Pediatric medulloblastomas are the most frequently diagnosed embryonal tumors of the central nervous system. Current therapies cause severe neurological and cognitive side effects including secondary malignancies. Cellular immunotherapy might be key to improve survival and to avoid morbidity. Efficient killing of tumor cells using immunotherapy requires to overcome cancer-associated strategies to evade cytotoxic immune responses. Here, we examined the immune response and immune evasion strategies in pediatric medulloblastomas. Cytotoxic T-cells, infiltrating medulloblastomas with variable activation status, showed no correlation with overall survival of the patients. We found limited numbers of PD1⁺ T-cells and complete absence of PD-L1 on medulloblastomas. Medulloblastomas downregulated immune recognition molecules MHC-I and CD1 d. Intriguingly, expression of granzyme inhibitors SERPINB1 and SERPINB4 was acquired in 23% and 50% of the tumors, respectively. Concluding, pediatric medulloblastomas exploit multiple immune evasion strategies to overcome immune surveillance. Absence of PD-L1 expression in medulloblastoma suggest limited or no added value for immunotherapy with PD1/PD-L1 blockers.

Cancer vaccines can be utilized in combination with checkpoint inhibitors to optimally stimulate the anti-tumor immune response. Uptake of vaccine antigen by antigen presenting cells (APCs) is a prerequisite for T cell priming, but often relies on non-specific mechanisms. Here, we have developed a novel vaccination strategy consisting of cancer antigen-containing liposomes conjugated with CD169- or DC-SIGN-specific nanobodies (single domain antibodies) to achieve specific uptake by APCs. Our studies demonstrate efficient nanobody liposome uptake by human and murine CD169⁺ and DC-SIGN⁺ APCs in vitro and in vivo when compared to control liposomes or liposomes with natural ligands for CD169 and DC-SIGN. Uptake of CD169 nanobody liposomes resulted in increased T cell activation by human APCs and stimulated naive T cell priming in mouse models. In conclusion, while nanobody liposomes have previously been utilized to direct drugs to tumors, here we show that nanobody liposomes can be applied as vaccination strategy that can be extended to other receptors on APCs in order to elicit a potent immune response against tumor antigens.

CD169 is a sialic acid-binding immunoglobulin-like lectin (Siglec-1, sialoadhesin) that is expressed by subsets of tissue-resident macrophages and circulating monocytes. This receptor interacts with α2,3-linked Neu5Ac on glycoproteins as well as glycolipids present on the surface of immune cells and pathogens. CD169-expressing macrophages exert tissue-specific homeostatic functions, but they also have opposing effects on the immune response. CD169⁺ macrophages act as a pathogen filter, protect against infectious diseases, and enhance adaptive immunity, but at the same time pathogens also exploit them to enable further dissemination. In cancer, CD169⁺ macrophages in tumor-draining lymph nodes are correlated with better clinical outcomes. In inflammatory diseases, CD169 expression is upregulated on monocytes and on monocyte-derived macrophages and this correlates with the disease state. Given their role in promoting adaptive immunity, CD169⁺ macrophages are currently investigated as targets for vaccination strategies against cancer. In this review, we describe the studies investigating the importance of CD169 and CD169⁺ macrophages in several disease settings and the vaccination strategies currently under investigation.

Figure: Background: Siglec-1 (Sialoadhesin/CD169) is expressed on myeloid cells and plays a key role in host defences by capturing incoming sialylated-pathogens such as Campylobacter jejuni . However, binding to Siglec-1 has also been exploited by pathogens such as SARS-CoV-2 for further dissemination. Results: Here we identified high-affinity VHHs also known as single-domain antibodies or Nanobodies that bind to Siglec-1 and allo- or orthosterically modulate ligand binding. VHH 2C2 was shown to bind directly to the ligand binding site of Siglec-1 and blocked binding of ganglioside liposomes and Campylobacter jejuni to monocyte-derived dendritic cells (moDCs) and ex vivo Siglec-1 ⁺ DCs. VHH 2C2 also blocked SARS-CoV-2 binding of moDCs. In contrast, the VHHs 1B5 and 1C1 interacted with Siglec-1 outside the ligand binding site and acted as positive allosteric modulators of Siglec-1 ligand interactions, as was illustrated by increased ganglioside liposome and Campylobacter jejuni binding by moDCs. Our data suggests that mechanistically, the VHH 1B5 and 1C1 interfere with the cis -binding sialic acids present on the Siglec-1-expressing cell and thereby enhance trans -interactions with ligands. Conclusion: In conclusion, we have isolated VHH that enhance or block Siglec-1 ligand binding to a variety of sialylated-pathogens enabling further interrogation of Siglec-1 function. Moreover, unlike conventional blocking antibodies targeting specific pathogens, Siglec-1 binding VHH could potentially serve as broad-spectrum pathogen blocking agents.

Cancer vaccines' efficacy to stimulate T cell responses in patients is currently sub-optimal, and this can be potentially overcome by the delivery of antigens directly to antigen-presenting cells (APCs). To achieve this, single domain antibodies (nanobodies, Nbs) specific for receptors on APCs can be conjugated to liposomes to deliver cancer antigens. Here, we generated three CD169-specific Nb liposomes containing 3 different Nb clones (1B5, 1C1, and 2C2) and one CLEC9A-specific Nb liposome. Blocking experiments with CD169-specific antibodies indicated distinct binding epitopes on CD169 for 2C2 compared with 1B5 and 1C1. Both 2C2 and 1C1 Nb liposomes showed stronger binding and uptake by CD169-overexpressing cell lines and monocyte derived dendritic cells compared to 1B5 liposomes. Analysis of primary blood-derived APCs demonstrated strong uptake of 2C2 and 1C1 Nb liposomes by Axl+ Siglec-6+ DCs, classical monocytes, and DC3s, whereas CLEC9A-specific Nb liposomes were taken up by CLEC9A-expressing XCR1+ DC1s. In contrast, when gp100-specific T cell activation was evaluated using gp100-containing CD169-specific Nb liposomes and moDCs, only 1B5 Nb liposomes led to an enhanced T cell response. Using enriched blood DCs, we also observed an increased T cell activation by gp100-containing CLEC9A-specific Nb liposomes. To conclude, CD169- and CLEC9A-specific Nb liposomes are specifically taken up by primary CD169- or CLEC9A-expressing human APCs, and, depending on the Nb, can lead to a higher T cell activation. These results highlight the potential of Nb liposomes to improve vaccination strategies for immunotherapy.

Dendritic cells (DCs) are important orchestrators of immune responses. Their development in the bone marrow is controlled by transcription factors, but epigenetic mechanisms remain poorly understood. DOT1L is emerging as a key epigenetic regulator in immune cells. By mapping DOT1L-mediated histone H3K79 methylation in canonical DC subsets, we observed that DOT1L modified common as well as DC subset-specific genes. In vitro- or in vivo- induced deletion of Dot1l followed by in vitro cell culture resulted in a decrease in myeloid progenitors and plasmacytoid DCs (pDCs) and an increase in cDC2s, while cDC1s remained unchanged. In vitro generated Dot1l- KO DCs were unable to produce IFNα upon stimulation. Moreover, transcriptomes of Dot1l -KO DC subsets exhibited enrichment of antigen presentation pathways and MHC class II surface levels were upregulated in pDCs. Mechanistically, inhibition of DOT1L linked the observed effects to its methyltransferase activity. Together, our data indicate that in DCs DOT1L differentially affects the development of canonical subsets and suppresses antigen presentation pathways.