Yo GABA GABA! A neurotransmitter interrupts DC-NK crosstalk

In this issue of the Journal of Leukocyte Biology, Bhandage and colleagues report the secretion of the neurotransmitter gamma aminobutyric acid (GABA) by natural killer (NK) cells that have been infected with the intracellular parasite Toxoplasma gondii. The NK cell-derived GABA can act in an autocrine/paracrine fashion to decrease NK cytotoxicity and modulate dendritic cell (DC) migration in vitro, leading the research team to hypothesize that the parasite may be utilizing the GABA signaling pathway to impair host immune defenses.

NK cells and DCs work synergistically to orchestrate the early type 1 immune response to viruses and intracellular pathogens.¹ Conserved molecular motifs expressed by pathogens and/or danger signals emanating from infection-related tissue damage bind to pattern recognition receptors expressed by DC. This causes DC activation and secretion of interleukin 12 (IL-12), which in turn activates NK cells. Activated NK cells augment cytotoxic pathways to increase immune surveillance of the surrounding tissue and secrete interferon gamma (IFN-γ), which promotes cell-intrinsic immune defenses in nearby cells. The DC-NK signaling axis not only helps protect the host against pathogen replication, but also guides the acquisition and presentation of antigen to pathogen-specific T cells, thereby supporting the development of a robust adaptive immune response.^2,3 The broad importance of DC-NK interactions for innate and adaptive type 1 immune defense makes both cell types attractive targets for disruption by invading pathogens, but the signaling pathways controlling their interactions are not well understood.

In the last decade, a growing appreciation has emerged for the role of neurotransmitters as signaling molecules controlling immune cell function.⁴ Primary and secondary lymphatic tissues are highly innervated, and signals from the nervous system including acetylcholine and catecholamine have been found to influence the function of T cells, DC, and macrophages. These signals can promote either pro- or anti-inflammatory function depending on the cell type, the signaling receptor, and immune context. Perhaps the most surprising development in this field is the recent finding that immune cells are also capable of synthesizing neurotransmitters themselves. Notably, Bhandage and colleagues find that human and NK cells not only can respond to GABA signaling, but can produce GABA at physiologically relevant levels. In conjunction with recent reports of GABA synthesis in antigen presenting cells (APC) and T cells,^5,6 this new study by Bhandage and colleagues establishes GABA as a key player in mediating neuroimmune communications.

The authors’ findings also suggest that GABA may be a key regulator of DC-NK interactions early during acute infection. They have previously demonstrated that myeloid CD11b+ DC infected by T. gondii secrete GABA, which acts in an autocrine/paracrine fashion to induce a hypermigratory DC phenotype.⁶ Importantly, injection of infected DC into naïve BALB/c mice leads to systemic infection, but dissemination is reduced by treatment of the DC with GABA synthesis inhibitors prior to inoculation. Bhandage et al. show that supernatant from infected DC can inhibit NK cytotoxicity in a GABA-dependent manner. Likewise, they show that supernatant from infected NK can induce a GABA-dependent hypermigratory DC phenotype in vitro. The implications of this effect will need to be examined in vivo in the future.

The authors hypothesize that their findings may reveal a unique mechanism of immune evasion/manipulation employed by T. gondii via the hijacking the host's native GABA signaling machinery. Toxoplasma gondii is an insidious pathogen, capable of infecting nearly any nucleated cell in an incredibly broad range of species. This apicomplexan parasite manipulates immune signaling in cells it infects via a suite of rhoptry proteins injected into the host cell during cellular invasion and adorned on the surface of the parasitophorous vacuole within the cell. Rhoptry proteins have been shown to block IL-12 secretion and inhibit activation of protective interferon response genes (IRGs) in infected cells.⁷ Bhandage and colleagues find that the induction of GABA production in NK cells and DC requires live parasites, suggesting that this behavior may also be controlled by these rhoptry proteins. Intriguingly, GABA secretion was not only increased in infected NK cells upon exposure to live parasites, but also increased in uninfected “bystander” NK cells, albeit less robustly. This suggests that this behavior may also be promoted by injection of T. gondii derived proteins into an uninfected cell by an extracellular parasite tachyzoite, a behavior known as “kiss and spit.”⁷

Many key questions regarding the role of GABA in the host response to T. gondii infection remain. Although NK cells are required for control of acute Toxoplasma infection, perforin-deficient mice survive this stage of infection,⁸ indicating that the protective function of NK cells in this model is dependent on their secretion of IFN-γ. The effect of GABA on cytokine production by NK cells is an important avenue for future research. Additionally, the relevance of GABA to NK-DC crosstalk in vivo is still unknown. Acute infection has been shown to generate clusters of NK cells and DC in close proximity to one another, raising the possibility that a secreted neurotransmitter could act at low concentrations across short distances between these cell types.² It is notable, however, that the effects of GABA signaling on APCs have been performed in myeloid CD11b+ DC, but BATF3-dependent “lymphoid” DC are the major source of IL-12 after T. gondii infection.⁹ Whether this population of DC produce GABA during T. gondii infection is unknown.

The finding that human and mouse NK cells constitutively express the molecular machinery for synthesis and secretion of GABA, as well as GABA signaling receptors, implies that this pathway influences NK function in noninfectious inflammatory settings. Indeed, potent immunosuppressive effects of GABA signaling have been reported in experimental autoimmune encephalitis, a murine model of multiple sclerosis,⁵ which suggests that this pathway may be relevant in human autoimmune diseases. Additionally, GABA is upregulated in a wide array of cancers and promotes tumor proliferation, migration, and invasion,¹⁰ leading one to speculate that this pro-tumor effect of GABA signaling may be further supported by reducing the cytotoxicity of tumor-infiltrating NK cells. With GABA now joining acetylcholine and the catecholamines as key signaling molecules regulating leukocyte function, it is time that we reconsider these factors as full-fledged members of the molecular immune signaling repertoire.

ACKNOWLEDGMENTS

The author would like to thank Elizabeth Woo for thoughtful discussion and Dr. Philip Coish for editing the manuscript. The graphical abstract was created using BioRender.com.

REFERENCES

Author notes