Dendritic cells are significantly reduced in non-Hodgkin's lymphoma and express less CCR7 and CD62L

Immune cells in the B-cell lymphoma microenvironment: From basic research to clinical applications

ABSTRACT

B-cell lymphoma is a group of hematological malignancies characterized by variable genetic and biological features and clinical behaviors. The tumor microenvironment (TME) is a complex network in tumors, which consists of surrounding blood vessels, extracellular matrix, immune and non-immune cells, and signaling molecules. Increasing evidence has shown that the TME, especially immune cells within, is a double-edged sword, acting either as a tumor killer or as a promoter of tumor progression. These pro-tumor activities are driven by subpopulations of immune cells that express typical markers but have unique transcriptional characteristics, making tumor-associated immune cells good targets for human anti-cancer therapy by ablating immunosuppressive cells or enhancing immune-activated cells. Thus, exploring the role of immune cells in the TME provides distinct insights for immunotherapy in B-cell lymphoma. In this review, we elucidated the interaction between immune cells and tumor cells and their function in the initiation, progression, and prognosis of B-cell lymphoma, from preclinical experiments to clinical trials. Furthermore, we outlined potential therapeutic approaches and discussed the potential clinical value and future perspectives of targeting immune cells in patients with B-cell lymphoma.

B cell/stromal cell crosstalk in health, disease, and treatment: Follicular lymphoma as a paradigm

Stromal cells organize specific anatomic compartments within bone marrow (BM) and secondary lymphoid organs where they finely regulate the behavior of mature normal B cells. In particular, lymphoid stromal cells (LSCs) form a phenotypically heterogeneous compartment including various cell subsets variably supporting B-cell survival, activation, proliferation, and differentiation. In turn, activated B cells trigger in-depth remodeling of LSC networks within lymph nodes (LN) and BM. Follicular lymphoma (FL) is one of the best paradigms of a B-cell neoplasia depending on a specific tumor microenvironment (TME), including cancer-associated fibroblasts (CAFs) emerging from the reprogramming of LN LSCs or poorly characterized local BM precursors. FL-CAFs support directly malignant B-cell growth and orchestrate FL permissive cell niche by contributing, through a bidirectional crosstalk, to the recruitment and polarization of immune TME subsets. Recent studies have highlighted a previously unexpected level of heterogeneity of both FL B cells and FL TME, underlined by FL-CAF plasticity. A better understanding of the signaling pathways, molecular mechanisms, and kinetic of stromal cell remodeling in FL would be useful to delineate new predictive markers and new therapeutic approaches in this still fatal malignancy.

Downregulation of USP18 reduces tumor-infiltrating activated dendritic cells in extranodal diffuse large B cell lymphoma patients

Extranodal diffuse large B cell lymphoma (EN DLBCL) often leads to poor outcomes, while the underlying mechanism remains unclear. As immune imbalance plays an important role in lymphoma pathogenesis, we hypothesized that immune genes might be involved in the development of EN DLBCL. Ninety-three differentially expressed immune genes (DEIGs) were identified from 1168 differentially expressed genes (DEGs) between tumor tissues of lymph node DLBCL (LN DLBCL) and EN DLBCL patients in TCGA database. Nine prognostic immune genes were further identified from DEIGs by univariate Cox regression analysis. A multivariate predictive model was established based on these prognostic immune genes. Patients were divided into high- and low-risk groups according to the median model-based risk score. Kaplan-Meier survival curves showed that patients in the high-risk group had a shorter survival time than those in the low-risk group (P < 0.001). Ubiquitin-specific peptidase 18 (USP18) was further recognized as the key immune gene in EN DLBCL on the basis of coexpression of differentially expressed transcription factors (DETFs) and prognostic immune genes. USP18 exhibited low expression in EN DLBCL, which was regulated by LIM homeobox 2 (LHX2) (R = 0.497, P < 0.001, positive). The potential pathway downstream of USP18 was the MAPK pathway, identified by gene set variation analysis (GSVA), gene set enrichment analysis (GSEA) and Pearson correlation analysis (R = 0.294, P < 0.05, positive). The "ssGSEA" algorithm and Pearson correlation analysis identified that activated dendritic cells (aDCs) were the cell type mostly associated with USP18 (R = 0.694, P < 0.001, positive), indicating that USP18 participated in DC-modulating immune responses. The correlations among key biomarkers were supported by multiomics database validation. Indeed, the USP18 protein was confirmed to be expressed at lower levels in tumor tissues in patients with EN DLBCL than in those with LN DLBCL by immunohistochemistry. In short, our study illustrated that the downregulation of USP18 was associated with reduced aDC number in the tumor tissues of EN DLBCL patients, indicating that targeting USP18 might serve as a promising therapy.

Activation of plasmacytoid dendritic cells promotes AML-cell fratricide

Acute myeloid leukemia (AML) is characterized by the proliferation of immature myeloid blasts and a suppressed immune state. Interferons have been previously shown to aid in the clearance of AML cells. Type I interferons are produced primarily by plasmacytoid dendritic cells (pDCs). However, these cells exist in a quiescent state in AML. Because pDCs express TLR 7–9, we hypothesized that the TLR7/8 agonist R848 would be able to reprogram them toward a more active, IFN-producing phenotype. Consistent with this notion, we found that R848-treated pDCs from patients produced significantly elevated levels of IFNβ. In addition, they showed increased expression of the immune-stimulatory receptor CD40. We next tested whether IFNβ would influence antibody-mediated fratricide among AML cells, as our recent work showed that AML cells could undergo cell-to cell killing in the presence of the CD38 antibody daratumumab. We found that IFNβ treatment led to a significant, IRF9-dependent increase in CD38 expression and a subsequent increase in daratumumab-mediated cytotoxicity and decreased colony formation. These findings suggest that the tolerogenic phenotype of pDCs in AML can be reversed, and also demonstrate a possible means of enhancing endogenous Type I IFN production that would promote daratumumab-mediated clearance of AML cells.

Angiogenesis in Lymph Nodes Is a Critical Regulator of Immune Response and Lymphoma Growth

Tumor-induced remodeling of the microenvironment in lymph nodes (LNs) includes the formation of blood vessels, which goes beyond the regulation of metabolism, and shaping a survival niche for tumor cells. In contrast to solid tumors, which primarily rely on neo-angiogenesis, hematopoietic malignancies usually grow within pre-vascularized autochthonous niches in secondary lymphatic organs or the bone marrow. The mechanisms of vascular remodeling in expanding LNs during infection-induced responses have been studied in more detail; in contrast, insights into the conditions of lymphoma growth and lodging remain enigmatic. Based on previous murine studies and clinical trials in human, we conclude that there is not a universal LN-specific angiogenic program applicable. Instead, signaling pathways that are tightly connected to autochthonous and infiltrating cell types contribute variably to LN vascular expansion. Inflammation related angiogenesis within LNs relies on dendritic cell derived pro-inflammatory cytokines stimulating vascular endothelial growth factor-A (VEGF-A) expression in fibroblastic reticular cells, which in turn triggers vessel growth. In high-grade B cell lymphoma, angiogenesis correlates with poor prognosis. Lymphoma cells immigrate and grow in LNs and provide pro-angiogenic growth factors themselves. In contrast to infectious stimuli that impact on LN vasculature, they do not trigger the typical inflammatory and hypoxia-related stroma-remodeling cascade. Blood vessels in LNs are unique in selective recruitment of lymphocytes via high endothelial venules (HEVs). The dissemination routes of neoplastic lymphocytes are usually disease stage dependent. Early seeding via the blood stream requires the expression of the homeostatic chemokine receptor CCR7 and of L-selectin, both cooperate to facilitate transmigration of tumor and also of protective tumor-reactive lymphocytes via HEV structures. In this view, the HEV route is not only relevant for lymphoma cell homing, but also for a continuous immunosurveillance. We envision that HEV functional and structural alterations during lymphomagenesis are not only key to vascular remodeling, but also impact on tumor cell accessibility when targeted by T cell-mediated immunotherapies.

Molecular regulation of dendritic cell development and function in homeostasis, inflammation, and cancer

Dendritic cells (DCs) are the principal antigen-presenting cells of the immune system and play key roles in controlling immune tolerance and activation. As such, DCs are chief mediators of tumor immunity. DCs can regulate tolerogenic immune responses that facilitate unchecked tumor growth. Importantly, however, DCs also mediate immune-stimulatory activity that restrains tumor progression. For instance, emerging evidence indicates the cDC1 subset has important functions in delivering tumor antigens to lymph nodes and inducing antigen-specific lymphocyte responses to tumors. Moreover, DCs control specific therapeutic responses in cancer including those resulting from immune checkpoint blockade. DC generation and function is influenced profoundly by cytokines, as well as their intracellular signaling proteins including STAT transcription factors. Regardless, our understanding of DC regulation in the cytokine-rich tumor microenvironment is still developing and must be better defined to advance cancer treatment. Here, we review literature focused on the molecular control of DCs, with a particular emphasis on cytokine- and STAT-mediated DC regulation. In addition, we highlight recent studies that delineate the importance of DCs in anti-tumor immunity and immune therapy, with the overall goal of improving knowledge of tumor-associated factors and intrinsic DC signaling cascades that influence DC function in cancer.

Role of the microenvironment across histological subtypes of NHL

Recent progress in next-generation sequencing strategies has revealed the genetic landscape of B-cell non-Hodgkin lymphoma, but the tumor microenvironment is increasingly recognized as crucial to sustaining malignant B-cell survival and growth, subclonal evolution, and drug resistance. The tumor niche is made up of a dynamic and organized network of strongly heterogeneous immune and stromal cell subsets characterized by specific phenotypic, transcriptomic, and functional features. Nonmalignant cell recruitment and plasticity are dictated by lymphoma B cells, which convert their surrounding microenvironment into a supportive niche. In addition, they are also influenced by the crosstalk between the various components of this niche. In agreement with this, the B-cell lymphoma subtype is a key determinant of the organization of the tumor niche, but genetic alteration patterns, tumor localization, stage of the disease, and treatment strategy may also modulate its composition and activity. Moreover, the complex set of bidirectional interactions between B cells and their microenvironment has been proposed as a promising therapeutic target with the aim of reinforcing antitumor immunity and/or of abbrogating the lymphoma-promoting signals delivered by the tumor niche.

Lymphoma: immune evasion strategies

While the cellular origin of lymphoma is often characterized by chromosomal translocations and other genetic aberrations, its growth and development into a malignant neoplasm is highly dependent upon its ability to escape natural host defenses. Neoplastic cells interact with a variety of non-malignant cells in the tumor milieu to create an immunosuppressive microenvironment. The resulting functional impairment and dysregulation of tumor-associated immune cells not only allows for passive growth of the malignancy but may even provide active growth signals upon which the tumor subsequently becomes dependent. In the past decade, the success of immune checkpoint blockade and adoptive cell transfer for relapsed or refractory lymphomas has validated immunotherapy as a possible treatment cornerstone. Here, we review the mechanisms by which lymphomas have been found to evade and even reprogram the immune system, including alterations in surface molecules, recruitment of immunosuppressive subpopulations, and secretion of anti-inflammatory factors. A fundamental understanding of the immune evasion strategies utilized by lymphomas may lead to better prognostic markers and guide the development of targeted interventions that are both safer and more effective than current standards of care.

Low expression of dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin in non-Hodgkin lymphoma and a significant correlation with β2-microglobulin

Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), a member of the C-type lectin superfamily, has been reported to bind to various pathogens and several tumor cells and to participate in immunoregulation. It is still unclear whether there is a significant association between the level of DC-SIGN and non-Hodgkin lymphoma (NHL). To investigate the clinical diagnostic significance of DC-SIGN in NHL, we conducted a study with 52 NHL patients and 104 healthy individuals. Enzyme-linked immunosorbent assay and tissue microarray technology were utilized for the analysis. The serum sDC-SIGN levels in the NHL patients were significantly lower than those in the healthy controls (P=0.0019). A cutoff value of 1.499 µg/ml for sDC-SIGN predicted the presence of NHL with 78.85% sensitivity and 53.85% specificity [area under the curve (AUC)=0.6531, P=0.0019]. The serum sDC-SIGN levels in NHL patients were also significantly correlated with β2-microglobulin (P=0.0062). Moreover, tissue microarray analysis demonstrated that the expression of DC-SIGN in the lymph nodes or tissues of 96 NHL patients was significantly lower than that in 18 normal lymph nodes (P<0.0001). However, the expression of DC-SIGN in NHL displayed no significant correlation with the expression of CD20 or CD79a. In conclusion, DC-SIGN may be a promising biological molecule for clinical research on NHL, whereas the underlying roles need to be investigated in additional studies.

Increased hyaluronan levels and decreased dendritic cell activation are associated with tumor invasion in murine lymphoma cell lines

Hyaluronan (HA), a component of the extracellular matrix surrounding tumors, modulates tumor progression and the immune response. Dendritic cells (DC) may tolerize or stimulate immunity against cancer. In this report, we study the association between tumor progression, HA levels and DC activation in a lymphoma model. Mice injected with the cells with highest invasive capacity (LBR-) presented increased HA in serum and lymph nodes, and decreased DC activation in infiltrated lymph nodes and liver. These findings could be related to lack of an effective antitumor immune response and suggest that serum HA levels could have a prognostic value in hematological malignancies.

From tumor immunosuppression to eradication: targeting homing and activity of immune effector cells to tumors

Unraveling the mechanisms used by the immune system to fight cancer development is one of the most ambitious undertakings in immunology. Detailed knowledge regarding the mechanisms of induction of tolerance and immunosuppression within the tumor microenvironment will contribute to the development of highly effective tumor eradication strategies. Research within the last few decades has shed more light on the matter. This paper aims to give an overview on the current knowledge of the main tolerance and immunosuppression mechanisms elicited within the tumor microenvironment, with the focus on development of effective immunotherapeutic strategies to improve homing and activity of immune effector cells to tumors.

Plasmacytoid dendritic cells and cancer

Cancer develops in a complex microenvironment comprising cancer cells, stromal cells, and host immune cells with their soluble products. The counteracting host-protective and tumor-promoting roles of different immune cell populations have been elegantly clarified in the last decade by pertinent genetically modified mouse models. Among cells with a potential role in cancer immunity, PDCs might represent important players as a result of their capacity to bring together innate and adaptive immunity. This review summarizes current knowledge about the role of PDCs in cancer immunity. PDCs have been documented in primary and metastatic human neoplasms; however, the clinical significance of this finding is still unknown. Once into the tumor bed, PDCs can be hijacked by the tumor microenvironment and lose their propensity to produce the required amount of endogenous I-IFN. However, when properly reprogrammed (i.e., by TLR agonists), PDCs might mediate tumor rejection in a clinical setting. Tumor rejection, at least partially, is driven by I-IFN and seems to require a cross-talk with other innate immune cells, including IFN DCs. The latter evidence, although still limited to skin cancers, can provide a leading model for developing adjuvant immune therapy for other neoplasms. To this end, the generation of appropriate mouse models to modulate the frequency and activation state of murine PDCs will also be of remarkable importance.

The host-tumor interface in B-cell non-Hodgkin lymphoma: a new world to investigate

Research on B-cell non-Hodgkin lymphoma focuses mainly on oncogenic events occurring in lymphoma cells, but recently a new component has appeared that may be crucial in lymphomagenesis: the tumor microenvironment. Indeed, compelling evidence demonstrates the key role played by nonmalignant bystander cells in the establishment and proliferation of the tumor. Among these cells, stromal cells, monocytes/macrophages, and T cells in lymphoid organs have all been described as contributing to tumor progression. Interactions linked to cell-cell intimate contacts-but also mediated through soluble mediators such as cytokines and chemokines-do form a specific network. All these interrelations directed by the tumor create a friendly environment for lymphoma cells that permits them to proliferate. Blocking the cross-talk between the tumor microenvironment and lymphoma cells may thus represent a promising new strategy for treating B-cell malignancies.

Distribution patterns of dendritic cells and T cells in diffuse large B-cell lymphomas correlate with prognoses

PURPOSE

Diffuse large B-cell lymphoma (DLBCL), the most common subtype of non-Hodgkin's lymphomas, accounts for 30% to 40% of all lymphoma cases. However, long-term survival by current chemotherapy was achieved in only 40% of patients, warranting the development of novel therapeutic strategies including T-cell immunotherapy. However, the level of baseline immune activation in DLBCL is unclear.

EXPERIMENTAL DESIGN

The density and distribution of dendritic cells and T cells in 48 cases of primary DLBCL was evaluated by immunohistochemistry.

RESULTS

Increased numbers of intratumoral CD1a+ dendritic cells and increased S100+ cells and CD45RO+ T cells around the edges of the tumors were seen in 10 of 48 (21%), 9 of 48 (19%), and 10 of 48 (21%) cases and these were correlated with a favorable prognosis (P = 0.015; P = 0.070, and P = 0.017, respectively), along with increased granzyme B+ T cells in tumor beds (P = 0.013). Increased peritumoral T cells were correlated with tumor expression of HLA-DR (r = 0.446; P = 0.002). Extranodal lymphomas showed fewer tumor-associated CD45RO+ T cells (r = -0.407; P = 0.001) and less conspicuous dendritic cell infiltrates.

CONCLUSIONS

In DLBCL, the presence of baseline antitumor immune response is associated with favorable clinical outcome, and thus adjuvant T-cell immunotherapy may further boost treatment responses.

IDO-expressing regulatory dendritic cells in cancer and chronic infection

Immune evasion and T cell tolerance induction have been associated both with malignant disease and chronic infection. In recent years, increasing evidence has been accumulated that antigen-presenting cells such as dendritic cells (DC) play a major role in immune regulation. They are not only involved in the induction of immunity but also can inhibit immune responses. Interesting parallels for major molecular mechanisms involved in turning DC from stimulatory to regulatory cells have been uncovered between malignant disease and chronic infection. Apparently, not only inhibitory cytokines such as IL-10 seem to play a role, but also metabolic mechanisms dysregulating tryptophan metabolism, thereby, leading to inhibition of T cells and pathogens. We focus here on recent findings establishing the tryptophan catabolizing enzyme indoleamine-pyrrole 2,3 dioxygenase (IDO) as a central feature of DC with regulatory function both in cancer and chronic infection. Induction of enzymatically active IDO can be triggered by various soluble and membrane-bound factors, and in general, require interferon (IFN) signaling. In addition, based on the most recently established link between tumor necrosis factor alpha (TNFalpha), prostaglandin E2 and IDO, a new model of regulation of IDO in context of cancer and infection is proposed. In light of the increasing use of anti-TNFalpha drugs, these findings are also of great interest to the clinician scientist.

CD25 and indoleamine 2,3-dioxygenase are up-regulated by prostaglandin E2 and expressed by tumor-associated dendritic cells in vivo: additional mechanisms of T-cell inhibition

Immune tolerance is a central mechanism counteracting tumor-specific immunity and preventing effective anticancer immunotherapy. Induction of tolerance requires a specific environment in which tolerogenic dendritic cells (DCs) play an essential role deviating the immune response away from effective immunity. It was recently shown that maturation of DCs in the presence of PGE2 results in upregulation of indoleamine 2,3-dioxygenase (IDO) providing a potential mechanism for the development of DC-mediated Tcell tolerance. Here, we extend these findings, demonstrating a concomitant induction of IDO and secretion of soluble CD25 after DC maturation in the presence of PGE2. While maturation of DCs induced IDO expression on transcriptional level, only integration of PGE2 signaling led to up-regulation of functional IDO protein as well as significant expression of cell-surface and soluble CD25 protein. As a consequence, T-cell proliferation and cytokine production were significantly inhibited, which was mediated mainly by IDO-induced tryptophan depletion. Of importance, we demonstrate that different carcinoma entities associated with elevated levels of PGE2 coexpress CD25 and IDO in peritumoral dendritic cells, suggesting that PGE2 might influence IDO expression in human DCs in the tumor environment. We therefore suggest PGE2 to be a mediator of early events during induction of immune tolerance in cancer.