Adrià-Arnau Martí i Líndez

A growing body of evidence indicates that, over the course of evolution of the immune system, arginine has been selected as a node for the regulation of immune responses. An appropriate supply of arginine has long been associated with the improvement of immune responses. In addition to being a building block for protein synthesis, arginine serves as a substrate for distinct metabolic pathways that profoundly affect immune cell biology; especially macrophage, dendritic cell and T cell immunobiology. Arginine availability, synthesis, and catabolism are highly interrelated aspects of immune responses and their fine-tuning can dictate divergent pro-inflammatory or anti-inflammatory immune outcomes. Here, we review the organismal pathways of arginine metabolism in humans and rodents, as essential modulators of the availability of this semi-essential amino acid for immune cells. We subsequently review well-established and novel findings on the functional impact of arginine biosynthetic and catabolic pathways on the main immune cell lineages. Finally, as arginine has emerged as a molecule impacting on a plethora of immune functions, we integrate key notions on how the disruption or perversion of arginine metabolism is implicated in pathologies ranging from infectious diseases to autoimmunity and cancer.

As sufficient extracellular arginine is crucial for T cell function, depletion of extracellular arginine by elevated arginase 1 (Arg1) activity has emerged as a hallmark immunosuppressive mechanism. However, the potential cell-autonomous roles of arginases in T cells have remained unexplored. Here, we show that the arginase isoform expressed by T cells, the mitochondrial Arg2, is a cell-intrinsic regulator of CD8+ T cell activity. Both germline Arg2 deletion and adoptive transfer of Arg2-/- CD8+ T cells significantly reduced tumor growth in preclinical cancer models by enhancing CD8+ T cell activation, effector function, and persistence. Transcriptomic, proteomic, and high-dimensional flow cytometry characterization revealed a CD8+ T cell-intrinsic role of Arg2 in modulating T cell activation, antitumor cytoxicity, and memory formation, independently of extracellular arginine availability. Furthermore, specific deletion of Arg2 in CD8+ T cells strongly synergized with PD-1 blockade for the control of tumor growth and animal survival. These observations, coupled with the finding that pharmacologic arginase inhibition accelerates activation of ex vivo human T cells, unveil Arg2 as a potentially new therapeutic target for T cell-based cancer immunotherapies.

Arginine depletion, an essential amino acid for T cells, is a major immunosuppressive mechanism for anti-tumoral T cells. Despite extensive characterisation of extracellular arginine depletion by the arginase enzyme Arg1, the role of the more ancestral Arg2 isoform in immunity, and especially in anti-tumor immunity, remained unaddressed. Preclinical murine melanoma and colorectal carcinoma models showed that, while Arg2-overexpression in tumor cells impaired adaptive anti-tumor responses, germ-line and CD8+ T cell-specific Arg2 deletion enhanced anti-tumor immune responses, reducing tumor growth. Notably, combination of Arg2 deletion and PD-1 blockade synergistically improved single-immunotherapy effects. Concomitantly, we also engineered a new mouse strain (Arg2em1Wreith), a tool for further comprehension of Arg2 post-transcriptional regulation by the immunorelevant microRNA-155. In conjunction with incipient data on human ARG2 inhibition in T cells, this thesis proposes Arg2 as a new molecular target for the improvement of T cell-based immunotherapies, at the forefront of cancer treatments in modern medicine.

Abstract Amoeboid cell migration is key to efficient T cell immunity. Spatial polarization of organelles within cells, including endo-lysosomes, is a prerequisite of migration. However, how ultrastructural polarization is linked to the signaling requirements governing T cell migration, remains unknown. Here we show that signaling molecules generated by endo-lysosome-localized kinases regulate velocity of amoeboid migration. Specifically, imaging of T cells identified accumulation of endo-lysosomes decorated with the lipid kinases VPS34–PIKfyve at the uropod of polarized cells. Activity of VPS34 and PIKfyve regulated speed, but not directedness, of migrating T cells. Mechanistically, PI(3,5)P 2 generated by the sequential action of VPS34 and PIKfyve mediated Ca 2+ efflux from lysosomes via the mucolipin TRP cation channel 1 (TRPML1), thus controlling activity of myosin IIA and hence the generation of propulsive force through retrograde actin flow. The VPS34–PIKfyve kinases also regulated velocity of myeloid cells, as well as of the amoeba Dictyostelium discoideum – establishing the axis as an evolutionary conserved speed control system of amoeboid cell migration. Graphical Abstract The VPS34–PIKfyve axis is active on endo-lysosomes at the uropod of migrating T cells. VPS34 and PIKfyve promote myosin IIA activation and retrograde action flow. Amoeboid cell migration speed is controlled by VPS34 and PIKfyve via TRPML1. Regulation of amoeboid migration speed is a conserved function of the VPS34–PIKfyve axis.