Human neutrophilic polymorphonuclear leukocytes (PMNs) are central to innate immunity and are responsible for clearance of pathogens. PMNs undergo a tightly regulated apoptosis program that allows for timely clearance of PMNs without extravasation of toxic intracellular contents. We investigated the rate of spontaneous apoptosis of human peripheral blood PMNs cultured at basal (37°C) and febrile-range (39.5°C) temperatures (FRT). We found that PMN apoptosis is accelerated at FRT, reaching ∼90% completion by 8 h at 39.5°C vs 18 h at 37°C based on morphologic criteria. Caspase-8 activation peaked within 15 min of PMN exposure to FRT, and subsequent activation of caspase-3 and -9, cleavage of the BH3 (Bcl-2 homology domain 3) only protein Bid, and mitochondrial release of cytochrome c were also greater in FRT-exposed PMNs. Inhibition of caspase-3, -8, and -9 conferred comparable protection from apoptosis in FRT-exposed PMNs. These results demonstrate that exposure to FRT enhances caspase-8 activation and subsequent mitochondrial-dependent and mitochondrial-independent apoptosis pathways. The PMN survival factors G-CSF, GM-CSF, and IL-8 each prolonged PMN survival at 37°C and 39.5°C, but did not reduce the difference in survival at the two temperatures. In a mouse model of intratracheal endotoxin-induced alveolitis, coexposure to FRT (core temperature ∼39.5°C) doubled the proportion of bronchoalveolar PMNs undergoing apoptosis compared with euthermic mice. This process may play an important role in limiting inflammation and tissue injury during febrile illnesses.
Neutrophilic polymorphonuclear leukocytes (PMNs)3 are phagocytic cells that constitute an integral component of innate immune defense (1). Early in the acute inflammatory response to infection and injury, PMNs migrate to sites of inflammation where they eliminate pathogens through phagocytosis and release of cytotoxic effector molecules into the phagolysosomes (2, 3). However, PMNs may also increase host collateral tissue injury through extravasation of these intracellular toxins into the extracellular microenvironment (4, 5) and by secreting inflammatory cytokines (6). The capacity of PMNs to injure tissue is limited, in part, by spontaneous, Fas-dependent PMN apoptosis (7, 8). Apoptosis not only allows PMNs to undergo cell death without loss of plasma membrane integrity and leakage of intracellular contents into the extracellular microenvironment, but phagocytosis of apoptotic PMNs by macrophages reprograms the macrophage gene expression program to an antiinflammatory profile (9). Thus, appropriate PMN apoptosis may be crucial for resolution of inflammation during infections (10, 11). Once PMNs have emigrated from the bone marrow, they survive for only 12–36 h (12). However, survival time is prolonged by exposure to soluble and cell-associated PMN survival factors, many of which are released during inflammatory reactions (13, 14, 15).
Fever, a temporary, regulated increase in core temperature, is widely thought to confer cytoprotection, but the mechanisms underlying these effects are incompletely understood (16). Our laboratory has focused on the immunomodulatory effects of febrile-range hyperthermia (FRH) and the participation of elements of the heat shock response in mediating these effects. We have shown that exposing mice to FRH (core temperature ∼39.5°C) accelerates pathogen clearance (17), but it also increases PMN accumulation and collateral tissue injury, especially in the lungs (18). We have shown that FRH augments expression of neutrophil chemoattractants and increases endothelial capacity for transendothelial migration of neutrophils (19, 20).
These observations led us to hypothesize that exposing PMNs to FRH prolongs PMN survival, which contributes to PMN accumulation. We tested this hypothesis in freshly isolated human PMNs cultured in vitro, in the absence and presence of defined exogenous PMN survival factors. Surprisingly, we found that culturing human PMNs at 39.5°C greatly accelerated caspase-dependent apoptotic cell death, thereby identifying a potentially important mechanism that may limit collateral tissue injury during febrile illnesses.
Materials and Methods
Rabbit anti-human caspase-3 Ab was purchased from Cell Signaling Technology. Mouse mAb recognizing the active cleavage product of human caspase-3 Ab was purchased from Chemicon International. Rabbit Ab raised against amino acids 120–180 in human Bid was purchased from Bethyl Laboratories. Mouse anti-human p38 MAPK Ab, goat anti-mouse IgG (HRP conjugated), and goat anti-rabbit IgG (HRP conjugated) were purchased from Santa Cruz Biotechnology. Peptide inhibitors of caspase-3 (Ac-DQMD-CHO), caspase-8 (Ac-IETD-CHO), and caspase-9 (Ac-LEHD-CHO) and rabbit polyclonal Ab raised against amino acids 120–180 of human Bid were purchased from Alexis Biochemicals. HEPES, EGTA, EDTA, 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF), leupeptin, and aprotinin were from Sigma-Aldrich. Recombinant human IL-8 (rhIL-8), rhG-CSF, and rhGM-CSF were purchased from BioSource International, Cell Sciences, and GenScript, respectively.
Isolation and culture of PMNs
All sample collection procedures were approved by and performed according to institutional guidelines (IRB/IACUC). Fifty milliliters of venous blood was collected from healthy subjects, anticoagulated with citrate, and PMNs were isolated by dextran sedimentation and density centrifugation through Histopaque-1083 (Sigma-Aldrich) at 20°C exactly as we have previously described (21). Contaminating erythrocytes were lysed by resuspending the pellet in 4°C water for 5 s, then adjusting the sodium chloride concentration to 150 mM with 3.5 M sodium chloride. The cells were collected by centrifugation at 700 × g for 15 min at 4°C, washed in 4°C Dulbecco’s PBS, and suspended in RPMI 1640 containing 10% FBS at 3 × 106 cells/ml. The resultant cell population comprised at least 95% PMNs based on their characteristic morphology. PMNs were cultured in polypropylene culture tubes and incubated in 5% CO2 atmosphere in incubators set at either 37°C or 39.5°C with 0.1°C precision and recalibrated before each experiment using an electronic thermometer (Fluke Instruments model 5211).
Morphologic analysis of apoptosis
Wright-Giemsa-stained cytospin preparations were analyzed blindly by one of us (A.N.) based on cell morphology. Apoptotic PMNs were identified based on replacement of the normal multisegmented nuclear architecture with condensed, spherical nuclei (22).
Flow cytometric quantification of apoptosis by TUNEL staining
An In Situ Cell Death Detection kit (Roche) was used to fluorescein label DNA strand breaks in PMNs according to the manufacturer’s instructions. Positive controls were obtained by treating PMNs with DNase I (300 U/ml) for 10 min at room temperature. PMNs were analyzed with a FACScan flow cytometer (BD Biosciences).
Caspase activity assays
Caspase-3, -8, and -9 activities in PMN cell lysates were measured using fluorometric activity measurement kits for caspase-3 (substrate, Ac-DEVD-7-amino-4-methyl-coumarin (AMC)), caspase-8 (substrate, IETD-7-amino-4-trifluoromethyl-coumarin (AFC)), and caspase-9 (substrate, Ac-LEHD-AFC) from Calbiochem following the manufacturer’s protocols. Briefly, to measure caspase-3 activity, PMNs were lysed in the supplier’s lysis buffer after incubation at 37°C or 39.5°C. Lysates were incubated with the fluorometric caspase-3 substrate Ac-DEVD-AMC for 2 h at 37°C and fluorescence was measured using excitation at 346 nm and measuring emission at 442 nm. To measure caspase-8 and -9, PMN lysates were incubated with the fluorometric substrates IETD-AFC or Ac-LEHD-AFC, respectively, for 2 h at 37°C and fluorescence was measured using excitation at 400 nm and measuring emission at 505 nm. Caspase-3 activity in intact PMNs was measured using the FLICA apoptosis detection kit (FAM-DEVD-FMK) from Immunochemistry Technologies. PMNs were incubated at 37°C or 39.5°C for the indicated time, the FLICA substrate was then added, and cells were incubated at 37°C for an additional 4 h. Fluorescence was measured using excitation and emission wavelengths of 488 nm and 530 nm, respectively. To analyze the temperature dependence of caspase-8 activity, the activity of recombinant caspase-8 was compared at 37°C and 39.5°C using a kit based on the colorimetric caspase-8 substrate Ac-IETD-pNA (CASP-8C, Sigma-Aldrich) according to the manufacturer’s protocol using an ELISA plate reader equipped with a plate warmer (VersaMax; Molecular Devices) set at either 37°C or 39.5°C.
PMNs were lysed in RIPA buffer (50 mM Tris-HCl (pH 7.4), 150 mM NaCl, 2 mM EDTA, 1% Nonidet P-40, 0.1% SDS) containing protease inhibitor cocktail and phosphatase inhibitor cocktail (both from Sigma-Aldrich), boiled for 5 min in loading buffer containing 2-ME, and resolved on 7.5, 10, 12, 15, or 10–20% Tris-HCl gels (Bio-Rad). The proteins were electrostatically transferred to Immobilon polyvinylidene difluoride membranes, blocked with TBST (25 mM Tris, 140 mM NaCl, 3 mM KCl, 0.05% Tween 20 (pH 8.0)) containing 5% nonfat milk, probed with primary Abs, then with secondary peroxidase conjugates, and detected by chemiluminescence (PerkinElmer) as we have described (23). The Western blots were imaged using a Fuji LAS-1000 gel documentation system and ImageQuant software.
Measurement of cytosolic cytochrome c
Following incubation, PMNs were washed twice with ice-cold PBS, resuspended in 100 μl of mitochondria buffer (20 mM HEPES (pH 7.5), 1 mM EGTA, 1 mM EDTA, 10 mM KCl, 1.5 mM MgCl2, 1 mM DTT, 250 mM sucrose, 0.1 mM AEBSF, 2 μg/ml leupeptin, 2 μg/ml pepstatin, and 2 μg/ml aprotinin), and lysed using a microfuge tube-pestle homogenizer (Kontes) on ice. After centrifugation at 800 × g for 20 min at 4°C to remove cell debris, mitochondria, and intact nuclei, supernatants were further clarified by centrifugation at 14,000 × g for 20 min at 4°C, and cytochrome c was analyzed using an ELISA kit (Calbiochem) and the manufacturer’s protocol.
Temperature clamping and intratracheal administration of LPS
Mice were adapted to standard plastic cages for at least 4 days before study. To avoid the influence of diurnal cycling, all experiments were started at approximately the same time each day (between 8:00 and 10:00 a.m.). Mice were placed in either 24°C (euthermia) or 34°C (FRH) infant incubators immediately after intratracheal instillation of 50 μg LPS (prepared by TCA extraction from Escherichia coli O111:B4 and 2,2,2-tribromoethyl; Sigma-Aldrich) in 50 μl PBS, exposures that we previously showed maintain core temperatures at ∼37°C and 39.5°C, respectively (18). Twenty-four hours after LPS instillation, the mice were euthanized by isoflurane inhalation and cervical dislocation, and lung lavage was performed with a total of 2 ml PBS as previously described (18). Cells were collected and analyzed for morphologic features of apoptosis and TUNEL staining as described above for human PMNs except that the cells were costained with PE-conjugated anti-Gr-1 (Serotec, cat. no. MCA2387PE) to facilitate gating in the flow cytometric analysis. All protocols were approved by the Institutional Animal Care and Use Committee of the University of Maryland, Baltimore.
Data are displayed as means ± SE. Differences between two groups were analyzed using Student’s t test. Differences among multiple groups were analyzed by applying the Tukey-Kramer honestly significant difference test to a one-way ANOVA using the JMP statistical software program (SAS Institute).
Exposure to 39.5°C accelerates PMN apoptosis
We compared the kinetics of spontaneous apoptosis in human PMNs cultured at 37°C and 39.5°C in RPMI 1640 containing 10% FBS, but without other exogenous survival factors. The percentage survival was calculated based on the characteristic morphologic features of apoptosis (22) (Fig. 1, A and B). PMNs in 39.5°C culture exhibited a much shorter lag time before onset of apoptosis, that is, 4 h compared with 8 h in 37°C PMN cultures. Once apoptosis began, it proceeded at similar rates at both temperatures. The times required for half the 39.5°C and 37°C PMNs to exhibit apoptotic morphology were ∼6.5 h and 11.5 h, respectively. We analyzed the effect of incubation temperatures intermediate to 37°C and 39.5°C on the rate of neutrophil apoptosis based on morphologic criteria and found the rank order of apoptosis rates to be 39.5°C > 39°C > 38°C > 37°C (Fig. 1 C).
We confirmed the morphologic analysis of PMN apoptosis by TUNEL staining and flow cytometry (Fig. 1 D). The appearance of TUNEL staining was delayed compared with onset of apoptotic morphology, but exhibited the same temperature dependence, occurring much earlier in 39.5°C than in 37°C PMN cultures. The lag time until appearance of TUNEL staining was markedly shortened in the 39.5°C PMNs, but then proceeded at similar rates in both 37°C and 39.5°C cells. The time required for 50% of the PMNs to exhibit TUNEL staining was ∼11 h in 39.5°C culture vs 18 h in 37°C culture.
Exposure to 39.5°C accelerates caspase-3 activation
Caspase-3, the predominant executioner caspase that mediates spontaneous PMN apoptosis (24, 25, 26), is cleaved to its active form by the actions of the initiator caspases, caspase-8 and -9. The kinetics of caspase-3 cleavage were analyzed by measuring the loss of full-length caspase-3 by Western blotting for full-length caspase-3 (Fig. 1,E) or by Western blotting with an Ab that recognizes the active caspase-3 cleavage product (Fig. 1,F). Compared with PMNs in 39.5°C culture, PMNs at 37°C exhibit a 4-h delay before loss of full-length caspase-3 was detectable (Fig. 1,E). Active caspase-3 forms were first detectable after 6 h at 39.5°C compared with 16 h at 37°C (Fig. 1 F).
The Western blot analysis of caspase-3 activation was confirmed using a functional assay of caspase-3 in PMN cell lysates based on cleavage of a caspase-3-specific fluorometric substrate (Ac-DEVD-AMC) (Fig. 1,G). This assay demonstrated a pattern that parallels onset of apoptotic morphology (Fig. 1,A). The lag time until caspase-3 activation was 4 h in the 39.5°C PMN cultures vs 12 h in the 37°C PMN cultures. The analysis of caspase-3 activity in PMN cell lysates was complemented by measuring caspase-3 activation in intact PMNs using an intracellular fluorometric caspase-3 substrate, FAM-DEVD-FMK, added after 8 h of incubation at 37°C or 39.5°C, the time of maximal difference in morphologic evidence of apoptosis (Fig. 1,A). Caspase-3 activity increased 4.6-fold vs baseline levels after 8 h of incubation at 39.5°C, but did not change from baseline in PMNs incubated in parallel at 37°C (Fig. 1 H).
Exposure to 39.5°C accelerates caspase-8 and -9 activation and Bid cleavage
Spontaneous PMN apoptosis is known to be triggered by activation of Fas, which activates the initiator caspase, caspase-8, and triggers the extrinsic apoptosis pathway (7, 8). Caspase-8 can directly cleave pro-caspase-3 to its active form. Alternatively, caspase-8 can indirectly activate caspase-3 by activating the intrinsic apoptosis pathway through the sequential cleavage of Bid, induction of mitochondrial cytochrome c leak, and activation of caspase-9.
Since caspase-8 and -9 may be activated without detectable proteolytic cleavage (27, 28), we analyzed activation of caspase-8 and -9 in PMNs using functional assays with fluorometric substrates (Ac-IETD-AFC for caspase-8; Ac-LEHD-AFC for caspase-9) (Fig. 2) rather than measuring cleavage by Western blotting. Caspase-8 activity peaked after 15 min incubation in 39.5°C PMNs at 2.3-fold above basal levels, and at least a 2-fold increase was sustained for 90 min (Fig. 2,A). In contrast, caspase-8 activity in the PMNs cultured at 37°C was not statistically different than baseline levels at any time during an 18-h incubation. Caspase-9 activity increased in PMNs cultured at both temperatures, but the increase in caspase-9 activity began earlier, 7 h vs 12 h, and reached similar maximal 6-fold increases earlier, 7 vs 18 h, at 39.5°C compared with 37°C cells (Fig. 2 B).
In addition to participating in the extrinsic apoptosis pathway by directly cleaving caspase-3 to its active form, caspase-8 can activate the intrinsic apoptosis pathway by cleaving the BH3 (Bcl-2 homology domain 3) only protein, Bid (29). The C-terminal tBid fragment that is generated translocates to mitochondria and stimulates release of cytochrome c (30, 31), a stimulus for caspase-9 activation (32, 33). The sequential activation of caspase-8 and caspase-9 in the 39.5°C PMN cultures suggests that this pathway may be activated in PMNs exposed to febrile-range (39.5°C) temperatures (FRT). To further analyze the potential participation of this pathway to PMN apoptosis at basal and febrile temperatures, we analyzed Bid cleavage in PMNs cultured at 37°C and 39.5°C by Western blotting and calculating the 15-kDa tBid/20-kDa full-length band density ratios for each lane (Fig. 3,A). The tBid/full-length Bid ratio increased gradually during the first 3 h and more rapidly during the subsequent 3–6 h in culture in the 39.5°C PMN cultures. In contrast, there was no detectable increase in tBid/full-length ratio during a 6-h incubation at 37°C. To confirm that Bid cleavage resulted in mitochondrial cytochrome c release, we analyzed cytosolic cytochrome c levels in PMNs cultured at 37°C and 39.5°C (Fig. 3 B). After 3 h of incubation, cytochrome c levels in mitochondria-free cytosol were 2-fold higher in PMNs cultured at 39.5°C than in cells cultured at 37°C.
Caspase-3, -8, and -9 inhibitors each reduce PMN apoptosis at both basal and febrile levels
To evaluate the contribution of extrinsic and intrinsic apoptosis pathways to PMN apoptosis at basal and febrile temperatures, we analyzed the antiapoptotic effects of caspase inhibitors in 37°C and 39.5°C PMN cultures (Fig. 4). Freshly isolated PMNs were treated with 10 μM concentration of each inhibitor, incubated at either 37°C or 39.5°C, and the kinetics of apoptosis analyzed based on morphologic criteria. Our preliminary dose-response studies demonstrated that inhibitor concentrations >10 μM increased cell death in human PMNs at both temperatures. Compared with vehicle-treated controls, each of the three inhibitors caused similar delays in appearance of apoptotic morphology in the 37°C PMN cultures, extending the time required for 50% apoptosis from 16 h to 19.5–21.5 h. As was found for the 37°C cells, each of the three inhibitors caused similar delays in apoptosis in the 39.5°C PMN cultures, extending the time required for 50% apoptosis from 3 to 8 h at 39.5°C. To confirm the antiapoptotic effects of the caspase inhibitors, we analyzed TUNEL expression in PMNs cultured with each of the three caspase inhibitors at 37°C for 18 h or at 39.5°C for 6 h. The three inhibitors exhibited similar antiapoptotic activity in the 37°C and 39.5°C PMN cultures, increasing survival 1.9- to 2.9-fold compared with DMSO-treated control cells.
The pro-survival cytokines G-CSF, GM-CSF, and IL-8 prolong PMN survival at both basal and febrile levels
Treatment with IL-8 (10 ng/ml), G-CSF (500 U/ml), and GM-CSF (500 U/ml) increased the 50% survival time of PMNs from 12 h to 20.5, 37, and 38.5 h, respectively, in 37°C culture (Fig. 5,A), and from 6.5 h to 10, 11.5, and 14 h, respectively, in 39.5°C culture (Fig. 5,B). An analysis of active caspase-3 levels using the fluorometric substrate, Ac-DEVD-AMC, confirmed the morphologic analysis (Fig. 5 C). IL-8, G-CSF, and GM-CSF reduced active caspase-3 levels by 83%, 74%, and 88% in 37°C PMN cultures and by 62%, 92%, and 88% in 39.5°C PMN cultures after 24 h in culture.
Effect of hyperthermia on PMN apoptosis in an in vivo lung injury model
To analyze the effect of hyperthermia on PMN apoptosis in a clinically relevant in vivo model, we analyzed the proportion of PMNs undergoing apoptosis in a previously described intratracheal LPS-challenged mouse model of pneumonia (18). We have previously shown that coexposing mice to FRH by increasing ambient temperature from 25°C to 34°C increased PMN accumulation in the bronchoalveolar compartment. In this study, we exposed mice to FRH for 24 h beginning immediately after intratracheal instillation of 50 μg LPS, then collected cells from lung lavage and analyzed them for TUNEL staining by flow cytometry and morphometric features of apoptosis by light microscopy (Table I). We found a similar effect of hyperthermia on PMN apoptosis in vivo as we found in human PMNs in vitro. When measured 24 h after instillation of LPS, approximately twice as many PMNs exhibited evidence of apoptosis in the warmer mice. The low numbers of PMNs in lung lavage did not permit a similar analysis in the absence of LPS challenge.
|Treatment .||% TUNEL-Positiveb .||% Apoptotic Morphologyb .|
|Euthermia||2.22 ± 0.05||11.8 ± 0.2|
|Hyperthermia||3.79 ± 0.45c||29.2 ± 2.7d|
|Treatment .||% TUNEL-Positiveb .||% Apoptotic Morphologyb .|
|Euthermia||2.22 ± 0.05||11.8 ± 0.2|
|Hyperthermia||3.79 ± 0.45c||29.2 ± 2.7d|
Mice received 50 μg LPS in 50 μl PBS via intratracheal instillation, then were maintained with core temperatures of 37°C (euthermia) or 39.5°C (hyperthermia) for 24 h. Lung lavage was collected and analyzed for apoptosis by TUNEL staining using flow cytometry and gating on GR-1-staining cells and for morphologic features of apoptosis.
Data from 4 mice/group are expressed as means ± SE of percentage PMNs that express TUNEL staining or apoptotic morphology.
p < 0.05 vs euthermia.
p < 0.03 vs euthermia.
We have previously demonstrated that coexposure to FRH increases PMN accumulation in mouse models of pneumonia and pulmonary oxygen toxicity (18, 20). Additional data in these mouse models and in in vitro cell culture models demonstrated that FRH exerts multiple effects that contribute to enhanced PMN recruitment, including 1) induction of G-CSF expression and expansion of the circulating PMN pool (34), 2) augmented expression of CXC chemokines (18, 20), and 3) increased endothelial capacity to facilitate PMN transmigration (19). In the present study, we tested the hypothesis that FRH would additionally contribute to PMN accumulation by prolonging PMN survival. We compared the kinetics of apoptosis in human PMNs cultured at 37°C and 39.5°C using apoptotic morphology, TUNEL staining, and caspase-3 activation as criteria. Characteristic morphologic changes, including cell rounding, shrinkage, blebbing, nuclear condensation, and chromatin fragmentation, remain the most specific criteria for identifying apoptotic cell death (35, 36, 37, 38). Since PMNs have multisegmented nuclei, nuclear condensation is easily assessed by light microscopy in these cells and correlates well with other characteristics of apoptotic death (22). We found that human PMNs incubated at 37°C in medium containing bovine serum without defined exogenous survival factors completed spontaneous apoptosis within 22 h. This spontaneous apoptosis rate is within the range of spontaneous PMN apoptosis rates reported by other laboratories (7, 14, 25, 39, 40). We found that the development of apoptotic morphology in the 37°C PMN cultures was accompanied by caspase-3 activation and was modestly attenuated by treatment with the caspase-3 inhibitory peptide, Ac-DQMD-CHO. These findings are also similar to those of previous studies of spontaneous apoptosis in human PMNs (25, 41). However, some investigators (40) have reported that treatment with pan-caspase inhibitors fails to block apoptosis. Collectively, these data suggest that caspase-dependent mechanisms may play only a minor role in spontaneous PMN cell death at 37°C.
Surprisingly, we found that the appearance of apoptotic morphology and TUNEL staining was greatly accelerated in PMNs incubated at 39.5°C and was similar to the reported rate of apoptosis in PMNs treated with TNF-α, a potent proapoptotic agonist (42). We used multiple complementary assays to demonstrate that PMNs incubated at 39.5°C exhibited greatly enhanced and accelerated activation of caspase-3 compared with 37°C PMN cultures. Full-length caspase-3 is cleaved to an active fragment by caspase-8 or -9. The time required for 50% reduction of full-length caspase-3 was 8 h in 39.5°C cells vs 14 h in 37°C cells. Utilizing an Ab specific for the active caspase-3 fragment, we showed that generation of the active caspase-3 fragment is detectable by 8 h at 39.5°C, but not until 16 h at 37°C. Casapase-3 activity in PMN lysates reached 50% maximal levels after only 4 h in 39.5°C cells vs 9 h in 37°C cells. Using an independent measurement of caspase-3 activity in intact PMNs with a cell-permeable fluorometric substrate, we showed that capsase-3 activity increased by 4.6-fold after 8 h of incubation at 39.5°C but had not yet increased above baseline levels in PMNs cultured for 8 h at 37°C. These independent measures of caspase-3 activation each show a similar pattern in which caspase-3 activation in PMNs occurs more rapidly at 39.5°C than at 37°C.
Although caspase-3 activation and onset of TUNEL staining, as well as nuclear condensation, occurred much earlier in 39.5°C PMN cultures, the caspase-3 inhibitory peptide, Ac-DQMD-CHO, exerted similar antiapoptotic activity in 37°C and 39.5°C PMN cultures. Collectively, these data indicate that the enhanced apoptosis caused by exposure to 39.5°C is caused, at least in part, by accelerated activation of caspase-3. This distinguishes the effects of FRT from the accelerated PMN death triggered by TNF-α, which proceeds in the presence of caspase inhibitors and lacks the classic morphologic features of apoptosis (42). Previous studies have demonstrated that the caspase-dependent apoptosis that does occur in cultured human PMNs is triggered in part by autocrine activation of the death domain receptor, Fas, and subsequent activation of caspase-8 (7, 8). Scaffidi et al. (43) described two pathways through which active caspase-8 causes caspase-3 activation. Type I cells exclusively use the extrinsic pathway in which capase-8 directly cleaves pro-caspase-3 to its active form, whereas type II cells utilize an alternative pathway in which caspase-8 activates the intrinsic apoptosis pathway by cleaving the pro-apoptotic Bcl family member, Bid. The C-terminal, BH3-containing Bid fragment generated by the actions of caspase-8, tBid, translocates to the mitochondria and triggers efflux of mitochondrial cytochrome c (29, 44). Subsequent studies of Fas-dependent apoptosis have demonstrated that Bid cleavage, mitochondrial leak of cyctochrome c, and activation of caspase-9 occur in both type I and type II cells, but that caspase-8 activation occurs more rapidly in type I cells (45). In the present study, caspase-8 was maximally activated within 15 min of warming PMNs to 39.5°C. Generation of tBid and release of mitochondrial cytochrome c were each detectable within 3 h of warming to 39.5°C. Caspase-9 activity occurred after 7 h of warming, but an upward trend in activity was evident within 5 h of warming. Caspase-3 was activated after 4 h, and morphologic characteristics of apoptosis appeared after 6 h of warming PMNs to 39.5°C. Treatment with the caspase-8 inhibitory peptide, Ac-IETD-CHO, reduced apoptosis in the 39.5°C PMN cultures as effectively as caspase-3 inhibition. In contrast, in 37°C PMN cultures, an increase in caspase-8 activation was not detectable during an 18-h incubation and activation of caspase-9 and caspase-3, and appearance of apoptosis did not occur until 12 h in culture. Treatment with peptide inhibitors of caspase-3, -8, and -9 each exerted similar antiapoptotic effects in 39.5°C and 37°C PMN cultures. Collectively, these data suggest that accelerated apoptosis in 39.5°C PMN cultures occurs, in part, via enhanced activation of caspase-8 and activation of type II apoptosis signaling pathway.
Although apoptosis is classically directed by activation of caspases, caspase-independent cell death with some features of apoptosis has been observed in several cell lines and may participate significantly in spontaneous death of PMNs cultured at 37°C (46). Furthermore, some proapoptotic agonists, such as TNF-α, have been shown to activate caspase-independent death pathways in PMNs (42). Although our results do not rule out smaller contributions by caspase-independent pathways, the protection conferred by caspase inhibitors against early onset of apoptosis in 39.5°C PMN cultures suggests that caspase-dependent apoptosis pathways are enhanced and predominate in PMNs exposed to FRT.
We extended the studies in human PMNs in vitro to a clinically relevant mouse model of pneumonia and lung injury. The mouse model not only analyzes the effect of hyperthermia on PMN apoptosis in vivo, but it analyzes PMNs that have emigrated from the vasculature into tissue, a process that can modify PMN apoptosis (47). When analyzed 24 h after LPS challenge, the proportion of PMNs at each temperature that exhibited TUNEL staining was much lower than the proportion of PMNs that exhibited morphologic features of apoptosis. A similar relationship between these two measures of apoptosis was found in our analysis of human PMNs in which morphologic features of apoptosis preceded TUNEL staining by several hours (compare Fig. 1, B and D). However, by utilizing either TUNEL staining or morphologic criteria, we found that the proportion of lung lavage PMNs exhibiting features of apoptosis was approximately twice as great in the FRH-exposed mice. The difference in the proportion of apoptotic PMNs in the warmer mice at this time point is even more meaningful when one considers the effect of FRH on the kinetics of PMN accumulation in lung. We previously showed that the total number of PMNs in lung lavage plateaus between 6 and 24 h after LPS challenge in euthermic mice, but PMNs continue to accumulate during this time in hyperthermic mice. This suggests that the PMNs recovered from the hyperthermic mice not only contain a greater proportion of apoptotic cells, but that these PMNs are younger compared with PMNs from euthermic mice. Taken together, these data suggest that the PMNs in the hyperthermic mice undergo earlier apoptosis.
Based on our previous in vivo studies of mouse lung injury that demonstrated enhanced PMN accumulation in FRT-exposed animals (18, 20), we predicted that PMN survival would be prolonged at FRT. A possible explanation for the disparity between our previous in vivo studies and the present study is that the enhanced generation of PMN survival factors that are stimulated by FRT might compensate for the direct proapoptotic effects of FRT on PMNs. We have shown that mice exposed to FRT increase expression of several known PMN survival factors, including G-CSF (34), GM-CSF (18), and CXC chemokines (18). In the present study, we show that supplementing 39.5°C PMN cultures with exogenous G-CSF, GM-CSF, or the principal human CXC chemokine, IL-8, restored PMN survival to 37°C levels. However, each of these cytokines exhibited greater antiapoptotic effects in 37°C than in 39.5°C PMN cultures. Because our analysis of PMN apoptosis in the in vivo LPS-challenged mouse model revealed a higher proportion of apoptotic PMNs in the lungs of hyperthermic than those of euthermic mice, we conclude that the enhanced expression of PMN survival factors that occurs in the warmer mice (18, 20) does not appear to compensate for the direct proapoptotic effects of hyperthermia.
In summary, we have shown that caspase-dependent apoptosis of human PMNs is accelerated at FRT (39.5°C) and that the effect is mediated by enhanced activation of caspase-8. The combined effect of FRT to enhance PMN recruitment (18, 20) and accelerate PMN apoptosis will result in accumulation of a younger PMN population at sites of infection and more rapid clearance of older PMNs through the apoptotic pathway. This will enable rapid pathogen elimination, minimize dysregulated release of toxic contents (7, 8) from older PMNs, and trigger the reprogramming of macrophages to an antiinflammatory cytokine expression profile (9). Such effects of accelerated PMN apoptosis may improve the balance between pathogen clearance and collateral tissue injury. However, the consequences of these effects of hyperthermia on PMN apoptotic signaling have not yet been elucidated in vivo.
The authors have no financial conflicts of interest.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
This work was supported by National Institutes of Health Grants GM066855, HL69057, and HL085256 (to J.D.H.) and GM069431 (to I.S.S.), and by Veterans Affairs Merit Review grants (to J.D.H. and I.S.S.).
Abbreviations used in this paper: PMN, polymorphonuclear leukocyte; AEBSF, 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride; AFC, 7-amino-4-trifluoromethyl-coumarin; AMC, 7-amino-4-methyl-coumarin; FRH, febrile-range hyperthermia; FRT, febrile-range (39.5°C) temperature; rh, recombinant human.