A new study has identified a subclass of macrophages which have possibly been disrupting drugs designed to target the anti-PD-1 checkpoint blockade in the deadly form of brain cancer, glioblastoma. By understanding the mechanisms, genetic targets, and now tumor microenvironments involved in this interaction, it may be possible to design more effective anticancer drugs.
“By studying the immune microenvironments across tumor types, we’ve identified a rational combination therapy for glioblastoma,” states first author Sangeeta Goswami, M.D., Ph.D.
Researchers have made this discovery in no small part by utilizing a novel research strategy which they call reverse translation. Instead of using the typical drug discovery model, where a hypothesis is generated through cell line and animal model experiments, and subsequently attempted to be translated to human clinical trials, the team started by analyzing human tumors to generate hypotheses for testing in the lab, in hopes of using these findings in human clinical trials. This thought process appears to be paying dividends in terms of the data and results that have been generated.
In the first phase of this study, the team analyzed a library of tumors to generate a hypothesis. This part of the project aimed to characterize the immune cells found in five tumor types using a combination of mass cytometry and single-cell RNA sequencing techniques. The researchers analyzed 94 human tumors across glioblastoma, non-small cell lung cancer and kidney, prostate and colorectal cancers to characterize clusters of immune cells.
The most surprising finding from this study was a metacluster of immune cells found predominantly among the 13 glioblastoma tumors. Cells in this cluster expressed CD68 (a cell marker for macrophages that can either aid or suppress the immune response). The CD68 metacluster also expressed high levels of CD73, as well as other immune-inhibiting molecules. The team confirmed these findings in nine additional glioblastomas.
The single-cell RNA sequencing portion of this experiment identified an immunosuppressive gene expression signature associated with the high-CD73-expressing macrophages. A refined gene signature for the cells was evaluated against 525 glioblastoma samples from The Cancer Genome Atlasand was correlated with decreased survival.
The team of researchers then identified macrophages that express high levels of CD73, a surface enzyme part of an immunosuppressive molecular pathway in the glioblastoma tumors they were studying. The strong presence of the CD73 macrophages was unique to glioblastoma among the five tumor types analyzed by the researchers.
The team confirmed these finding by conducting a mass cytometry cluster analysis on five glioblastoma tumors treated with the PD-1 checkpoint inhibitor pembrolizumab and compared the results to seven untreated tumors. They identified three CD73-expressing macrophage clusters that persisted despite pembrolizumab treatment. Sharma and colleagues note the prevalence of CD73-expressing macrophages likely contributed to the lack of tumor-killing T cell responses and poor clinical outcome.
After establishing the cells’ presence in human tumors and correlating them with decreased survival, the researchers took their hypothesis to a mouse model of glioblastoma. They found that combining anti-PD-1 and anti-CTLA-4 immunotherapies in CD73 knockout mice stifled tumor growth and increased survival.
In the second phase of the study, the team treated two groups of mice with either PD-1 inhibitors or a combination of PD-1 and CTLA-4 immune checkpoint inhibitor drugs. Mice with intact CD73 treated with the combination showed an increased survival rate compared to untreated mice, while mice with CD73 knocked out lived even longer after combination therapy. There was no survival benefit from anti-PD-1 alone, a result that was previously seen in human clinical trials.
“Based on our data and earlier studies, we propose a combination therapy strategy to target CD73 plus dual blockade of PD-1 and CTLA-4,” the team concludes in the paper, noting that anti-CD73 antibodies have yielded promising results in early studies. A mouse model of glioblastoma that can show knocking out CD73 alone can slow tumor growth and increased survival is very relevant for human studies.
“We’re working with pharmaceutical companies that are developing agents to target CD73 to move forward with a glioblastoma clinical trial in combination with anti-PD-1 and anti-CTLA-4 checkpoint inhibitors,” said Padmanee Sharma, M.D., Ph.D., and co-author in the study.
To effectively use immunotherapy in additional cancer types, Sharma says researchers need to realize immune microenvironments differ from cancer to cancer. “Understanding what’s different in immune niches across cancers provides clues and targets for treating tumors,” Sharma says. “That’s why we did this study.”
It will be very interesting to see the results of this newly proposed treatment method in human clinical trials.