Real-World Effectiveness of Nirmatrelvir/Ritonavir on Coronavirus Disease 2019-Associated Hospitalization Prevention: A Population-based Cohort Study in the Province of Quebec, Canada.

BACKGROUND: Nirmatrelvir/ritonavir has shown to reduce COVID-19 hospitalization and death before Omicron, but updated real-world evidence studies are needed. This study aimed to assess whether nirmatrelvir/ritonavir reduces the risk of COVID-19-associated hospitalization among high-risk outpatients. METHODS: A retrospective cohort study of outpatients with SARS-CoV-2 between March 15 and 15 October 2022, using data from the Quebec clinico-administrative databases. Outpatients treated with nirmatrelvir/ritonavir were compared with infected ones not receiving nirmatrelvir/ritonavir using propensity-score matching. Relative risk (RR) of COVID-19-associated hospitalization within 30 days was assessed using a Poisson regression. RESULTS: A total of 8402 treated outpatients were matched to controls. Regardless of vaccination status, nirmatrelvir/ritonavir treatment was associated with a 69% reduced RR of hospitalization (RR: .31; 95% CI: .28; .36; number needed to treat [NNT] = 13). The effect was more pronounced in outpatients with incomplete primary vaccination (RR: .04; 95% CI: .03; .06; NNT = 8), while no benefit was found in those with a complete primary vaccination (RR: .93; 95% CI: .78; 1.08). Subgroups analysis among high-risk outpatients with a complete primary vaccination showed that nirmatrelvir/ritonavir treatment was associated with a significant decrease in the RR of hospitalization in severely immunocompromised outpatients (RR: .66; 95% CI: .50; .89; NNT = 16) and in high-risk outpatients aged ≥70 years (RR: .50; 95% CI: .34; .74; NNT = 10) when the last dose of the vaccine was received at least 6 months ago. CONCLUSIONS: Nirmatrelvir/ritonavir reduces the risk of COVID-19-associated hospitalization among incompletely vaccinated high-risk outpatients and among some subgroups of completely vaccinated high-risk outpatients.

S100A9 induces differentiation of acute myeloid leukemia cells through TLR4.

S100A8 and S100A9 are calcium-binding proteins predominantly expressed by neutrophils and monocytes and play key roles in both normal and pathological inflammation. Recently, both proteins were found to promote tumor progression through the establishment of premetastatic niches and inhibit antitumor immune responses. Although S100A8 and S100A9 have been studied in solid cancers, their functions in hematological malignancies remain poorly understood. However, S100A8 and S100A9 are highly expressed in acute myeloid leukemia (AML), and S100A8 expression has been linked to poor prognosis in AML. We identified a small subpopulation of cells expressing S100A8 and S100A9 in AML mouse models and primary human AML samples. In vitro and in vivo analyses revealed that S100A9 induces AML cell differentiation, whereas S100A8 prevents differentiation induced by S100A9 activity and maintains AML immature phenotype. Treatment with recombinant S100A9 proteins increased AML cell maturation, induced growth arrest, and prolonged survival in an AML mouse model. Interestingly, anti-S100A8 antibody treatment had effects similar to those of S100A9 therapy in vivo, suggesting that high ratios of S100A9 over S100A8 are required to induce differentiation. Our in vitro studies on the mechanisms/pathways involved in leukemic cell differentiation revealed that binding of S100A9 to Toll-like receptor 4 (TLR4) promotes activation of p38 mitogen-activated protein kinase, extracellular signal-regulated kinases 1 and 2, and Jun N-terminal kinase signaling pathways, leading to myelomonocytic and monocytic AML cell differentiation. These findings indicate that S100A8 and S100A9 are regulators of myeloid differentiation in leukemia and have therapeutic potential in myelomonocytic and monocytic AMLs.

DAMP molecule S100A9 acts as a molecular pattern to enhance inflammation during influenza A virus infection: role of DDX21-TRIF-TLR4-MyD88 pathway.

Pathogen-associated molecular patterns (PAMPs) trigger host immune response by activating pattern recognition receptors like toll-like receptors (TLRs). However, the mechanism whereby several pathogens, including viruses, activate TLRs via a non-PAMP mechanism is unclear. Endogenous "inflammatory mediators" called damage-associated molecular patterns (DAMPs) have been implicated in regulating immune response and inflammation. However, the role of DAMPs in inflammation/immunity during virus infection has not been studied. We have identified a DAMP molecule, S100A9 (also known as Calgranulin B or MRP-14), as an endogenous non-PAMP activator of TLR signaling during influenza A virus (IAV) infection. S100A9 was released from undamaged IAV-infected cells and extracellular S100A9 acted as a critical host-derived molecular pattern to regulate inflammatory response outcome and disease during infection by exaggerating pro-inflammatory response, cell-death and virus pathogenesis. Genetic studies showed that the DDX21-TRIF signaling pathway is required for S100A9 gene expression/production during infection. Furthermore, the inflammatory activity of extracellular S100A9 was mediated by activation of the TLR4-MyD88 pathway. Our studies have thus, underscored the role of a DAMP molecule (i.e. extracellular S100A9) in regulating virus-associated inflammation and uncovered a previously unknown function of the DDX21-TRIF-S100A9-TLR4-MyD88 signaling network in regulating inflammation during infection.

Dendritic cells derived from hemozoin-loaded monocytes display a partial maturation phenotype that promotes HIV-1 trans-infection of CD4+ T cells and virus replication.

Coinfection of HIV-1 patients with Plasmodium falciparum, the etiological agent of malaria, results in a raise of viral load and an acceleration of disease progression. The primary objective of this study was to investigate whether the malarial pigment hemozoin (HZ), a heme by-product of hemoglobin digestion by malaria parasites, can affect HIV-1 transmission by monocytes-derived dendritic cells (DCs) to CD4(+) T cells when HZ is initially internalized in monocytes before their differentiation in DCs. We demonstrate in this study that HZ treatment during the differentiation process induces an intermediate maturation phenotype when compared with immature and fully mature DCs. Furthermore, the DC-mediated transfer of HIV-1 is enhanced in presence of HZ, a phenomenon that may be linked with the capacity of HZ-loaded cells to interact and activate CD4(+) T cells. Altogether our findings suggest a new mechanism that could partially explain the increased HIV-1 virus production during a coinfection with P. falciparum. Understanding the multifaceted interactions between P. falciparum and HIV-1 is an important challenge that could lead to the development of new treatment strategies.

Inefficient fusion due to a lack of attachment receptor/co-receptor restricts productive human immunodeficiency virus type 1 infection in human hepatoma Huh7.5 cells.

Since the widespread use of the highly active antiretroviral therapy, the incidence of liver disease has increased to become a leading cause of death among human immunodeficiency virus type 1 (HIV-1)-infected individuals. It can be proposed that the ability of HIV-1 to infect hepatocytes could influence liver diseases. Although the presence of HIV-1 was identified in hepatocytes from HIV-1 seropositive patients, the susceptibility of hepatocytes to HIV-1 infection in vitro remains controversial. We present evidence here that human hepatoma cells are not productively infected with CD4-dependent HIV-1 strains because of inefficient fusion related to an absence of cell surface CD4 and CXCR4. However, these cells display an increased susceptibility to infection with a CD4-independent viral isolate through an interaction with galactosyl ceramide, an alternate receptor for HIV-1. This study provides further understanding of the susceptibility of human hepatocytes to HIV-1 infection. However, in vivo investigations are recommended to consolidate these data.

Malaria hemozoin modulates susceptibility of immature monocyte-derived dendritic cells to HIV-1 infection by inducing a mature-like phenotype.

Together, Plasmodium falciparum (P. falciparum) and HIV-1 infections cause more than four million deaths a year. There is still limited information about the putative impact of the malaria pigment hemozoin (HZ) on the dissemination of HIV-1. As so, we propose a premise where HZ present in human dendritic cells (DCs) could modulate HIV-1 transfer to CD4(+) T cells. We report here that HZ promotes transmission of HIV-1 by immature monocyte-derived DCs (iMDDCs). Moreover, we noted that in the presence of HZ, iMDDCs were less permissive to productive HIV-1 infection. The HZ-dependent modulation of the interaction between iMDDCs and HIV-1 seems to be partly due to a decreased expression of CCR5 and also to the induction of a more mature phenotype as proven by microscopy and flow cytometry analyses. Therefore, exposure of iMDDCs to malaria pigments provokes their maturation rendering them more potent to trans-infect CD4(+) T cells with HIV-1.

TLR2 and TLR4 triggering exerts contrasting effects with regard to HIV-1 infection of human dendritic cells and subsequent virus transfer to CD4+ T cells.

BACKGROUND: Recognition of microbial products through Toll-like receptors (TLRs) initiates inflammatory responses orchestrated by innate immune cells such as dendritic cells (DCs). As these cells are patrolling mucosal surfaces, a portal of entry for various pathogens including human immunodeficiency virus type-1 (HIV-1), we investigated the impact of TLR stimulation on productive HIV-1 infection of DCs and viral spreading to CD4+ T cells. RESULTS: We report here that engagement of TLR2 on DCs increases HIV-1 transmission toward CD4+ T cells by primarily affecting de novo virus production by DCs. No noticeable and consistent effect was observed following engagement of TLR5, 7 and 9. Additional studies indicated that both HIV-1 infection of DCs and DC-mediated virus transmission to CD4+ T cells were reduced upon TLR4 triggering due to secretion of type-I interferons. CONCLUSION: It can thus be proposed that exposure of DCs to TLR2-binding bacterial constituents derived, for example, from pathogens causing sexually transmissible infections, might influence the process of DC-mediated viral dissemination, a phenomenon that might contribute to a more rapid disease progression.

LFA-1 antagonists as agents limiting human immunodeficiency virus type 1 infection and transmission and potentiating the effect of the fusion inhibitor T-20.

Adhesion molecules are known to play major roles in the initiation and stabilization of cell-to-cell contacts during the immunological response. Human immunodeficiency virus type 1 (HIV-1) exploits those interactions to facilitate infection and propagation processes. The primary objective of the present study was to investigate the ability of antagonists specific for lymphocyte function-associated antigen 1 (LFA-1) to diminish HIV-1 infection and transmission. We demonstrate here that LFA-1 antagonists can significantly reduce HIV-1 replication in primary human cells and virus propagation by affecting cell-to-cell interactions. Moreover, the inhibition of LFA-1-mediated adhesion events also potentiates the antiviral efficacy of the peptide fusion inhibitor T-20. Altogether, our data suggest that LFA-1 antagonists represent promising antiviral agents. Antiadhesion therapy could be considered a complementary strategy targeting cellular functions essential for HIV-1 spreading and against which the combined therapy currently used displays a limited efficacy.

Enhanced myelopoiesis and aggravated arthritis in S100a8-deficient mice.

Expressed strongly by myeloid cells, damage-associated molecular pattern (DAMP) proteins S100A8 and S100A9 are found in the serum of patients with infectious and autoimmune diseases. Compared to S100A9, the role of S100A8 is controversial. We investigated its biological activity in collagen-induced arthritis using the first known viable and fertile S100a8-deficient (S100a8-/-) mouse. Although comparable to the wild type (WT) in terms of lymphocyte distribution in blood and in the primary and secondary lymphoid organs, S100a8-/- mice had increased numbers of neutrophils, monocytes and dendritic cells in the blood and bone marrow, and these all expressed myeloid markers such as CD11b, Ly6G and CD86 more strongly. Granulocyte-macrophage common precursors were increased in S100a8-/- bone marrow and yielded greater numbers of macrophages and dendritic cells in culture. The animals also developed more severe arthritic disease leading to aggravated osteoclast activity and bone destruction. These findings were correlated with increased inflammatory cell infiltration and cytokine secretion in the paws. This study suggests that S100A8 is an anti-inflammatory DAMP that regulates myeloid cell differentiation, thereby mitigating the development of experimental arthritis.

Cross-Reactivity to Cephalosporins and Carbapenems in Penicillin-Allergic Patients: Two Systematic Reviews and Meta-Analyses.

BACKGROUND: There is no recent systematic review on the risk of cross-reactivity to cephalosporins and carbapenems in penicillin-allergic patients despite many new studies on the subject. All past reviews have several limitations such as not including any patient with a T-cell-mediated penicillin allergy. OBJECTIVES: To determine the risk of cross-reactivity to cephalosporins and carbapenems in patients with a proven IgE- or T-cell-mediated penicillin allergy. To measure the association between R1 side chain similarity on cephalosporins and penicillins and the risk of cross-reactivity. METHODS: MEDLINE and EMBASE were searched from January 1980 to March 2019. Studies had to include at least 10 penicillin-allergic subjects whose allergy had been confirmed by a positive skin test (ST) or drug provocation test (DPT) result. Cross-reactivity had to be assessed to at least 1 cephalosporin or carbapenem through ST or DPT. Both random-effects and fixed-effect models were used to combine data. A bioinformatic model was used to quantify the similarity between R1 side chains. RESULTS: Twenty-one observational studies on cephalosporin cross-reactivity involving 1269 penicillin-allergic patients showed that the risk of cross-reactivity varied with the degree of similarity between R1 side chains: 16.45% (95% CI, 11.07-23.75) for aminocephalosporins, which share an identical side chain with a penicillin (similarity score = 1), 5.60% (95% CI, 3.46-8.95) for a few cephalosporins with an intermediate similarity score (range, 0.563-0.714), and 2.11% (95% CI, 0.98-4.46) for all those with low similarity scores (below 0.4), irrespective of cephalosporin generation. The higher risk associated with aminocephalosporins was observed whether penicillin allergy was IgE- or T-cell-mediated. Eleven observational studies on carbapenem cross-reactivity involving 1127 penicillin-allergic patients showed that the risk of cross-reactivity to any carbapenem was 0.87% (95% CI, 0.32-2.32). CONCLUSIONS: Although it remains possible that these meta-analyses overestimated the risk of cross-reactivity, clinicians should consider the increased risk of cross-reactivity associated with aminocephalosporins, and to a lesser extent with intermediate-similarity-score cephalosporins, compared with the very low risk associated with low-similarity-score cephalosporins and all carbapenems when using beta-lactams in patients with a suspected or proven penicillin allergy.

Secretion of S100A8, S100A9, and S100A12 by Neutrophils Involves Reactive Oxygen Species and Potassium Efflux.

S100A8/A9 (calprotectin) and S100A12 proinflammatory mediators are found at inflammatory sites and in the serum of patients with inflammatory or autoimmune diseases. These cytoplasmic proteins are secreted by neutrophils at sites of inflammation via alternative secretion pathways of which little is known. This study examined the nature of the stimuli leading to S100A8/A9 and S100A12 secretion as well as the mechanism involved in this alternative secretion pathway. Chemotactic agents, cytokines, and particulate molecules were used to stimulate human neutrophils. MSU crystals, PMA, and H2O2 induced the release of S100A8, S100A9, and S100A12 homodimers, as well as S100A8/A9 heterodimer. High concentrations of S100A8/A9 and S100A12 were secreted in response to nanoparticles like MSU, silica, TiO2, fullerene, and single-wall carbon nanotubes as well as in response to microbe-derived molecules, such as zymosan or HKCA. However, neutrophils exposed to the chemotactic factors fMLP failed to secrete S100A8/A9 or S100A12. Secretion of S100A8/A9 was dependent on the production of reactive oxygen species and required K(+) exchanges through the ATP-sensitive K(+) channel. Altogether, these findings suggest that S100A12 and S100A8/A9 are secreted independently either via distinct mechanisms of secretion or following the activation of different signal transduction pathways.

Dectin-1/TLR2 and NOD2 agonists render dendritic cells susceptible to infection by X4-using HIV-1 and promote cis-infection of CD4(+) T cells.

HIV-1 pathogenesis is intimately linked with microbial infections and innate immunity during all stages of the disease. While the impact of microbial-derived products in transmission of R5-using virus to CD4(+) T cells by dendritic cells (DCs) has been addressed before, very limited data are available on the effect of such compounds on DC-mediated dissemination of X4-tropic variant. Here, we provide evidence that treatment of DCs with dectin-1/TLR2 and NOD2 ligands increases cis-infection of autologous CD4(+) T cells by X4-using virus. This phenomenon is most likely associated with an enhanced permissiveness of DCs to productive infection with X4 virus, which is linked to increased surface expression of CXCR4 and the acquisition of a maturation profile by DCs. The ensuing DC maturation enhances susceptibility of CD4(+) T cells to productive infection with HIV-1. This study highlights the crucial role of DCs at different stages of HIV-1 infection and particularly in spreading of viral strains displaying a X4 phenotype.

An inflammation loop orchestrated by S100A9 and calprotectin is critical for development of arthritis.

OBJECTIVE: The S100A9 and S100A8 proteins are highly expressed by neutrophils and monocytes and are part of a group of damage-associated molecular pattern molecules that trigger inflammatory responses. Sera and synovial fluids of patients with rheumatoid arthritis (RA) contain high concentrations of S100A8/A9 that correlate with disease activity. METHODS: In this study, we investigated the importance of S100A9 in RA by using neutralizing antibodies in a murine lipopolysaccharide-synchronized collagen-induced arthritis model. We also used an in vitro model of stimulation of human immune cells to decipher the role played by S100A9 in leukocyte migration and pro-inflammatory cytokine secretion. RESULTS: Treatment with anti-S100A9 antibodies improved the clinical score by 50%, diminished immune cell infiltration, reduced inflammatory cytokines, both in serum and in the joints, and preserved bone/collagen integrity. Stimulation of neutrophils with S100A9 protein led to the enhancement of neutrophil transendothelial migration. S100A9 protein also induced the secretion by monocytes of proinflammatory cytokines like TNFα, IL-1ß and IL-6, and of chemokines like MIP-1α and MCP-1. CONCLUSION: The effects of anti-S100A9 treatment are likely direct consequences of inhibiting the S100A9-mediated promotion of neutrophil transmigration and secretion of pro-inflammatory cytokines from monocytes. Collectively, our results show that treatment with anti-S100A9 may inhibit amplification of the immune response and help preserve tissue integrity. Therefore, S100A9 is a promising potential therapeutic target for inflammatory diseases like rheumatoid arthritis for which alternative therapeutic strategies are needed.

Innate lymphoid cells promote anatomical containment of lymphoid-resident commensal bacteria.

The mammalian intestinal tract is colonized by trillions of beneficial commensal bacteria that are anatomically restricted to specific niches. However, the mechanisms that regulate anatomical containment remain unclear. Here, we show that interleukin-22 (IL-22)-producing innate lymphoid cells (ILCs) are present in intestinal tissues of healthy mammals. Depletion of ILCs resulted in peripheral dissemination of commensal bacteria and systemic inflammation, which was prevented by administration of IL-22. Disseminating bacteria were identified as Alcaligenes species originating from host lymphoid tissues. Alcaligenes was sufficient to promote systemic inflammation after ILC depletion in mice, and Alcaligenes-specific systemic immune responses were associated with Crohn's disease and progressive hepatitis C virus infection in patients. Collectively, these data indicate that ILCs regulate selective containment of lymphoid-resident bacteria to prevent systemic inflammation associated with chronic diseases.

Human hepatoma cells transmit surface bound HIV-1 to CD4+ T cells through an ICAM-1/LFA-1-dependent mechanism.

BACKGROUND: During the viremic phase of human immunodeficiency virus type-1 (HIV-1) infection, hepatocytes are likely to be constantly exposed to circulating virions. Knowing that a contact between hepatocytes and CD4+ T lymphocytes is favoured by the local slow blood flow present within the liver, we hypothesize that hepatic cells can act as a viral reservoir and thus contribute to HIV-1 propagation. RESULTS: We report that human hepatoma cells bind and internalize HIV-1 particles. Infection of CD4+ T cells was found to be much more efficient following a contact with virus-loaded hepatocytes than with cell-free virus. Additional studies suggest that infection of CD4+ T cells in trans with hepatocytes carrying virus is primarily due to surface bound HIV-1 particles and relies on LFA-1/ICAM-1 interactions. CONCLUSION: This work represents the first demonstration by which circulating CD4+ T cells can be potentially infected with HIV-1 through a contact with hepatocytes in the liver.

TLR5 stimulation is sufficient to trigger reactivation of latent HIV-1 provirus in T lymphoid cells and activate virus gene expression in central memory CD4+ T cells.

When effector CD4+ T cells carrying integrated HIV-1 proviruses revert back to a resting memory state, the virus can remain silent in those cells for years. Following re-exposure to the nominal antigen or in response to other stimuli (e.g. pro-inflammatory cytokines), these cells can begin to produce virus. Here we demonstrate that TLR5 stimulation induces activation of NF-kappaB and reactivate latent HIV-1 in CD4+ T lymphoid cells. Interestingly, we report also that TLR5 engagement leads to virus gene expression in quiescent central memory CD4+ T cells, a cell population recognized as a major reservoir in infected individuals. This study supports the hypothesis that translocation of microbes that can engage pathogen recognition receptors might play a dominant role in chronic immune activation seen in HIV-1-infected individuals and promote virus replication and dissemination.

Ingestion of the malaria pigment hemozoin renders human macrophages less permissive to HIV-1 infection.

Few studies have investigated the pathophysiologic mechanisms responsible for what seems to be a possible interaction between Plasmodium falciparum, the causative agent of malaria, and HIV-1 in dually infected patients. It has been shown that Plasmodium parasites detoxify heme molecules into a pigment called hemozoin (HZ), which can significantly modulate the immune system. The primary objective of this study was to determine whether exposure of human primary monocyte-derived macrophages (MDMs) to the malaria pigment influences the process of HIV-1 infection. We report here that HIV-1 replication is significantly diminished in HZ-loaded MDMs. The HZ-mediated reduction in virus replication is due to a block at a step in the virus life cycle occurring between the completion of full-length reverse transcripts and integration of viral DNA within the host chromosome. Understanding the pathological mechanisms involved in P. falciparum and HIV-1 co-infection is of high importance because of possible therapeutic ramifications.

Virus-associated host CD62L increases attachment of human immunodeficiency virus type 1 to endothelial cells and enhances trans infection of CD4+ T lymphocytes.

Previous studies have identified several host-derived cell-surface proteins incorporated within emerging human immunodeficiency virus type 1 (HIV-1) particles. Some of these molecules play a role in different steps of the virus life cycle and are often advantageous for the virus. We report here that the leukocyte L-selectin (also called CD62L) remains functional when inserted within the envelope of HIV-1. Indeed, we demonstrate that adsorption of virions to endothelial cells is enhanced upon acquisition of host-derived CD62L. The more important binding of CD62L-bearing HIV-1 particles resulted in a more efficient virus transmission to CD4(+) T lymphocytes. Capture and eventual transfer of such CD62L-bearing virions by the endothelium could play a role in the pathogenesis of HIV-1 infection.

TLR2 signaling renders quiescent naive and memory CD4+ T cells more susceptible to productive infection with X4 and R5 HIV-type 1.

It has been recently demonstrated that circulating microbial products are responsible for a systemic immune activation in individuals infected with HIV-type 1. Bacterial products carry structural conserved motifs recognized by TLRs. Some TLR members are expressed in primary human CD4+ T cells but the precise functional role played by these pattern recognition receptors is still imprecise. In this study, we report that engagement of TLR2 in quiescent naive and memory CD4+ T cells leads to the acquisition of an effector-like phenotype. Interestingly, engagement of TLR2 renders both cell subsets more susceptible to productive infection with X4 virions and a higher virus production was seen with R5 viruses. It can be proposed that exposure of resting CD4+ T cells to pathogen-derived products that can engage TLR2 induces the acquisition of an effector-like phenotype in naive and memory CD4+ T lymphocytes, a phenomenon that might result in an acceleration of virus replication, immune dysregulation, and HIV-type 1-mediated disease progression.

Tetraspanin CD81 provides a costimulatory signal resulting in increased human immunodeficiency virus type 1 gene expression in primary CD4+ T lymphocytes through NF-kappaB, NFAT, and AP-1 transduction pathways.

The tetraspanin superfamily member CD81 has been shown to form microdomains in the plasma membrane and to participate in the recruitment of numerous adhesion molecules, receptors, and signaling proteins in the central zone of the immune synapse. Beside its structural role, CD81 also delivers a cosignal for T cells to trigger cytokine production and cellular proliferation, thus suggesting a key role in some fundamental biological functions. It has been shown that signaling events initiated through the T-cell receptor (TCR)/CD3 complex and the coactivator CD28 positively affect human immunodeficiency virus type 1 (HIV-1) gene expression, but no study had investigated the putative costimulatory activity of CD81 on HIV-1 transcriptional activity. We observed that CD81 engagement potentiates TCR/CD3-mediated signaling, resulting in an enhancement of HIV-1 transcription and de novo virus production in both established Jurkat cells and primary CD4+ T lymphocytes at a magnitude that approximates that with CD28. These observations were made by using transiently transfected plasmids (i.e., nonintegrated viral DNA) and fully competent viruses (i.e., stably integrated provirus). Moreover, the CD81-mediated enhancement of HIV-1 gene expression is linked with increased nuclear translocation of transcription factors known to positively regulate virus transcription, i.e., NF-kappaB, NFAT, and AP-1. These findings suggest that engagement of CD81 decreases the signaling threshold required to initiate TCR/CD3-mediated induction of integrated HIV-1 proviral DNA in primary CD4+ T cells.