August

August 2021

The following articles have been recommended for further reading in the field of cancer immunotherapy by Dr. Sjoerd H. van der Burg, co-Section Editor for the Basic Tumor Immunology Section.

“Transcriptional programs of neoantigen-specific TIL in anti-PD-1-treated lung cancers” by Justina X. Caushi et al

High tumor mutational burden predicts response to immune checkpoint blockade (ICB), however, the vast majority of tumor-infiltrating lymphocytes (TILs) do not recognize mutation-associated neoantigens (MANA) and the contribution of the small set of MANA-specific T cells to ICB efficacy remains unknown. Justina Caushi and colleagues used transcriptionally profiled MANA-specific T cells in tissue and blood samples from human patients with resectable non-small cell lung cancer (NSCLC) following neoadjuvant PD-1 blockade to define characteristics associated with major pathologic response. Notably, MANA-specific and viral-specific (including Epstein-Barr and influenza viruses) TCRs were found at similar frequencies in samples from patients both with and without major pathologic response to ICB. Refined clustering of unique CD8⁺ T cell clonotypes demonstrated phenotypic overlap between influenza- and MANA-specific T cells with a tissue resident memory cell (TRM) signature. However, MANA-specific TILs expressed lower levels of genes encoding cytotoxic proteins and upregulated expression of checkpoint and T cell exhaustion genes compared to influenza-specific T cells. Functional assays demonstrated a higher avidity/affinity for MHC-neoantigen peptide complexes in MANA-specific TCRs from patients who achieved major pathologic response compared to those that did not. Furthermore, a lack of major pathological response was associated with higher MANA-specific TIL expression of genes associated with T cell dysfunction such as TOX2, CTLA4, HAVCR2, and ENTPD1. Conversely, elevated expression of genes linked to effector function including IL7R, TCF7, and GZMK were seen in MANA-specific TILs from patients with major response. In one patient with major pathologic response, mobilization of MANA-specific TRM cells into the peripheral blood was seen, with reprogramming to an effector signature after resection.

Why this matters: This study confirms the specific expression of CD39 on tumor-specific T cells and suggests that T cells with neoantigen-specific TCRs, unlike some of those reactive to tumor-associated antigens, may be ineffective because of lower affinity cognate interactions.

“Tissue resident macrophages provide a pro-tumorigenic niche to early NSCLC cells” by Maria Casanova-Acebes et al 

Macrophages arise from either self-renewing cells (tissue-resident macrophages, TRMs) or from hematopoietic stem cells (monocyte-derived macrophages, MDMs) and the contribution of each lineage to immunosuppression in the tumor microenvironment remains unclear. Maria Casanova-Acebes and colleagues demonstrated distinct temporal and spatial dynamics for TRMs and MDMs in non-small cell lung cancer (NSCLC). Using previously published single cell RNA sequencing homologous populations of TRMs and MDMs were identified in human NSCLC and murine models. Lineage tracing in lung tumor-bearing mice showed that TRMs clustered near tumor cells during the first 15 days of tumor development and then redistributed to the tumor microenvironment (TME) periphery thereafter. More advanced tumor tissue, on the other hand, was dominated by MDMs. More than 1,600 genes were differentially expressed in TRMs isolated from tumor-bearing mice, including modulators of the TME such as peptidases, chemoattractants, and regulators of Wnt signaling. Epigenetic profiling revealed open chromatin at MHC class II loci in TRMs from early NSCLC tumors. In vitro, tumor cells cultured with TRMs induced a molecular program associated with cell migration and epithelioid-mesenchymal transition, which was associated with enhanced invasion in 3D-matrigel assays. Furthermore, in co-culture, tumor-associated TRMs strongly induced expression of CD73 and CTLA-4 on regulatory T cells (Tregs). In vivo, early depletion of TRMs led to higher numbers of CD3⁺ T cells and CD8⁺ T cells, a higher ratio of CD8⁺ T cells to Tregs, and decreased tumor size. Depleting established tumor tissue of TRMs, however, was not associated with a change in tumor burden, number of Tregs, CD8⁺ to Treg ratio, or CD3⁺ T cell counts.

Why this matters: Tumor-infiltrating macrophages are generally considered to confer poor prognosis to patients, however, they consist of many different functional subsets. Together with the article of Chow et al. (below) these studies suggest an important role for specific sets of tissue-resident macrophages— rather than inflammatory macrophages—in promoting tumor growth and immune evasion early during tumor development. Subsequently this role may be taken over by other subsets as depletion of TRMs at later tumor stages is not effective.

“Tim-4⁺ cavity-resident macrophages impair anti-tumor CD8⁺ T cell immunity” by Andrew Chow et al

Body cavity resident macrophages express high levels of the phosphatidylserine (PS) receptor Tim-4. Andrew Chow and colleagues sought to determine the mechanisms by which these cells may impair anti-tumor activity in metastases to body cavities. In multiple cohorts of human patients, metastases to serous body cavities was associated with worse progression-free and overall survival after immune checkpoint blockade. Depletion of Tim-4⁺ but not Tim-4^- macrophages delayed tumor growth in mouse models of peritoneal carcinomatosis established with MC38 colon cancer cells. In three murine models of peritoneal carcinomatosis, abrogation of Tim-4 (either genetically or via anti-Tim-4 antibody) did not decrease tumor burden or improve survival. In combination with PD-1 blockade, however, Tim-4 abrogation was associated with increased CD8⁺ T cells in the peritoneal cavity and improved tumor control and survival despite no effects on overall numbers of Tim-4⁺ cells. In peritoneal lavage samples from MC38-bearing mice and pleural effusion specimens from human patients with non-small cell lung cancer, high levels of the Tim-4 ligand PS were associated with a transcriptional program consistent with a highly cytotoxic state with higher expression of the proliferation marker Mki67, cytotoxic molecules, and activation/exhaustion genes, results confirmed by flow cytometry. CellTrace Violet labeling of flow sorted CD8⁺ T cells demonstrated preferential accumulation on Tim-4⁺ macrophages in vivo, suggestive of preferential adhesion. In vitro co-culture of both murine and human activated PS^high CD8⁺ T cells with Tim-4⁺ peritoneal macrophages led to sequestration and suppression of proliferation of the CD8⁺ T cells.

Why this matters: Tumor-infiltrating macrophages are generally considered to confer poor prognosis to patients, however, they consist of many different functional subsets. Together with the article of Casanova-Acebes et al. (above) these studies suggest an important role for specific sets of tissue-resident macrophages—rather than inflammatory macrophages—in promoting tumor growth and immune evasion early during tumor development. Here, the capacity of Tim-4 expressing body cavity resident macrophages to sequester cytotoxic PS-enriched CD8⁺ T cells outside the tumor cell nests may form a mechanism underlying the so-called altered-immunosuppressed T cell-infiltration pattern often observed in cancer. Tim-4 blockade may be a viable strategy to enhance the efficacy of immunotherapy.

“Loss of fragile site-associated tumor suppressor promotes antitumor immunity via macrophage polarization” by Lijuan Zhang et al

The function of the enzyme fragile site-associated tumor suppressor (FATS) in immune cells is unknown. Lijuan Zhang and colleagues mechanistically characterize a macrophage-mediated contribution of FATS to immunosuppression in the tumor microenvironment in mice. Compared to WT, reduced tumor growth was seen in melanoma- and pancreatic cancer-bearing Fats^-/- mice. FATS deficiency led to upregulation of MHCII and anti-tumor M1 markers along with reduced expression of tumor-promoting M2 markers in tumor-associated macrophages (TAMs). Serum levels of proinflammatory cytokines such as interferon gamma, IL-12, and IL-1 beta were also increased in Fats^-/- mice. Depletion of macrophages in wild-type mice impaired B16 tumor growth while macrophage depletion in Fats^-/- mice rescued B16 tumor growth, consistent with a functional M2 and M1 type of macrophage, respectively. Bone marrow-derived macrophages (BMDM) from Fats^-/- mice were skewed toward M1 polarization and expressed higher levels of CD80/86 and MHCI/II when exposed to B16 cell supernatant. Following LPS stimulation, increased phosphorylation of IκBα was seen in Fats^-/- macrophages and co-immunoprecipitations revealed interactions between FATS and IκBα, suggesting inhibition of the TLR4-NF-κB activation pathway. In vivo, the percentages and numbers of CD4⁺ T cells and CD8⁺ T cells were significantly increased in tumor-bearing Fats^-/- mice along with elevated CD4⁺ T cell levels of interferon gamma and tumor necrosis factor with a concomitant reduction in regulatory T cells. Depletion of CD4⁺ T cells equalized tumor growth in WT and Fats^-/- BMDMs, whereas adoptive transfer of FATS knockdown BMDMs led to significantly reduced tumor growth in wild-type mice pre-challenged with B16.

Why this matters: Tumor macrophages were shown to be key in the promotion or control of tumor-growth in the same tumor model, depending on the type of macrophage present, which was regulated by the expression of the enzyme FATS. This confirms other recent publications on the effector role of macrophages in tumors and importantly provides a target to capitalize on their tumor-controlling role.