Keeping in Check Neuropilin-1 for Effective Antitumor Therapy

The use of immune checkpoint blockade (ICB) therapy which aims to reinvigorate the immune response against tumors is one of the most remarkable achievements of modern medicine. To highlight this success, it’s worthy to look at the past. Prior to the advent of ICB therapy, the diagnosis of metastatic melanoma for almost all patients carried a typical death sentence with a life expectancy ranging from a few months to a year from diagnosis. ICB therapy has changed this dramatically, and now in the case of metastatic melanoma, in some cases almost half of the patients survive the disease for at least five years from diagnosis. Similar success stories in other types of advanced cancers, including non-small cell lung cancer, renal cell carcinoma, and Hodgkin’s lymphoma, to name a few, are also attributed to ICB therapy. 

Still, one of the puzzles of ICB therapy is that even in the best scenarios, only a fraction of patients responds positively to treatment. Sadly, ICB therapy does little for the other cancer patients who eventually succumb to the disease. In order to understand the variation in who responds positively and who fails to respond to ICB therapy, we must understand the agents involved in these treatments. ICB therapy primarily involves antibodies that target molecules expressed by immune cells (and in some cases also by the cancerous cells) and these include cytotoxic T lymphocyte antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed death-ligand 1 (PD-L1). ICB therapy is thought to remove a “molecular brake” and thus permits cytotoxic CD8+ T cells to destroy cancer cells more effectively. These CD8+ T cells are critical in clearing damaged cells from our bodies that have become cancerous (or also become infected by an intracellular pathogen, like a virus). A common observation though, is that in a chronic disease, like cancer, CD8+ T cells often become dysfunctional (also termed “exhausted”) and no longer are able clear the cancerous cells effectively. ICB therapy, thus, relieves this exhausted state of the cancer fighting CD8+ T cells, although as already mentioned, it does not work for some patients. 

One possibility explaining the lack of efficacy in ICB therapy could be that the “molecular brake” in CD8+ T cells is perhaps not adequately relieved just by targeting of the molecules mentioned above (CTLA-4, PD-1 or PD-L1). Indeed, other molecules that are associated with this exhausted or dysfunctional state of the cancer fighting CD8+ T cells may also prevent effective antitumor immunity and their targeting may provide better efficacy than current ICB therapy. A remarkable study from Dr. Dario Vignali’s lab at the University of Pittsburgh led by Dr. Chang Liu and published recently in the Nature Immunology journal sought to address the role of one of these molecules, called Neuropilin-1 (NRP-1). NRP-1 is a cellular receptor that has many functions in development, vascularization, and neuronal axon guidance. In the immune system, NRP-1 also plays a role in immunoregulation as its expressed by regulatory T cells (Tregs) and facilitates their functions. Recent evidence suggested that NRP-1 may also promote the dysfunctional state of CD8+ T cells in tumors as its expression associates well with the exhausted phenotype. To this end, Liu et al. examined the role of NRP-1 by using mice lacking NRP-1 specifically in CD8+ T cells.

Liu et al. found that NRP-1 was not required for tumor growth during a primary tumor, meaning that mice lacking NRP-1 in CD8+ T cells succumbed to tumors as rapidly as normal mice. However, the authors found that NRP-1 played a critical role in enabling tumor growth during a secondary response to the tumor. In this case, the authors challenged the mice with tumors which were removed after 12 days and then the mice were re-challenged 1 month or 2 months later with the same tumor. If the mice lacked CD8+ T cells with NRP-1 during the re-challenge phase, they were significantly more likely to survive and clear their secondary tumors than normal mice (which have NRP-1-expressing CD8+ T cells). Additionally, mice lacking NRP-1 specifically in the CD8+ T cells exhibited better tumor clearance to an inferior dose of anti-PD-1 ICB therapy. These results indicate that NRP-1 on CD8+ T cells is detrimental for secondary or “memory” responses in antitumor immunity and for positive responses to ICB therapy.

Indeed, the authors found that NRP-1 limited a memory precursor pool (so-called “stem-like” CD8+ T cells) of the cancer fighting CD8+ T cells in mice, although the underlying molecular mechanisms for this phenomenon remained unclear. The authors did find that expression of NRP-1 in CD8+ T cells promoted more exhaustion and cell death, and this was associated with decreased expression of the c-Jun transcription factor. This decreased expression of c-Jun is thought to contribute to the dysfunctional state of exhausted T cells. To emphasize relevance to human cancers, the authors found that NRP-1 expression on a specific memory subset of CD8+ T cells (effector memory; TEM) in blood was associated with more aggressive head and neck squamous cell carcinoma in a clinical cohort. Additionally, in a second cohort, patients who showed poor responses to ICB therapy in advanced skin cancers displayed more NRP-1 on TEM’s among CD8+ T cells in blood compared to those who showed positive responses. Altogether, these results identify NRP-1 as a specific checkpoint that regulates memory CD8+ T cell responses to tumors and which may also regulate the response to ICB therapy in cancer patients.

Despite the rigorous work by Liu et al. in establishing NRP-1 as a “memory checkpoint” for cancer fighting CD8+ T cells, some questions remained unanswered. For one, whether signaling by NRP-1 intrinsically limits the memory precursors of the cancer fighting CD8+ T cells was not specifically addressed. NRP-1 is surface expressed by the T cells and is thought to bind to the Semaphorin-3A/-4A and the Vascular Endothelial Growth Factor (VEGF) ligands. Whether these ligands are involved in limiting the antitumor memory T cell response would be an exciting avenue of future work, as these ligands may also be relevant therapeutic targets. Moreover, NRP-1 is also expressed by Tregs which also are implicated in antitumor immunity. NRP1-expressing Tregs may be immunosuppressive in the tumor and detrimental in the response to ICB therapy. As well as through the reinvigoration of the memory precursors of cancer fighting CD8+ T cells, as proposed by this paper, perhaps the selective targeting of NRP-1 may also be beneficial through a mechanism that involves Tregs. Last, this paper raises hope that NRP-1 targeting may improve responses to ICB therapy, which would represent a significant advance in treatment of advanced cancers.

 By Carlos Castellanos

Link to the Study: https://www.nature.com/articles/s41590-020-0733-2

Ribbon Diagram of the Crystal Structure of the b1 domain of Human Neuropilin-1, a memory checkpoint for CD8+ T cells in antitumor immunity (Source: Wikipedia)