Fas death pathway is considered the most important in CD4+ T-cells depletion in PLWHIV. It has been extensively documented that the FasL is augmented in CD4+ T-cells, monocytes, macrophages and NK cells. (9, 10). In the present study was found a non-significant increase in the FasL expression on CD3+ CD4+ T-cells and CD14+ CD4+ monocytes from PLWHIV. In a study performed by Sloand in 1997, was observed a similar difference in FasL expression, however, they considered all the positive T-cells without making a distinction between CD4+ and CD8+ T-cells and since it has been documented that CD8+ T-cells have a notably augmented expression of FasL, this could explain the discrepancy of our study with the aforementioned study (20). In concordance with existing literature, our results confirm that FasL expression was higher in monocytes compared to lymphocytes, additionally the expression was heightened on monocytes from PLWHIV compared to HIV- controls. Furthermore, elevated levels of sFas and sFasL as well as a positive correlation of sFasL with the viral load has been previously reported (21, 22), undoubtedly a marker of interest in PLHIV before ART. Thus, in PLWHIV an increased FasL turnover towards soluble sFasL should be considered (21).
The expression and MFI of Fas was elevated in both study groups with a non-significantly difference. However, it was observed a subtle elevation in PLWHIV compared to HIV- controls. Sloand et.al. observed something similar, finding an expression of up to 60% in T-cells from PLWHIV. Additionally, in Sloand study included subjects with a CD4+ T-cells count < 200 cells/µL, which showed a broader receptor expression compared to subjects who had a higher cell count, data in accordance with our results since PLWHIV in this study had more than 200 cells/µL (20). Thus, cells from PLWHIV would be more susceptible to apoptosis through the Fas pathway due to the expression of both molecules, Fas and specially FasL on these cells, particularly in monocytes. It is important to consider that the expression of receptors and ligands of death pathways, is not necessarily correlated to apoptosis sensitivity, the function can be modified by inhibitory mechanisms, such as: soluble protein blocking of ligands or receptors, among them Bcl-2 family inhibitors. Additionally, it has been previously described that some HIV viral proteins have both pro and anti-apoptotic effects, which can affect the extrinsic pathway sensibility independently of receptor expression. HIV protease as well as Tat and Env, can cause Caspase 8 excision (23), as well as the increase of pro apoptotic proteins such as Bax (24) and the decrease of FLIP expression (25, 26). Regarding functional studies of Fas and TRAIL pathways, we did not found an increased in apoptosis sensibility in PLWHIV, contrary to previous reports (20, 27). However, the methodologies used were different. Estaquier, et. al. performed light microscopy with chromatin condensation, and they used purified CD4+ and CD8+ T-cells instead PBMCs (27). Sloand et.al. used DNA quantification by cytometry to evaluate the functionality of apoptotic pathways, and additionally, the study population had a low CD4+ T-cell count, which would make them more sensitive to the inducted apoptosis (20).
The TRAIL pathway has been less studied in the context of HIV infection. It has been observed that the use of recombinant ligand can cause apoptosis only to infected cells without significantly affecting the functionality of uninfected T-cells (28). It should be noted that this is the first study that to evaluate the expression of TRAIL in cells from PLWHIV before ART, and interestingly it was observed a significant elevation of DR5 expression on CD14+ CD4+ monocytes and CD3+ CD4+ T-cells of PLWHIV although in the last population there was only an increasing trend. In the case of DR4 and ligand TRAIL, the expression turned out to be almost null in both study groups. Previously, it has been described a high expression of TRAIL/Apo2L as well as the decoy receptor 2 (DCR2) and DR5 at level of RNA and surface expression in CD4+ and CD8+ T-cells from PLWHIV (15). Additionally, in vitro and ex vivo models have shown that the viral infection or viral proteins as gp120 and Tat induce the expression of TRAIL, as well as the four receptors of TRAIL on T-cells, monocytes, DC and PBMCs (10, 15, 17, 29, 30). More recently, it has been identified a TRAIL splice variant that is produced by HIV infected and non-infected cells with the capacity of to generate resistance to apoptosis via TRAIL since it binds and directs proteasomal degradation to DR1 and DR2 receptors (31). It has been previously described that in T-cell from elite controllers (19) as well as slow progressors (32) the expression of DR5 is diminished and even similar to the control group (this could be to the low rate of infection found in this subjects). In the case of our population of PLWHIV it is not possible to consider them slow progressors, keeping in mind the estimated time of infection in them is below two years and that to be included in this group they would have to keep a CD4+ T-cells count above 350 for more than 10 years without treatment and maintain a low viral load. Observations made by Herbeuval et. al. consider DR5 as a possible marker of HIV infection (33), hypothesis that is supported by this study since DR5 expression was significantly increased in CD14+ CD4+ monocytes of PLWHIV and that its expression was positively correlated with the time of infection. Regarding the functional assays, we did not find an increased in the apoptosis induced by the TRAIL ligand. Contrary to the results reported by Lum in 2001 (15), who with a similar methodology and Hoechst stain, reported an elevated induced apoptosis in cells from PLWHIV, however, they do not refer to the CD4 count they considered. As mentioned previously, this parameter is fundamental in sensitivity to apoptosis.
Regarding the entry coreceptors was not observed any significant difference, just an increased expression of CCR5 on PLWHIV monocytes. However it was found a negative correlation between this coreceptor expression in both CD3+ CD4+ T-cells and CD14+ CD4+ monocytes from PLWHIV and the time of infection, which agrees with the natural tropism history in which, the R5 tropism is gradually lost due to the depletion of the cells which express CCR5 receptor (34). In studies performed with both lymphocytic cell lines and primary lymphocytes cultures, using as stimulus the viral protein gp120 directed to CD4 and CXCR4 was reported that, binding of the glycoprotein with the coreceptor CXCR4 increased the expression of the TRIAL receptor, while the single binding with CD4 was not capable of inducing this phenomenon (35). In a similar work, with cells that selectively expressed CD4 and CXCR4 proteins with the glycoprotein gp120, it was observed that binding of the gp120 to CXCR4 caused caspase dependent apoptosis, assessed using an inhibitor caspases 1, 3, 7 and 8, which blocked apoptosis induced by gp120 union (36). Thus, hypothesizing that the expression and stimulation of CXCR4 is necessary to modify the expression of TRAIL receptors and the activity of TRAIL pathway. In this sense, in the context of HIV infection, is known that the CXCR4 expression is late, and associated with X4 strains and a rapid depletion of CD4+ T-cells in approximately 50% of infected individuals. This event may be associate with the gain in the expression and function of the TRAIL pathway in late stages of HIV infection, hypothesis that could support the inactivity of the pathway reported in this study (37). Thus, several mechanisms of proapoptotic effects have been proposed for the glycoprotein gp120, which include the upregulation of Fas, FasL, TRAIL and TNF-α expression (10). Regarding our study, even if CXCR4 was not found significantly elevated, probably due to the inclusion of PLWHIV in early stage of infection, a positive correlation between CXCR4 and DR5 expression (p < 0.001) in both CD3+ CD4+ T-cells (r = 0.86; p < 0.001) and CD14+ CD4+ monocytes (r = 0.68; p < 0.005) was found. This result could indicate that even if the expression of this co-receptor was not so different in both groups, stimulation with gp120 could induce an increase in TRAIL receptor expression, activation of axis TRAIL pathway and cell death independently of the entry receptor CD4, and this in turn could cause an augmented apoptosis sensibility in subjects which are going through a X4 tropism change mediated by the TRAIL pathway.
Immune activation is an essential topic in the pathogenesis of the disease due to it has been recognized that in HIV infection an activation state is related to a more efficient viral replication, an increase in the expression of more virulent strains, an impaired hematopoiesis, and apoptosis of CD4+ T-cells. Furthermore, a state of chronic activation favors the viral transmission, the progression of the disease, and in turn decreases the survival of patients (38). It was measured the expression of CD80, CD86 and HLA-DR on CD14+ CD4+ monocytes from PLWHIV and HIV- controls, without finding significant differences in the expression of any activation marker on these cells. According to our results, Stiksrud, et.al. found no difference in HLA-DR expression between ART-naïve PLWHIV and the control group, however they reported an increase in the value of MFI of HLA-DR in total monocytes as well as in intermediate monocytes (CD14++ CD16+) from immunological non-responders (INRs) vs Immunological Responders (IR) (39). Again, the difference of their investigation with our study is the CD4+ T-cell count because they found more immune activation in PLWHIV with poor reconstitution (INR). This could contribute to the events typical of this group of PLWHIV, including, a high rate of apoptosis (40). In vitro assays have reported an increase in the expression of CD80 and CD86 after exposure to HIV infection (41), Otherwise, in relation to apoptosis mediated by CD80 and CD86 expression, it is known that at least in B cells, CD80 and CD86 regulate the expression of pro-apoptotic as well as Fas and FasL expression and anti-apoptotic molecules; respectively. In macrophages the viability is improve after CD86 stimulation (42, 43). Thus, the level of expression and function of CD80 and CD86, could have a greater impact in the progression of the disease in more advanced stages of the HIV infection. In relation to CD3+ CD4+ T-cells, we evaluated the expression of CD25, CD69 and CD38, finding only a significant increase in the expression of CD38 in cells from PLWHIV respect to HIV- controls. CD69 and CD25 have been classified as early activation markers on lymphocytes; CD69 is expressed 2–3 hours after stimulation reaching its maximum levels within 12–24 hours and is involved in the transmission of costimulatory signals, while the CD25 is part of the IL-2 receptor and is expressed around 24–48 hours after mitogenic stimulation (44–46). In relation to CD69 and in accordance with our results, no significant differences were reported between PLWHIV and healthy controls (44). The presence of CD69 is related to insufficient activation signals, a decreased proliferation (47), death of monocytes (48) and apoptosis of activated T-cells (44, 49). With respect to CD25 expression, the reports are contradictory; there are studies that, according to our results, do not find a significant difference in the expression of CD25 on CD4+ T-cells; contradictory, Ramilo et. al. through an in vitro assay shown a significant increase in CD25 in PLWHIV (50). By maintaining the expression of CD25, the virus ensures a productive infection (35), however in HIV infection where is perpetuated a repeated stimulation of CD4+ T-cells, also is favored an activation induced cell death (AICD) result of the co-expression of Fas and FasL (51).
CD38 is an enzyme that hydrolyzes nicotinamide adenine dinucleotide (NAD+) to nicotinamide and ADP-ribose favoring the cellular metabolic rate in a state of activation (52). CD38 is a marker of cellular activation (53, 54) and its elevated expression on CD8+ T-cells and in the context of HIV infection, is considered a marker predictor in the progression to AIDS and also it has been postulated as a marker of clinical management in PLWHIV (55). The elevated expression of CD38 on CD4+ T-cells also is related to a poor prognosis in these subjects since it has been demonstrated that a decline important of CD4+ T-cells correspond to CD4+ CD38+ HLA-DR+ T-cell, moreover the loss of CD38+ CD4+ T-cells occurs in subjects with advanced infection (56–58). Several mechanisms have been proposed, through which is induced a loss of CD38+ CD4+ T-cells in HIV infection, such as the intracellular decrease in NAD+ promoting the Walburg effect, an increase in the release of cytoplasmic Ca+ 2, thereby affecting the integrity of mitochondrial membrane and a low proliferation rate, therefore CD38 seems to be more involved in cell death via intrinsic (59). On the other hand, the enzymatic activity of CD38 may protect lymphocytes from apoptosis, increasing the expression of molecules such as members of Bcl-2 family and NAm, this last inhibits HIV-1 replication at a post integrational level, and prevents apoptosis in T cells of PLWHIV(59).
Regarding the inflammatory cytokines, we did not observe a significant difference between IL1-β, IL-6, IL-8 and TNF-α, whereas in the case of IL-18 it was observed a significant elevation of the cytokine in sera of PLWHIV (p < 0.05, Fig. 8E). Diverse studies of cell cultures infected in vitro and monocytes cultures derived from patients, as well as measurements made on PLWHIV serum, which include untreated subjects, have reported elevated levels of cytokines such as: IL1-β, IL-6, IL-8 and TNF-α, while, levels that decrease after the initiation of ART, which might contribute to suppression of HIV replication and restoration of CD4+ T-cells. At this point, it is important to mention that the detection methods were different from the one used in this study (60, 61). Recently, in previous study by our research group, it was reported that IL-1β and IL-8 are elevated in serum from INR ART-treated chronic PLWHIV compared to HIV- controls (62). Otherwise, some investigations with the same interest groups as those in this study and, using a similar technique, did not report a significant difference in cytokine levels between PLWHIV before ART in an early and late infection; interestingly, and in accordance with our results, this study reports a significant increase of IL-18 in PLWHIV before ART, levels that are decreased, not at level of HIV- control, in subjects who starting their ART (63). Thus, cytokine alterations vary over the course of HIV disease progression. In these sense it has been reported that in the acute phase of HIV infection, elevated plasma levels of inflammatory cytokines are associated with a high viral load and can estimate the time at which the CD4+ T-cells count will drop < 350 cells/µL; in this study, PLWHIV showed a CD4+ T-cells count > 400 cells/µL; a phenomenon that could be related to the fact that PLWHIV in these study still do not show a significant elevation of most of the cytokines evaluated (64). The elevation of IL-18 in these PLWHIV could contribute to stimulate innate immunity and Th1 response by its ability to induce IFN-γ from T and natural killer (NK) cells. However, it has been hypothesized that chronic excessive production of IL-18 contributes to AIDS pathogenesis, since in vitro studies have shown that IL-18 improves HIV-1 replication in both monocytes and T-cells. In association with decreased levels of IL-12, the IL-18 promotes the differentiation of Th2 CD4+ T-cells in the later stages of the infection; in addition, it has been reported that IL-18 induce destruction of the central nervous system and other tissues, thus it has been suggested that elevated levels of IL-18 are associated with and may contribute to a virological treatment failure and disease progression in PLWHIV (30, 65). It has been reported that, IL-18 boosts FasL expression on NK and CD8+ T-cells, additionally binding with Fas can induce IL-18 production(8). In a study using PBMCs from PLWHIV, TRAIL expression was evaluated after IL-18 stimulation, in which an increase in ligand expression was observed as well as coreceptor CXCR4 (30). On the other hand, in the present study we found a positive correlation with IL-18 levels and DR5 expression, both CD3+ CD4+ T-cells (r = 0.56; p < 0.05) and monocytes (r = 0.636; p < 0.05), (Fig. 9A and B; respectively). Thus, this receptor increase could be modulated by IL-18 secretion.
Finally, regarding the assays using the recombinant ligands and the Caspase 8 inhibitor. It was found that the Fas pathway behaves in a mostly homogeneous manner. In the CD3+ CD4+ T-cells, greater apoptosis was observed in PLWHIV. The functional activity of Fas pathway in this group could be explained by a c-FLIP overexpression, this molecule blocks the union of the pro-caspase 8 to the DISC and by that causes an apoptosis inhibition (66). In the case of CD14+ CD4+ monocytes from HIV- controls the apoptosis decreased significantly when the ligand was added, a cellular response that was not observed in CD14+ CD3+ monocytes from PLWHIV. Instead, it was observed a slightly higher apoptosis, due to which it can be assume that cell death is carried out by a Fas independent mechanism.
In the case of the TRAIL pathway, and specifically in CD3+ CD4+ T-cells in early apoptosis a behavior very similar to Fas pathway was presented. However, it was observed a significant increase in late apoptosis in CD3+ CD4+ T-cells and C14+ CD4+ monocytes from PLWHIV and HIV- controls, cellular event that in turn is decreased in a significant manner after adding the inhibitor. Therefore, it can be speculated that TRAIL pathway modulates the apoptosis in this population, and it is functional in PLWHIV contrary to what was observed on the Fas pathway. In the case of the CD14+ CD4+ monocytes from PLWHIV, there was an increase in early apoptosis in response to recombinant TRAIL ligand, an effect that was not inhibited by caspase 8 blockade. TRAIL activity independent to Caspase 8, has been reported in a mechanism that promotes necroptotic cell death by activation of RIPK (67), this could explain the lack of inhibition once we block caspase activity, adding to this, phosphatidyl serine externalization has been reported in initial stages of necroptosis (68) which explains why we can detect it by using Annexin V.