CD4+ T cell immunity against cutaneous melanoma encompasses multifaceted MHC II-dependent responses

Whereas CD4+ T cells conventionally mediate antitumor immunity by providing help to CD8+ T cells, recent clinical studies have implied an important role for cytotoxic CD4+ T cells in cancer immunity. Using an orthotopic melanoma model, we provide a detailed account of antitumoral CD4+ T cell responses and their regulation by major histocompatibility complex class II (MHC II) in the skin. Intravital imaging revealed prominent interactions of CD4+ T cells with tumor debris-laden MHC II+ host antigen-presenting cells that accumulated around tumor cell nests, although direct recognition of MHC II+ melanoma cells alone could also promote CD4+ T cell control. CD4+ T cells stably suppressed or eradicated tumors even in the absence of other lymphocytes by using tumor necrosis factor-α and Fas ligand (FasL) but not perforin-mediated cytotoxicity. Interferon-γ was critical for protection, acting both directly on melanoma cells and via induction of nitric oxide synthase in myeloid cells. Our results illustrate multifaceted and context-specific aspects of MHC II-dependent CD4+ T cell immunity against cutaneous melanoma, emphasizing modulation of this axis as a potential avenue for immunotherapies.

CD4+ T cell calibration of antigen-presenting cells optimizes antiviral CD8+ T cell immunity

Antiviral CD8+ T cell immunity depends on the integration of various contextual cues, but how antigen-presenting cells (APCs) consolidate these signals for decoding by T cells remains unclear. Here, we describe gradual interferon-α/interferon-β (IFNα/β)-induced transcriptional adaptations that endow APCs with the capacity to rapidly activate the transcriptional regulators p65, IRF1 and FOS after CD4+ T cell-mediated CD40 stimulation. While these responses operate through broadly used signaling components, they induce a unique set of co-stimulatory molecules and soluble mediators that cannot be elicited by IFNα/β or CD40 alone. These responses are critical for the acquisition of antiviral CD8+ T cell effector function, and their activity in APCs from individuals infected with severe acute respiratory syndrome coronavirus 2 correlates with milder disease. These observations uncover a sequential integration process whereby APCs rely on CD4+ T cells to select the innate circuits that guide antiviral CD8+ T cell responses.

Microbial Metabolites in the Maturation and Activation of Dendritic Cells and Their Relevance for Respiratory Immunity

The respiratory tract is a gateway for viruses and bacteria from the external environment to invade the human body. Critical to the protection against these invaders are dendritic cells (DCs) - a group of highly specialized myeloid cells that monitors the lung microenvironment and relays contextual and antigenic information to T cells. Following the recognition of danger signals and/or pathogen molecular associated patterns in the lungs, DCs undergo activation. This process arms DCs with the unique ability to induce the proliferation and differentiation of T cells responding to matching antigen in complex with MHC molecules. Depending on how DCs interact with T cells, the ensuing T cell response can be tolerogenic or immunogenic and as such, the susceptibility and severity of respiratory infections is influenced by the signals DCs receive, integrate, and then convey to T cells. It is becoming increasingly clear that these facets of DC biology are heavily influenced by the cellular components and metabolites produced by the lung and gut microbiota. In this review, we discuss the roles of different DC subsets in respiratory infections and outline how microbial metabolites impact the development, propensity for activation and subsequent activation of DCs. In particular, we highlight these concepts in the context of respiratory immunity.

Epigenetic Activation of Plasmacytoid DCs Drives IFNAR-Dependent Therapeutic Differentiation of AML

Pharmacologic inhibition of epigenetic enzymes can have therapeutic benefit against hematologic malignancies. In addition to affecting tumor cell growth and proliferation, these epigenetic agents may induce antitumor immunity. Here, we discovered a novel immunoregulatory mechanism through inhibition of histone deacetylases (HDAC). In models of acute myeloid leukemia (AML), leukemia cell differentiation and therapeutic benefit mediated by the HDAC inhibitor (HDACi) panobinostat required activation of the type I interferon (IFN) pathway. Plasmacytoid dendritic cells (pDC) produced type I IFN after panobinostat treatment, through transcriptional activation of IFN genes concomitant with increased H3K27 acetylation at these loci. Depletion of pDCs abrogated panobinostat-mediated induction of type I IFN signaling in leukemia cells and impaired therapeutic efficacy, whereas combined treatment with panobinostat and IFNα improved outcomes in preclinical models. These discoveries offer a new therapeutic approach for AML and demonstrate that epigenetic rewiring of pDCs enhances antitumor immunity, opening the possibility of exploiting this approach for immunotherapies.

SIGNIFICANCE

We demonstrate that HDACis induce terminal differentiation of AML through epigenetic remodeling of pDCs, resulting in production of type I IFN that is important for the therapeutic effects of HDACis. The study demonstrates the important functional interplay between the immune system and leukemias in response to HDAC inhibition. This article is highlighted in the In This Issue feature, p. 1397.

Ptpn2 and KLRG1 regulate the generation and function of tissue-resident memory CD8+ T cells in skin

Tissue-resident memory T cells (T_RM cells) are key elements of tissue immunity. Here, we investigated the role of the regulator of T cell receptor and cytokine signaling, Ptpn2, in the formation and function of T_RM cells in skin. Ptpn2-deficient CD8⁺ T cells displayed a marked defect in generating CD69⁺ CD103⁺ T_RM cells in response to herpes simplex virus type 1 (HSV-1) skin infection. This was accompanied by a reduction in the proportion of KLRG1⁻ memory precursor cells and a transcriptional bias toward terminal differentiation. Of note, forced expression of KLRG1 was sufficient to impede T_RM cell formation. Normalizing memory precursor frequencies by transferring equal numbers of KLRG1⁻ cells restored T_RM generation, demonstrating that Ptpn2 impacted skin seeding with precursors rather than downstream T_RM cell differentiation. Importantly, Ptpn2-deficient T_RM cells augmented skin autoimmunity but also afforded superior protection from HSV-1 infection. Our results emphasize that KLRG1 repression is required for optimal T_RM cell formation in skin and reveal an important role of Ptpn2 in regulating T_RM cell functionality.

Flexible Usage and Interconnectivity of Diverse Cell Death Pathways Protect against Intracellular Infection

Programmed cell death contributes to host defense against pathogens. To investigate the relative importance of pyroptosis, necroptosis, and apoptosis during Salmonella infection, we infected mice and macrophages deficient for diverse combinations of caspases-1, -11, -12, and -8 and receptor interacting serine/threonine kinase 3 (RIPK3). Loss of pyroptosis, caspase-8-driven apoptosis, or necroptosis had minor impact on Salmonella control. However, combined deficiency of these cell death pathways caused loss of bacterial control in mice and their macrophages, demonstrating that host defense can employ varying components of several cell death pathways to limit intracellular infections. This flexible use of distinct cell death pathways involved extensive cross-talk between initiators and effectors of pyroptosis and apoptosis, where initiator caspases-1 and -8 also functioned as executioners when all known effectors of cell death were absent. These findings uncover a highly coordinated and flexible cell death system with in-built fail-safe processes that protect the host from intracellular infections.

Microbiota-Derived Short-Chain Fatty Acids Promote the Memory Potential of Antigen-Activated CD8+ T Cells

Interactions with the microbiota influence many aspects of immunity, including immune cell development, differentiation, and function. Here, we examined the impact of the microbiota on CD8⁺ T cell memory. Antigen-activated CD8⁺ T cells transferred into germ-free mice failed to transition into long-lived memory cells and had transcriptional impairments in core genes associated with oxidative metabolism. The microbiota-derived short-chain fatty acid (SCFA) butyrate promoted cellular metabolism, enhanced memory potential of activated CD8⁺ T cells, and SCFAs were required for optimal recall responses upon antigen re-encounter. Mechanistic experiments revealed that butyrate uncoupled the tricarboxylic acid cycle from glycolytic input in CD8⁺ T cells, which allowed preferential fueling of oxidative phosphorylation through sustained glutamine utilization and fatty acid catabolism. Our findings reveal a role for the microbiota in promoting CD8⁺ T cell long-term survival as memory cells and suggest that microbial metabolites guide the metabolic rewiring of activated CD8⁺ T cells to enable this transition.

Molecular dissection of plasmacytoid dendritic cell activation in vivo during a viral infection

Plasmacytoid dendritic cells (pDC) are the major source of type I interferons (IFN-I) during viral infections, in response to triggering of endosomal Toll-like receptors (TLRs) 7 or 9 by viral single-stranded RNA or unmethylated CpG DNA, respectively. Synthetic ligands have been used to disentangle the underlying signaling pathways. The adaptor protein AP3 is necessary to transport molecular complexes of TLRs, synthetic CpG DNA, and MyD88 into endosomal compartments allowing interferon regulatory factor 7 (IRF7) recruitment whose phosphorylation then initiates IFN-I production. High basal expression of IRF7 by pDC and its further enhancement by positive IFN-I feedback signaling appear to be necessary for robust cytokine production. In contrast, we show here that in vivo during mouse cytomegalovirus (MCMV) infection pDC produce high amounts of IFN-I downstream of the TLR9-to-MyD88-to-IRF7 signaling pathway without requiring IFN-I positive feedback, high IRF7 expression, or AP3-driven endosomal routing of TLRs. Hence, the current model of the molecular requirements for professional IFN-I production by pDC, established by using synthetic TLR ligands, does not strictly apply to a physiological viral infection.

Fluid absorption modulates Peyer’s patch homeostasis and mucosal antibody responses

Peyer’s patches (PPs) are B cell-rich lymphoid tissues situated throughout the small intestine which play an important role in mucosal antibody responses. PP architecture and stromal cell composition closely resemble that of peripheral lymph nodes despite geographical and functional differences. Notably, fibroblastic reticular cells (FRCs) located in small intestinal PPs form a network of collagen-rich reticular fibers similar to conduits found in lymph nodes. In the PP, these conduits extend from the basement membrane of the epithelial cell lining into the PP follicle, and terminate along blood and efferent lymphatic vessels.

Unlike lymph nodes, PPs lack a conventional source of afferent lymph that would normally contribute fluid flow through the conduit network. Instead fluid flow through PP conduits depends largely on water absorbed across the intestinal epithelium. We find that by disrupting water absorption, we can limit or prevent the contribution of absorbed lumenal fluids to the flow of PP conduits. Disruption of fluid absorption subsequently has profound effects on the structural integrity of the high endothelial venule (HEV) and surrounding perivascular FRCs. These alterations correlate with a striking defect in the recruitment of naïve lymphocytes to the PP.

Prolonged blockade of water absorption additionally impacts mucosal antibody responses. Antigen-specific fecal IgA titers are reduced, and germinal center responses are decreased in scale. These findings reveal a critical role for fluid absorption and conduit-mediated fluid flow in the maintenance of PP function and may have important implications for mucosal health under conditions of fluid malabsorption, long-term laxative use, and diarrhoeal diseases.