Editorial: Immune Checkpoint Molecules and Cancer Immunotherapy

Enhanced development of functional human NK cells in NOD-scid-IL2rgnull mice expressing human IL15

Human innate immunity plays a critical role in tumor surveillance and in immunoregulation within the tumor microenvironment. Natural killer (NK) cells are innate lymphoid cells that have opposing roles in the tumor microenvironment, including NK cell subsets that mediate tumor cell cytotoxicity and subsets with regulatory function that contribute to the tumor immune suppressive environment. The balance between effector and regulatory NK cell subsets has been studied extensively in murine models of cancer, but there is a paucity of models to study human NK cell function in tumorigenesis. Humanized mice are a powerful alternative to syngeneic mouse tumor models for the study of human immuno-oncology and have proven effective tools to test immunotherapies targeting T cells. However, human NK cell development and survival in humanized NOD-scid-IL2rg^null (NSG) mice are severely limited. To enhance NK cell development, we have developed NSG mice that constitutively expresses human Interleukin 15 (IL15), NSG-Tg(Hu-IL15). Following hematopoietic stem cell engraftment of NSG-Tg(Hu-IL15) mice, significantly higher levels of functional human CD56+ NK cells are detectable in blood and spleen, as compared to NSG mice. Hematopoietic stem cell (HSC)-engrafted NSG-Tg(Hu-IL15) mice also supported the development of human CD3+ T cells, CD20+ B cells, and CD33+ myeloid cells. Moreover, the growth kinetics of a patient-derived xenograft (PDX) melanoma were significantly delayed in HSC-engrafted NSG-Tg(Hu-IL15) mice as compared to HSC-engrafted NSG mice demonstrating that human NK cells have a key role in limiting the tumor growth. Together, these data demonstrate that HSC-engrafted NSG-Tg(Hu-IL15) mice support enhanced development of functional human NK cells, which limit the growth of PDX tumors.

Upregulation of Multiple CD8+ T Cell Exhaustion Pathways Is Associated with Recurrent Ocular Herpes Simplex Virus Type 1 Infection

A large proportion of the world's population harbors latent HSV type 1 (HSV-1). Cross-talk between antiviral CD8⁺ T cells and HSV-1 appear to control latency/reactivation cycles. We found that compared with healthy asymptomatic individuals, in symptomatic (SYMP) patients, the CD8⁺ T cells with the same HLA-A*0201-restricted HSV-1 epitope specificities expressed multiple genes and proteins associated to major T cell exhaustion pathways and were dysfunctional. Blockade of immune checkpoints with anti-LAG-3 and anti-PD-1 antagonist mAbs synergistically restored the frequency and function of antiviral CD8⁺ T cells, both 1) ex vivo, in SYMP individuals and SYMP HLA-A*0201 transgenic mice; and 2) in vivo in HSV-1-infected SYMP HLA-A*0201 transgenic mice. This was associated with a significant reduction in virus reactivation and recurrent ocular herpetic disease. These findings confirm antiviral CD8⁺ T cell exhaustion during SYMP herpes infection and pave the way to targeting immune checkpoints to combat recurrent ocular herpes.

Enhanced expression of CD39 and CD73 on T cells in the regulation of anti-tumor immune responses

Synthesis of extracellular adenosine by the ectonucleotidases CD39 and CD73 represents an important pathway of immune suppression in the tumor microenvironment. Using two mouse models (RET transgenic melanoma and Panc02 orthotopic pancreatic adenocarcinoma), we identified an elevated frequency of ectonucleotidase-expressing T cells in tumors and spleens. Importantly, these ectonucleotidase-positive T cells also showed a pronounced expression of PD-1. Conversely, the PD-1⁺ T cell subsets in tumors contained substantially larger proportions of ectonucleotidase-expressing cells compared to their counterparts lacking PD-1 expression. Our in vitro experiments showed that the activation of normal T cells resulted in an increase in the CD39 expression. CD39⁺ and CD73⁺ T cells displayed effector or memory phenotypes and produced IFN-γ, thereby linking ectonucleotidase expression to T cell effector functions. An accumulation of conventional and regulatory T cells expressing CD39 and/or CD73 was also detected in the peripheral blood of patients with melanoma and pancreatic cancer. Moreover, we demonstrated a significant association between low frequencies of circulating CD73⁺CD8⁺ T cells and CD73⁺CD4⁺ regulatory T cells and better overall survival of melanoma patients. Tumor-derived soluble factors (in particular, TGF-β) significantly enhanced the frequencies of ectonucleotidase-expressing cells in mice. Our findings suggest that the upregulation of ectonucleotidase expression in T cells promotes extracellular adenosine accumulation and represents an important mechanism of homeostatic immune auto-regulation, which could be hijacked by tumors to evade anti-cancer immunity. Targeting CD39 and CD73 can open new avenues for cancer immunotherapy.