The peptide, Pep24, was selected from a peptide library in a dominant effector genetics approach for its ability to represses host pathways that are important for HIV-1 replication in T cells. Pep24 was used as ‘bait’ to identify the host factor JAB1 as an important component that regulates HIV-1 replication in T cells. JAB1 (also known as CSN5) is involved in the activation of c-Jun, a component of the AP-1 heterodimeric transcription factor 
. Pep24 inhibited relocalization of JAB1 post-LFA-1 engagement, downstream signaling pathways, and, therefore, HIV-1 replication (). As JAB1 enhanced HIV-1 replication only in PHA-activated primary CD4+
T cells, JAB1 is a mediator of HIV-1 replication only after appropriate signaling primes its activity.
Proposed model for the inhibition of signalling pathway by Pep24.
JAB1 interacts with the β subunit of the cytoplasmic domain of the integrin, LFA-1, and contributes to integrin-mediated signal transduction 
. The interaction of LFA-1 with ICAM-1, ICAM-2, or ICAM-3 is critical for formation of the immunological synapse during full T cell activation 
. The kinetics of induction of LFA-1 are complex. In resting T cells, a low affinity form of LFA-1 is present at the cell surface. The interaction of specific antigen and TCR induces intracellular signals to convert LFA-1 to a high-affinity state 
that, in concert with engagement of ICAM ligands, can transduce signals that allow for JAB1 release. After appropriate engagement, JAB1 redistributes to the cytoplasm and nucleus where it can activate AP-1 transcriptional activity required for T cell activation 
. Interestingly LFA-1 was previously implicated as a promoter of HIV-1 transmission during virus entry and replication 
A mutant of LFA-1 that cannot bind with high affinity to ICAM-1 fails to promote HIV-1 replication in spite of the fact that this mutant LFA-1 promoted virus entry into cells 
, suggesting that an LFA-1 mediated signal is required to promote a replicative environment for HIV-1 in T cells. It has also been reported that HIV-1 infected, Nef-expressing macrophages shed soluble CD23 and soluble ICAM that are HIV-1 replication permissivity factors for primary resting T cells 
. This supports our finding that the ICAM/LFA-1 interaction and the ensuing activation of the JAB1 signaling pathway are critical for HIV-1 replication in T cells. Taken together, current data suggest that the formation of the APC::T cell junction, LFA-1 engagement, and subsequent signaling that releases JAB1 and activates the transcriptional regulators AP1 and NFAT (initial immutable events during T cell activation) leads to a molecular outcome that facilitates HIV-1 replication.
JAB1 is thought to interact with many proteins such as p27kip1
, macrophage migration inhibitory factor 1 (MIF1), c-Jun, LFA-1, Bcl-3, and steroid receptor co-activator (SRC-1) 
. The functions of JAB1 in these interactions are not well understood. JAB1/CSN5 has been shown to contain a JAMM domain and support an isopeptidase in the COP9 signalosome in the removal of the ubiquitin-like Nedd8 from the Cul1 subunit of SCF ubiquitin ligase 
. It is known that JAB1 engagement of p27kip1
allows a cell to proceed through the G1 phase of the cell cycle 
. Based on these results, we hypothesized that the binding to JAB1 by Pep24 led to a blockade of JAB1 function. If the interaction of JAB1 and Pep24 prevents the binding to p27kip1
by JAB1 there should be cell cycle arrest. However, this was not observed in Pep24-expressing T cell (data not shown). This suggests that Pep24 binds to a region of JAB1 that effects regulation of c-Jun but not p27kip1
. It is also possible that the peptide binds to a region of JAB1 that must be post-translationally modified to allow its release from LFA-1. If so, the Pep24-JAB1 interaction site could be a locus for design of small molecule regulators of specific T cell functions, including those that either distinctly, or spatially, inhibit this aspect of T cell activation, to block HIV-1 replication. Importantly, JAB1 has not previously been identified as a potential drug target. As the JNK inhibitor SP600125 
blocked HIV-1 replication in a dose-dependent manner, specific MAP kinase-related events downstream of JAB1 facilitate HIV-1 replication.
In this study, intracellular peptide display technology was used to identify host factors critical to HIV-1 replication pathways. Host factors defined in this manner are candidates for drug development. Inhibitors that block virus access to necessary host factors may be less likely to result in drug resistance than drugs targeting viral proteins. This dominant effector screening approach might also be applied as an adjunct to classical analysis of signaling systems to provide a series of inhibitors that define complementation groups. A unique involvement of LFA-1 signaling via JAB1 that is involved in T cell activation and enables HIV-1 replication in cells was demonstrated. The intriguing role of JAB1 in early T cell activation events that drive HIV-1 replication underscores a need for new tools for identification of early and late host processes required for pathogen parasitism.