CALHM6: function in the innate immune system and role in infectious diseases
Keywords: immunological synapse
macrophages
innate immunity
CALHMs
ATP channels
listeria monocytogenes
NK cells

Innate myeloid cells, such as macrophages and dendritic cells (DC), regulate the type, magnitude, and duration of the immune response. Macrophages and DCs do so, by instructing the functions of effector cells, such as natural killer (NK) cells. NK cells' effector functions are important in the killing of virus or bacterial-infected and malignantly transformed cells. NK cells also participate in the production of cytokines like IFN-γ, IL-10 and TNF after viral, fungal or bacterial infection.
CALHMs (calcium homeostasis modulators) are a family of membrane proteins recently identified as voltage-gated oligomeric ion- and ATP channels. CALHM1 and 3 specifically, form a channel that localises in the synapse between taste bud cells and neurons, which it is important for the formation of a synaptic structure. Interestingly, a single member of the CALHM family, CALHM6 is expressed almost exclusively in immune cells. Like other CALHMs, CALHM6 forms a higher order complex, composed of up to 11 oligomers, with a channel-like structure. Previous studies show that CALHM6 is required for the instruction of antitumour effector responses in NK cells, but its role in infections and its mechanism of action remains unknown.
In this work, we show that in myeloid cells, CALHM6 is highly induced on myeloid cells in response to pathogen-derived signals and is inhibited by anti-inflammatory cytokines. Also, while the structure of CALHM6 resembles a higher oligomeric version of the CALHM1/3 channel, we show that its gating is different as it does not release ATP in response to the same stimuli that open CALHM1/3.
To understand the function of CALHM6, we generated CALHM6 deficient mice (Calhm6-/-). To test whether CALHM6 plays a role in the immune response to infections we challenged WT and Calhm6-/- mice with Listeria monocytogenes (L. monocytogenes) an intracellular gram-positive bacterium that elicits a strong innate immune response and requires NK cells derived IFN-γ for its clearance by activated macrophages. Calhm6-/- mice fail to control L. monocytogenes burden at peak infection. This is not caused by an intrinsic failure of macrophages in killing Listeria but it is likely driven by an early deficiency in NK-derived IFN-γ, which causes a delay in the kinetic of the innate immune response.
It is well appreciated that the most efficient activation of NK cells in vivo requires their direct priming/interaction with activated macrophages/DC. Calhm6-/- mice are specifically unable to activate NK cells in a DC- and macrophage-dependent manner. Bone marrow-derived macrophages from Calhm6-/- mice fail to upregulate mRNA expression of synaptic-associated genes when stimulated with pro-inflammatory stimuli. This together with our findings that CALHM6 relocates to the macrophage-NK cell interface in vitro, suggests that CALHM6 localises in the immunological synapse between innate and NK cells, where it forms a channel that is important in the innate cell-mediated instruction of NK cell activation.