Apoptotic caspase inhibits innate immune signaling by cleaving NF-κBs in both Mammals and Flies

Host organisms use different innate immune mechanisms to defend against pathogenic infections, while tight control of innate immunity is essential for proper immune induction and balance. Here, we reported that apoptotic induction or caspase-3 overexpression caused dramatic reduction of differently triggered cytokine signalings in human cells, murine primary cells and mouse model, while the loss of caspase-3 or inhibiting apoptosis markedly enhances these immune signalings. Furthermore, caspase-3 can mediate the cleavage of NF-κB members p65/RelA, RelB, and c-Rel via its protease activity. And the caspase-3-resistant p65/RelA, RelB, or c-Rel mutant mostly restored the caspase-3-induced suppression of cytokine production. Interestingly, we further uncovered that apoptotic induction also dramatically inhibited Toll immune signaling in Drosophila, and the Drosophila effector caspases, drICE and DCP-1, also mediated the degradation of DIF, the NF-κB of Toll signaling. Together, our findings demonstrate apoptotic effector caspases, including mammalian caspase-3 and fly drICE/DCP-1, can function as repressors of NF-κB-mediated innate immune signalings.Subject terms: Immunology, Innate immunity     

Syphilitic infection impairs immunity by inducing both apoptosis and pyroptosis of CD4+ and CD8+ T lymphocytes

Syphilis is an important health problem worldwide; however, few studies have probed the impact of syphilitic infection on T cell turnover. The mechanisms behind the frequency of T cell subset changes and the associations between these subsets during syphilitic infection remain unclear. Herein, we used a cell-staining method and flow cytometry to explore changes in T cell subpopulations and potential contribution of apoptosis and pyroptosis that triggered therein. We investigated caspase-1-mediated pyroptosis and caspase-3-mediated apoptosis of CD4⁺ and CD8⁺ T cells, the major effector lymphocytes with pivotal roles in the pathogenesis of infectious diseases. We found that the levels of caspase-1 and caspase-3 increased in both the circulation and intracellularly in CD4⁺ and CD8⁺ T cells. Caspase-1 showed a continual increase from early latent stage infection through to phase 2 disease, whereas caspase-3 increased through to phase 1 disease but declined during phase 2. In addition, serum levels and intracellular expression of caspase-1 and caspase-3 were positively correlated. Overall, this study increases our understanding of how syphilitic infection influences CD4⁺ and CD8⁺ T-cell turnover, which may help with designing novel and effective strategies to control syphilis infection and prevent its transmission.     

Delayed plumage maturation in green‐backed flycatchers (Ficedula elisae): An evidence of female mimicry

Delayed plumage maturation (DPM) is the delayed acquisition of an adult color and pattern of plumage until after the first potential breeding period. Among the hypotheses proposed to explain DPM, the female mimicry hypothesis (FMH) has received considerable attention. FMH predicts that after‐second‐calendar‐year (ASY) males should attack ASY males more than second‐calendar‐year (SY) males and females, while no difference between the two latter. Few studies have been thought as support for FMH, while in fact most of them give no conclusive evidence to the assumption of FMH that ASY males are unable to distinguish SY males from females. Thus, other support besides behavioral experiments, such as it is physiologically impossible to discriminate SY males and females from avian perspective, is important in testing FMH. We studied color differences in six plumage patches between ASY male, SY male and female by analyzing reflectance spectra in the avian visual system, and tested the prediction of FMH by conducting intrusion experiments on territorial males with three kinds of conspecifics (i.e., ASY males, SY males and females) as territory “intruders” in the green‐backed flycatcher (Ficedula elisae). It is physiologically impossible for ASY males to distinguish SY males and females through the plumage color of crown, mantle, rump, and throat patches, moreover difficult to distinguish through breast and belly patches even under reasonable viewing conditions. ASY males were more aggressive toward ASY males than SY males and females with no differences between the two latter. SY males showed a slightly but not significantly higher attack intensity on ASY males than SY males and females. Our results suggest that female mimicry is more likely to be the explanation for DPM in the green‐backed flycatcher. To our knowledge, this is the first study combining avian visual system and behavioral experiments in testing hypotheses of DPM.     

Intercellular transfer of activated STING triggered by RAB22A-mediated non-canonical autophagy promotes antitumor immunity

STING, an endoplasmic reticulum (ER) transmembrane protein, mediates innate immune activation upon cGAMP stimulation and is degraded through autophagy. Here, we report that activated STING could be transferred between cells to promote antitumor immunity, a process triggered by RAB22A-mediated non-canonical autophagy. Mechanistically, RAB22A engages PI4K2A to generate PI4P that recruits the Atg12–Atg5–Atg16L1 complex, inducing the formation of ER-derived RAB22A-mediated non-canonical autophagosome, in which STING activated by agonists or chemoradiotherapy is packaged. This RAB22A-induced autophagosome fuses with RAB22A-positive early endosome, generating a new organelle that we name Rafeesome (RAB22A-mediated non-canonical autophagosome fused with early endosome). Meanwhile, RAB22A inactivates RAB7 to suppress the fusion of Rafeesome with lysosome, thereby enabling the secretion of the inner vesicle of the autophagosome bearing activated STING as a new type of extracellular vesicle that we define as R-EV (RAB22A-induced extracellular vesicle). Activated STING-containing R-EVs induce IFNβ release from recipient cells to the tumor microenvironment, promoting antitumor immunity. Consistently, RAB22A enhances the antitumor effect of the STING agonist diABZI in mice, and a high RAB22A level predicts good survival in nasopharyngeal cancer patients treated with chemoradiotherapy. Our findings reveal that Rafeesome regulates the intercellular transfer of activated STING to trigger and spread antitumor immunity, and that the inner vesicle of non-canonical autophagosome originated from ER is secreted as R-EV, providing a new perspective for understanding the intercellular communication of organelle membrane proteins.Subject terms: Macroautophagy, Endosomes, Multivesicular bodies, Small GTPases, Cancer microenvironment     

Identification of FkpA as a Key Quality Control Factor for the Biogenesis of Outer Membrane Proteins under Heat Shock Conditions

The outer membrane proteins (OMPs) of Gram-negative bacterial cells, as well as the mitochondrion and chloroplast organelles, possess unique and highly stable β-barrel structures. Biogenesis of OMPs in Escherichia coli involves such periplasmic chaperones as SurA and Skp. In this study, we found that the ΔsurA Δskp double-deletion strain of E. coli, although lethal and defective in the biogenesis of OMPs at the normal growth temperature, is viable and effective at the heat shock temperature. We identified FkpA as the multicopy suppressor for the lethal phenotype of the ΔsurA Δskp strain. We also demonstrated that the deletion of fkpA from the ΔsurA cells resulted in only a mild decrease in the levels of folded OMPs at the normal temperature but a severe decrease as well as lethality at the heat shock temperature, whereas the deletion of fkpA from the Δskp cells had no detectable effect on OMP biogenesis at either temperature. These results strongly suggest a functional redundancy between FkpA and SurA for OMP biogenesis under heat shock stress conditions. Mechanistically, we found that FkpA becomes a more efficient chaperone for OMPs under the heat shock condition, with increases in both binding rate and affinity. In light of these observations and earlier reports, we propose a temperature-responsive OMP biogenesis mechanism in which the degrees of functional importance of the three chaperones are such that SurA > Skp > FkpA at the normal temperature but FkpA ≥ SurA > Skp at the heat shock temperature.