Compared to IFN-λ1 or IFN-λ2, IFN-λ3 is thought to be more important for anti-HCV therapy, as its genetic variation of three single-nucleotide polymorphisms (SNPs) (rs12979860, rs12980275, and rs8099917) in upstream of the IFN-λ3 (IL-28B) gene is highly associated with treatment outcomes of Peg-IFNα-2a plus ribavirin therapy 
. Although the anti-HIV function of IFN-λ1 or IFN-λ2 has been demonstrated 
, it remains to be determined whether IFN-λ3 has the ability to suppress HIV. In the present study, we have provided the experimental evidence that similar to IFN-λ1 and IFN-λ2, IFN-λ3 treatment resulted in the inhibition of HIV infection of macrophages. When macrophages were pretreated once with IFN-λ3, a nearly complete inhibition of HIV of macrophages was observed (). The presence of IFN-λ3 in the cultures is necessary to maintain HIV suppression, as the withdrawal of IFN-λ3 from the cultures resulted in a rebound of viral replication (). This inhibitory effect of IFN-λ3 treatment was seen even after HIV infection had been initiated in macrophages (). We also demonstrated that IFN-λ3 appeared to be more potent in inhibiting HIV replication in macrophages than IFN-λ1 or IFN-λ2 (). This enhanced ability to inhibit HIV could be due to its stronger effects on several key cellular antiviral factors in macrophages than IFN-λ1 or IFN-λ2 ().
IFN-λ functionally resembles type I IFNs in antiviral protection 
. Despite of the overlapping antiviral functions of IFN-λ with type I IFNs, the more restriction pattern of IFN-λ receptor (IL-28Rα) expression suggests that IFN-λ has a more specialized role in host antiviral defense against viruses in a subset of cells 
. In addition, the mechanisms by which IFN-λ establishes an antiviral state are not as well characterized as those for IFN-α/β. Our studies showed that IFN-λ3 through multiple mechanisms suppressed HIV infection and replication. We first investigated whether IFN-λ3 activates JAK-STAT signaling pathway, inducing the antiviral state in macrophages. IFN-λ3 treatment of macrophages upregulated the expression of either key factors of JAK-STAT pathway, such as STAT1 and STAT2 (, ), or the known ISGs in the downstream of JAK-SATA pathway, including ISG-56, MxA, and OAS-1 (). The role of JAK-STAT pathway in the anti-HIV activity of IFN-λ3 was further confirmed by the observation that to block of JAK-STAT signaling pathway by the JAK inhibitor could compromise the IFN-λ3 action on HIV and significantly decrease the expression of IFN-λ3-induced antiviral factors (). We also demonstrated that APOBEC3G and APOBEC3F, the specific anti-HIV cellular factors, were induced by IFN-λ through JAK-STAT pathway ( and ). In addition, tetherin, a transmembrane protein that specifically inhibits HIV infection by preventing its release from infected cells, was also induced by IFN-λ through JAK-STAT pathway ( and ). Thus, JAK-STAT pathway plays a crucial role in the anti-HIV activity of IFN-λ3 in macrophages, which is in agreement with the previous findings, showing that IFN-λ3, through the activation of JAK-STAT pathway, exerts its anti-HCV function in human hepatocytes 
TLRs are crucial in the innate immune responses to pathogens, because they recognize and respond to PAMPs, which leads to activation of intracellular signaling pathways. Among the eleven identified human TLRs, three TLRs (TLR- 3, 7, and 9) play a key role in virus-mediated innate immunity, as they specifically recognizes viral RNA or DNA and initiate antiviral signaling pathways in macrophages 
. Our studies demonstrated that IFN-λ3 could significantly induce the expression of TLR3 as well as TLR3-associated adaptors such as MyD88 and TRIF (, ). These observations provide an additional mechanism for IFN-λ3-mediated anti-HIV action, as TLR3 signaling has a crucial role in early innate immune response to viral infections, including HIV 
Our further investigation showed that IFN-λ3 induced the expression of IRFs (IRF-1, 3, 5, 7 and 9) in macrophages infected with or without HIV ( and ). The IRFs are important regulators in the type I and III IFN-mediated antiviral immunity. These regulators not only recognize the elements of DNA promoter to modulate type I and III IFN genes, but also regulate the IFN-stimulated response element (ISRE) in some of ISGs, leading to induction of an antiviral state 
. Recent studies 
have further demonstrated an essential role of IRF-3, IRF-5, and IRF-7 in the activation of type I and III IFN expression, which is critical for the production of down-stream antiviral factors such as ISG-56, MxA, OAS-1, tetherin ().
Taken together, our data have provided the compelling evidence that IFN-λ3 through the activation of TLR3 and JAK-STAT pathways inhibited HIV infection of macrophage. Although the precise cellular and molecular mechanisms by which IFN-λ3 inhibits HIV replication remain to be determined, the induction of multiple cellular restriction factors against HIV should account for much of IFN-λ3-mediated anti-HIV activity. These anti-HIV mechanisms of the IFN-λ3 action offer an attractive alternative for HIV treatment, as it would be extremely difficult for HIV to develop resistance to the IFN-λ3 actions that suppress virus at various steps of its replication in the context of host cell innate immunity. However, future studies are needed in order to determine the impact of IFN-λ3 on HIV in ex vivo and in vivo systems. These additional studies shall explore the potential for developing IFN-λ3-based therapy for people infected with HIV.