Probing infectious disease by single-cell RNA sequencing: Progresses and perspectives

The increasing application of single-cell RNA sequencing (scRNA-seq) technology in life science and biomedical research has significantly increased our understanding of the cellular heterogeneities in immunology, oncology and developmental biology. This review will summarize the development of various scRNA-seq technologies; primarily discussing the application of scRNA-seq on infectious diseases, and exploring the current development, challenges, and potential applications of scRNA-seq technology in the future.

A correlation of measles specific antibodies and the number of plasmacytoid dendritic cells is observed after measles vaccination in 9 month old infants

Measles virus (MeV) represents one of the main causes of death among young children, particularly in developing countries. Upon infection, MeV controls both interferon induction (IFN) and the interferon signaling pathway which results in a severe host immunosuppression that can persists for up to 6 mo after infection. Despite the global biology of MeV infection is well studied, the role of the plasmacytoid dendritic cells (pDCs) during the host innate immune response after measles vaccination remains largely uncharacterized. Here we investigated the role of pDCs, the major producers of interferon in response to viral infections, in the development of adaptive immune response against MeV vaccine. We report that there is a strong correlation between pDCs population and the humoral immune response to Edmonston Zagreb (EZ) measles vaccination in 9-month-old mexican infants. Five infants were further evaluated after vaccination, showing a clear increase in pDCs at baseline, one week and 3 months after immunization. Three months postvaccination they showed increase in memory T-cells and pDCs populations, high induction of adaptive immunity and also observed a correlation between pDCs number and the humoral immune response. These findings suggest that the development and magnitude of the adaptive immune response following measles immunization is directly dependent on the number of pDCs of the innate immune response.

CpG oligodeoxynucleotides as mucosal adjuvants

Bacterial DNA comprising palindromic sequences and containing unmethylated CpG is recognized by toll-like receptor 9 of plasmacytoid dendritic cells (pDCs) and induces the production of interferon-α and chemokines, leading to the activation of a Th1 immune response. Therefore, synthetic equivalents of bacterial DNA (CpG oligodeoxynucleotides) have been developed for clinical applications. They are usually phosphorothioated for in vivo use; this approach also leads to adverse effects as reported in mouse models.Mucosal vaccines that induce both mucosal and systemic immunity received substantial attention in recent years. For their development, phosphodiester-linked oligodeoxynucleotides, including the sequence of a palindromic CpG DNA may be advantageous as adjuvants because their target pDCs are present right there, in the mucosa of the vaccination site. In addition, the probability of adverse effects is believed to be low. Here, we review the discovery of such CpG oligodeoxynucleotides and their possible use as mucosal adjuvants.

Dendritic cell defects in the colorectal cancer

Colorectal cancer (CRC) results from the accumulation of both genetic and epigenetic alterations of the genome. However, also the formation of an inflammatory milieu plays a pivotal role in tumor development and progression. Dendritic cells (DCs) play a relevant role in tumor by exerting differential pro-tumorigenic and anti-tumorigenic functions, depending on the local milieu. Quantitative and functional impairments of DCs have been widely observed in several types of cancer, including CRC, representing a tumor-escape mechanism employed by cancer cells to elude host immunosurveillance.

Understanding the interactions between DCs and tumors is important for comprehending the mechanisms of tumor immune surveillance and escape, and provides novel approaches to therapy of cancer. This review summarizes updated information on the role of the DCs in colon cancer development and/or progression.

Tumor-infiltrating plasmacytoid dendritic cells promote immunosuppression by Tr1 cells in human liver tumors

CD4⁺ type 1 T regulatory (Tr1) cells have a crucial role in inducing tolerance. Immune regulation by these cells is mainly mediated through the secretion of high amounts of IL-10. Several studies have suggested that this regulatory population may be involved in tumor-mediated immune-suppression. However, direct evidence of a role for Tr1 cells in human solid tumors is lacking. Using ex vivo isolated cells from individuals with hepatocellular carcinoma (HCC; n = 39) or liver metastases from colorectal cancer (LM-CRC; n = 60) we identify a CD4⁺FoxP3^-IL-13^-IL-10⁺ T cell population in tumors of individuals with primary or secondary liver cancer that is characterized as Tr1 cells by the expression of CD49b and the lymphocyte activation gene 3 (LAG-3) and strong suppression activity of T cell responses in an IL-10 dependent manner. Importantly, the presence of tumor-infiltrating Tr1 cells is correlated with tumor infiltration of plasmacytoid dendritic cells (pDCs). pDCs exposed to tumor-derived factors enhance IL-10 production by Tr1 cells through up-regulation of the inducible co-stimulatory ligand (ICOS-L). These findings suggest a role for pDCs and ICOS-L in promoting intra-tumoral immunosuppression by Tr1 cells in human liver cancer, which may foster tumor progression and which might interfere with attempts of immunotherapeutic intervention.

Lymphocyte activation gene-3 (LAG-3, CD223) in plasmacytoid dendritic cells (pDCs): a molecular target for the restoration of active antitumor immunity

We have recently reported that lymphocyte activation gene-3 (LAG-3,CD223) mediates the alternative, IFNα-deficient activation of plasmacytoid dendritic cells (pDCs) at tumor sites. Our findings define a novel tumor-driven strategy that promotes immunosuppression by pDCs, and we have provided more detailed information regarding the immunomodulatory role of of LAG-3. The translational relevance of our results for the treatment of tumors and autoimmune diseases is discussed herein.