I signals mediate thymic atrophy in viral infections, with more severe and prolonged loss of thymic output and unique kinetics and subtypes of IFN-α/β expression in chronic infection compared to acute infection. Loss of thymic output was linked to inhibition of early stages of thymopoiesis (DN1-DN2 transition, and DN3 proliferation) and pronounced apoptosis during the late DP stage. Notably, infection-associated thymic defects were largely abrogated upon ablation of IFNαβR and partially mitigated in the absence of CD8 T cells, thus implicating direct as well as indirect effects of IFN-I on thymocytes. These findings provide mechanistic underpinnings for immunotherapeutic strategies targeting IFN-1 signals to manipulate disease outcomes during chronic infections and cancers.

Differential interleukin-2 (IL-2) signaling and production are associated with disparate effector and memory fates. Whether the IL-2 signals perceived by CD8 T cells come from autocrine or paracrine sources, the timing of IL-2 signaling and their differential impact on CD8 T cell responses remain unclear. Using distinct models of germline and conditional IL-2 ablation in post-thymic CD8 T cells, this study shows that paracrine IL-2 is sufficient to drive optimal primary expansion, effector and memory differentiation, and metabolic function. In contrast, autocrine IL-2 is uniquely required during primary expansion to program robust secondary expansion potential in memory-fated cells. This study further shows that IL-2 production by antigen-specific CD8 T cells is largely independent of CD4 licensing of dendritic cells (DCs) in inflammatory infections with robust DC activation. These findings bear implications for immunizations and adoptive T cell immunotherapies, where effector and memory functions may be commandeered through IL-2 programming.

Subacute necrotizing encephalopathy, or Leigh syndrome (LS), is the most common pediatric presentation of genetic mitochondrial disease. LS is a multi-system disorder with severe neurologic, metabolic, and musculoskeletal symptoms. The presence of progressive, symmetric, and necrotizing lesions in the brainstem are a defining feature of the disease, and the major cause of morbidity and mortality, but the mechanisms underlying their pathogenesis have been elusive. Recently, we demonstrated that high-dose pexidartinib, a CSF1R inhibitor, prevents LS CNS lesions and systemic disease in the Ndufs4(-/-) mouse model of LS. While the dose-response in this study implicated peripheral immune cells, the immune populations involved have not yet been elucidated. Here, we used a targeted genetic tool, deletion of the colony-stimulating Factor 1 receptor (CSF1R) macrophage super-enhancer FIRE (Csf1rΔFIRE), to specifically deplete microglia and define the role of microglia in the pathogenesis of LS. Homozygosity for the Csf1rΔFIRE allele ablates microglia in both control and Ndufs4(-/-) animals, but onset of CNS lesions and sequalae in the Ndufs4(-/-), including mortality, are only marginally impacted by microglia depletion. The overall development of necrotizing CNS lesions is not altered, though microglia remain absent. Finally, histologic analysis of brainstem lesions provides direct evidence of a causal role for peripheral macrophages in the characteristic CNS lesions. These data demonstrate that peripheral macrophages play a key role in the pathogenesis of disease in the Ndufs4(-/-) model.

Robust T cell responses are crucial for effective anti-tumor responses and often dictate patient survival. However, in the context of solid tumors, both endogenous T cell responses and current adoptive T cell therapies are impeded by the immunosuppressive tumor microenvironment (TME). A multitude of inhibitory signals, suppressive immune cells, metabolites, hypoxic conditions and limiting nutrients are believed to render the TME non-conducive to sustaining productive T cell responses. In this study we conducted an in-depth phenotypic and functional comparison of tumor-specific T cells and tumor-nonspecific bystander memory T cells within the same TME. Using two distinct TCR transgenic and solid-tumor models, our data demonstrate that despite exposure to the same cell-extrinsic factors of the TME, the tumor-nonspecific bystander CD8 T cells retain the complete panoply of memory markers, and do not share the same exhaustive phenotype as tumor-reactive T cells. Compared to tumor-specific T cells, bystander memory CD8 T cells in the TME also retain functional effector cytokine production capabilities in response to ex vivo cognate antigenic stimulation. Consistent with these results, bystander memory T cells isolated from tumors showed enhanced recall responses to secondary bacterial challenge in a T cell transplant model. Importantly, the tumor-resident bystander memory cells could also efficiently utilize the available resources within the TME to elaborate in situ recall effector functions following intra-tumoral peptide antigen injection. Additionally, CRISPR-Cas9 gene deletion studies showed that CXCR3 was critical for the trafficking of both tumor antigen-specific and bystander memory T cells to solid tumors. Collectively, these findings that T cells can persist and retain their functionality in distinct solid tumor environments in the absence of cognate antigenic stimulation, support the notion that persistent antigenic signaling is the central driver of T cell exhaustion within the TME. These studies bear implications for programming more efficacious TCR- and CAR-T cells with augmented therapeutic efficacy and longevity through regulation of antigen and chemokine receptors.