Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Tumour-associated hybrid neutrophils (APC-TANs)

• Glioblastoma (GB)

• Skull bone marrow

Main Findings

1. Tumour associated neutrophils (TANs) are perivascular durable residents of the Glioblastoma (GB) tumour microenvironment (TME) arising from rather immature neutrophil precursors coming from the skull marrow.

2. A major subpopulation of GB TANs is capable of antigen processing and MHCII-dependent T cell activation, exhibiting a non-canonical antigen presenting cell (APC) phenotype that cannot be fully acquired by peripheral blood neutrophils (PBNs).

3. Trajectory and pseudotime analysis reveal that hybrid TAN development occurs intratumorally from immature precursors where the presence of T cells plays a significant role.

4. T cell and TANs co-infiltration of GB engages these two populations in a positive feedback loop required for neutrophils to acquire this APC-TAN phenotype that consequently boosts MHCII-dependent T cell-mediated antitumoral immunity. Absence of T cells in the GB TME shifts TANs towards a protumoral phenotype and establishes oncogenic PDGF/HIF1α signalling supporting tumorigenesis via promoting angiogenesis and GB stem cell enrichment.

5. Ablation of skull bone marrow immune cell progenitors by radiation results in diminished APC-TANs in the GB TME that correlates with increased GBM tumour growth and decreased survival in C57BL6/J and BALB/c mice.

This preprint first reports mechanistic functions of hybrid neutrophils as a long-lasting GB TME population that is capable of antigen processing and MHCII-dependent T cell activation. These hybrid neutrophils were shown to acquire dendritic cell features in the presence of T cell co-infiltration, whereas in the absence of T cells, pro-oncogenic signalling predominates in TANs further leading to GB progression. The study also shows that immature PBN lineage-independent neutrophils in skull marrow give rise to these antigen-presenting TANs that mature and acquire dendritic phenotype intratumorally. Moreover, it was suggested that only bone marrow neutrophil (BMN) progenitors, and not conventional PBNs have the plasticity to be “reprogrammed” to acquire this APC-TAN phenotype.

Importantly, this study suggests that CXCR4 antagonist-stimulated egress of immature neutrophils (together with other APCs) from the skull marrow could be a potent therapeutic strategy reducing GB growth and prolonging lifespan.

Limitations

• The study only briefly touches upon preferential chemotaxis of IDH (isocitrate dehydrogenase)-wild type GB towards neutrophils versus IDH-mutant GBs. The discovery of a GB chemotactic source attracting neutrophils could bring more knowledge for designing therapeutic strategies.

• There is also little to no knowledge whether the presence or absence of APC-TANs in GB also affects lymphocytes or other myeloid populations, which are known to potentially exert pro- or anti-tumoral immune response.

• The study has shown that using tumour-conditioned media, it is possible to ex vivoreprogram immature murine bone marrow neutrophils (BMNs) to express MHCII, representing APC-TAN phenotype observed in vivo. Validation of these experiments using human bone marrow-derived neutrophils (BMNs) would be valuable.

• While this study also suggests that CXCR4 antagonist might be used to increase the migration of immature neutrophils to the GB tumours and should be reconsidered as a therapeutic strategy, it would be interesting how CXCR4 also affects TANs and T cell infiltration and interactions, and possibly other players (e.g., macrophages, DCs) in the immune response in GB.

• Moreover, irradiation effects on depleting immune cell progenitors from skull marrow was mainly shown in murine models. Further validation in resected GB tumours from patients who have received radiation would grain more insight on the span of APC-TAN depletion in human GB.

Significance/Novelty

1. Skull marrow serves as a reservoir for immature neutrophils that migrate to the GB TME and give rise to a heterogeneous TAN population, including a major subpopulation of APC-TANs. Besides TANs, this paper also shows that skull marrow supplies the GB TME with other antigen-presenting myeloid populations, such as MHCIIhi macrophages that are consistent with the newest literature.

2. This APC-TAN subpopulation, and its co-infiltration with T cells, are crucial for antitumoral immune response against GBM.

3. Skull marrow ablation with radiotherapy depletes immune cell progenitor reservoirs and promotes GB tumour growth mainly due to the elimination of immature neutrophils.

To date resection, radiation and chemotherapy remain the standard treatment options for GB patients only mitigating symptoms and prolonging survival by months. Immunotherapies in the form of CAR-T cells, dendritic cell vaccines or immune checkpoint blockage have led to disappointing results in clinical trials. Therefore, it is imperative that we understand the unique TME that may lead to treatment failure as well developing novel immunotherapies that overcome treatment resistance.

Mechanistic knowledge of APC-TANs and their capability to process and present antigens might encourage different or revised therapeutic strategies, that could be effective against GB tumours. Moreover, possible intratumoral delivery of autologous T cells directly into GB TME might be a way to break through the oncogenic signalling in TANs that predominates due to insufficient T cell infiltration. Thus, such therapeutic strategies, combined with e.g., CXCR4 antagonists or other egressing drugs could slow down tumour progression and increase the lifespan of patients. Or, in operated patients, such treatment strategies could be used instead of radiotherapy or chemotherapy, as a less aggressive option.

Credit

Reviewed by Austeja Baleviciute as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.

The author declares no conflict of interests in relation to their involvement in the review.