HIV-1 p17 matrix protein enhances type I interferon responses through the p17-OLA1-STING axis.

Stimulator of IFN genes (STING; also known as STING1) is an important adaptor protein for detecting cytosolic double-stranded DNA, which can come from HIV infection. Several HIV proteins, such as p6, Vpx and Vif, can influence STING-mediated innate immunity, but the function of p17 is still unknown. In this study, we find that HIV-1 p17, but not HIV-2 p17 or SIV p17, promotes STING signaling induced by cyclic GMP-AMP (cGAMP) treatment. Mechanistically, HIV-1 p17 binds to Obg-like ATPase 1 (OLA1) and inhibits the regulation of STING by OLA1. Here, OLA1 interacts with STING and inhibits the translocation and phosphorylation of STING upon cGAMP stimulation. Furthermore, compared with HIV-2 and SIV, the ATPase and GTPase activities of OLA1 are only promoted by HIV-1 p17. Our study shows that the p17 of HIV-1, but not HIV-2 or SIV, promotes STING-mediated innate immunity by interfering the interaction between OLA1 and STING, thus providing a new clue for specific immune activation of HIV-1.

ALG2 regulates type I interferon responses by inhibiting STING trafficking.

Stimulator of IFN genes (STING), an endoplasmic reticulum (ER) signaling adaptor, is essential for the type I interferon response to cytosolic double-stranded DNA. Translocation from the ER to perinuclear vesicles following cyclic GMP-AMP (cGAMP) binding is a critical step for STING to activate downstream signaling molecules, which leads to the production of interferon and pro-inflammatory cytokines. Here, we found that apoptosis-linked gene 2 (ALG2, also known as PDCD6) suppressed STING signaling induced by herpes simplex virus-1 (HSV-1) infection or cGAMP presence. Knockout of ALG2 markedly increased the expression of type I interferons upon cGAMP treatment or HSV-1 infection in THP-1 monocytes. Mechanistically, ALG2 associated with the C-terminal tail of STING and inhibited its trafficking from the ER to the perinuclear region. Furthermore, the ability of ALG2 to coordinate Ca2+ was crucial for its regulation of STING trafficking and DNA-induced innate immune responses. This work suggests that ALG2 is involved in DNA-induced innate immune responses by regulating STING trafficking.

ALG2 Influences T cell apoptosis by regulating FASLG intracellular transportation.

In the immune system, T lymphocytes undergo rapid clonal expansion upon pathogen infection. Following pathogen clearance, most of proliferated T cells will be eliminated by the apoptosis pathway to keep the balance of immune cells. FASLG, by interacting with its cognate receptor FAS, plays a major role in controlling the T cell death. FASLG is a type II transmembrane protein, with its C-terminal extracellular domain responsible for interacting with FAS. The N-terminal cytosolic region, despite short and intrinsically disordered, plays critical roles on the protein stability and transportation. The correct localization, either on the plasma membrane or secreted with exosome, or shed into the extracellular region after protease cleavage, has a great impact on the proper function of FASLG. Following synthesis, FASLG is transported by intracellular vesicle transportation system to the final destination. In this report, ALG2, a molecule identified in the T cell apoptosis and shown to be involved in vesicle trafficking previously, was found to interact with FASLG and regulate FASLG transportation. Therefore, we identified a new regulating factor for FASLG function within T cells and also revealed a new pathway for ALG2 involvement in T cell apoptosis.

Protocol using lentivirus to establish THP-1 suspension cell lines for immunostaining and confocal microscopy.

THP-1, a monocyte cell line growing in suspension, is widely used in immunology research. However, establishing suspension cell lines and performing confocal microscopy can be challenging. Here, we present a protocol to efficiently generate THP-1 cell lines using lentivirus and perform immunostaining and confocal microscopy. We detail steps for virus production, THP-1 cell infection and clone selection, fixing the suspension cells to the glass slide for immunostaining, and subsequent confocal microscopy. This protocol can be applied to other suspension cells. For complete details on the use and execution of this protocol, please refer to Ji et al. (2021).1.

Newly synthesized AIFM1 determines the hypersensitivity of T lymphocytes to STING activation-induced cell apoptosis.

STING is a well-known signaling adaptor essential for sensing cytosolic dsDNA to produce type I interferon. Although the detailed underlying mechanisms remain enigmatic, recent studies show that STING activation can lead to T lymphocyte apoptosis. Here, we report that AIFM1 facilitates STING activation-induced cell apoptosis in T lymphocytes. Mechanistically, AIFM1 is upregulated after STING activation in T cells but not in HEK293T-STING and THP-1 cells, rendering T cells more sensitive to apoptosis. In contrast to the canonical role of AIFM1 in the caspase-independent parthanatos, the function of AIFM1 is operated by the formation of an AIFM1/IRF3/BAX complex and mitochondrial outer membrane permeabilization, which cause cytochrome c release and caspase activation. Furthermore, supplementation with newly synthesized AIFM1 can reconstitute STING activation-induced cell apoptosis in HEK293T-STING and THP-1 cells. Our study identifies AIFM1 as a key regulating factor determining the hypersensitivity of T lymphocytes to STING activation-induced cell apoptosis.

ALG-2 couples T cell activation and apoptosis by regulating proteasome activity and influencing MCL1 stability.

T cell homeostasis is critical for the proper function of the immune system. Following the sharp expansion upon pathogen infection, most T cells die in order to keep balance in the immune system, a process which is controlled by death receptors during the early phase and Bcl-2 proteins in the later phase. It is still highly debated whether the apoptosis of T cells is determined from the beginning, upon activation, or determined later during the contraction. MCL1, a Bcl-2 family member, plays a pivotal role in T cell survival. As a fast turnover protein, MCL1 levels are tightly regulated by the 26S proteasome-controlled protein degradation process. In searching for regulatory factors involved in the actions of MCL1 during T cell apoptosis, we found that ALG-2 was critical for MCL1 stability, a process mediated by a direct interaction between ALG-2 and Rpn3, a key component of the 26S proteasome. As a critical calcium sensor, ALG-2 regulated the activity of the 26S proteasome upon increases to cytosolic calcium levels following T cell activation, this consequently influenced the stability of MCL1 and accelerated the T cell "death" process, leading to T cell contraction and restoration of immune homeostasis. Our study provides support for the notion that T cells are destined for apoptosis after activation, and echoes the previous study about the function of ALG-2 in T cell death.