ABCA1, apoA-I, and BTN3A1: A Legitimate Ménage à Trois in Dendritic Cells

Abstract

Human Vγ9Vδ2 T cells have the capacity to detect supra-physiological concentrations of phosphoantigens (pAgs) generated by the mevalonate (Mev) pathway of mammalian cells under specific circumstances. Isopentenyl pyrophosphate (IPP) is the prototypic pAg recognized by Vγ9Vδ2 T cells. B-cell derived tumor cells (i.e., lymphoma and myeloma cells) and dendritic cells (DCs) are privileged targets of Vγ9Vδ2 T cells because they generate significant amounts of IPP which can be boosted with zoledronic acid (ZA). ZA is the most potent aminobisphosphonate (NBP) clinically available to inhibit osteoclast activation and a very potent inhibitor of farnesyl pyrophosphate synthase in the Mev pathway. ZA-treated DCs generate and release in the supernatants picomolar IPP concentrations which are sufficient to induce the activation of Vγ9Vδ2 T cells. We have recently shown that the ATP-binding cassette transporter A1 (ABCA1) plays a major role in the extracellular release of IPP from ZA-treated DCs. This novel ABCA1 function is fine-tuned by physical interactions with IPP, apolipoprotein A-I (apoA-I), and butyrophilin-3A1 (BTN3A1). The mechanisms by which soluble IPP induces Vγ9Vδ2 T-cell activation remain to be elucidated. It is possible that soluble IPP binds to BTN3A1, apoA-I, or other unknown molecules on the cell surface of bystander cells like monocytes, NK cells, Vγ9Vδ2 T cells, or any other cell locally present. Investigating this scenario may represent a unique opportunity to further characterize the role of BTN3A1 and other molecules in the recognition of soluble IPP by Vγ9Vδ2 T cells.

Keywords: ATP-binding cassette transporter A1; Vγ9Vδ2 T cells; apolipoprotein A-I; butyrophilin-3A1; isopentenyl pyrophosphate; phosphoantigens.

Figure 1

Proposed model of ABCA1, apoA-I, and isopentenyl pyrophosphate (IPP) interactions. (A) It is unknown whether intracellular IPP binds to intracellular ABCA1 domains as it does with the intracellular B30.2 domain of BTN3A1 (see Figure 2). IPP is extruded across ABCA1 pore and reaches the extracellular environment. Limited trypsin-digestion cleaves ABCA1 into four fragments, corresponding to different extracellular and intracellular domains (51). The schematic diagram of trypsin-limited digestion of ABCA1 and the molecular sizes of the fragments produced are shown. We have found that IPP is associated with the amino-terminal extracellular portion of ABCA1 (31). Interestingly, apoA-I has been reported to interact with the same portion (52). We propose that IPP and apoA-I meet and associate within the amino-terminal portion of ABCA1 in the ECD1. IPP locally competes with cholesterol and other phospholipids for apoA-I binding and transportation. Local concentrations and the oxidized status of apoA-I, especially if IPP-producing cells are embedded in an inflammatory microenvironment, may favor IPP binding vs other metabolites. It is also possible that IPP is released in the extracellular space unbound to apoA-I. It is currently unknown whether IPP/apoA-I is more resistant to degradation by serum nucleotide pyrophosphatases than soluble IPP and more effective in the activation of Vγ9Vδ2 T cells faraway from IPP-producing cells. (B) ABCA1 is schematically represented from left to the right without any extra-loaded molecule, loaded with apoA-I only, with IPP only, and with IPP/apoA-I + IPP. It has been shown that apoA-I and IPP can bind to ABCA1, and that ABCA1 can bind to BTN3A1 (31). It is currently unknown whether ABCA1 has different affinity for BTN3A1 depending on IPP and/or apoA-I loading. Arrows: moleculare weight of fragments derived from trypsin-cleavage sites; COOH, carboxyterminal domain; ECD1, extracellular domain 1; ECD2, extracellular domain 2; NBD, nucleotide binding domain; NH2, amino-terminal domain.

Figure 2

Leads of the ménage à trois between ABCA1, apoA-I, and BTN3A1. (A) BTN3A1 inactive and active dimer conformations are represented from left to the right. Models are derived from crystallographic-based models using recombinant proteins and/or immobilized Vγ9Vδ2 TCRs, and tested using Fluorescence Resonance Energy Transfer-based measurements or proximity-ligation assays. These models are inspiring but still unproved in living cells under physiological or pathological conditions. The inactive conformations include both the head-to-tail conformation (left) and a V-shaped conformation (right) (65). Active conformations are characterized by loss of the head-to-tail conformation or by a rotational shift in the V-shaped dimer induced by the agonistic 20.1 mAb (which binds to extracellular IGHV-like domains), isopentenyl pyrophosphate (IPP) (which binds to the intracellular B30.2 domain), or by BTN3A1/BTN3A2 interactions as recently reported by Vantourout et al. (56). (B) Hypothetical configuration of ABCA1/apoA-I/BTN3A1 interactions are represented from left to right. Left: in the absence of zoledronic acid (ZA)-induced supra-physiological IPP concentrations, BTN3A1 maintains the inactive dimer conformation (for simplicity only the head-to-tail dimer is shown); ABCA1 in cooperation with apoA-I is mainly committed to extrude cholesterol (blue dots). ABCA1 and BTN3A1 are not physically associated under these conditions. Middle: hypothetical configuration of ABCA1/apoA-I/BTN3A1 interactions driven by ZA-induced intracellular IPP accumulation (red dots). ABCA1 and apoA-I expressions are increased, BTN3A1 expression is not increased, but BTN3A1 acquires the active dimer conformation because IPP is bound to the intracellular B30.2 domain; the mutually supportive cooperation between ABCA1, apoA-I, and active BTN3A1 leads to the release of picomolar IPP amounts in the extracellular fluids. Whether extracellular IPP (red dots) binds to extracellular IGHV-like domain of BTN3A1 (as shown) and participate to the activation of Vγ9Vδ2 T cells according to the antigen-presentation presentation model proposed by Vavassori (57) is unknown; Right: hypothetical scenario in Btn3a1-silenced ZA-treated dendritic cells. Desertion of BTN3A1 from the ménage à trois decreases the efficiency of extracellular IPP release by ABCA1/apoA-I even if they remain upregulated (31). These data indicate that the expression of BTN3A1 is useful but dispensable and that the main role is played by ABCA1/apoA-I; (C) hypothetical models of ABCA1/apoA-I/BTN3A1-BTN3A2 interactions are represented from left to right. Active BTN3A1 conformation is induced by interactions between BTN3A1/BTN3A2 and IPP bound to the B30.2 domains of BTN3A1. No data are currently available to support the hypothesis that BTN3A2 is physically bound to ABCA1. Left: ABCA1 and apoA-I expressions are increased, and BTN3A1 acquires the active dimer conformation because of BTN3A1/BTN3A2/IPP interactions. It is unknown whether this is the most effective complex to extrude IPP. Middle: in the experiments reported in Ref. (31), we have silenced BTN3A1 expression only and we know that this complex is still able to release IPP although with a lower efficiency [see also (B), right panel]. One possible explanation is that BTN3A2 partially substitutes for BTN3A1. Right: it is conceivable, although yet unproved, that desertion of both BTN3A1 and BTN3A2 from the complex compromises even more extracellular IPP release. (D) It is currently unknown whether unloaded ABCA1 or IPP/apoA-I-loaded ABCA1 can switch BTN3A1 from its inactive conformation to the active dimer conformation in the absence of IPP bound to the intracellular B30.2 domain. For simplicity only the head-to-tail dimer is shown. It is also unknown whether unloaded or IPP/apoA-I-loaded ABCA1 can overcome the inactive BTN3A1 conformation locked by the antagonist 103.2 mAb.