The mosquito effect: regulatory and effector T cells acquire cytoplasmic material from tumor cells through intercellular transfer

The phenomenon of intercellular transfer of cellular material, including membranes, cytoplasm, and even organelles, has been observed for decades. The functional impact and molecular mechanisms of such transfer in the immune system remain largely elusive due to the absence of a robust in vivo model. Here, we introduce a new tumor mouse model, where tumor cells express the soluble ultra-bright fluorescent protein ZsGreen, which allows detection and measurement of intercellular transfer of cytoplasm from tumor cells to infiltrating immune cells. We found that in addition to various types of myeloid lineage cells, a large fraction of T regulatory cells and effector CD8 T cells acquire tumor material. Based on the distribution of tumor-derived ZsGreen, the majority of T cells integrate captured cytoplasm into their own, while most myeloid cells store tumor material in granules. Furthermore, scRNA-seq analysis revealed significant alterations in transcriptomes of T cells that acquired tumor cell cytoplasm, suggesting potential impact on T cell function. We identified that the participation of T cells in intercellular transfer requires cell-cell contact and is strictly dependent on the activation status of T lymphocytes. Finally, we propose to name the described phenomenon of intercellular transfer for tumor infiltrating T cells the "mosquito effect".

Let-7 enhances murine anti-tumor CD8 T cell responses by promoting memory and antagonizing terminal differentiation

The success of the CD8 T cell-mediated immune response against infections and tumors depends on the formation of a long-lived memory pool, and the protection of effector cells from exhaustion. The advent of checkpoint blockade therapy has significantly improved anti-tumor therapeutic outcomes by reversing CD8 T cell exhaustion, but fails to generate effector cells with memory potential. Here, using in vivo mouse models, we show that let-7 miRNAs determine CD8 T cell fate, where maintenance of let-7 expression during early cell activation results in memory CD8 T cell formation and tumor clearance. Conversely, let-7-deficiency promotes the generation of a terminal effector population that becomes vulnerable to exhaustion and cell death in immunosuppressive environments and fails to reject tumors. Mechanistically, let-7 restrains metabolic changes that occur during T cell activation through the inhibition of the PI3K/AKT/mTOR signaling pathway and production of reactive oxygen species, potent drivers of terminal differentiation and exhaustion. Thus, our results reveal a role for let-7 in the time-sensitive support of memory formation and the protection of effector cells from exhaustion. Overall, our data suggest a strategy in developing next-generation immunotherapies by preserving the multipotency of effector cells rather than enhancing the efficacy of differentiation.

LKB1 isoform expression modulates T cell plasticity downstream of PKCθ and IL-6

Following activation, CD4 T cells undergo metabolic and transcriptional changes as they respond to external cues and differentiate into T helper (Th) cells. T cells exhibit plasticity between Th phenotypes in highly inflammatory environments, such as colitis, in which high levels of IL-6 promote plasticity between regulatory T (Treg) cells and Th17 cells. Protein Kinase C theta (PKCθ) is a T cell-specific serine/threonine kinase that promotes Th17 differentiation while negatively regulating Treg differentiation. Liver kinase B1 (LKB1), also a serine/threonine kinase and encoded by Stk11, is necessary for Treg survival and function. Stk11 can be alternatively spliced to produce a short variant (Stk11_S) by transcribing a cryptic exon. However, the contribution of Stk11 splice variants to Th cell differentiation has not been previously explored. Here we show that in Th17 cells, the heterogeneous ribonucleoprotein, hnRNPLL, mediates Stk11 splicing into its short splice variant, and that Stk11_S expression is diminished when Hnrnpll is depleted using siRNA knock-down approaches. We further show that PKCθ regulates hnRNPLL and, thus, Stk11_S expression in Th17 cells. We provide additional evidence that exposing induced (i)Tregs to IL-6 culminates in Stk11 splicing downstream of PKCθAltogether our data reveal a yet undescribed outside-in signaling pathway initiated by IL-6, that acts through PKCθ and hnRNPLL to regulate Stk11 splice variants and facilitate Th17 cell differentiation. Furthermore, we show for the first time, that this pathway can also be initiated in developing iTregs exposed to IL-6, providing mechanistic insight into iTreg phenotypic stability and iTreg to Th17 cell plasticity.

Pathogenic Th17 cell differentiation is negatively regulated by let-7 microRNAs in a mouse model of multiple sclerosis

Autoimmune disorders such as multiple sclerosis (MS) are caused by proinflammatory events mediated by pathogenic Th17 cells. In MS, these cells arise in response to autoantigen recognition and exposure to the cytokines IL-1β and IL-23, migrate to the central nervous system (CNS) by following gradients of CCR2- and CCR5-cognate chemokines, and secrete GM-CSF. GM-CSF is essential for disease development, as it promotes the activation, differentiation, and recruitment of peripheral inflammatory myeloid cells to the CNS that directly demyelinate neurons and damage axons. Th17 cell pathogenicity in MS has been correlated with microRNA (miRNA) dysregulation, which leads to aberrant post-transcriptional regulation of gene expression and enhanced autoreactive phenotype. We found that the lethal-7 (let-7) miRNA family is abundantly expressed in naive CD4+ T cells, but gets dramatically downregulated over time following antigen encounter, indicating that let-7 may control the differentiation of pathogenic Th17 cells. To investigate a potential regulatory role for let-7 in Th17 cell autoreactivity, we used experimental autoimmune encephalomyelitis (EAE), the animal model of MS. Specifically, we demonstrated that let-7 confers protection from EAE by negatively regulating the proliferation, IL-1β/IL-23-dependent differentiation, and CCR2/CCR5-dependent migration of pathogenic Th17 cells to the CNS. Conversely, absence of let-7 led to enhanced Th17 cell autoreactivity and aggravated disease. Therefore, our results identify a novel regulatory role for let-7 miRNAs in pathogenic Th17 differentiation during EAE development, suggesting a promising therapeutic application for the treatment of MS-related autoimmune diseases.

Non-coding RNAs in CD8 T cell biology

CD8 T cells are among the most vigorous soldiers of the immune system that fight viral infections and cancer. CD8 T cell development, maintenance, activation and differentiation are under the tight control of multiple transcriptional and post-transcriptional networks. Over the last two decades it has become clear that non-coding RNAs (ncRNAs), which consist of microRNAs (miRNAs) and long ncRNAs (lncRNAs), have emerged as global biological regulators. While our understanding of the function of specific miRNAs has increased since the discovery of RNA interference, it is still very limited, and the field of lncRNAs is just starting to blossom. Here we will summarize our knowledge on the role of ncRNAs in CD8 T cell biology, including differentiation into memory and exhausted cells.

Differentiation of Pathogenic Th17 Cells Is Negatively Regulated by Let-7 MicroRNAs in a Mouse Model of Multiple Sclerosis

Multiple sclerosis (MS) is a disabling demyelinating autoimmune disorder of the central nervous system (CNS) which is driven by IL-23- and IL-1β-induced autoreactive Th17 cells that traffic to the CNS and secrete proinflammatory cytokines. Th17 pathogenicity in MS has been correlated with the dysregulation of microRNA (miRNA) expression, and specific miRNAs have been shown to promote the pathogenic Th17 phenotype. In the present study, we demonstrate, using the animal model of MS, experimental autoimmune encephalomyelitis (EAE), that let-7 miRNAs confer protection against EAE by negatively regulating the proliferation, differentiation and chemokine-mediated migration of pathogenic Th17 cells to the CNS. Specifically, we found that let-7 miRNAs may directly target the cytokine receptors Il1r1 and Il23r, as well as the chemokine receptors Ccr2 and Ccr5. Therefore, our results identify a novel regulatory role for let-7 miRNAs in pathogenic Th17 differentiation during EAE development, suggesting a promising therapeutic application for disease treatment.

Survival of Naïve T Cells Requires the Expression of Let-7 miRNAs

Maintaining the diversity and constant numbers of naïve T cells throughout the organism's lifetime is necessary for efficient immune responses. Naïve T cell homeostasis, which consists of prolonged survival, occasional proliferation and enforcement of quiescence, is tightly regulated by multiple signaling pathways which are in turn controlled by various transcription factors. However, full understanding of the molecular mechanisms underlying the maintenance of the peripheral T cell pool has not been achieved. In the present study, we demonstrate that T cell-specific deficiency in let-7 miRNAs results in peripheral T cell lymphopenia resembling that of Dicer1 knockout mice. Deletion of let-7 leads to profound T cell apoptosis while overexpression prevents it. We further show that in the absence of let-7, T cells cannot sustain optimal levels of the pro-survival factor Bcl2 in spite of the intact IL-7 signaling, and re-expression of Bcl2 in let-7 deficient T cells completely rescues the survival defect. Thus, we have uncovered a novel let-7-dependent mechanism of post-transcriptional regulation of naïve T cell survival in vivo.

Modulation of let-7 miRNAs controls the differentiation of effector CD8 T cells

The differentiation of naive CD8 T cells into effector cytotoxic T lymphocytes upon antigen stimulation is necessary for successful antiviral, and antitumor immune responses. Here, using a mouse model, we describe a dual role for the let-7 microRNAs in the regulation of CD8 T cell responses, where maintenance of the naive phenotype in CD8 T cells requires high levels of let-7 expression, while generation of cytotoxic T lymphocytes depends upon T cell receptor-mediated let-7 downregulation. Decrease of let-7 expression in activated T cells enhances clonal expansion and the acquisition of effector function through derepression of the let-7 targets, including Myc and Eomesodermin. Ultimately, we have identified a novel let-7-mediated mechanism, which acts as a molecular brake controlling the magnitude of CD8 T cell responses.

DOI:http://dx.doi.org/10.7554/eLife.26398.001

Suppression of primary allogenic response by CD8+ memory cells

Immune response to allogenic tumor cells is associated with the appearance of long-living CD8+ memory cells capable of rapid restimulation and lysis of tumor cells in case of repeated injection of these cells. In order to acquire the effector function, allorestricted memory cells need antigen restimulation for 2 days, which is a specific feature of central memory cell population. These cells can suppress proliferation of naive splenocytes in vitro. In mixed lymphocyte cultures containing memory cells, antigen stimulation induces more intensive IL-10 production and deeper suppression of IL-2 production in comparison with cultures containing naive cells. The conditions for activation of naive cells during secondary immune response are not optimal.

Cross reactivity of T cell receptor on memory CD8+ cells isolated after immunization with allogeneic tumor cells

Experiments on mice deficient in expression of class I major histocompatibility complex molecules showed that memory CD8+ cells recognizing the alloantigen by the direct allogeneic recognition mechanism selectively proliferated in response to heated allogeneic cells. Adoptive transfer of memory cells from mice expressing green fluorescent protein transgene to wild-type animals showed for the first time that long-living memory cells suppress the response of naive T cells and abolish their involvement in the pool of memory cells. The pool of long-living memory T cells was obtained in vitro with heated allogeneic stimulators. Apart from immunizing alloantigen, this clone recognized foreign molecules of the major histocompatibility complex. Cloning and sequencing of rearranged regions in memory T cells showed that two alpha-chains and one functional beta-chain are rearranged in cells of this pool. Only one alpha-chain was capable of forming protein product, which determines expression of only one form of T cell receptor. Experimental data directly confirm the hypothesis about degeneracy of recognition of allelic products of major histocompatibility complex molecules by T cell receptors. Suppression of the response of naive cells by memory cells probably underlies a previously unknown type of polarization of the immune response and determines clonal dominance and peripheral selection of T lymphocytes.