Given immunomodulation and immunocompromising effect of opiates, abuse of opiates has been suggested as a cofactor in promoting HIV disease progression. However, much remain to be learned about the mechanisms of opiate-mediated broad influence on host immunity related to control of viral replication. In this study, we showed that morphine significantly inhibited endogenous type I (IFN-α/β) and III (IFN-λ1) IFN expression (), which was associated with increased susceptibility of macrophages to HIV and SIV infection and enhanced virus replication. This morphine effect is specific through the opioid receptor, as the suppression of IFN expression by morphine could be abrogated by naltrexone (). These findings support the earlier reports showing that morphine suppresses Sendai virus-induced IFN-α production by peripheral blood mononuclear cells and monocytes 
. Our earlier study also showed that morphine inhibited endogenous IFN-α expression and enhanced complete hepatitis C virus replication in human hepatocytes 
. A novel finding of this study is that morphine inhibited IFN-λ expression in macrophages (). IFN-λ has been shown to inhibit replication of a number of viruses, including HIV 
. Thus, the finding that morphine inhibited endogenous IFN-λ expression in macrophages provides a sound mechanism for the morphine action on HIV or SIV.
In order to further investigate the mechanism(s) responsible for the action of morphine, we examined the effect of morphine on the expression of TLRs and RIG-I, which recognize viral infections and activate IFN pathway signaling 
. A recent study showed that purified genomic RNA from HIV induced a RIG-I dependent type I IFN response 
. Thus, to suppress RIG-I expression by morphine should impair intracellular innate immunity, providing a favorable environment for viral replication. In addition to its negative effect on RIG-I expression, morphine also suppressed the expression of IRF-7, the key regulator of type I IFNs 
. Similar to type I IFNs, IFN- λ1 is also activated by both IRF-3 and IRF-7 
. IRFs not only recognize the elements in the IFN promoter to modulate the expression of type I IFN genes selectively, but also regulate the IFN-stimulated response element (ISRE) in some of IFN-stimulated genes (ISGs), leading to induction of an antiviral state 
. We were particularly interested in IRF-3 and IRF-7, as IRF-3 and IRF-7 are the key regulators of type I IFN gene expression induced by viruses 
. IRF-7 is the master regulator of type I IFN-dependent immune response, as it not only induces IFN-α expression, but also activates many antiviral ISGs 
. Therefore, the suppression of IRF-7 expression in macrophages by morphine treatment explains inhibitory effect of morphine on both type I and type III IFN expression.
APOBEC3 family members are cellular cytidine deaminases that have the ability to inhibit the mobility of HIV 
. Among the APOBEC3 family members, APOBEC3G, APOBEC3F and APOBEC3H have been identified to have the ability to restrict HIV replication in both CD4+
T cells and macrophages 
. APOBEC3G can either edit the newly synthesized viral DNA or have an inhibitory effect through lethal editing of nascent reverse transcripts of the HIV life cycle 
. APOBEC3F also encodes an antiretroviral protein that is selectively packaged into HIV virions and profoundly inhibits HIV infectivity 
. APOBEC3B and APOBEC3C have been shown to act as the potent inhibitors of SIV replication 
. Thus, the suppression of several key members of APOBEC3 family in macrophages by morphine justifies the enhancing effect of morphine action on HIV or SIV infection and replication.
To further explore the mechanisms involved in morphine-mediated enhancement of AIDS virus infection of macrophages, we attempted to determine whether morphine modulates the expression of the negative regulators of the JAK-STAT signaling pathway. It is known that the JAK-STAT signaling pathway is the major pathway for IFN-mediated signaling and activation of gene expression 
. IFNs through binding to their specific receptors activate JAK-STAT pathway, which regulates the expression of immune system genes 
. Morphine treatment not only induced the expression of SOCS-1, SOCS-2 and SOCS-3, but also enhanced the expression of PIAS-1, PIAS-3, PIAS-X and PIAS-Y, the potent suppressors of the JAK-STAT signaling cascade 
. These findings support our earlier in vivo
investigation, showing that the heroin users had significantly higher levels of SOCS and PIAS than the control subjects 
Taken together, our study provides compelling experimental evidence that morphine enhances AIDS virus replication in macrophages through the modulation of multiple factors in IFN signaling pathway at both cellular and molecular levels. Although additional mechanisms might also be involved in the morphine action on AIDS virus, to suppress the expression of endogenous IFNs and IFN-inducible antiviral genes should account for much of morphine-mediated HIV or SIV enhancement in macrophages. Because morphine exerts a profound and detrimental effects on host cell innate immunity that has a critical role in restricting HIV or SIV replication in macrophages, it is likely that opiate abuse has the ability to alter the course of HIV disease progression.