ADP-Ribosylation of P2X7: A Matter of Life and Death for Regulatory T Cells and Natural Killer T Cells

Abstract

ADP-ribosyltransferases comprise a family of enzymes originally discovered as bacterial toxins and later characterised also in mammals. In mice, the ADP-ribosyltransferase ARTC2.2 is expressed at the surface of T lymphocytes and has been studied extensively. In the presence of extracellular NAD(+), ARTC2.2 ADP-ribosylates several cell surface target proteins and thereby regulates their function. P2X7, an ATP-gated cation channel, has been discovered as a prominent ARTC2.2 target at the surface of mouse T cells. ADP-ribosylation of P2X7 in the presence of low micromolar extracellular NAD(+) induces long-lasting P2X7 activation and triggers cell death. Regulatory T cell subsets (Tregs and NKT cells) are remarkably sensitive to NAD(+)-induced cell death (NICD). Thus, liberation of endogenous NAD(+) by stressed cells is now viewed as a danger signal promoting immune responses by hindering regulatory T cells. This review will highlight the recent discoveries on the in vivo role of the ARTC2.2/P2X7 pathway triggered by the endogenous release of extracellular NAD(+), the relative sensitivity of lymphocytes subsets to this regulatory pathway and its pharmacological manipulation using camelid-derived ARTC2.2-blocking nanobodies.

ENPP1 processes protein ADP‐ribosylation in vitro

“…Our biochemical data suggest that ENPP1 may be involved in the metabolism of extracellular PAR through its hydrolysis and release of phosphoribosylated proteins, PRAMP and, contextually, free PAR – a molecule that has previously been described as an extracellular stimulus driving inflammatory signalling . Moreover, colocalization of ARTCs and ENPP1 on the extracellular membrane may suggest a physiological function for ENPP1 in the regulation of immune cell activity and survival.…”

Daratumumab induces CD38 internalization and impairs myeloma cell adhesion

“…50 CD38 participates in a number of enzymatic activities: 51 1) regulating calcium homeostasis within the cell through synthesis of cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) in a pH-dependent process; 10 and 2) breaking down extracellular nicotinamide adenine dinucleotide (NAD+) or forming intracellular nicotinamide (NAM) or nicotinamide mononucleotide (NMN). 9 NAD+ hydrolysis by CD38 generates adenosine diphosphate ribose (ADPR), directly or through the cADPR intermediate, which is ultimately converted to adenosine (ADO), 52 a nucleotide that influences immune cell functions [11][12][13] and is present in large amounts in the MM marrow 14 and in microvesicles (MVs) enriched with CD38, isolated from BM plasma of MM patients. 53 Generation of adenosine affects the MM microenvironment but also CD38 has direct immunologic activities on surrounding cells by associating with the T-cell receptor, 54 the B-cell receptor complex, 55 and with CD16 on natural killer cells.…”

“…8 CD38 participates in a number of enzymatic activities, including breaking down extracellular nicotinamide adenine dinucleotide (NAD+) 9 and regulating calcium homeostasis, 10 which influences immune cell functions. [11][12][13] It is present in large amounts in the marrow of MM patients. 14 In support of its function as an ectoenzyme, it has been previously reported that a fraction of the entire CD38 surface molecule can be internalized by endocytosis in a number of leukemia-and lymphoma-derived cell lines.…”

ARTC2.2/P2RX7 Signaling during Cell Isolation Distorts Function and Quantification of Tissue-Resident CD8+ T Cell and Invariant NKT Subsets

“…Indeed, previous studies have shown extensive cell death of T cell populations under these circumstances, especially cells expressing high levels of ARTC2.2 and P2RX7, like CD4 + T regulatory cells (T reg ) (1). Moreover, even cells that survive isolation steps may be compromised for in vitro functional assays (13).…”

“…To tackle this issue, ARTC2.2-specific antagonist nanobodies to block the ARTC2.2/P2RX7 signaling axis were developed (9). Previous studies successfully used this strategy to recover lymphocytes with high expression of ARTC2.2, including T reg and invariant NKT cells (iNKT) (13). Two recent reports showed that ARTC2.2 blockade also prevents the death of liver CD8 + tissue-resident memory T cells (T RM ) during tissue preparation (14,15).…”