Microbial exposure alters HIV-1-induced mucosal CD4+ T cell death pathways Ex vivo
© Steele et al.; licensee BioMed Central Ltd. 2014
Received: 6 December 2013
Accepted: 1 February 2014
Published: 4 February 2014
Early HIV-1 infection causes massive CD4+ T cell death in the gut and translocation of bacteria into the circulation. However, the programmed cell death (PCD) pathways used by HIV-1 to kill CD4+ T cells in the gut, and the impact of microbial exposure on T cell loss, remain unclear. Understanding mucosal HIV-1 triggered PCD could be advanced by an ex vivo system involving lamina propria mononuclear cells (LPMCs). We therefore modeled the interactions of gut LPMCs, CCR5-tropic HIV-1 and a commensal gut bacterial species, Escherichia coli. In this Lamina Propria Aggregate Culture (LPAC) model, LPMCs were infected with HIV-1BaL by spinoculation and cultured in the presence or absence of heat killed E.coli. CD4+ T cell numbers derived from flow cytometry and viable cell counts were reported relative to mock infection. Viable cells were identified by viability dye exclusion (AqVi), and intracellular HIV-1 Gag p24 protein was used to identify infected cells. Annexin V and AqVi were used to identify apoptotic versus necrotic cells. Caspase-1 and Caspase-3 activities were blocked using specific inhibitors YVAD and DEVD, respectively.
CD4+ T cell depletion following HIV-1 infection was reproducibly observed by 6 days post infection (dpi). Depletion at 6 dpi strongly correlated with infection frequency at 4 dpi, was significantly blocked by Efavirenz treatment, and was primarily driven by p24-negative cells that were predominantly necrotic. HIV-1 infection significantly induced CD4+ T-cell intrinsic Caspase-1 activity, whereas Caspase-1 inhibition, but not Caspase-3 inhibition, significantly blocked CD4+ T cell depletion. Exposure to E.coli enhanced HIV-1 infection and CD4+ T depletion, and significantly increased the number of apoptotic p24+ cells. Notably, CD4+ T cell depletion in the presence of E.coli was partially blocked by Caspase-3, but not by Caspase-1 inhibition.
In the LPAC model, HIV-1 induced Caspase-1 mediated pyroptosis in bystander CD4+ T cells, but microbial exposure shifted the PCD mechanism toward apoptosis of productively infected T cells. These results suggest that mucosal CD4+ T cell death pathways may be altered in HIV-infected individuals after gut barrier function is compromised, with potential consequences for mucosal inflammation, viral dissemination and systemic immune activation.
KeywordsHuman Immunodeficiency Virus Programmed cell death Microbial translocation
HIV-1 infection is characterized by high levels of virus replication, gradual peripheral CD4+ T cell depletion, and aberrantly high immune activation. Chronic immune activation may result from pathogenic events in the gut mucosa established during early infection, when viral replication in the intestinal mucosa results in massive killing of lamina propria (LP) CD4+ T cells, enteropathy, inflammation, and microbial translocation [1, 2]. However, the mechanisms and pathways involved in HIV-associated LP CD4+ T cell depletion remain key unanswered questions in basic HIV research [3, 4].
Major efforts to investigate the mechanisms of HIV-1 mediated CD4+ T cell depletion have been made using ex vivo models of HIV-1 infection in primary human CD4+ T cells or cell lines. Ex vivo modeling studies of HIV-1 infection of primary human CD4+ T cells indicated that HIV-1-mediated killing could occur in both productively-infected and bystander, or nonproductively-infected, cells. CXCR4-tropic (X4) HIV-1 was found to kill resting spleen and tonsil CD4+ T cells ex vivo through abortive infection , whereas double-stranded DNA breaks occurring during HIV-1 integration were responsible for the death of productively-infected CD4+ T cells from peripheral blood . However, it remains unclear whether the death of productively-infected or bystander cells is primarily responsible for driving human LP CD4+ T cell depletion. Interestingly, earlier studies in the SIV/rhesus macaque model also suggested that LP CD4+ T cell death could occur in both productively infected  and bystander  cells to drive depletion. Unraveling the mechanisms underlying HIV-1 mediated LP CD4+ T cell depletion may require the use of primary human LP CD4+ T cell lymphocytes.
Unlike peripheral blood or lymphoid CD4+ T cells, LP CD4+ T cells are predominantly of a recently activated, CCR5hi effector memory phenotype . These cells are highly susceptible to infection by CCR5-tropic HIV-1 strains, which are found in over 90% of chronically HIV-infected patients, and account for most transmitted viruses [10, 11]. The LP CD4+ T cell pool in the gut-associated lymphoid tissue (GALT) is a heterogeneous population comprised of multiple T helper (Th) subsets that have diverse functions in host defense . In particular, the loss of mucosal IL-17 producing T cells (Th17), which play a role in defense against extracellular pathogens, has been closely linked to pathogenic SIV and HIV infection [13–15]. The gut microbiome also plays an important role in establishing the LP microenvironment. In HIV-1 infection, translocation of microbial products strongly correlates with increased immune activation [16–18]. In fact, commensal Gram-negative Escherichia coli (E.coli) was detected in the LP of rhesus macaques during early stages of pathogenic SIV infection . Thus, adequately modeling HIV interactions in the LP must account for any effects of commensal bacteria. We previously showed that commensal E.coli activates resident LP CD4+ T cells in an MHC Class II-dependent fashion , and increases HIV-1 replication in human LP CD4+ T cells ex vivo. However, the impact of microbial exposure on the magnitude and mechanisms of LP CD4+ T cell death remains unknown.
To date, very limited information exists as to which cell death pathway(s) are triggered in human LP CD4+ T cells by R5-tropic HIV-1 infection. Most programmed cell death (PCD) pathways are dependent on proteolytic enzymes known as caspases. In the canonical PCD pathway, apoptosis, cells undergo cell shrinkage, blebbing and DNA fragmentation but retain plasma membrane integrity . Apoptosis can be triggered by either extrinsic (e.g. Fas/FasL) or intrinsic stimuli, but both pathways converge on the effector molecule Caspase-3 . Apoptotic cells are generally disposed of in vivo in a non-inflammatory manner through the exposure of phosphotidylserine (PS) from the inner leaflet of the plasma membrane to the cell surface [24–26]. There is a substantial body of literature suggesting that apoptosis is aberrantly triggered or has become dysregulated during HIV-1 infection [27–29]. In contrast to apoptosis, pyroptosis is a highly inflammatory form of PCD that involves oncosis, plasma membrane rupture, and the rapid release of cytoplasmic contents into the surrounding environment [22, 30]. Pyroptosis has been linked to the ‘inflammasome’, a multimeric complex containing active Caspase-1 and pattern recognition receptors such as NLRP3 [30, 31]. In addition to mediating pyroptosis, Caspase-1 processes pro-IL-1β to the mature secreted form that could contribute to inflammation and epithelial barrier dysfunction [32, 33]. Interestingly, increased Caspase-1 activity has been documented in HIV infection ex vivo in the Human Lymphoid Aggregate Culture (HLAC) model and in primary peripheral blood T cells from HIV infected patients [5, 34, 35]. It remains unknown whether Caspase-1 plays a role in HIV-1 mediated LP CD4+ T cell death.
In this report, we used the Lamina Propria Aggregate Culture (LPAC) model to identify the PCD pathway(s) triggered in primary LP CD4+ T cells ex vivo by infection with an R5 tropic HIV-1 strain. We further assessed the impact of commensal E. coli exposure on the magnitude and mechanisms of HIV-mediated LP CD4+ T cell depletion. We provide evidence for augmented HIV-1 mediated LP CD4+ T cell death and a shift in the PCD pathway following microbial exposure.
CD4+ T cell depletion strongly correlates with productive infection in the LPAC model
HIV-1 infection increases the frequency of CD4 T cells with apoptotic and necrotic phenotypes
We next compared the p24+ and p24neg populations to gain insight into their respective contributions to total CD4+ T cell loss. The percentage of cells with an apoptotic phenotype was greater in the p24+ population, whereas the percentage of cells with a necrotic phenotype was greater in p24neg population (Additional file 2: Figure S2A). However, since there were more p24neg cells than p24+ cells in the culture, the absolute numbers of apoptotic p24+ and p24neg cells were similar at 4 dpi (Figure 3C), suggesting that apoptosis contributed equally to depletion in both subsets. In sharp contrast, the absolute number of p24neg cells with a necrotic phenotype was 34-fold greater than the number of necrotic p24+ cells (Figure 3C). Presented another way, 63% of the CD4+ T cells committed to dying (AnnexinV+) were p24neg and had a necrotic phenotype (Figure 3D), indicating that necrotic cell death in the p24neg population accounted for the majority of depletion during HIV-1 infection. The p24+ cells accounted for only 13% of the total dying cells (Figure 3D), but it was the magnitude of p24+ cell death that correlated with HIV-1 mediated LP CD4+ T cell depletion measured at 6 dpi (compare Figure 3E and F).
HIV-1 promotes LP CD4+ T cell death predominantly through Caspase-1-mediated pyroptosis
Irreversible peptide inhibitors were used to block the function of Caspase-1 (pyroptosis; YVAD) and Caspase-3 (apoptosis; DEVD) to determine their relative effects on HIV-1 induced depletion. To avoid toxicity, we used the highest possible dose (25 μM) of each inhibitor at a non-toxic DMSO concentration and added each inhibitor (25 μM) at days 0 and 2 (Figure 1A). Depletion was determined relative to mock infection and normalized to the DMSO vehicle control (100% depletion) to determine the percent protection from depletion with each inhibitor for each donor (Figure 4D; representative donor). Neither DEVD nor YVAD significantly altered the frequency of p24+ CD4+ T cells compared to the DMSO control, indicating that HIV-1 replication was not impeded by the presence of drug (Additional file 3: Figure S3A). In 6 donors, HIV-1 infection significantly depleted LP CD4+ T cells by 4 dpi relative to mock infection with DMSO (Figure 4E). DEVD did not significantly inhibit LP CD4+ T cell depletion (Figure 4E). However, YVAD significantly attenuated HIV-1 mediated depletion from 36% to 18% (Figure 4E) indicating a role for Caspase-1 activity in HIV-1 mediated LP CD4+ T cell death.
Exposure to commensal E. colialters the kinetics and increases the magnitude of HIV-1 mediated LP CD4+ T cell depletion in the LPAC model
Pathogenic lentivirus infection is associated with extensive depletion of the gut Th17 subset which plays a key role in bacterial defense [13, 14, 38–40], raising the possibility that the Th subsets in the LP may have different susceptibilities to HIV-1 mediated killing. We therefore investigated the susceptibility of Th1 (IFN-γ+IL-17-), Th17 (IL-17+IFN-γ-), Th1/17 (IFN-γ+ IL-17+) and non-Th1/17 (IFN-γ-IL-17-, double negative, DN) subsets to undergo HIV-1 mediated death at 6 dpi in the presence or absence of bacteria. The Th subsets were identified by intracellular cytokine staining by flow cytometry following PMA/Ionomycin stimulation. At 6 dpi, R5-tropic HIV-1 depleted Th1, Th17 and Th1/17 cells in the LPAC model to a similar extent (Figure 5D). In contrast, DN cells were relatively resistant to HIV-1 mediated killing (Figure 5D). Commensal E. coli exposure led to 2 to 3-fold increased LP T cell depletion in the Th1, Th17 and DN subsets compared to HIV-1 alone (Figure 5D). Remarkably, E. coli enhanced depletion in the Th1/17 depletion by 10-fold (Figure 5D). As with HIV-1 infection alone (Figure 2B), the level of productive infection was strongly associated with the level of depletion following E. coli exposure (Figure 5E; Spearman r for samples with E.coli = 0.5760; p = 0.017).
Exposure to E. coliincreases the death of productively infected cells through increased apoptosis
DEVD and YVAD were used to determine whether Caspase inhibition could block HIV-1 associated LP CD4+ T cell death in the presence of E. coli. The inhibitors did not significantly decrease the LP CD4+ T cell number in the mock plus E. coli condition (Additional file 3: Figure S3C-D). LP CD4+ T cell depletion in the presence of HIV-1 plus E. coli was significantly blocked by DEVD compared to the DMSO control (Figure 6E). Surprisingly, Caspase-1 inhibition by YVAD did not significantly prevent LP CD4+ T cell depletion with virus and E. coli present (Figure 6E). The results suggest that in the presence of E.coli, HIV-1 mediated LP CD4+ T cell death is Caspase-3, but not Caspase-1, dependent.
Counteracting inflammation and immune activation in HIV-1 infection may require resolving critical knowledge gaps about how HIV-1 causes extensive LP CD4+ T cell death during primary infection. Here, we demonstrate the versatility of the LPAC model in addressing these critical questions regarding the mechanisms underlying R5-tropic HIV-1 mediated mucosal CD4+ T cell death. We provide evidence that HIV-1 induces pyroptosis of non-productively infected LP CD4+ T cells based on the detection of active Caspase-1 in CD4+ T cells and the inhibition of LP CD4+ T cell depletion by blocking Caspase-1 activity. Surprisingly, our results with primarily effector memory LP CD4+ T cells mirrored published findings based on X4-tropic HIV-1 infection of HLAC cells , which are predominantly resting CD4+ T cells. Thus, despite the important differences in CD4+ T cell phenotypes between the two ex vivo models, pyroptosis appears to be a critical HIV-1-induced PCD pathway in non-productively infected CD4+ T cells.
The molecular trigger(s) for Caspase-1 mediated death in the LPAC model remain unknown, but recent high-profile studies suggest plausible mechanisms [5, 6, 41, 42]. One possibility is that the accumulation of incomplete HIV-1 reverse transcripts in abortively infected LP CD4+ T cells triggered death, analogous to that reported in the HLAC model . In this model, IFI16 has been identified as the DNA sensor that triggers pyroptosis in the bystander T cells , and p24+ cells were shown to be the killing unit rather than the free virus . Notably, in the LPAC model, it was the number of dying p24+, but not p24neg cells, which significantly correlated with depletion. If a similar PCD mechanism operates in the LPAC as in the HLAC model, each p24+ cell could induce cell death in variable number of bystander cells, thereby weakening the association between p24neg cells and depletion. However, LP CD4+ T cells are more susceptible to HIV-1 infection than resting HLAC CD4+ T cells due to higher activation levels and expression of HIV-1 co-receptors [9, 43]. Thus, at this time, we cannot confirm that abortive infection triggers pyroptosis in the gut and cannot exclude alternative triggers for Caspase-1 activity. Nabel and colleagues recently reported decreased HIV-1 gene expression following DNA-PK recognition of double-stranded DNA breaks during integration . Thus, an alternative hypothesis is that HIV-1 gene expression is being quickly shut off in permissive LP CD4+ T cells upon integration induced cell death, and permissive cells dying post-HIV-1 integration could appear as p24neg. Loss of viral protein expression in permissive cells could also potentially explain the strong association we observed between the frequency of productively infected cells and depletion. However, to our knowledge, there is no established link between DNA-PK and Caspase-1 activities.
Caspase-1 activity may be a potential link between HIV-1-mediated LP CD4+ T cell death and HIV-1-mediated mucosal inflammation. In antigen presenting cells, Caspase-1 cleaves pro-IL-18 and pro-IL-1β to their active forms . Increased IL-1β expression was observed in the LP during acute pathogenic SIV, but not during non-pathogenic infection, suggesting that Caspase-1 activity could be a mediator of lentivirus pathogenesis . IL-1β, other pro-inflammatory cytokines, and HIV-1 gp120 have been shown to cause tight junction breakdown between intestinal epithelial cells in vitro[32, 44]. Furthermore, pyroptosis releases cellular contents into the environment that can act as danger-associated molecular patterns (DAMPs), thereby triggering inflammation in the local microenvironment [30, 31]. Thus, Caspase-1 activity in LP CD4+ T cells could precipitate epithelial barrier dysfunction by releasing either traditional inflammatory cytokines or cellular DAMPs.
Epithelial barrier breakdown results in microbial translocation during the later stages of acute SIV infection , suggesting that studies in the LPAC model are incomplete without taking enteric bacteria into account. We previously reported  that exposure to commensal bacteria increased productive infection in LP CD4+ T cells in vitro, likely by enhancing T cell activation. We now link these findings to increased LP CD4+ T cell depletion in the presence of commensal bacteria. In addition, the LPAC data for Th subsets recapitulated the high susceptibility of IL-17-producing cells to HIV-1-mediated depletion observed in vivo and highlighted a role for microbial exposure in exacerbating this process [13, 14, 38–40].
Exposure to E. coli increased overall LP CD4 T cell depletion by increasing the number of productively infected cells dying by apoptosis. The involvement of apoptosis as the dominant PCD pathway in the presence of E. coli was further supported by significant inhibition of HIV-1-mediated depletion by blocking Caspase-3 activity. Several viral proteins such as protease, gp120, Tat, Nef and Vpr have been reported to facilitate apoptotic CD4+ T cell death [27, 28]. Enhanced HIV-1 infection following microbial exposure could lead to increased cellular levels of these viral proteins, thereby triggering apoptosis. Indirectly, microbial products may also sensitize LP CD4+ T cells to undergo apoptosis at a lower signaling threshold. For example, LPS and inflammatory cytokines have been reported to increase tryptophan catabolism by DCs , potentially causing local tryptophan depletion, which then sensitizes activated T cells to apoptosis [45, 46].
The data establish the Lamina Propria Aggregate Culture (LPAC) model as a robust and versatile platform for studying the impact of R5-tropic HIV-1 infection and commensal microbial species on mucosal CD4+ T cell death. Using the LPAC model, we provide evidence for Caspase-1 mediated T cell pyroptosis as a primary mechanism for LP CD4+ T cell death in bystander cells. However, upon exposure to commensal E. coli, LP CD4+ T cell death was augmented and shifted to Caspase-3 mediated apoptosis due to a significant increase in productively-infected cells. The results suggest a biphasic model of LP CD4+ T cell death during acute HIV infection, in which distinct CD4+ T cell death pathways, demarcated by intestinal barrier dysfunction and microbial translocation, may converge to drive mucosal inflammation, viral dissemination and systemic immune activation.
Intestinal tissue samples
Macroscopically normal human jejunum tissue samples, that would otherwise be discarded, were obtained in a de-identified manner from 17 patients undergoing elective abdominal surgery. Those with a history of inflammatory bowel disease, recent chemotherapy, radiation, or other immunosuppressive drugs were excluded from the study. The use of discarded tissue was granted exempt status by the Colorado Multiple Institutional Review Board and patients signed a pre-operative consent form allowing its unrestricted use.
Preparation of LPMCs from intestinal samples
The primary LP cells were obtained using a two-step fractionation process as previously described [20, 21]. EDTA was used to separate the epithelial cells and intraepithelial lymphocytes, followed by Collagenase D (Roche Catalog #: 11088882001) treatment to release the LPMCs. The cells were cryopreserved in RPMI + 10% DMSO + 10% FBS.
Preparation of HIV-1 stocks
MOLT4-CCR5 cells (AIDS Research and Reference Reagent Program; ARRP Catalog# 510) were grown at 0.5 to 1 million (M)/ml in RPMI containing 10% FBS, 1% penicillin/streptomycin/glutamine and 1 mg/ml G418 (propagation media) at 37°C to 50 M total cells. Majority (>90%) of MOLT4-CCR5 cells expressed CD4 and CCR5 by flow cytometry. 25 M MOLT4-CCR5 cells were infected with the R5-tropic HIV-1 Ba-L strain (ARRP Catalog# 4984) with 2 μg/ml polybrene for 2 h. The cells were washed with propagation media and grown at 0.5 to 1 M cells/ml. Supernatants containing virus were collected at 9 dpi and concentrated by ultracentrifugation at 141,000×g for 2 h. The concentration of p24 in the supernatant was determined by HIV Gag p24 ELISA (Perkin Elmer, Walthm, MA). Virus stocks were frozen in single use aliquots at −80°C.
Preparation of heat-killed Eschericia coli
E. coli stocks (ATCC #25922, Manassass, VA) were grown as described previously  and kept at −80°C in single use aliquots.
LPMC infection assay
LPMCs were thawed using a standard protocol. Briefly, 1 ml aliquots of cells were quick thawed at 37°C. The following amounts of thaw media (90 ml RPMI + 10% FBS + 1% penicillin/streptomycin/glutamine + 100 μl DNAse) were added at 1 min intervals: 100 μl, 200 μl, 400 μl, 800 μl, 1.6 ml, 3.2 ml (twice). The cells were centrifuged at 1500 rpm for 5 mins and then washed once in 10 ml of thaw media. The thawed LPMCs were immediately resuspended in complete RPMI (RPMI + 10% human AB serum, 1% penicillin/streptomycin/glutamine, 500 μg/ml Zosyn) without any additional growth factors. HIV-1 (80 ng p24) per 1 M LPMCs was used to infect primary LPMCs at 10 M/ml in complete RPMI. LPMCs were mock infected in parallel. Infection by spinoculation was performed at 1200×g for 2 h at room temperature . After spinoculation, the supernatant containing free virus was discarded; the cell pellet was resuspended in complete RPMI at 1 M LPMCs/ml and then seeded in a 48-well plate. Heat-killed E. coli was added to LPMCs at a ratio of 5 E.coli: 1 LPMC where indicated. Fungizone (1.25 μg/ml) was added at 1 dpi. The LPMCs were harvested at indicated time points.
LPMC Infection in the presence of drugs
For infections in the presence of Efavirenz (ARRP Catalog #4624), LPMCs were pre-incubated with Efavirenz (1 nM or 10 nM) for 2 h at 37°C and infected as described above and cultured in media containing the indicated Efavirenz dose. A second dose of Efavirenz was added at 3 dpi. At 6 dpi, the LPMCs were collected and analyzed as described.
Inhibitors of Caspase-3 (DEVD-fmk; DEVD (Calbiochem)) and Caspase-1 (YVAD-cmk; YVAD (Calbiochem)) were used to inhibit PCD ex vivo in donor samples that showed depletion at 4 dpi. The YVAD motif is predominately cleaved by Caspase-1 but does have some cross reactivity (340-fold less affinity) with other group I caspases, Caspase-4 and Caspase-5 . The inhibitors were resuspended in DMSO at 50 mM and stored at −20°C per the manufacturer’s instructions. The dose used in primary LPMCs was based on optimization studies to balance maximum inhibition of the pathway with minimum toxicity. The 4 dpi time point was chosen so that consistent depletion was observed but DMSO exposure was minimal. Following HIV-1 spinoculation (0 dpi), the inhibitors were added to the appropriate wells at a 25 μM concentration. An additional dose of each inhibitor was added to the cultures at 2 dpi without changing the media. At 4 dpi, LPMCs were collected for analysis.
Mitogen Stimulation for Th Subset Identification
6 dpi LPMCs were collected, counted, resuspended at 1 M/ml in complete RPMI, and stimulated with 100 ng/ml PMA (Sigma-Aldrich), 100 μg/ml Ionomycin (Sigma- Aldrich) and 1 μg/ml Brefeldin A (Golgi Plug, BD Biosciences) for 5 h at 37°C in 5% CO2. LPMCs capable of making IFN-γ and IL-17 were identified by flow cytometry .
Antibodies for flow cytometry
Aqua Viability Dye (AqVi; Invitrogen) exclusion was used to identify viable cells. The antibodies used for these studies were: CD3-ECD (Beckman Coulter), CD8-APC (BD Pharmingen), CD19-APCH7 (BD Biosciences), HIV-1 p24-PE (Beckman Coulter), IL-17-V450 (BD biosciences), IFN-γ-AF700 (BD biosciences) and AnnexinV-Pacific Blue (Life Technologies). Isotype controls were used to establish the gates for IL-17 (Mouse IgG1- V450, BD Biosciences) and IFNγ (Mouse IgG1-AF700, BD biosciences).
Extracellular and Intracellular Staining
LPMCs were stained with cocktails of antibodies against markers expressed on the cell surface for 20 minutes in PBS + 1% BSA + 2 mM EDTA (FACS buffer) at 4°C. The LPMCs were then incubated with AqVi per the manufacturer’s instructions. The LPMCs were then fixed and permeabilized using the Invitrogen Medium A/Medium B system .
The best practices protocol from eBioscience was followed to detect AnnexinV + LPMCs in conjunction with intracellular markers incorporating minor modifications following the AqVi step in the standard protocol. After the LPMCs were incubated with AqVi, they were washed once with 2 ml of 1× AnnexinV binding buffer (BD Pharmingen). The LPMCs were resuspended in 1× AnnexinV binding buffer at a concentration of 1-3 M cells/ml and incubated, at room temperature for 15 min, with 5 μL of AnnexinV. Unbound AnnexinV was washed away using 2 ml 1× AnnexinV binding buffer. The cells were then fixed and stained for intracellular p24 using the Medium A/Medium B system. In optimization experiments, the early apoptotic phenotype was not detectable beyond 4 dpi. Therefore, we selected samples that consistently depleted at 4 dpi to determine the phenotype of the dying CD4+ T cells.
Fluorogenic Detection of Caspase-1 Activity
2 dpi and 6 dpi LPMCs were collected and counted using trypan blue. The LPMCs were resuspended in 75 uL of 10 mM CaspaLux-1 substrate (Oncoimmunin) and incubated for 30 mins at 37°C in 5% CO2. The LPMCs were washed once with 1 mL of the provided FACS buffer and surface stained as described above. The LPMCs were fixed with 4% PFA for 15 mins at room temperature, washed once with 2 mL of FACS buffer, and acquired in 1% PFA on the LSRII within 60 mins.
Flow cytometric acquisition and analysis
Acquisition occurred within 24 hours for all samples and within 2 h for samples stained with AnnexinV. All samples were collected on an LSR II flow cytometer (BD Biosciences) and analyzed using BD FACS DIVA version 6.1.3.
To quantify depletion, the number of viable T cells in an HIV-1 infected well was reported as a percent of the T cells in the mock infected control well at 4 or 6 dpi. The LPMCs were harvested and counted using trypan blue to determine the number of live cells in the well. However, because the LPAC model is a mixed cell culture model, the number of live cells counted by trypan blue does not equate to the number of viable CD4+ T cells per well. To determine the number of viable CD4+ T cells per well, flow cytometry was used to determine the percentage of the viable cells (analogous to the trypan blue count) that were CD3+CD8- in a tight lymphocyte gate that excluded both cellular debris and epithelial cells. The trypan blue count was then multiplied by the percentage of viable CD3+CD8- T cells measured by flow cytometry. For example, if 100,000 cells were counted by trypan blue and 40% were viable CD3+CD8- T cells by flow cytometry, we would report out 40,000 CD4+ T cells for that well. The same method was used to determine the number of Th1, Th17, Th1/17, and apoptotic or necrotic CD4+ T cells. CD4+ T cell depletion was determined in the presence and absence of E.coli.
Statistical analysis was conducted using GraphPad Prism version 5 for Windows (GraphPad, San Diego, CA). Non-parametric statistics were used in these analyses.
CCW was supported by funding from the National Institutes of Health (R01 DK088663 and AI108404). MLS was supported by the University of Colorado Department of Medicine Early Career Scholar Program and startup funds. AKS was supported by funds from the Colorado HIV-1 Research Training Program (NIH 2T32AI007447-21). We acknowledge Zachary Dong, Daniel Hecht, and David Shugarts for their assistance with the study. Finally, we would like to acknowledge the AIDS reagent program for providing the virus stock and Efavirenz used in these studies.
- Brenchley JM, Schacker TW, Ruff LE, Price DA, Taylor JH, Beilman GJ, Nguyen PL, Khoruts A, Larson M, Haase AT, Douek DC: CD4+ T cell depletion during all stages of HIV disease occurs predominantly in the gastrointestinal tract. J Exp Med. 2004, 200: 749-759. 10.1084/jem.20040874.PubMed CentralView ArticlePubMedGoogle Scholar
- Sankaran S, Guadalupe M, Reay E, George MD, Flamm J, Prindiville T, Dandekar S: Gut mucosal T cell responses and gene expression correlate with protection against disease in long-term HIV-1-infected nonprogressors. Proc Natl Acad Sci U S A. 2005, 102: 9860-9865. 10.1073/pnas.0503463102.PubMed CentralView ArticlePubMedGoogle Scholar
- Chase A, Zhou Y, Siliciano RF: HIV-1-induced depletion of CD4+ T cells in the gut: mechanism and therapeutic implications. Trends Pharmacol Sci. 2006, 27: 4-7. 10.1016/j.tips.2005.11.005.View ArticlePubMedGoogle Scholar
- Thomas C: Roadblocks in HIV research: five questions. Nat Med. 2009, 15: 855-859. 10.1038/nm0809-855.View ArticlePubMedGoogle Scholar
- Doitsh G, Cavrois M, Lassen KG, Zepeda O, Yang Z, Santiago ML, Hebbeler AM, Greene WC: Abortive HIV infection mediates CD4 T cell depletion and inflammation in human lymphoid tissue. Cell. 2010, 143: 789-801. 10.1016/j.cell.2010.11.001.PubMed CentralView ArticlePubMedGoogle Scholar
- Cooper A, Garcia M, Petrovas C, Yamamoto T, Koup RA, Nabel GJ: HIV-1 causes CD4 cell death through DNA-dependent protein kinase during viral integration. Nature. 2013, 498: 376-379. 10.1038/nature12274.View ArticlePubMedGoogle Scholar
- Mattapallil JJ, Douek DC, Hill B, Nishimura Y, Martin M, Roederer M: Massive infection and loss of memory CD4+ T cells in multiple tissues during acute SIV infection. Nature. 2005, 434: 1093-1097. 10.1038/nature03501.View ArticlePubMedGoogle Scholar
- Li Q, Duan L, Estes JD, Ma ZM, Rourke T, Wang Y, Reilly C, Carlis J, Miller CJ, Haase AT: Peak SIV replication in resting memory CD4+ T cells depletes gut lamina propria CD4+ T cells. Nature. 2005, 434: 1148-1152.PubMedGoogle Scholar
- Kunkel EJ, Boisvert J, Murphy K, Vierra MA, Genovese MC, Wardlaw AJ, Greenberg HB, Hodge MR, Wu L, Butcher EC, Campbell JJ: Expression of the chemokine receptors CCR4, CCR5, and CXCR3 by human tissue-infiltrating lymphocytes. Am J Pathol. 2002, 160: 347-355. 10.1016/S0002-9440(10)64378-7.PubMed CentralView ArticlePubMedGoogle Scholar
- Wilkin TJ, Su Z, Kuritzkes DR, Hughes M, Flexner C, Gross R, Coakley E, Greaves W, Godfrey C, Skolnik PR, et al: HIV type 1 chemokine coreceptor use among antiretroviral-experienced patients screened for a clinical trial of a CCR5 inhibitor: AIDS Clinical Trial Group A5211. Clin Infect Dis. 2007, 44: 591-595. 10.1086/511035.View ArticlePubMedGoogle Scholar
- Keele BF, Giorgi EE, Salazar-Gonzalez JF, Decker JM, Pham KT, Salazar MG, Sun C, Grayson T, Wang S, Li H, et al: Identification and characterization of transmitted and early founder virus envelopes in primary HIV-1 infection. Proc Natl Acad Sci U S A. 2008, 105: 7552-7557. 10.1073/pnas.0802203105.PubMed CentralView ArticlePubMedGoogle Scholar
- Smith PM, Garrett WS: The gut microbiota and mucosal T cells. Front Microbiol. 2011, 2: 111-PubMed CentralView ArticlePubMedGoogle Scholar
- Brenchley JM, Paiardini M, Knox KS, Asher AI, Cervasi B, Asher TE, Scheinberg P, Price DA, Hage CA, Kholi LM, et al: Differential Th17 CD4 T-cell depletion in pathogenic and nonpathogenic lentiviral infections. Blood. 2008, 112: 2826-2835. 10.1182/blood-2008-05-159301.PubMed CentralView ArticlePubMedGoogle Scholar
- Ciccone EJ, Read SW, Mannon PJ, Yao MD, Hodge JN, Dewar R, Chairez CL, Proschan MA, Kovacs JA, Sereti I: Cycling of gut mucosal CD4+ T cells decreases after prolonged anti-retroviral therapy and is associated with plasma LPS levels. Mucosal Immunol. 2010, 3: 172-181. 10.1038/mi.2009.129.PubMed CentralView ArticlePubMedGoogle Scholar
- Favre D, Mold J, Hunt PW, Kanwar B, Loke P, Seu L, Barbour JD, Lowe MM, Jayawardene A, Aweeka F, et al: Tryptophan catabolism by indoleamine 2,3-dioxygenase 1 alters the balance of TH17 to regulatory T cells in HIV disease. Sci Transl Med. 2010, 2: 32-a36View ArticleGoogle Scholar
- Brenchley JM, Price DA, Schacker TW, Asher TE, Silvestri G, Rao S, Kazzaz Z, Bornstein E, Lambotte O, Altmann D, et al: Microbial translocation is a cause of systemic immune activation in chronic HIV infection. Nat Med. 2006, 12: 1365-1371.View ArticlePubMedGoogle Scholar
- Jiang W, Lederman MM, Hunt P, Sieg SF, Haley K, Rodriguez B, Landay A, Martin J, Sinclair E, Asher AI, et al: Plasma levels of bacterial DNA correlate with immune activation and the magnitude of immune restoration in persons with antiretroviral-treated HIV infection. J Infect Dis. 2009, 199: 1177-1185. 10.1086/597476.PubMed CentralView ArticlePubMedGoogle Scholar
- Pandrea I, Gaufin T, Brenchley JM, Gautam R, Monjure C, Gautam A, Coleman C, Lackner AA, Ribeiro RM, Douek DC, Apetrei C: Cutting edge: experimentally induced immune activation in natural hosts of simian immunodeficiency virus induces significant increases in viral replication and CD4+ T cell depletion. J Immunol. 2008, 181: 6687-6691.PubMed CentralView ArticlePubMedGoogle Scholar
- Estes JD, Harris LD, Klatt NR, Tabb B, Pittaluga S, Paiardini M, Barclay GR, Smedley J, Pung R, Oliveira KM, et al: Damaged intestinal epithelial integrity linked to microbial translocation in pathogenic simian immunodeficiency virus infections. PLoS Pathog. 2010, 6: e1001052-10.1371/journal.ppat.1001052.PubMed CentralView ArticlePubMedGoogle Scholar
- Howe R, Dillon S, Rogers L, McCarter M, Kelly C, Gonzalez R, Madinger N, Wilson CC: Evidence for dendritic cell-dependent CD4(+) T helper-1 type responses to commensal bacteria in normal human intestinal lamina propria. Clin Immunol. 2009, 131: 317-332. 10.1016/j.clim.2008.12.003.PubMed CentralView ArticlePubMedGoogle Scholar
- Dillon SM, Manuzak JA, Leone AK, Lee EJ, Rogers LM, McCarter MD, Wilson CC: HIV-1 infection of human intestinal lamina propria CD4+ T cells in vitro is enhanced by exposure to commensal Escherichia coli. J Immunol. 2012, 189: 885-896. 10.4049/jimmunol.1200681.PubMed CentralView ArticlePubMedGoogle Scholar
- Kroemer G, Galluzzi L, Vandenabeele P, Abrams J, Alnemri ES, Baehrecke EH, Blagosklonny MV, El-Deiry WS, Golstein P, Green DR, et al: Classification of cell death: recommendations of the Nomenclature Committee on Cell Death 2009. Cell Death Differ. 2009, 16: 3-11. 10.1038/cdd.2008.150.PubMed CentralView ArticlePubMedGoogle Scholar
- Galluzzi L, Vitale I, Abrams JM, Alnemri ES, Baehrecke EH, Blagosklonny MV, Dawson TM, Dawson VL, El-Deiry WS, Fulda S, et al: Molecular definitions of cell death subroutines: recommendations of the Nomenclature Committee on Cell Death 2012. Cell Death Differ. 2012, 19: 107-120. 10.1038/cdd.2011.96.PubMed CentralView ArticlePubMedGoogle Scholar
- Fink SL, Cookson BT: Apoptosis, pyroptosis, and necrosis: mechanistic description of dead and dying eukaryotic cells. Infect Immun. 2005, 73: 1907-1916. 10.1128/IAI.73.4.1907-1916.2005.PubMed CentralView ArticlePubMedGoogle Scholar
- Silva MT, do Vale A, dos Santos NM: Secondary necrosis in multicellular animals: an outcome of apoptosis with pathogenic implications. Apoptosis. 2008, 13: 463-482. 10.1007/s10495-008-0187-8.View ArticlePubMedGoogle Scholar
- Opferman JT, Korsmeyer SJ: Apoptosis in the development and maintenance of the immune system. Nat Immunol. 2003, 4: 410-415. 10.1038/ni0503-410.View ArticlePubMedGoogle Scholar
- Alimonti JB, Ball TB, Fowke KR: Mechanisms of CD4+ T lymphocyte cell death in human immunodeficiency virus infection and AIDS. J Gen Virol. 2003, 84: 1649-1661. 10.1099/vir.0.19110-0.View ArticlePubMedGoogle Scholar
- Arnoult D, Viollet L, Petit F, Lelievre JD, Estaquier J: HIV-1 triggers mitochondrion death. Mitochondrion. 2004, 4: 255-269. 10.1016/j.mito.2004.06.010.View ArticlePubMedGoogle Scholar
- Gougeon ML, Piacentini M: New insights on the role of apoptosis and autophagy in HIV pathogenesis. Apoptosis. 2009, 14: 501-508. 10.1007/s10495-009-0314-1.View ArticlePubMedGoogle Scholar
- Miao EA, Rajan JV, Aderem A: Caspase-1-induced pyroptotic cell death. Immunol Rev. 2011, 243: 206-214. 10.1111/j.1600-065X.2011.01044.x.PubMed CentralView ArticlePubMedGoogle Scholar
- Bergsbaken T, Fink SL, Cookson BT: Pyroptosis: host cell death and inflammation. Nat Rev Microbiol. 2009, 7: 99-109. 10.1038/nrmicro2070.PubMed CentralView ArticlePubMedGoogle Scholar
- Al-Sadi R, Ye D, Said HM, Ma TY: Cellular and molecular mechanism of interleukin-1beta modulation of Caco-2 intestinal epithelial tight junction barrier. J Cell Mol Med. 2011, 15: 970-982. 10.1111/j.1582-4934.2010.01065.x.PubMed CentralView ArticlePubMedGoogle Scholar
- Roselli M, Finamore A, Britti MS, Mengheri E: Probiotic bacteria Bifidobacterium animalis MB5 and Lactobacillus rhamnosus GG protect intestinal Caco-2 cells from the inflammation-associated response induced by enterotoxigenic Escherichia coli K88. Br J Nutr. 2006, 95: 1177-1184. 10.1079/BJN20051681.View ArticlePubMedGoogle Scholar
- Scheuring UJ, Sabzevari H, Corbeil J, Theofilopoulos AN: Differential expression profiles of apoptosis-affecting genes in HIV-infected cell lines and patient T cells. AIDS. 1999, 13: 167-175. 10.1097/00002030-199902040-00004.View ArticlePubMedGoogle Scholar
- Sloand EM, Kumar PN, Kim S, Chaudhuri A, Weichold FF, Young NS: Human immunodeficiency virus type 1 protease inhibitor modulates activation of peripheral blood CD4(+) T cells and decreases their susceptibility to apoptosis in vitro and in vivo. Blood. 1999, 94: 1021-1027.PubMedGoogle Scholar
- Rhee SS, Marsh JW: Human immunodeficiency virus type 1 Nef-induced down-modulation of CD4 is due to rapid internalization and degradation of surface CD4. J Virol. 1994, 68: 5156-5163.PubMed CentralPubMedGoogle Scholar
- Vanden Berghe T, Vanlangenakker N, Parthoens E, Deckers W, Devos M, Festjens N, Guerin CJ, Brunk UT, Declercq W, Vandenabeele P: Necroptosis, necrosis and secondary necrosis converge on similar cellular disintegration features. Cell Death Differ. 2010, 17: 922-930. 10.1038/cdd.2009.184.View ArticlePubMedGoogle Scholar
- Favre D, Lederer S, Kanwar B, Ma ZM, Proll S, Kasakow Z, Mold J, Swainson L, Barbour JD, Baskin CR, et al: Critical loss of the balance between Th17 and T regulatory cell populations in pathogenic SIV infection. PLoS Pathog. 2009, 5: e1000295-10.1371/journal.ppat.1000295.PubMed CentralView ArticlePubMedGoogle Scholar
- Gosselin A, Monteiro P, Chomont N, Diaz-Griffero F, Said EA, Fonseca S, Wacleche V, El-Far M, Boulassel MR, Routy JP, et al: Peripheral blood CCR4 + CCR6+ and CXCR3 + CCR6 + CD4+ T cells are highly permissive to HIV-1 infection. J Immunol. 2010, 184: 1604-1616. 10.4049/jimmunol.0903058.PubMed CentralView ArticlePubMedGoogle Scholar
- Monteiro P, Gosselin A, Wacleche VS, El-Far M, Said EA, Kared H, Grandvaux N, Boulassel MR, Routy JP, Ancuta P: Memory CCR6 + CD4+ T cells are preferential targets for productive HIV type 1 infection regardless of their expression of integrin beta7. J Immunol. 2011, 186: 4618-4630. 10.4049/jimmunol.1004151.View ArticlePubMedGoogle Scholar
- Doitsh G, Galloway NL, Geng X, Yang Z, Monroe KM, Zepeda O, Hunt PW, Hatano H, Sowinski S, Munoz-Arias I, Greene WC: Cell death by pyroptosis drives CD4 T-cell depletion in HIV-1 infection. Nature. 2013, 505: 509-514. 10.1038/nature12940.View ArticleGoogle Scholar
- Monroe KM, Yang Z, Johnson JR, Geng X, Doitsh G, Krogan NJ, Greene WC: IFI16 DNA sensor is required for death of lymphoid CD4 T cells abortively infected with HIV. Science. 2013, 343: 428-432.PubMed CentralView ArticlePubMedGoogle Scholar
- Aziz S, Fackler OT, Meyerhans A, Muller-Lantzsch N, Zeitz M, Schneider T: Replication of M-tropic HIV-1 in activated human intestinal lamina propria lymphocytes is the main reason for increased virus load in the intestinal mucosa. J Acquir Immune Defic Syndr. 2005, 38: 23-30. 10.1097/00126334-200501010-00005.View ArticlePubMedGoogle Scholar
- Nazli A, Chan O, Dobson-Belaire WN, Ouellet M, Tremblay MJ, Gray-Owen SD, Arsenault AL, Kaushic C: Exposure to HIV-1 directly impairs mucosal epithelial barrier integrity allowing microbial translocation. PLoS Pathog. 2010, 6: e1000852-10.1371/journal.ppat.1000852.PubMed CentralView ArticlePubMedGoogle Scholar
- Fallarino F, Grohmann U, Vacca C, Bianchi R, Orabona C, Spreca A, Fioretti MC, Puccetti P: T cell apoptosis by tryptophan catabolism. Cell Death Differ. 2002, 9: 1069-1077. 10.1038/sj.cdd.4401073.View ArticlePubMedGoogle Scholar
- Lee GK, Park HJ, Macleod M, Chandler P, Munn DH, Mellor AL: Tryptophan deprivation sensitizes activated T cells to apoptosis prior to cell division. Immunology. 2002, 107: 452-460. 10.1046/j.1365-2567.2002.01526.x.PubMed CentralView ArticlePubMedGoogle Scholar
- Frleta D, Ochoa CE, Kramer HB, Khan SA, Stacey AR, Borrow P, Kessler BM, Haynes BF, Bhardwaj N: HIV-1 infection-induced apoptotic microparticles inhibit human DCs via CD44. J Clin Invest. 2012, 122: 4685-4697. 10.1172/JCI64439.PubMed CentralView ArticlePubMedGoogle Scholar
- Gasper-Smith N, Crossman DM, Whitesides JF, Mensali N, Ottinger JS, Plonk SG, Moody MA, Ferrari G, Weinhold KJ, Miller SE, et al: Induction of plasma (TRAIL), TNFR-2, Fas ligand, and plasma microparticles after human immunodeficiency virus type 1 (HIV-1) transmission: implications for HIV-1 vaccine design. J Virol. 2008, 82: 7700-7710. 10.1128/JVI.00605-08.PubMed CentralView ArticlePubMedGoogle Scholar
- O’Doherty U, Swiggard WJ, Malim MH: Human immunodeficiency virus type 1 spinoculation enhances infection through virus binding. J Virol. 2000, 74: 10074-10080. 10.1128/JVI.74.21.10074-10080.2000.PubMed CentralView ArticlePubMedGoogle Scholar
- Garcia-Calvo M, Peterson EP, Leiting B, Ruel R, Nicholson DW, Thornberry NA: Inhibition of human caspases by peptide-based and macromolecular inhibitors. J Biol Chem. 1998, 273: 32608-32613. 10.1074/jbc.273.49.32608.View ArticlePubMedGoogle Scholar
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