Characterization of monoclonal antibodies to the plasma cell alloantigen ENPP1

ENPP1 Immunobiology as a Therapeutic Target

ENPP1 (ecto-nucleotide pyrophosphatase/phosphodiesterase) participates in the hydrolysis of different purine nucleotides in an array of physiologic processes. However, ENPP1 is frequently overexpressed in local relapses and tumor metastases, which are associated with poor prognosis and survival in a range of solid tumors. ENPP1 promotes an immunosuppressive tumor microenvironment (TME) by tilting the balance of ATP/adenosine (Ado) in conjunction with other components (CD38, CD39/ENTPD1, and CD73/NT5E). Moreover, ENPP1 intersects with the stimulator of interferon genes (STING), impairing its robust immune response through the hydrolysis of the effector 2´,3´-cyclic GMP-AMP. Thus, ENPP1 blockade emerges as a unique target eliciting immune remodeling and leveraging the STING pathway. Several ENPP1 inhibitors have shown an immunostimulatory effect, and their combination with other therapeutic modalities, such as immune-checkpoint blockade, STING activation, DNA damage response (DDR) inhibitors, and radiotherapy (RT), represents a promising avenue to boost antitumor-immune responses and to improve current clinical outcomes in several tumors. This comprehensive review summarizes the current state of the art and opens new perspectives for novel treatment strategies.

Tumor ENPP1(CD203a)/Haptoglobin Axis Exploits Myeloid-Derived Suppressor Cells to Promote Post-Radiotherapy Local Recurrence in Breast Cancer

Locoregional failure (LRF) in breast cancer patients post-surgery and post-irradiation (IR) is linked to a dismal prognosis. In a refined new model, we identified Enpp1 (Ectonucleotide pyrophosphatase /phosphodiesterase 1/CD203a) to be closely associated with LRF. Enpp1high circulating tumor cells (CTC) contribute to relapse by a self-seeding mechanism. This process requires the infiltration of PMN-MDSC and neutrophil extracellular traps (NET) formation. Genetic and pharmacological Enpp1 inhibition or NET blockade extend relapse-free survival. Furthermore, in combination with fractionated irradiation (FD), Enpp1 abrogation obliterates LRF. Mechanistically, Enpp1-generated adenosinergic metabolites enhance Haptoglobin (Hp) expression. This inflammatory mediator elicits myeloid invasiveness and promotes NET formation. Accordingly, a significant increase in ENPP1 and NET formation is detected in relapsed human breast cancer tumors. Moreover, high ENPP1 or HP levels are associated with poor prognosis. These findings unveil the ENPP1/HP axis as an unanticipated mechanism exploited by tumor cells linking inflammation to immune remodeling favoring local relapse.

ENPP1, an Old Enzyme with New Functions, and Small Molecule Inhibitors-A STING in the Tale of ENPP1

Ectonucleotide pyrophosphatase/phosphodiesterase I (ENPP1) was identified several decades ago as a type II transmembrane glycoprotein with nucleotide pyrophosphatase and phosphodiesterase enzymatic activities, critical for purinergic signaling. Recently, ENPP1 has emerged as a critical phosphodiesterase that degrades the stimulator of interferon genes (STING) ligand, cyclic GMP-AMP (cGAMP). cGAMP or analogs thereof have emerged as potent immunostimulatory agents, which have potential applications in immunotherapy. This emerging role of ENPP1 has placed this "old" enzyme at the frontier of immunotherapy. This review highlights the roles played by ENPP1, the mechanism of cGAMP hydrolysis by ENPP1, and small molecule inhibitors of ENPP1 with potential applications in diverse disease states, including cancer.

ATP-degrading ENPP1 is required for survival (or persistence) of long-lived plasma cells

Survival of antibody-secreting plasma cells (PCs) is vital for sustained antibody production. However, it remains poorly understood how long-lived PCs (LLPCs) are generated and maintained. Here we report that ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) is preferentially upregulated in bone marrow LLPCs compared with their splenic short-lived counterparts (SLPCs). We studied ENPP1-deficient mice (Enpp1^−/−) to determine how the enzyme affects PC biology. Although Enpp1^−/− mice generated normal levels of germinal center B cells and plasmablasts in periphery, they produced significantly reduced numbers of LLPCs following immunization with T-dependent antigens or infection with plasmodium C. chabaudi. Bone marrow chimeric mice showed B cell intrinsic effect of ENPP1 selectively on generation of bone marrow as well as splenic LLPCs. Moreover, Enpp1^−/− PCs took up less glucose and had lower levels of glycolysis than those of wild-type controls. Thus, ENPP1 deficiency confers an energetic disadvantage to PCs for long-term survival and antibody production.

Plasma cell alloantigen ENPP1 is expressed by a subset of human B cells with potential regulatory functions

Plasma cell alloantigen 1 (PC1), also known as ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1), is an enzyme involved primarily in hydrolysis of adenosine triphosphate at the cell surface. Although the expression pattern of PC1 is relatively broad, its expression in B cells is found at significant levels only in terminally differentiated germinal center B cells, plasma cells and a subset of B-1a cells in mice. Here we describe studies designed to determine whether expression of PC1 might define novel populations of human B cells with similarities to mouse B cells. We found that PC1 is expressed in small populations of human B lineage cells in peripheral blood, cord blood, tonsils, bone marrow and pediatric peritoneal fluid, with the highest levels in plasma cells. The characteristics of human PC1(+) B cells differ from mouse peritoneal B-1a subsets and from features of the human CD20(+)CD27(+)CD43(+)CD70(-) B-cell subset proposed to be human B-1 cells. Expression of PC1 was greatly increased in B cells stimulated with the combination of CD40 ligand, interleukin (IL)-4 and IL-21. In addition, PC1(+) B cells activated CD4(+) T regulatory cells. ENPP1 thus defines a subset of human B cells that differs significantly from mouse peritoneal B-1a and proposed human B-1 cells.

New insights into heterogeneity of peritoneal B-1a cells

Peritoneal B-1a cells are characterized by their expression of CD5 and enrichment for germline-encoded IgM B cell receptors. Early studies showing expression of a diverse array of VDJ sequences among purified B-1a cells provided a molecular basis for understanding the heterogeneity of the B-1a cell repertoire. Antigen-driven positive selection and the identification of B-1a specific progenitors suggest multiple origins of B-1a cells. The introduction of new markers such as PD-L2, CD25, CD73, and PC1 (plasma cell alloantigen 1, also known as ectonucleotide phosphodiesterase/pyrophosphatase 1) further helped to identify phenotypically and functionally distinct B-1a subsets. Among many B-1a subsets defined by these new markers, PC1 is unique in that it subdivides B-1a cells into PC1(hi) and PC1(lo) subpopulations with distinct functions, such as production of natural IgM and gut IgA, response to the pneumococcal antigen PPS-3, secretion of interleukin-10, and support for T helper 1 (TH 1) cell differentiation. RNA sequencing of these subsets revealed differential expression of genes involved in cellular movement and immune cell trafficking. We will discuss these new insights underlying the heterogeneous nature of the B-1a cell repertoire.

Expression of plasma cell alloantigen 1 defines layered development of B-1a B-cell subsets with distinct innate-like functions

Innate-like B-1a cells contribute significantly to circulating natural antibodies and mucosal immunity as well as to immunoregulation. Here we show that these classic functions of B-1a cells segregate between two unique subsets defined by expression of plasma cell alloantigen 1 (PC1), also known as ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1). These subsets, designated B-1a.PC1(lo) and B-1a.PC1(hi), differ significantly in IgH chain utilization. Adoptively transferred PC1(lo) cells secreted significantly more circulating natural IgM and intestinal IgA than PC1(hi) cells. In contrast, PC1(hi) cells produced more IL-10 than PC1(lo) cells when stimulated with LPS and phorbol 12-myristate 13-acetate (PMA). PC1(hi) cells were also more efficient than PC1(lo) cells in regulating Th1 cell differentiation, even though both B-1a subsets were comparably active in stimulating T-cell proliferation. Furthermore, PC1(lo) cells generated antigen-specific IgM responses to pneumococcal polysaccharide antigens, whereas PC1(hi) cells do not. We found that PC1(lo) cells develop from an early wave of B-1a progenitors in fetal life, whereas PC1(hi) cells are generated from a later wave after birth. We conclude that identification of B-1a.PC1(lo) and B-1a.PC1(hi) cells extends the concept of a layered immune system with important implications for developing effective vaccines and promoting the generation of immunoregulatory B cells.

Visinin-like protein-1 overexpression is an indicator of lymph node metastasis and poor prognosis in colorectal cancer patients

Lymph node metastasis is an important factor determining outcome from colorectal cancer (CRC). Identification of molecular markers useful to predict lymph node metastasis is urgently needed. Our objective was to identify genes useful for characterization and prediction of lymph node metastasis in CRC. Gene expression profiles of cancer were determined by human U133 Plus 2.0 GeneChip® in 24 CRC patients, and patients with and without lymph node metastasis were compared. We focused on the visinin-like protein-1 (VSNL-1) gene and evaluated VSNL-1 mRNA expression levels with reverse transcriptase-polymerase chain reaction and immunohistochemical methods. Immunohistochemical evaluation of VSNL-1 mRNA expression was also performed in 143 other CRC patients to determine clinicopathological significance of VSNL-1. Twenty-four novel discriminating genes showed expression significantly different between patients with and without lymph node metastasis. Mean level of VSNL-1 mRNA expression in tumor tissue with lymph node metastasis was significantly higher than that in tissue without lymph node metastasis. Immunohistochemical examination demonstrated immunoreactivity for VSNL-1 in cytoplasm of the cancer cells with lymph node metastasis. High VSNL-1 expression was significantly associated with lymphatic invasion in stage II disease (p = 0.0061) and number of lymph node metastases in stage III disease (p = 0.0461). Patients with high VSNL-1 expression had significantly poorer prognosis than those with low expression in stage III disease (p = 0.045). This study is the first to demonstrate a prognostic role for VSNL-1 at the mRNA level, suggesting the possible usefulness of VSNL-1 as a predictor of lymph node metastasis and poor prognosis in CRC.