The MOR was originally discovered in the central nervous system. However, during the last two decades, the expression of the MOR in the immune system has been well characterized (6
). Activation of the MOR by agonists such as morphine can result in immunosuppression (18
). Many individuals infected with the HIV-1 virus are opioid abusers (32
), and even HIV-1 patients who are not drug abusers sometimes receive treatment with a MOR agonist, including morphine, for pain management (32
). In addition, in vitro studies by Guo et al. demonstrated that morphine increases HIV-1 infection of human monocyte-derived macrophages (20
HIV-1 infection is marked by an array of pathologies and a variety of secondary infections (25
), and HIV-positive patients often contract bacterial infections (25
). Few studies, thus far, have examined the interactive effects of bacterial endotoxins and viral proteins in the course of HIV infection using either animal models or human subjects. In this study, we used both the HIV-1Tg rat model and human cell cultures to investigate the interaction between the bacterial endotoxin LPS and the HIV-1 glycoprotein gp120 on MOR expression.
Highly active antiretroviral therapy (HAART) consists of inhibitors that target viral entry, reverse transcriptase, and viral protease to control viral replication, restore immunity, and delay disease progression, but it cannot eliminate the infection (1
). The clinical challenge in this post-HAART era is, therefore, the persistent infection that results from the presence of HIV-1 viral proteins in the host (25
). The development of various manifestations of human HIV-1 infection in the HIV-1Tg rat, without viral replication, indicates that the presence of viral proteins in the host is sufficient to affect the target cells, including the immune cells such as T cells and macrophages, and cause the clinical progression to AIDS. Thus, the HIV-1Tg rat appears to mimic the condition of patients given HAART, who have limited (controlled) viral replication but persistent HIV infection that eventually advances to AIDS.
In this study, we were particularly interested in observing the effects of viral proteins such as gp120 on immune cells in vivo in the absence of viral replication. Macrophages have long been known to act as reservoirs for the HIV-1 virus, where the virus can replicate and remain latent (39
). These cells have been shown to harbor the HIV-1 virus in both the peripheral blood and bone marrow as well as in target organs such as the brain, lungs, lymph nodes, and skin (22
). In contrast, the macrophages in the HIV-1Tg rat model do not act as viral reservoirs; however, viral proteins are still being expressed in blood and tissues. Thus, the peritoneal macrophages in the HIV-1Tg rat can be used to help delineate the effects of viral proteins such as gp120 on immune function in the absence of viral replication.
Our data show that the basal level of MOR mRNA in the peritoneal macrophages of the HIV-1Tg rats is significantly higher than that in the F344 control animals, indicating that the HIV-1 transgene with the gag-pol defect may itself increase MOR expression. As gag and pol are mainly responsible for HIV viral replication, our results suggest that one or more of the other seven remaining viral proteins may be involved in the up-regulation of MOR expression. In addition, since the macrophages do not act as the viral reservoir as in the patients infected with HIV-1, the information obtained using peritoneal macrophages can apply to any cells on which the HIV viral proteins exert their actions.
Immunocompromised HIV-positive patients often contract secondary bacterial infections (25
). LPS, a bacterial endotoxin, is commonly used to study a host's response to a bacterial infection, including the production of inflammatory cytokines. We have previously shown that systemic treatment with a nonpyrogenic dose of LPS increases the serum levels of proinflammatory cytokines, including TNF-α, IL-1β, and IL-6, in healthy Sprague-Dawley rats (30
). In this study, we found that in F344 control rats, there were comparable increases in the serum levels of IL-1β and TNF-α (1- to 1.5-fold increases, respectively) following treatment with the same nonpyrogenic dose of LPS (30
). However, in the HIV-1Tg rats, the same dose of LPS induced a much greater increase in serum IL-1β and TNF-α levels (7- and 38-fold increases, respectively). In contrast to the robust increase in serum IL-1β and TNF-α levels induced by LPS in HIV-1Tg rats, the LPS-induced increases in the serum level of IL-10, an anti-inflammatory cytokine, in the HIV-1Tg rats and F344 control animals were more comparable (0.8- versus 1.5-fold increases, respectively). Taken together, our data suggest that there may be an imbalance between the levels of production of pro- and anti-inflammatory cytokines in response to bacterial endotoxin in HIV-1-infected individuals. Such an imbalance has been reported to be related to the progression of HIV-1 infection to AIDS (8
), and, thus, this imbalance can have detrimental consequences.
We and others have demonstrated that the MOR is constitutively expressed in peritoneal macrophages isolated from rats and mice (37
; unpublished data). In this study, although MOR expression in the peritoneal macrophages of both HIV-1Tg rats and F344 control animals was increased following systemic treatment with LPS, the LPS-induced up-regulation of MOR expression was significantly greater in the HIV-1Tg rats than in the F344 control animals. This is the first in vivo study to demonstrate that not only does LPS up-regulate the MOR but the LPS-induced up-regulation of the MOR is greater in the presence of HIV proteins.
In this study, we used an ex vivo approach to show that serum prepared from the HIV-1Tg rats up-regulated MOR mRNA in the peritoneal macrophages isolated from the F344 control animals. Furthermore, preincubation of HIV-1Tg serum with a monoclonal antibody to human gp120 abolished the up-regulation of MOR mRNA in the F344 peritoneal macrophages. These data not only confirm our observation that MOR mRNA levels are significantly higher in the HIV-1Tg rats than in the F344 animals but also suggest that circulating viral proteins in the HIV-1Tg rat, specifically gp120, may be involved in the up-regulation of the MOR.
Many laboratories, including our own, routinely use HL-60 promyelocytic leukemia cells as an in vitro macrophage cell model because of their ability to differentiate into macrophage-like cells in response to treatment with TPA (4
). We have previously shown that TPA-HL-60 cells respond to gp120 stimulation by secreting TNF-α and up-regulating MOR expression (3
). To demonstrate that our observations in the HIV-1Tg rat model could have clinical implications, we showed that pretreatment with gp120 potentiates LPS-induced up-regulation of MOR expression in human TPA-HL-60 cells. Taken together, both our in vitro and ex vivo studies suggest that there may be some underlying molecular and cellular mechanisms that would explain why HIV-1-infected individuals who use opioids are significantly more susceptible to opportunistic bacterial infections. There have been a number of studies showing that the use or abuse of morphine can have adverse effects on immune function (16
). Several previous studies have shown that exposure to opioids induces septic shock, which leads to a compromised immune response to bacterial infection (16
). Morphine use can result in immunosuppression, which can further aggravate the progression of HIV infection to full-blown AIDS (2
). Our results show that the MOR is up-regulated in the presence of HIV proteins such as gp120. Furthermore, these results indicate that the HIV-infected individual could be placed in an immune deficit situation via the actions of HIV proteins on MOR expression. Since HIV-infected patients often contract bacterial infections and, in addition, may also be given a MOR agonist for pain management, our data provide important information with significant clinical implications.