Introduction

STING drives cytosolic DNA-induced type I IFNs production ¹. Recent research revealed that STING promotes inflammation in a variety of inflammatory diseases including nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, kidney injury, neurodegenerative diseases, cardiovascular diseases, obesity, diabetes, and aging ^2–9. The type I IFNs-independent function of STING has also emerged ^9–12. For example, initially described as a type I interferonopathy ¹³, recent studies in STING-associated vasculopathy with onset in infancy (SAVI) mouse models showed that SAVI is largely independent of type I IFNs ^14–17. In a STING N153S mouse model of SAVI, crossing N153S mice to IRF3/IRF7, and IFNAR1 knockout mice, N153S mice still developed spontaneous lung diseases ¹⁴. In another example of STING-dependent, type I IFNs-independent inflammatory disease, DNase II-deficient mice developed autoinflammatory arthritis that can be rescued by STING deficiency ¹⁸. But Dnase II^−/− mice with the STING-S365A mutation still exhibited severe polyarthritis ^19,20. The STING-S365A mutant cannot recruit IRF3 and produce IFNβ ^10,11,19,20. How STING drives inflammation in vivo, independent of type I IFNs, remains unknown. The activation of NFκB or NLRP3 inflammasome by STING has been proposed ^9,21.

Characterized as an innate immune sensor, STING expression is, paradoxically, high in CD4 T cells ^13,22. Furthermore, STING activation kills mouse and human CD4 T cells ex vivo ^23–25. SAVI patients and mouse model had CD4 T cellpenia ^13,25. STING was first discovered as MPYS for its cell growth inhibition and cell death function in mouse B lymphoma cells ²⁶. STING-mediated cell death is cell type dependent. For example, while STING activation kills human endothelial cells, primary and cancerous T cells, it does not kill mouse MEFs, BMDCs, or BMDMs ^27–29. Second, STING-mediated cell death is type I IFNs-independent ^25,27,29. Multiple cell death pathways, i.e., apoptosis, necroptosis, pyroptosis, ferroptosis, and PANoptosis, are proposed ^29–31. Last, the in vivo biological significance of STING-mediated CD4 T cell death is not clear ^25,29. WT and STING^−/− mice have similar thymic and pLN CD4⁻CD8⁻, CD4⁺CD8⁺, and single positive CD4 and CD8 T cell populations ^23,24. The expression of CD4 and CD8 TCR are comparable ^23,24. The naive, memory, or regulatory T cell populations are similar in WT and STING^−/− mice that expressed comparable levels of CD69 and CD25 following TCR activation ^23,24. Together, the absence of STING in mice does not alter T cell development or TCR signaling ²³. In humans, SAVI patients with constitutively activated STING have low CD4 T cell numbers ¹³, and type I IFNs are dispensable for STING-mediated human CD4 T cell death ²⁵. Notably, SAVI patients (N154S or V155M) had normal counts of CD8 T and B cells ¹³.

The human TMEM173 gene is highly heterogeneous ^32,33. ∼50% of people in the U.S. carry at least one copy of non-WT TMEM173 allele ³². Among them, the R71H-G230A-R293Q (HAQ) is the 2^nd most common TMEM173 allele carried by ∼23% of people in the U.S. ³². However, in East Asians, WT/HAQ (34.3%), not WT/WT (22.0%), is the most common TMEM173 genotype ³³. Critically, the HAQ allele was positively selected in modern humans outside Africa ³⁴. Anatomically modern humans outside Africa are descendants of a single Out-of-Africa Migration 50,000∼70,000 years ago. ∼1.4% of Africans have the HAQ allele, while ∼63.9% of East Asians are HAQ carriers ³⁴. Haplotype analysis revealed that HAQ was derived from G230A-R293Q (AQ) allele ³⁴. Importantly, the AQ allele was negatively selected outside Africa ³⁴. ∼40.1% of Africans are AQ carriers, while ∼0.4% of East Asians have the AQ allele ³⁴. Importantly, TMEM173 alleles often have a dominant negative effect likely because the protein STING exists as a homodimer ^26,35. SAVI is an autosomal dominant inflammatory disease ¹³. WT/HAQ individuals had reduced Pneumovax23®-induced antibody responses compared to WT/WT individuals (NCT02471014) ³⁶. Notably, AQ responds to CDNs and produce type I IFNs in vivo and in vitro ^34,37,38, but the AQ allele was negatively selected in non-Africans ³⁴. In contrast, the HAQ allele, defective in CDNs-type I IFNs responses ^{32,33,36,39,40}, was positively selected in non-Africans ³⁴ indicating that the CDNs-type I IFNs independent function of STING was essential for the survival of early modern humans outside of Africa.

In this study, we discovered, surprisingly, that the HAQ, AQ splenocytes are resistant to STING-mediated cell death. We generated HAQ/SAVI(N153S) and AQ/SAVI(N153S) mice and found that the HAQ, AQ alleles prevent CD4 T cellpenia, increase/restore T-regs and alleviate/stop tissue inflammation in SAVI mice, thus providing evidence for the in vivo significance of type I IFNs-independent, STING-mediated CD4 T cell death.

Results

STING activation kills mouse spleen CD4, CD8 T and CD19 B cells ex vivo

We first used the synthetic non-CDNs STING agonist diABZI ⁴¹ to induce lymphocyte death because diABZI induces cell death without the needs for lipid transfection or detergent for cell permeabilization ^28,42 and diABZI is in clinical trials (NCT05514717). Splenocytes from C57BL/6N mice were treated with diABZI in culture, and cell death was determined by Annexin V and Propidium Iodide stain. Splenocyte cell death could be detected as early as 5hrs post diABZI treatment (Figure S1A). Dosage responses showed that ∼25ng/ml diABZI could kill 70% of splenocytes (Figure S1B). Similarly, STING agonists DMXAA, and synthetic CDNs RpRpssCDA, killed mouse spleen CD4, CD8 T cells, and CD19 B cells (Figure 1A). Thus, STING activation readily induces mouse lymphocyte death ex vivo.

Fig. 1.

TBK1 activation is required for STING-mediated mouse spleen cell death ex vivo

STING activation can lead to apoptosis, pyroptosis, necroptosis, or ferroptosis ^{25–31,42,43}. We then treated mouse splenocytes with apoptosis, pyroptosis, necroptosis inhibitors, STING inhibitors H-151, C-176, palmitoylation inhibitor 2-bromopalmitate (2-BP), followed by diABZI stimulation. Inhibitors for NLRP3 (MCC950), RIPK1 (Necrostatin-1), RIPK3 (GSK872), Caspase-1 (VX-795), Caspase-3 (Z-DEVD-FMK), Caspase 1,3,8,9 (Q-VD-Oph), ferroptosis (liproxstatin-1) did not affect diABZI-induced splenocyte cell death ex vivo (Figure S1B, S1C). The STING inhibitors H-151, C-176, and 2-BP also could not prevent diABZI-induced cell death (Figure S1C) though they inhibited diABZI-induced IFNβ production (Figure S1D). Instead, the TBK1 inhibitor BX-795 abolished diABZI-induced splenocyte death (Figure S1C).

BX-795 is a multi-kinase inhibitor including 3-phosphoinositide-dependent protein kinase 1 (PDK1) and TBK1 (IC_50s = 6 and 11 nM, respectively). However, the treatment of PDK1 inhibitor GSK2334470 (IC50 = 10nM) did not prevent diABZI-induced splenocyte death (Figure 1B). In contrast, GSK8612, a highly potent and selective inhibitor for TBK1, prevented diABZI-induced splenocyte death (Figure 1B). Thus, TBK1 activation is likely critical for STING-mediated lymphocyte cell death ex vivo.

HAQ, AQ, Q293 STING knock-in mouse splenocytes are resistant to STING-mediated cell death ex vivo

HAQ and AQ are common human TMEM173 alleles ^32–34. Previously, we reported that HAQ knock-in mice are defective in CDNs-induced immune responses, while CDNs responses in AQ knock-in mice are similar to WT mice ³⁴. We treated splenocytes from HAQ and AQ mice with diABZI ex vivo and found, surprisingly, that both HAQ and AQ splenocytes were resistant to diABZI-induced cell death (Figure 1C, 1D). In comparison, IFNAR1^-/- splenocytes were killed by diABZI, confirming that STING-mediated lymphocytes death are type I IFNs-independent (Figure 1C, 1D) ^25,27,29.

HAQ and AQ share the common A230 and Q293 residues changes. We thus generated a Q293 TMEM173 knock-in mouse. Notably, the Q293 splenocytes were resistant to STING agonists 2’3’-cGAMP, RpRpssCDA, and diABZI-induced cell death (Figure 1E, 1F). Thus, the residue 293 of STING is critical for its cell death function.

WT/HAQ, WT/AQ mouse splenocytes are partially resistant to STING-mediated cell death ex vivo

WT/HAQ (34.3%), not WT/WT (22.0%), is the most common human STING genotype in East Asians, while WT/AQ (28.2%) is the 2^nd most common STING genotype in Africans ³³. We generated WT/HAQ, WT/AQ mice and treated the splenocytes with mouse STING agonist DMXAA. WT/HAQ and WT/AQ splenocytes were protected from 25µg/ml DMXAA-induced cell death (Figure 1G). 100µg/ml DMXAA could kill WT/HAQ and WT/AQ splenocytes albeit less than WT/WT cells (Figure 1G). Thus, the HAQ and AQ alleles are dominant and likely impact STING activation even in heterozygosity.

STING activation kills primary human CD4 T cells, but not CD8 T or CD19 B cells

STING agonists-based clinical trials in humans have been disappointing (NCT02675439, NCT03010176, NCT05514717) ^44,45. We showed that the human TMEM173 gene might undergo natural selection during the out-of-Africa migration ³⁴ sensitive to evolutionary pressure. Thus, we investigated STING-mediated death in primary human lymphocytes.

Human explant lung cells from the WT(R232)/WT(R232) donors were treated with STING agonists 2’3-c GAMP, RpRpssCDA, diABZI for 24hrs in culture. Lymphocyte cell death was determined by Propidium Iodide staining. Different from mouse lymphocytes, diABZI and RpRpssCDA killed human CD4 T but not CD8 T or CD19 B cells (Figure 2A, 2B). Human CD8 T and CD19 B cells are resistant to 500ng/ml diABZI-induced cell death (Figure S2A).

Fig. 2.

WT/HAQ human CD4 T cells are resistant to low-dose of diABZI-induced cell death

WT/HAQ mouse splenocytes are resistant to low dose diABZI-induced cell death. To extend our observation into primary human T cells, we obtained lung explants from WT/WT and WT/HAQ individuals (Figure S2B) and treated them with diABZI in culture. 25ng/ml diABZI killed WT/WT, but not WT/HAQ, human lung CD4 T cells (Figure 2C).

diABZI induces cell death in STING-KO human THP-1 cells reconstituted with WT human STING (R232) but not HAQ, AQ or Q293 human STING allele

To further determine cell death influenced by human TMEM173 alleles HAQ, AQ and Q293, we used the STING-KO THP-1 cell line because STING agonist induces type I IFNs and cell death in STING-KO THP-1 cells expressing WT human STING ³¹ (Figure S2C, S2D). We, thus, generated stable THP-1 STING-KO lines expressing HAQ, AQ, WT or Q293 TMEM173 allele. Cell death was determined by Annexin-V staining. diABZI killed THP-1 STING-KO lines expressing WT but not HAQ, AQ, or Q293 TMEM173 allele (Figure 2D, 2E). No cell death was induced in the Q293 THP-1 cells stimulated by 20 to 200 ng/ml of diABZI (Figure 2F). diABZI also did not induce STING activation in Q293 THP-1 cells (Figure 2G). Notably, 50ng/ml diABZI induced p-IRF3 activation and type I IFNs in AQ THP-1 cells, but not HAQ THP-1 cells (Figure 2H, 2I) indicating that the STING-cell death and STING-IRF3-Type I IFNs pathways can be uncoupled.

HAQ and AQ alleles rescue the lymphopenia and suppress myeloid cell expansion in SAVI(N153S) mice

The in vivo significance of the STING/MPYS-cell death is unclear. Furthermore, multiple cell death pathways, i.e., apoptosis, necroptosis, pyroptosis, ferroptosis, and PANoptosis, are proposed ^29–31. The uncertainty likely results from studies using different cell types (primary cells vs cancer cell lines); species (human vs mouse); STING agonists (cGAMP, which requires cell permeabilization by detergents or lipid transfection, vs diABZi, DMXAA that can directly cross the membrane) ^{23,24,27,28,46}. Critically, which mechanism is relevant in vivo, causing T cellpenia is not known. To clarify the in vivo significance and mechanisms of STING-mediated cell death, we turned to SAVI mice.

SAVI is an autosomal dominant, inflammatory disease caused by one copy of a gain-of-function STING mutant ¹³. CD4 T cellpenia was found in SAVI patients and SAVI mouse models ^13,14. STING activation in SAVI mice is independent of ligands and happens in vivo. We thus generated HAQ/SAVI(N153S) and AQ/SAVI(N153S) mice aiming to establish the in vivo significance and mechanism of STING-cell death.

First, HAQ/SAVI(N153S) and AQ/SAVI(N153S) mice had reduced splenomegaly compared to WT/SAVI(N153S) mice though their spleens were still larger than the littermates WT/HAQ and WT/AQ (Figure 3A, 3B). Next, HAQ/SAVI(N153S) and AQ/SAVI(N153S) mice had similar spleen B cells and CD4 T cells numbers as the WT/HAQ, WT/AQ littermates (Figure 2B, 2C). Their CD8⁺ T cells were lower than their WT littermates but much higher than the WT/SAVI(N153S) mice (Figure 3D). Third, spleen myeloid cell numbers, i.e., neutrophils, Ly6C^hi monocytes and F4/80 macrophages, were all reduced by half compared to WT/SAVI(N153S) mice (Figure 3E∼3H). Notably, the HAQ/SAVI(N153S) and AQ/SAVI(N153S) mice also had restored bone marrow monocyte (Figure S3). Thus, HAQ and AQ alleles prevent lymphopenia and suppress myeloid cells expansion in SAVI(N153S) mice.

Fig. 3.

The HAQ allele alleviates and the AQ allele prevents SAVI(N153S) disease in mice

SAVI(N153S) disease is characterized by early onset, failure to thrive (low body weight), persistent lung inflammation, decreased lung function, and young death in humans and mouse models ^13,17,46,47. The HAQ/SAVI(N153S) mice weighed more and had an improved lifespan than the WT/SAVI(N153S) mice (Figure 4A, 4B). The lifespan, airway resistance and tissue inflammation (lung, liver) were also improved in HAQ/SAVI(N153S) mice compared to the WT/SAVI(N153S) mice (Figure 4C, 4D, 4J, 4K, S4). However, the pulmonary artery pressure was still elevated in HAQ/SAVI(N153S) mice (Figure 4E). Remarkably, the AQ/SAVI(N153S) mice had similar body weight and lifespan as the WT/AQ mice (Figure 4F, 4G). The airway resistance, pulmonary artery pressure, and tissue inflammation in AQ/SAVI(N153S) were similar to the WT/AQ littermates (Figure 4H, 4I, 4J, 4K, S4). Thus, the AQ allele prevents inflammatory SAVI disease in mice.

Fig. 4.

diABZI induces similar STING, TBK1, IRF3, NFκB activation in the AQ/SAVI(N153S) and WT/SAVI(N153S) BMDM

SAVI was characterized as type I interferonopathy ¹⁷. However, several studies showed that type I IFN signaling and IRF3 activation were dispensable for SAVI disease ^14,15,47,48. AQ allele prevents SAVI disease (Figure 4). However, diABZI-treated AQ/SAVI(N153S) and WT/SAVI(N153S) BMDM had similar TBK1-IRF3 activation and IFNβ production (Figure 5A, 5D). diABZI treatment caused IκBα degradation, and similar TNF production in WT/SAVI and AQ/SAVI BMDM (Figure 5B, 5E). Furthermore, diABZI activation led to STING protein degradation in WT/SAVI and AQ/SAVI BMDM (Figure 5C). Last, using cleavable crosslinker dithiobis (succinimidyl propionate (DSP), we showed that STING in WT/SAVI, AQ/SAVI BMDM forms similar dimer in situ (Figure 5F). Thus, the AQ/SAVI BMDM had similar STING degradation, TBK1, IRF3, NFκB activation, and dimerization as the WT/SAVI BMDM.

Fig. 5.

The HAQ allele increased, and the AQ allele restored T-regs in SAVI(N153S) mice

IFNγ was proposed to drive SAVI disease ^15,35,48. We confirmed that WT/SAVI CD4 T cells were enriched with IFNγ⁺ cells (Figure 6A). However, WT/SAVI mice have CD4 T cellpenia. Thus, the total numbers of spleen IFNγ⁺ CD4 T cells were comparable in WT/SAVI and AQ/SAVI mice (Figure 6A). In contrast, the HAQ/SAVI mice had decreased IFNγ⁺ CD4 T cells (Figure 6A).

Fig. 6.

The induction of Foxp3 expression in T-reg cells during ongoing autoimmune inflammation resolved inflammation and pathology in mice ⁴⁹. CD4 T cellpenia depletes CD4 T-regs. Indeed, WT/SAVI mice had ∼20-fold reduction of spleen FoxP3⁺ T-regs compared to AQ/SAVI or WT/WT littermates (Figure 6B). The HAQ/SAVI mice also had ∼10-fold more T-regs than the WT/SAVI littermate (Figure 6B).

Discussion

This study, using the HAQ, AQ, SAVI(N153S) TMEM173 knock-in mice, reveals the in vivo significance and mechanism of STING-mediated CD4 T cell death. HAQ, AQ alleles prevent CD4 T cellpenia, and increase/restore CD4 T-regs in SAVI mice. The results are consistent with previous finding that the impaired CD4 T cell proliferation by the SAVI(V155M) mutant could be rescued by the addition of the HAQ allele in vitro ²⁴. STING has been increasingly implicated in inflammatory diseases such as nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, cardiomyopathy, obesity, diabetes, neurodegenerative diseases, aging, and kidney injury, many of which are independent of type I IFNs ^2,3,9. It is tempting to suggest that STING activation in CD4 T cells leads to CD4 T-regs depletion that break tissue tolerance and exacerbates tissue inflammation.

Human immunodeficiency virus (HIV) primarily infects CD4 T cells and might activate the STING pathway in CD4 T cells ^50–55. The loss of CD4 T cells is the hallmark of untreated HIV infection ^56,57, and the measurement of CD4 T cell count is a central part of HIV care. We found that HAQ and AQ CD4 T cells are resistant to STING-mediated cell death. Mogensen and colleagues reported that HAQ/HAQ was enriched in HIV-infected long-term nonprogressors ³⁹. These HAQ/HAQ individuals had reduced inhibition of CD4 T cell proliferation and a reduced immune response to DNA and HIV ³⁹. It is likely that HIV infection activates STING-cell death pathway in CD4 T cells. In HAQ/SAVI and AQ/SAVI mice, one copy of HAQ, AQ allele suppressed CD4 T cell death. HAQ, AQ carriers might have fewer HIV-induced CD4 T cell death, thus being long-term nonprogressors in HIV infection-induced Acquired immunodeficiency syndrome (AIDS) ³⁹. Targeting STING to prevent CD4 T cell death might be a valid therapy for AIDS. Activating the STING pathway is a promising strategy for cancer immunotherapy ^58–64.

Multiple STING agonists are in clinical trials ^44,45. Recently, the safety issue emerged in some STING agonist trials ^44,45. For example, in the STING agonist, ADU-S100, clinical trial, Grade 3/4 treatment-related adverse events were reported in 12.2% of 41 pretreated patients (NCT02675439)^44,45. The National Institutes of Health defines grade 3 as “incapacitating; unable to perform usual activities; requires absenteeism or bed rest.” In a clinical trial using STING antibody-drug-conjugate (ADC) that conjugates diABZI to anti-HER2 Ab, a Grade 5 (fatal) serious adverse event was recorded and deemed related to the STING-ADC (NCT05514717). SAVI disease, driven by overreacting STING, is often fatal. AQ, to a less degree, HAQ, suppress mortality in SAVI mice. Future STING clinical trials should be based on human TMEM173 genotype to achieve safe and effective responses.

Mechanistically, apoptosis, pyroptosis, ferroptosis, necroptosis, and PANoptosis have all been reported in STING-mediated cell death ^{25–31,42,43}. Different cell types and STING agonists used likely contributed to the inconsistency and complexity. Here, we focused on lymphopenia in the SAVI mice that avoids ligand-dependent, non-physiological dosage in STING-mediated cell death. HAQ and AQ alleles could prevent CD4 T cellpenia in the SAVI mice strongly indicating that residue A230 or Q293 prevent STING-mediated CD4 T cell death in vivo. Splenocyte from Q293 mice were resistant to STING agonists-induced cell death ex vivo. Thus, it is likely that the Q293 residue is critical for STING-mediate lymphopenia. Notably, Q293 is outside the C-terminal tail (CTT) (residues 341–379 of human STING) critical for TBK1 recruitment and IRF3 phosphorylation ⁶⁵ or miniCTT domain (aa343–354) ²⁴, or the UPR motif (aa322–343) ⁴⁶ important for T cells death in vitro. Further studies are needed to understand how the aa293 of STING mediates cell death in vivo. Noteworthy, AQ/SAVI cells had similar TBK1-IRF3, NFκB activation and STING degradation as the WT/SAVI cells. Yet, AQ/SAVI mice did not have CD4 T cellpenia as WT/SAVI mice suggesting that the canonical STING-TBK1-IRF3/NFκB pathway, likely STING oligomerization, is not sufficient for the induction of cell death at the physiological condition.

We used the WT/N153S knock-in SAVI mouse model that spontaneously develop lung inflammation, T cell cytopenia, and early mortality, mimicking pathological findings in human SAVI patients¹⁶. Using the WT/N153S SAVI mouse model and human Jurkat T cell line, it was proposed that STING activation causes chronic ER stress and unfolded protein response, leading to T cell death by apoptosis ⁴⁶. Furthermore, the study showed that crossing WT/N153S mice to the OT-I mice reduced ER stress and restored CD8⁺, but not CD4⁺, T cells ⁴⁶. The restoration of CD8⁺T cells reduces inflammation and lung disease ⁴⁶. However, human WT/N154S SAVI patients have normal CD8⁺ T cells numbers ¹³, and primary human CD8⁺ T cells are largely resistant to STING-agonists-induced cell death ex vivo (Figure 2A) ²⁵. Thus, it is puzzling how restoring CD8⁺ T cells can rescue SAVI phenotypes since the SAVI patients already have normal CD8⁺ T cells numbers.

Finally, it is unexpected that both HAQ and AQ alleles are resistant to cell death. Our previous studies showed that the HAQ and AQ alleles have opposite functions ³⁴. AQ-STING, not HAQ-STING, responds to CDNs ^{32–34,36–40,66}. AQ mice are lean while HAQ mice are fat ³⁴. Most importantly, HAQ was positively selected, while AQ was negatively selected, in modern humans outside Africans ³⁴. Thus, the death pathway of STING is also distinct from the STING function that was naturally selected. Together, we propose that there are at least three unique pathways of STING/MPYS: STING-Type I IFNs, STING-cell death, and STING-fatty acid metabolism.

The limitations of the study

The poor transferability of mouse to humans is a major issue in STING research ^44,45. The present study used AQ/SAVI and HAQ/SAVI mice. Confirmation is needed in humans with the identification and evaluation of people who are AQ/SAVI, HAQ/SAVI.

Materials and Methods

Experimental Design

The study was designed to reveal (i) the in vivo significance of the type I IFNs-independent, STING-dependent cell death function; (ii) the interplay between common STING alleles HAQ, AQ and the rare, gain-of-function SAVI STING mutation; (iii). The driver for the inflammatory SAVI disease. Mouse splenocytes, primary human lung cells, human THP-1 cells and HAQ, AQ, SAVI knock-in mice were used to establish the in vivo significance and human relevance. All the repeats were biological replications that involve the same experimental procedures on different mice. Where possible, treatments were assigned blindly to the experimenter by another individual in the lab. When comparing samples from different groups, samples from each group were analyzed in concert, thereby preventing any biases that might arise from analyzing individual treatments on different days. All experiments were repeated at least twice.

Mice

WT/SAVI(N153S) mice were purchased from The Jackson Laboratory. HAQ, AQ mice were previously generated in the lab ^33,34. The Q293 mice were generated by Cyagen Biosciences. Briefly, the linearized targeting vector was transfected into JM8A3.N1 C57BL/6N embryonic stem cells. A positive embryonic stem clone was subjected to the generation of chimera mice by injection using C57BL/6J blastocysts as the host. Successful germline transmission was confirmed by PCR sequencing. The heterozygous mice were bred to Actin-flpase mice [The Jackson Laboratory, B6.Cg-Tg (ACTFLPe)9205Dym/J] to remove the neo gene and make the Q293 knock-in mouse. Age- and gender-matched mice (2 – 6 month old, both male and female) were used for indicated experiments. WT/SAVI (male) x WT/HAQ (female), WT/SAVI (male) x WT/AQ (female) breeders were set up to generate HAQ/SAVI, AQ/SAVI mice. Mice were housed at 22°C under a 12-h light-dark cycle with ad libitum access to water and a chow diet (3.1 kcal/g, Teklad 2018, Envigo, Sommerset, NJ) and bred under pathogen-free conditions in the Animal Research Facility at the University of Florida. Littermates of the same sex were randomly assigned to experimental groups. All mouse experiments were performed by the regulations and approval of the Institutional Animal Care and Use Committee at the University of Florida, IACUC202200000058.

Reagents

Recombinant human IFNβ (R&D, cat no. 8499-IF-010/CF), diABZI (Invivogen, cat no. 2138299-34-8), 2’3’-cGAMP (Invivogen, cat no. tlrl-nacga23-02), DMXAA (Invivogen, cat no. tlrl-dmx), H151 (Invivogen, cat no. inh-h151), RpRpSSCDA (Biolog, cat no. C118), THP1-Dual™ KO-STING Cells (Invivogen, cat no. thpd-kostg). All other chemical inhibitors are from Selleckchem. Mouse TNF alpha ELISA Ready Set Go. (eBioscience, cat no. 88-7324). Mouse IFN-Beta ELISA Kit (PBI, cat no. 42400).

Histology

Lungs and livers were fixed in 10% formalin, paraffin-embedded, and cut into 4-µm sections. Lung, liver sections were then stained for hematoxylin-eosin. All staining procedures were performed by the histology core at the University of Florida. Briefly, tissue sectins were immersed Harris Hematoxylin for 10 seconds, then washed with tap water. Cleard sections were re-immersed in EOSIN stain for ∼30 seconds. The sections were washed with tap water until clear, then dehydrate in ascending alcohol solutions (50%,70%,80%,95% x 2, 100% x 2). Afterwards, the sections werer cleared with xylene (3 - 4 x). The sections were mounted on glass slide with permount organic mounting medium for visulization.

Lung Function

Pulmonary function was evaluated using an isolated, buffer-perfused mouse lung apparatus (Hugo Sachs Elektronik, March-Huggstetten, Germany), as previously described ⁶⁷. Briefly, mice were anesthetized with ketamine and xylazine and a tracheostomy was performed, and animals were ventilated with room air at 100 breaths/min at a tidal volume of 7 μl/g body weight with a positive end-expiratory pressure of 2 cm H2O using a pressure-controlled ventilator (Hugo Sachs Elektronik, March-Huggstetten, Germany).

Isolation of lung cells

Cells were isolated from the lung as previously described ⁶⁸. The lungs were perfused with ice-cold PBS and removed. Lungs were digested in DMEM containing 200μg/ml DNase I (Roche, 10104159001), 25μg/ml Liberase TM (Roche, 05401119001) at 37°C for 2 hours. Red blood cells were then lysed and a single cell suspension was prepared by filtering through a 70-µm cell strainer.

BMDM activation

BMDMs were induced from mouse bone marrow cells cultured in RPMI 1640 (cat#11965; Invitrogen) with 10% FBS, 2 mM L-glutamine, 1 mM sodium pyruvate, 10 mM HEPES buffer, 1% nonessential amino acids, 50 mM 2-ME, 1% Pen/Strep, with 20ng/ml M-CSF (Kingfisher, RP0407M) for 10 days ³³. STING agonists were added into the culture (no transfection or membrane permeabilization).

Flow cytometry

Single cell suspensions were stained with fluorescent-dye-conjugated antibodies in PBS containing 2% FBS and 1mM EDTA. Surface stains were performed at 4°C for 20 min. For intracellular cytokine or transcription factor staining of murine and human cells, cells were fixed and permeabilized with the Foxp3 staining buffer set (eBioscience, cat no 00-5523-00). Cells were washed and stained with surface markers. Cells were then fixed and permeabilized (eBioscience, cat no. 00-5523-00) for intracellular cytokine stain. Data were acquired on a BD LSRFortessa and analyzed using the FlowJo software package (FlowJo, LLC). Cell sorting was performed on the BD FACSAriaIII Flow Cytometer and Cell Sorter. More information on the antibodies used can be found in supplementary table S1.

Human lung explants

Human lung explants were procured at the Lung Transplant Center, Division of Pulmonary, Critical Care and Sleep Medicine, Department of Medicine, University of Florida. Donor and patients consent was obtained for a research protocol (UF IRB201902955-Treatment with IFNβ Induces Tolerogenic Lung Dendritic Cells in Human advanced lung disease). Healthy donor lungs were surgically removed postmortem, perfused, small pieces were cut from the right middle and lower lobes for research purpose, and stored in cold Perfadex ® at 4°C for no more than 12 hrs before processing. Ex planted lungs from emphysema lung transplant patients were stored in cold Perfadex ® at 4°C for no more than 12 hrs before the process. No lung explants were procured from prisoners.

Statistical Analysis

To gain statistical power, we employ three∼four mice/groups to characterize lung immunity. Ten mice/group to monitor animal health. The statistical justification for group size was calculated using the SAS program to calculate the animal numbers. The analysis was carried out using a standard error of 0.5 for immunological assays, and a power of 0.9. All data are expressed as means ± SEM. Statistical significance was evaluated using Prism 9.0 software. Comparisons between two groups were analyzed by performing an unpaired Student’s t test. Comparisons between more than two groups were analyzed by performing a one-way analysis of variance (ANOVA) with Tukey’s multiple comparisons test.

Acknowledgements

None

Funding

National Institutes of Health grant HL152163 (L.J.).

Author contributions

Conceptualization: LJ

Methodology: AAT, LAS, ATP, AME

Investigation: AAT, LAS, ATP, AKS, AJB, LJ

Supervision: LJ

Writing—original draft: LJ

Writing—review & editing: LJ, AJB, AKS

Competing interests

Authors declare that they have no competing interests.

Data and materials availability

All data are available in the main text or the supplementary materials.