Hematopoietic stem and progenitor cells directly participate in host immune response.

The properties of hematopoietic stem and progenitor cells (HSPCs), including self-renewal and pluripotency, have been extensively studied. These features have been explored in the management of several haematological disorders and malignancies. Although their role as precursors of innate immune cells is well understood, little is known about their direct participation in host immune response. In this review, we explicate the direct role of HSPCs in the host immune response and highlight therapeutic options for the infectious disease burden that is currently ravaging the world, including COVID-19.

Macrophage-secreted granulin supports pancreatic cancer metastasis by inducing liver fibrosis.

Pancreatic ductal adenocarcinoma (PDAC) is a devastating metastatic disease for which better therapies are urgently needed. Macrophages enhance metastasis in many cancer types; however, the role of macrophages in PDAC liver metastasis remains poorly understood. Here we found that PDAC liver metastasis critically depends on the early recruitment of granulin-secreting inflammatory monocytes to the liver. Mechanistically, we demonstrate that granulin secretion by metastasis-associated macrophages (MAMs) activates resident hepatic stellate cells (hStCs) into myofibroblasts that secrete periostin, resulting in a fibrotic microenvironment that sustains metastatic tumour growth. Disruption of MAM recruitment or genetic depletion of granulin reduced hStC activation and liver metastasis. Interestingly, we found that circulating monocytes and hepatic MAMs in PDAC patients express high levels of granulin. These findings suggest that recruitment of granulin-expressing inflammatory monocytes plays a key role in PDAC metastasis and may serve as a potential therapeutic target for PDAC liver metastasis.

Enhanced suppressive capacity of tumor-infiltrating myeloid-derived suppressor cells compared with their peripheral counterparts.

Although the main site of action for myeloid-derived suppressor cells (MDSCs) is most likely the tumor microenvironment, so far the study of these cells has been largely restricted to spleen-derived MDSCs. In this study, we compared the suppressive capacity of splenic and tumor-derived MDSCs in different subcutaneous mouse tumor models. We investigated which suppressive mechanisms were involved. Finally, we investigated whether MDSCs and regulatory T cells (Treg ) cooperate in the suppression of T-cell responses. In all models, splenic granulocytic MDSCs (grMDSC) strongly suppress CD4(+) T-cell proliferation while the suppressive effect on CD8(+) T cells is less pronounced. Splenic monocytic MDSCs (moMDSC) have a lower suppressive capacity, compared to grMDSC, on both CD4(+) and CD8(+) T-cell proliferation. Both grMDSC and moMDSC isolated from the tumor have a much stronger suppressive activity compared to MDSCs isolated from the spleen of tumor-bearing mice, associated with a higher NO2 (-) production by the tumor-derived moMDSC and arginase activity for both subsets. The expression of CD80 is also elevated on tumor-derived grMDSC compared with their peripheral counterparts. Direct contact with tumor cells is required for the upregulation of CD80 and CD80(+) MDSCs are more suppressive than CD80(-) MDSCs. Coculture of Treg and MDSCs leads to a stronger suppression of CD8(+) T-cell proliferation compared to the suppression observed by Treg or MDSCs alone. Thus, we showed that tumor-infiltrating MDSCs possess a stronger suppressive capacity than their peripheral counterparts and that various suppressive mechanisms account for this difference.

The role of SMAC mimetics in regulation of tumor cell death and immunity.

Mimetics of second mitochondria-derived activator of caspases (SMAC) enhance tumor cell death in a variety of cancers. Several molecular mechanisms of action have been identified. However, it was only recently that the modus of action was linked to stimulation of anti-tumor immunity. Here we comment on these findings, highlighting several remaining questions.

Proinflammatory characteristics of SMAC/DIABLO-induced cell death in antitumor therapy.

Molecular mimetics of the caspase activator second mitochondria-derived activator of caspase (SMAC) are being investigated for use in cancer therapy, but an understanding of in vivo effects remains incomplete. In this study, we offer evidence that SMAC mimetics elicit a proinflammatory cell death in cancer cells that engages an adaptive antitumor immune response. Cancer cells of different histologic origin underwent apoptosis when transduced with lentiviral vectors encoding a cytosolic form of the SMAC mimetic LV-tSMAC. Strikingly, treatment of tumor-bearing mice with LV-tSMAC resulted in the induction of apoptosis, activation of antitumor immunity, and enhanced survival. Antitumor immunity was accompanied by an increase of tumor-infiltrating lymphocytes displaying low PD-1 expression, high lytic capacity, and high levels of IFN-γ when stimulated. We also noted in vivo a decrease in regulatory T cells along with in vitro activation of tumor-specific CD8(+) T cells by dendritic cells (DC) isolated from tumor draining lymph nodes. Last, tumor-specific cytotoxic T cells were also found to be activated in vivo. Mechanistic analyses showed that transduction of cancer cells with LV-tSMAC resulted in exposure of calreticulin but not release of HMGB1 or ATP. Nevertheless, DCs were activated upon engulfment of dying cancer cells. Further validation of these findings was obtained by their extension in a model of human melanoma using transcriptionally targeted LV-tSMAC. Together, our findings suggest that SMAC mimetics can elicit a proinflammatory cell death that is sufficient to activate adaptive antitumor immune responses in cancer.

Lentiviral vectors for anti-tumor immunotherapy.

It is generally accepted that active therapeutic immunization approaches hold great promise for treating malignant tumors. In recent years, lentiviral vectors have emerged as promising tools for anti-tumor immunotherapy due to their capacity to transduce a wide range of different dividing and non-dividing cell types, including tumor cells and dendritic cells (DC). The latter are considered to be the key regulators of immunity and are therefore applied as 'nature's adjuvant' in terms of eliciting strong antigen-specific cytotoxic T lymphocyte responses against tumor antigens. Therefore, lentiviral vectors have been carefully examined as gene transfer vehicles, be it for ex vivo or in vivo modification of DC and have been demonstrated to induce potent T cell mediated immune responses that can control tumor growth. Here, we review the use of lentivirally transduced DC and lentiviral vectors - as such - as an anti-tumor immunotherapeutic. Furthermore, we focus on the DC modulatory capacity of lentiviral vectors and the various efforts that have been made to improve the overall performance and safety of in vivo administration of lentiviral vectors. In conclusion, this review highlights the potential of lentiviral vectors as a generally applicable 'off-the-shelf' therapeutic for anti-tumor immunotherapy.

Activation of immature monocyte-derived dendritic cells after transduction with high doses of lentiviral vectors.

Dendritic cells (DCs) are an attractive tool for immunomodulation, targeting mature DCs (mDCs) for immunization or immature/semimature DCs (iDCs) for tolerization. Therefore, introducing antigens into DCs has become a prime topic in various immunological disciplines. Numerous studies have shown that lentiviruses are an efficient vehicle for this purpose. This study evaluates the effects of lentiviral transduction on iDC activation. Immature DCs are efficiently transduced with increasing doses of lentivirus without affecting cell viability. Transduction at low multiplicities of infection (MOIs) did not result in phenotypical or functional maturation. Higher doses of lentivirus, however, resulted in upregulation of adhesion, costimulatory, and HLA molecules, as well as in increased allostimulatory capacity and secretion of interleukin (IL)-6, IL-8, and tumor necrosis factor-alpha. Production of IL-12 p70, IL-10, and interferon-alpha was observed only at extremely high doses. Protein kinase R phosphorylation on transduction at an MOI of 150 was demonstrated by Western blotting. A Toll-like receptor (TLR)-driven luciferase reporter assay showed dose-dependent activation of TLR2, TLR3, and TLR8, which was independent of the pseudotype, production, or transduction protocol and was abrogated on heat inactivation. These data show that lentiviral vectors provide not only the antigen but also appropriate activation signals to iDCs, favoring their use for immunotherapy and vaccine development.