Extracellular HIV-1 Tat protein activates phosphatidylinositol 3- and Akt/PKB kinases in CD4+ T lymphoblastoid Jurkat cells

The biological basis for the pleiotropic activity of extracellular human immunodeficiency virus (HIV)-1 Tat protein on lymphoid T cell survival is not well understood. We have here demonstrated that the addition in culture of 0.1–10 nM Tat protein to 36-h serum-starved lymphoblastoid Jurkat T cells rapidly stimulates the catalytic activity of phosphatidylinositol 3-kinase (PI 3-K). The peak of activation was observed 30 min after Tat addition. Extracellular Tat also stimulated the catalytic activity of the Akt/PKB kinase, a major target of PI 3-K lipid products. Pretreatment of serum-starved Jurkat cells with 100 nM wortmannin (WT) or 10 μM LY294002, two unrelated pharmacological inhibitors of PI 3-K, markedly suppressed the catalytic activity of both PI 3-K and Akt/PKB in Jurkat cells. Moreover, at low concentrations (0.1–1 nM), extracellular Tat showed a small but reproducible protection of Jurkat cells from apoptosis induced by serum deprivation (p < 0.05), while the combination of Tat plus 100 nM WT significantly (p < 0.05) increased the percentage of apoptosis with respect to cells left untreated or treated with Tat alone. Taken together, these data suggest that the anti-apoptotic activity of low concentrations of Tat protein on Jurkat cells is mediated by a PI 3-kinase/Akt pathway.     

Inhibition of intra- and extra-cellular Tat function and HIV expression by pertussis toxin B-oligomer

HIV-1-transactivating factor Tat contributes to virus replication and to the onset of AIDS-associated pathologies by targeting different infected and uninfected cell types. We previously demonstrated that the B-oligomer of pertussis toxin (PTX-B) inhibits HIV infection and replication in primary T cells and macrophages and Tat-dependent HIV-1 long terminal repeat (LTR) transactivation inT lymphoid Jurkat cells. Here we demonstrate that PTX-B inhibits Tat-dependent NF-kappaB activation and HIV-1 LTR-transactivation in non-permissive epithelial HL3T1 cells in a phosphatidylinositol 3'-kinase-dependent way. PTX-B exerts its inhibition both when Tat is produced endogenously in transfected cells and in cells incubated with the extracellular Tat protein. In this latter case, PTX-B does not interfere with extracellular Tat uptake by cells. PTX-B inhibited also interleukin-8 secretion and virus expression stimulated in chronically infected U1 promonocytic cells by intra- and/or extracellular Tat. The genetically modified holotoxin PT-9 K/129G retains the capacity to inhibit Tat transactivating activity and HIV replication in both HIV-permissive and non-permissive cells. Inconclusion, PTX-B acts as a "pleiotropic" inhibitor of Tat, and this may significantly contribute to the broad spectrum of anti-HIV-1 effects exerted by PTX-B in different cell types, and suggests PTX-B and its derivatives as prototypic for the development of anti-Tat drugs.     

Expression and function of a CD5 cDNA in human and murine T cells

In order to define the function of the CD 5 (T1, Leu 1, Tp 67 in the human; Ly-1 in the mouse) molecule, a cDNA clone of human CD 5 was expressed in a CD5-deficient Jurkat cell line and in a murine T cell hybridoma. A Jurkat subclone (Jurkat 9.9) produced interleukin 2 (IL 2) in response to anti-CD 3 monoclonal antibody (mAb) cross-linked to solid support. IL 2 production was enhanced by co-culture with the anti-CD 5 mAb OKT 1. A CD 5-deficient mutant clone Jurkat 1.15 was generated by treatment with ethyl methanesulfonate followed by selection with anti-CD 5 mAb plus complement. Jurkat 1.15 did not demonstrate enhancement of IL 2 production by OKT 1 in the presence of cross-linked anti-CD 3 mAb. A cDNA encoding human CD 5 was introduced into a defective retrovirus which was used to infect Jurkat 1.15. A Jurkat clone stably expressing CD 5 was established. In response to OKT 1, a rise in intracellular calcium was observed in both the parent Jurkat 9.9 and the CD 5+ infectant but not in the CD 5- mutant or a G 418- resistant control. Furthermore, expression of CD 5 restored the augmentation of IL 2 production by OKT 1 in response to cross-linked anti-CD 3 mAb. A murine T cell hybridoma By 155.16 which produces IL 2 in response to HLA-DR antigens was also infected with the CD 5-recombinant retrovirus and three stable CD 5+ infectants were generated. These hybridomas showed enhancement of IL 2 production by stimulation with OKT 1 in the presence of suboptimal concentrations of soluble anti-murine CD 3 mAb. These results provide further evidence that CD 5 provides a co-stimulatory signal for T cell activation.     

CD59锚定蛋白对CD3介导Jurkat细胞活化信号转导中的增强效应

目的用前期构建好的CD59pSUPER-RNAi质粒转染Jurkat细胞,探讨CD59特异性沉默前后CD59对CD3介导Jurkat细胞活化信号转导的相关作用。方法采用Lipofectamine2000转染Jurkat细胞,经G418筛选建立稳定转染细胞系,利用RT-PCR检测转染后CD59在基因水平的表达抑制效果。实验分为未转染的Jurkat细胞组(A组),转染空质粒pSUPER组(B组)和转染CD59pSUPER-RNAi质粒的Jurkat细胞组(C组),用噻唑蓝(MTT)比色法,Western blot技术和激光共聚焦扫描显微镜分别检测交联CD59单抗与固相化CD3单抗联合作用对3组Jurkat细胞的增殖效应,ZAP-70酪氨酸磷酸化的水平及细胞浆内钙离子的变化。结果重组载体转染后,经G418筛选得到稳定转染细胞系,RT-PCR结果表明转染CD59pSUPER-RNAi质粒的Jurkat细胞组CD59分子的表达受到抑制。转染干扰质粒Jurkat细胞组CD59与CD3联合作用后细胞增殖能力,ZAP-70酪氨酸磷酸化水平及钙离子浓度均明显低于未转染组和转染空质粒组(P<0.05);但未转染组和转染空质粒组之间无差异。结论 CD59对CD3介导T细胞活化信号转导有增强效应。     

Inhibition of the protein kinase C pathway promotes anti-CD95-induced apoptosis in Jurkat T cells

The role of the basal activity of the serine/threonine protein kinase, protein kinase C (PKC) in the regulation of anti-CD95-induced apoptosis in Jurkat T cells was investigated. The PKC-specific inhibitor GF 109203X and the proposed cPKC-specific inhibitor Go 6976, in a concentration-dependent manner, increased the percentage of cells undergoing apoptosis induced by anti-CD95 mAb as demonstrated by propidium iodide (PI) staining, TUNEL assay and DNA fragmentation by gel electrophoresis. Furthermore, Go 6976 and GF 109203X abrogated phorbol myristate acetate-induced inhibition of anti-CD95-induced apoptosis. To examine the molecular mechanism by which PKC modulates anti-CD95-induced apoptosis, the effects of Go 6976 on known effector and regulatory molecules of cell death were studied. Increased recruitment of cells undergoing apoptosis was associated with enhanced anti-CD95-induced proteolytic cleavage of the most receptor-proximal cysteine protease caspase-8, subsequent cleavage and activation of the machinery protease caspase-3, and cleavage of the caspase substrates DNA-dependent protein kinase catalytic subunit, poly-(ADP-ribose) polymerase and lamin B1. CD95 and FADD protein levels in Jurkat T cells were not altered by Go 6976 treatment. In addition, Go 6976 did not alter protein levels and subcellular distribution of the anti-apoptotic molecules Bcl-2 and Bcl-xL. These data suggest indirectly that basal PKC activity acts at an early stage in the anti-CD95-induced caspase pathway to attenuate subsequent activation of downstream effector molecules and associated apoptosis in Jurkat T cells.      

HIV-1 Tat increases the adhesion of monocytes and T-cells to the endothelium in vitro and in vivo: implications for AIDS-associated vasculopathy

HIV-1-infected patients exhibit severe damages of the aortic endothelium, develop angioproliferative lesions such as Kaposi's sarcoma (KS), and have an increased risk of cardiovascular diseases and atherosclerosis. An increased adhesion of leukocytes to the endothelium is a common pathogenic parameter of AIDS-associated vascular diseases. Here we show that the HIV-1 Tat protein, a regulatory protein of HIV-1 released by infected cells, and TNF-alpha, a cytokine increased in sera and tissues of HIV-1-infected patients, activate synergistically the adhesion of leukocytes to endothelial cells both in vitro and in vivo. This effect is selectively mediated by HIV-1 Tat, since HIV-1 Nef, another HIV-1 regulatory protein, and the HIV-1 envelope protein gp41, had no effect. In vitro adhesion assays with PBMC and quantitative cell type analysis of adherent cells by FACS demonstrated that HIV-1 Tat selectively activates the adhesion of T-cells and monocytes but not of B-cells. Intravital microscopic studies in mice confirmed the synergistic activity of HIV-1 Tat and TNF-alpha on leukocyte adhesion to the endothelium in vivo. These data indicate that HIV-1 Tat in cooperation with TNF-alpha may contribute to the vascular damage and cardiovascular diseases observed in AIDS patients but also to the prominent extravasation of T-cells and monocytes which is a key process in the formation and progression of KS lesions.     

Expression of RB2/p130 tumor-suppressor gene in AIDS-related non-Hodgkin's lymphomas: implications for disease pathogenesis

In this study we examined 21 cases of AIDS-related lymphomas for genomic organization and expression of RB2/p130 oncosuppressor gene and compared the results with the proliferative features of these neoplasms. We found no mutations in the RB2/p130 gene and unusually high percentages of cells expressing nuclear pRb2/p130 in tumors with a high proliferative activity, such as AIDS-related lymphomas. These findings might suggest that a molecular mechanism usually observed in viral-linked oncogenesis could be involved. We performed in vitro and in vivo binding assays to investigate whether the human immunodeficiency virus (HIV) gene product Tat and Rb2/p130 could interact. The results of these assays revealed that the HIV-1 Tat protein binds specifically to pRb2/p130. This may result in the inactivation of its oncosuppressive properties and the induction of genes needed to proceed through the cell cycle including p107, cyclin A, and cyclin B. Using single-cell polymerase chain reaction (PCR) assay, we found HIV-1 DNA in the neoplastic cells of only 2 of the 21 cases examined, whereas PCR on whole tissue revealed HIV-1 DNA in all of the cases. Furthermore, a diffuse and nuclear stain was observed in tissue sections with anti-Tat monoclonal antibody. These findings are in accordance with the notion that soluble Tat protein could function as a biologically active extracellular protein released by infected cells and taken up readily by uninfected B cells. In conclusion, our results seem to suggest that pRb2/p130 oncosuppressor protein may be a target in the interaction between the HIV-1 gene products and host proteins.     

Human immunodeficiency virus type-1 Tat protein induces secretory leukocyte protease inhibitor expression in African green monkey but not human cells

African monkeys are resistant to HIV-1 infection due to intrinsic restriction mechanisms found in their cells. However, although they can be infected by monkey-adapted modified HIV-1 particles that are designed to overcome known restriction factors, virus numbers drop to undetectable levels in immunocompetent animals. These results indicate the possibility of the presence of yet unidentified factor(s) that restrict HIV-1 in old-world monkey (OWM) cells after integration of the viral genome into the host cell chromosome. In the light of these findings, we hypothesized that OWMs might have evolved resistance mechanism(s) against HIV-1 by switching specific gene(s) on in response to the synthesis of viral proteins in infected cells. In an attempt to mimic post-infection status, we expressed HIV-1 Tat gene in African green monkey cells and compared the whole proteome with normal cells and identified secretory leukocyte protease inhibitor (SLPI), a protein with known extracellular anti-HIV-1 activity, as an over-expressed protein in the presence of HIV-1 Tat protein by 2D-PAGE and mass spectrometry analysis. We also showed that overexpression of SLPI in the presence of HIV-1 Tat was specific to monkey cells. Our results also suggest that SLPI had a previously undiscovered intracellular anti-HIV activity in addition to its extracellular activity.

     

G1/S cell cycle arrest provoked in human T cells by antibody to CD26

CD26 is T cell costimulatory molecule with dipeptidyl peptidase IV (DPPIV) enzyme activity located in its extracellular region. The expression of CD26 is enhanced after activation of T cells, while it is preferentially expressed on a subset of CD4+ memory T cells in the resting state. In this paper, we demonstrate that binding of the soluble anti-CD26 monoclonal antibody (mAb) 1F7 inhibits human T-cell growth and proliferation in both CD26-transfected Jurkat T-cell lines and human T-cell clones by inducing G1/S arrest, which is associated with enhancement of p21Cip1 expression. This effect depends on the DPPIV enzyme activity of the CD26 molecule. Moreover, we show that expression of p21Cip1 after treatment with the anti-CD26 mAb 1F7 appears to be induced through activation of extracellular signal-regulated kinase (ERK) pathway. These data thus suggest that anti-CD26 treatment may have potential use in the clinical setting involving activated T cell dysregulation, including autoimmune disorders and graft-vs.-host disease.     

Proliferative response of human CD4+ T lymphocytes stimulated by the lectin jacalin

The Galβ(1–3)GalNAc-binding lectin jacalin is known to specifically induce the proliferation of human CD4⁺ T lymphocytes in the presence of autologous monocytes and to interact with the CD4 molecule and block HIV-1 infection of CD4⁺ cells. We further show that jacalin-induced proliferation is characterized by an unusual pattern of T cell activation and cytokine production by human peripheral blood mononuclear cells (PBMC). A cognate interaction between T cells and monocytes was critical for jacalin-induced proliferation, and human recombinant interleukin (IL)-1 and IL-6 did not replace the co-stimulatory activity of monocytes. Blocking studies using monoclonal antibodies (mAb) point out the possible importance of two molecular pathways of interaction, the CD2/LFA-3 and LFA-1/ICAM-1 pathways. One out of two anti-CD4 mAb abolished jacalin responsiveness. Jacalin induced interferon-γ and high IL-6 secretion, mostly by monocytes, and no detectable IL-2 synthesis or secretion by PBMC. In contrast, jacalin-stimulated Jurkat T cells secreted IL-2. CD3^? Jurkat cell variants failed to secrete IL-2, suggesting the involvement of the T cell receptor/CD3 complex pathway in jacalin signaling. IL-2 secretion by CD4^? Jurkat variant cells was delayed and lowered. In addition to CD4, jacalin interacts with the CD5 molecule. Jacalin-CD4 interaction and the proliferation of PBMC, as well as IL-2 secretion by Jurkat cells were inhibited by specific jacalin-competitive sugars.     

Both CD28 ligands CD80 (B7-1) and CD86 (B7-2) activate phosphatidylinositol 3-kinase, and wortmannin reveals heterogeneity in the regulation of T cell IL-2 secretion

In this report, the co-stimulatory signals provided by CD80(B7-1) or CD86 (B7-2) were compared to CD28 ligation by mAb.We demonstrate that while both anti-CD3 and anti-CD28 antibodiesinduced activation of phospholnositide (PI) 3-kinase, the kineticsof activation differed. Anti-CD28 produced a sustained activationof PI 3-kinase while anti-CD3 induced activation was transient.Both B7-1 and B7-2 could induce prolonged activation of PI 3-kinase.The co-stimulatory effects of B7-1 and B7-2 were dependent onCD28 cross-linking, based on complete inhibition of PI 3-kinaseactivation by CD28 antibody Fab fragments. While Jurkat T cellsco-stimulated with anti-CD3 and B7-1 or B7-2 secreted high levelsof IL-2, there were distinct effects of anti-CD28 mAb and B7-1or B7-2 on IL-2 secretion in conjunction with protein kinaseC activation. To assess functional effects of CD28 ligation,pharmacologic inhibitors of PI 3-kinase were evaluated. In Jurkatcells, efficient inhibition of PI 3-kinase activation afterB7-2 stimulation was achieved using wortmannin; however, weobserved a surprising increase in IL-2 secretion after B7 oranti-CD28 stimulation. The effect of wortmannin was concentrationdependent. Moreover, the effect was specific for receptor-mediatedactivation as wortmannin did not enhance phorbol ester pluslonomycin-induced IL-2 secretion. Another inhibitor of PI 3-kinase,LY294002, also resulted in augmentation of anti-CD28-inducedIL-2 secretion by Jurkat cells. The effects of wortmannin onIL-2 secretion were also examined in primary T cells. In markedcontrast, wortmannin resulted in a potent inhibition of anti-CD3plus B7-1 or anti-CD28-induced IL-2 secretion while phorbolester plus lonomycin-induced IL-2 secretion was wortmannin resistant.Together these observations demonstrate that signal transductionby both B7-1 and B7-2 involves PI 3-kinase, and that PI 3-kinaseor other wortmannin-sensitive targets are important for IL-2secretion. Finally, treatment of Jurkat cells with PI 3-kinaseinhibitors alone was sufficient to induce low levels of IL-2secretion. This is consistent with the notion that a wortmannin-sensitivetarget such as PI 3-kinase may down-regulate IL-2 secretionin Jurkat cells.     

PD-L2:PD-1 involvement in T cell proliferation, cytokine production, and integrin-mediated adhesion

The B7 family member programmed death ligand 2 (PD-L2) has been implicated in both positive and negative regulation of T cell activity. In this study, we demonstrate that on human T cells, PD-L2 acts only as a negative regulator of T cell activity, inhibiting proliferation, IL-2 production, and IFN-gamma production via its interaction with programmed death-1 (PD-1). This study also shows a novel role for PD-1 in inhibiting beta1 and beta2 integrin-mediated adhesion. PD-L2 inhibition of T cell function involves modulation of the phosphoinositide 3-OH kinase (PI 3-K)/AKT and extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) pathways, with PD-L2 inhibiting anti-CD3-induced AKT phosphorylation within minutes and ERK phosphorylation after hours. Analysis of phosphatase activity of Src homology 2 domain-containing tyrosine phosphatase (SHP)-1 and SHP-2 in response to anti-CD3 mAb or anti-CD3 mAb + PD-L2 stimulation revealed that while SHP-1 phosphatase activity is not affected by stimulation, SHP-2 phosphatase activity is significantly increased by anti-CD3 mAb + PD-L2 stimulation. Anti-CD3 mAb + PD-L2 stimulation also increased the level of SHP-2 associated with the PD-1 receptor. These results suggest that catalytically active SHP-2 associated with the PD-1 receptor is involved in modulating T cell function.     

Positive signal transduction via surface CD4 molecules does not need coexpression of the CD3/TcR complex.

We previously reported that the human CD4 molecule is capable of transducing a positive signal when activated by an anti-CD4 mAb B66. This antibody, in contrast to many other anti-CD4 mAb, induced IL2 production and proliferation of resting CD4+ peripheral blood T lymphocytes in the absence of any other signal. We further reported that anti-CD4 mAb B66 was able to induce IL2 production in murine T-cell hybridoma cells transfected with full-length human CD4 cDNA. In the present study, we extend these findings by demonstrating that anti-CD4 mAb B66 was able to induce Ca2+ mobilization and IL2 production in a CD3/TcR- variant 31-13, of the CD3/TcR+ Jurkat cell line. We further showed that anti-CD4 mAb B66 was able to activate CD4+ cells from the promonocytic cell line U937. In these cells, mAb B66 induced Ca2+ mobilization when cross-linked with a second antibody and, in addition, the production of large quantities of IL1 beta was measured. In essence, our findings provide direct evidence that cross-linking of CD4 may cause T-cell activation in the absence of the coexpression of the CD3/TcR molecular complex and that, in addition, CD4 might transduce a positive signal in CD4+ cells of the myeloid lineage.     

Residues Y429 and Y463 of the human CD5 are targeted by protein tyrosine kinases

The human CD5 lymphocyte cell surface co-receptor modulates activation and differentiation responses mediated by the antigen-specific receptor of T and B cells. CD5 is phosphorylated following lymphocyte activation; however, the exact sites and kinases involved are yet to be determined. Jurkat T cell transfectants expressing tyrosine-mutated CD5 molecules have been used to show that residues Y429 and Y463 are targeted in vivo by protein tyrosine kinases following cell stimulation with anti-CD3 mAb or pervanadate. This is in agreement with data from direct in vitro kinase assays using purified recombinant Lck and Fyn protein tyrosine kinases. The analysis of Lck- and CD3-deficient Jurkat cells shows that tyrosine phosphorylation of CD5 requires Lck activity. We propose that T cell activation mediates CD5 tyrosine phosphorylation at residues Y429 and Y463 mainly through the activation of Lck.     

CD46 RNAi对CD59介导的T细胞信号转导的作用研究

目的构建携带针对CD46基因的pSUPER retro RNAi逆转录病毒载体,研究糖基磷脂酰肌醇(GPI)锚定蛋白CD59与CD46在介导T细胞信号转导中的相关性。方法将能转录产生靶向CD46小发夹RNA(shRNA)的寡核苷酸序列,克隆入逆转录病毒载体pSUPER retro,转化大肠杆菌JM109并转染Jurkat细胞。将Jurkat细胞分为未转染的Jurkat细胞组(Ⅰ组)、转染空质粒的Jurkat细胞组(Ⅱ组)、转染CD59干扰质粒的Jurkat细胞组(Ⅲ组)及转染CD46干扰质粒的Jurkat细胞组(Ⅳ组)。用RT-PCR、Western blot技术检测各组细胞中的CD59、CD46基因的表达水平。用噻唑蓝(MTT)比色法检测CD46与CD59联合作用对4组Jurkat细胞的增殖效应。结果重组载体经PCR及限制性内切酶酶切鉴定初步成功后送测序,结果表明序列正确,构建成功,稳定转染后,Ⅳ组细胞CD46分子的表达被成功抑制,Ⅲ组细胞CD59分子的表达被抑制。Ⅰ组和Ⅱ组细胞CD46与CD59单抗联合作用后,增殖能力明显高于Ⅲ组、Ⅳ组(P<0.05);但Ⅰ组和Ⅱ组,Ⅲ组和Ⅳ组之间无差异。结论 CD59可增强CD46对T细胞信号转导的效应。     

CD59锚定蛋白对CD55介导的T细胞信号转导的增强效应

目的:研究糖基磷脂酰肌醇(GPI)锚定蛋白CD59对CD55介导T细胞信号转导的增强效应。方法:实验分为未转染的Jurkat细胞组(Ⅰ组)、转染空质粒的Jurkat细胞组(Ⅱ组)及转染CD59干扰质粒的Jurkat细胞组(Ⅲ组)。用RT-PCR检测3组细胞中的CD59基因表达水平。用噻唑蓝(MTT)比色法、免疫印迹技术和激光共聚焦扫描显微镜分别检测CD55与CD59联合作用对3组Jurkat细胞的增殖效应,Src家族酪氨酸激酶(SrcPTK)磷酸化的水平及细胞内钙离子的变化。结果:稳定转染后,Ⅲ组细胞CD59分子的表达被成功抑制。Ⅰ组和Ⅱ组细胞CD55与CD59联合作用后增殖能力,SrcPTK磷酸化水平及钙离子浓度均明显高于Ⅲ组(P<0.05);但Ⅰ组和Ⅱ组之间无差异。结论:CD59可增强CD55对T细胞信号转导的效应。     

Synergistic neurotoxicity of opioids and human immunodeficiency virus-1 Tat protein in striatal neurons in vitro

Human immunodeficiency virus (HIV) infection selectively targets the striatum, a region rich in opioid receptor-expressing neural cells, resulting in gliosis and neuronal losses. Opioids can be neuroprotective or can promote neurodegeneration. To determine whether opioids modify the response of neurons to human immunodeficiency virus type 1 (HIV-1) Tat protein-induced neurotoxicity, neural cell cultures from mouse striatum were initially characterized for mu and/or kappa opioid receptor immunoreactivity. These cultures were continuously treated with morphine, the opioid antagonist naloxone, and/or HIV-1 Tat (1-72) protein, a non-neurotoxic HIV-1 Tat deletion mutant (TatDelta31-61) protein, or immunoneutralized HIV-1 Tat (1-72) protein. Neuronal and astrocyte viability was examined by ethidium monoazide exclusion, and by apoptotic changes in nuclear heterochromatin using Hoechst 33342. Morphine (10nM, 100nM or 1microM) significantly increased Tat-induced (100 or 200nM) neuronal losses by about two-fold at 24h following exposure. The synergistic effects of morphine and Tat were prevented by naloxone (3microM), indicating the involvement of opioid receptors. Furthermore, morphine was not toxic when combined with mutant Tat or immunoneutralized Tat. Neuronal losses were accompanied by chromatin condensation and pyknosis. Astrocyte viability was unaffected. These findings demonstrate that acute opioid exposure can exacerbate the neurodegenerative effect of HIV-1 Tat protein in striatal neurons, and infer a means by which opioids may hasten the progression of HIV-associated dementia.