Increased frequency of immunoglobulin (Ig)A-secreting cells following Toll-like receptor (TLR)-9 engagement in patients with Kawasaki disease.

Kawasaki disease (KD) is an acute vasculitis affecting mainly infants and children. Human B cells express Toll-like receptor (TLR)-9, whose natural ligands are unmethylated cytosine-guanine dinucleotide (CpG) motifs characteristic of bacterial DNA. The aim of this study was to clarify the pathogenesis of KD analysing the activation status of peripheral blood mononuclear cells (PBMC), focusing on B lymphocyte activation and functions. Ten patients and 10 age-matched healthy donors were recruited from the Bambino Gesù Hospital of Rome, Italy and enrolled into this study. We determined phenotype profile and immunoglobulin (Ig) production of PBMC from KD patients and age-matched controls. We found that the frequency of CD19(+) B lymphocytes and CD19(+) /CD86(+) activated B lymphocytes from KD patients during the acute phase before therapy was increased significantly. Moreover, B lymphocytes of acute-phase KD patients were more prone to CpG oligodeoxynucleotide (ODN) activation compared with the age-matched controls, as assessed by a significant increase of the number of IgA-secreting cells (SC). In the same patients we found a marked increase of IgM, IgG, interleukin (IL)-6 and tumour necrosis factor (TNF)-α production compared with the control group. In addition, in two convalescent KD patients, conventional treatment with intravenous immunoglobulin (IVIG) restored the normal frequency of CD19(+) B cells, the number of IgA-, IgM- and IgG-SC and the production of IL-6 and TNF-α. Our findings indicate that the percentages of peripheral B lymphocytes of acute-phase KD patients are increased and are prone to bacterial activation in terms of increased numbers of IgA-SC and increased production of IL-6 and TNF-α inflammatory cytokines. Thus, our data support the hypothesis of an infectious triggering in KD.

IFN-alpha amplifies human naive B cell TLR-9-mediated activation and Ig production.

TLRs are a family of molecules that function as sensors for the detection of pathogens. TLR-9, expressed on B cells and pDCs, recognizes CpG motifs of unmethylated bacterial DNA and plays a role in the development of autoimmunity. The present study was designed to investigate the effects of IFN-alpha in combination with CpG ODN on the activation of CD27(-) naïve B cells and on Ig production. We provide evidence that CpG ODN not only induces a total and T-dependent, specific IgM response by naïve B cells but also their phenotypic differentiation in plasma cells, as demonstrated by the up-regulation of CD38 expression. We found that TLR-9 stimulation with CpG ODN induces IL-1beta, TNF-alpha, IL-10, and IL-6 production. Interestingly, we also found that CpG ODN induces naïve B cell maturation into memory cells, as demonstrated by the induction of CD27, AID mRNA expression, and IgG production. More importantly, our results demonstrate that IFN-alpha amplifies the inductive effect of CpG ODN on naïve B activation and on Ig production through a mechanism involving TLR-9/MyD88-dependent signaling. Moreover, we found that IFN-alpha enhances the frequency of CpG ODN-induced memory B cells. Our results may contribute to clarify the events promoting IFN-alpha-induced amplification of naïve B cell activation via TLR-9 for a better understanding of the pathogenesis of autoimmune disorders and may guide treatments targeting this pathway within B cells.

The Rac-activating toxin cytotoxic necrotizing factor 1 oversees NK cell-mediated activity by regulating the actin/microtubule interplay.

The cell cytoskeleton is widely acknowledged as a master for NK cell function. Specifically, actin filaments guide the NK cell binding to target cells, engendering the formation of the so-called immunological synapse, while microtubules direct the killer behavior. All these cytoskeleton-dependent activities are competently governed by the Rho GTPases, a family of regulatory molecules encompassing the three different subfamilies, Rho, Rac, and Cdc42. By using a Rac GTPase-activating bacterial protein toxin from Escherichia coli named cytotoxic necrotizing factor 1 (CNF1), we obtained results supporting the activation of Rac GTPase as a booster for effector cell-binding efficiency, recruitment ability, and, consequently, cytotoxicity. In particular, the augmented killer capacity of CNF1-treated NK cells was associated with the increased expression of certain cell adhesion or activation-associated molecules and the reshaping of the actin and microtubule networks. Importantly, CNF1 counteracted the activity exerted by toxins disrupting the cytoskeletal architecture. Hence, the activation of Rho GTPases, particularly Rac, induced by CNF1, appears to orchestrate a dynamic cross talk between microtubules and actin filaments, leading to a fruitful NK cell activity and polarization state. Our findings suggest that protein toxins might be viewed as modulators of NK cell cytotoxic activity and could possibly be regarded as useful pharmacological tools for certain Rho-linked immune diseases in the near future.

N-acetylcysteine inhibits the induction of an antigen-specific antibody response down-regulating CD40 and CD27 co-stimulatory molecules.

We investigated the effect of N-acetylcysteine (NAC) on normal human B cell functions. We found that NAC significantly inhibited both the induction of the specific antibody response to the T-dependent antigen Candida albicans and T-dependent pokeweed mitogen (PWM)-induced polyclonal Ig production. NAC did not induce either cell death due to a non-specific toxicity or apoptosis. The NAC-induced inhibitory effect might be a functional consequence of: (i) a down-regulation of the expression on the B cell surface of CD40 and CD27 co-stimulatory molecules and (ii) a down-regulation of interleukin (IL-4) production. In contrast, NAC up-regulated interferon-gamma (IFN-gamma) production. NAC did not induce any effect on the T cell-independent B cell polyclonal activation system. These results indicate that NAC down-regulates T dependent B cell activation and leads to T helper cell type 1 (Th1) polarization.

Redox imbalance and immune functions: opposite effects of oxidized low-density lipoproteins and N-acetylcysteine.

This study investigates the in vitro effects of oxidized low-density lipoproteins (ox-LDL), 'physiological' pro-oxidants, N-acetylcysteine (NAC), a free radical scavenger and glutathione precursor, and their combination on human peripheral blood mononuclear cell functions. We found that treatment with ox-LDL induced a significant down-regulation of proliferative response to mitogens, antigens and interleukin-2. Lipid extracts from ox-LDL were able to reproduce the same effect as the lipoprotein. On the other hand, NAC exposure induced a significant up-regulation of proliferative responses to all the stimuli used. Moreover, we showed that natural killer (NK) cell-mediated cytotoxic activity was significantly down-regulated by ox-LDL while treatment with NAC induced a significant up-regulation of NK-cell activity. Finally, we found that ox-LDL and NAC exerted opposite effects on the cytokine network, interfering both at the protein secretion level and the messenger RNA synthesis level. More importantly, when NAC was used in combination with ox-LDL the proliferative responses, NK-cell-mediated cytotoxic activity and cytokine production were restored to values comparable to controls. These data indicate that ox-LDL and NAC modulate immune functions, exerting opposite effects reflecting their pro-oxidant and antioxidant behaviours. Our results add new insights to the key role played by redox imbalance as a modulator of immune system homeostasis and suggest that an antioxidant drug such as NAC could be useful against pathologies associated with an increase in lipid peroxidation.

HIV-1 Nef protein inhibits the in vitro induction of a specific antibody response to Candida albicans by an early up-regulation of IL-15 production.

We have previously demonstrated that exogenous Nef protein induced activation of normal human T cells up-regulating IL-15 production by monocytes. Since HIV-1 infection results in the early impairment of immune functions we decided to evaluate if Nef is able to modulate the induction of a specific antibody response. Human peripheral blood mononuclear cells from healthy donors were induced in vitro to mount a specific antibody response to the Candida albicans antigen. We show that Nef inhibited, in a dose-dependent manner, the induction of the anti-C. albicans antibody response. The ability of an anti-Nef antibody to prevent such inhibition indicates that the effect was indeed Nef-specific. In the Nef-treated cultures an early increase of IL-15 production was observed and the addition of anti-IL-15 antibody abrogated the Nef-induced inhibitory effect. Moreover the addition of IL-15 to the cultures inhibited, as well as Nef, the induction of the specific antibody response. Thus, our results suggest that Nef may inhibit the induction of a specific antibody response by an early up-regulation of IL-15 production. A better comprehension of this phenomenon may be important for unravelling some aspects of the B cell defects in HIV infection.

Subcellular alterations induced by UV-oxidized low-density lipoproteins in epithelial cells can be counteracted by alpha-tocopherol.

Oxidized LDL (ox-LDL) have been involved in the pathogenesis of several human diseases including dermatological pathologies. Oxidative modification of low-density lipoproteins (LDL) is accompanied by both extensive degradation of its polyunsaturated fatty acids and production of lipoperoxides. These highly reactive products induce an intracellular oxidative stress with a variety of cytotoxic effects. In order to evaluate cellular damage induced by oxidative stress in epidermal cells, a human epidermoid carcinoma cell line in culture (A 431) was used as experimental model. Cell treatment with UV-oxidized LDL resulted in cytostatic and cytotoxic effects characterized by morphological and functional alterations: inhibition of cell proliferation, modifications of cytoskeleton network, microtubular derangement, loss of cell-cell and cell-substrate contacts, cell detachment and cell death by apoptosis. The ox-LDL-induced alterations were almost completely prevented by pre-incubating cells with alpha-tocopherol. The results presented here could be of relevance for a better comprehension of the pathogenic mechanisms of several human diseases, including dermatological pathologies, and could indicate that antioxidants such as alpha-tocopherol could represent an important therapeutic challenge in the maintenance of cell and tissue homeostasis in the long run.

Short synthetic peptides derived from viral proteins compete with HIV gp120 for the binding to CD4 receptors.

In the complex mechanism of adhesion, internalization, and infection of cells by human immunodeficiency virus (HIV) viral particles, a determinant role is played by the viral envelope glycoprotein gp120, which binds to CD4 receptors of T cells and monocytes. We tested the ability of a panel of 7- to 12-residue synthetic peptides, selected from the region 414-434 of the HIV-1 gp120, to inhibit the binding of the viral protein to CD4 receptors of cultured human lymphoid cells. The assay was based on the observation that the binding of gp120 to the receptors interferes with the binding of a specific anti-CD4 monoclonal antibody, as a result of the masking of the antibody epitope; thus, we tested whether preincubation of cells with the peptides before gp120 addition might restore the recognition of the CD4 molecule by the antibody. High expression of CD4 receptors was thus assumed as indication that the binding of the viral protein had been inhibited. Maximum activity was displayed by a 9-residue peptide located near the amino terminal end of the 414-434 fragment. In addition, several fragments deduced from other viral proteins, possessing partial amino acid sequence homology with the HIV gp120 fragment, exhibited a similar type of interaction with the CD4 receptor. All active peptides contain the Cys residue (position 423 of gp120). This residue is essential, although not sufficient, for inhibiting gp120 binding, as few other amino acid residues within the fragment play a complementary role in increasing or decreasing the inhibitory ability.

Induction of interleukin-15 production by HIV-1 nef protein: a role in the proliferation of uninfected cells.

Several recent reports have provided evidence that Nef enhances human immunodeficiency virus HIV infectivity, and in vitro experiments with the nef gene have demonstrated the possible role of Nef in modulating immune responses. Exogenous Nef has been demonstrated to induce proliferation of normal human peripheral blood mononuclear cells (PBMC) and to enhance HIV-1 replication. The aim of this study was to evaluate the biological mechanisms by which Nef, used as exogenous protein, modulates cellular activation. We showed that exogenous Nef protein induces the proliferation of unstimulated and suboptimally stimulated normal human PBMC, while it has no effect on the proliferation of optimally stimulated PBMC. Moreover, the activating effect of exogenous Nef on PBMC proliferation was associated with an increase of IFN-gamma, TNF-alpha, and IL-6 production, while, surprisingly, IL-2 production was not affected by Nef. More importantly we showed, for the first time, that Nef exerts its activating effects on PBMC proliferation through IL-15 synthesis induction by monocyte/macrophage population. In conclusion, we found that exogenous Nef protein (i) induces activation of normal PBMC, increasing their proliferative response; (ii) modulates cytokine production; (iii) exerts its activating effects through IL-15 synthesis induction; and (iv) exerts these effects entering monocyte/macrophages. Our results might suggest that Nef enhances the rate of viral replication by a novel mechanism involving the production of IL-15.

Human intestinal lamina propria lymphocytes are naturally permissive to HIV-1 infection.

The presence of HIV-1 in the intestinal mucosa of AIDS patients has been reported and human intestinal lamina propria lymphocytes (LPL) have been proposed as important targets for HIV-1 infection. However, little information is available concerning the permissiveness of human intestinal CD4+ T lymphocytes to HIV-1 infection. Here, we show that human LPL, in contrast to autologous peripheral blood lymphocytes (PBL), are permissive to both X4 T-tropic and R5 M-tropic strains of HIV-1, as well as to clinical isolates, in the absence of exogenous stimuli. Flow cytometry showed that the vast majority of T LPL were CD45RO+ and CD69+, and that CD4+ T LPL highly expressed CC chemokine receptor 5 (CCR5) as compared to PBL, while CX chemokine receptor 4 was equally expressed on LPL and PBL. Exogenous RANTES and macrophage inflammatory protein-1alpha (natural CCR5 ligands) virtually abolished the entry of the R5 M-tropic strain HIV-1 into human LPL. Thus, we infer that human intestinal CD4+ T lymphocytes are naturally susceptible to HIV-1 infection, due to their physiological state of activation and to marked expression of HIV-1 coreceptors, independently of the route of primary (either mucosal or parental) infection and the shifts of the virus phenotype occurring during the course of AIDS.

Lack of immunotoxicity of saquinavir (Ro 31-8959) used alone or in double or triple combination with AZT and ddC.

Saquinavir (Ro 31-8959; SQV) has been demonstrated to be a potent inhibitor of human immunodeficiency virus (HIV) proteinases and acts synergistically with dideoxynucleoside analogues. The aim of this study was to investigate the in vitro immunomodulatory effects of SQV on normal human peripheral blood mononuclear cells (PBMC) and on lamina propria mononuclear cells (LPMC). We used the drug either alone or in double and triple combination with AZT and ddC to assess whether SQV enhances the immunomodulatory effects induced by AZT and ddC that we previously observed. We demonstrated that SQV did not induce any modulation of the proliferative response either in PBMC or in LPMC. Similarly, NK cell-mediated cytotoxic activity and cytokine production were not modified by SQV. More importantly, SQV/AZT, SQV/ddC, and SQV/AZT/ddC combinations did not strengthen neither the inhibition of PBMC and LPMC proliferative response or the modulation of cytokine production induced by AZT, ddC, and AZT/ddC. On the other hand, the increased IL-2 production induced by AZT and ddC was not observed adding SQV to the dideoxynucleoside analogues. In conclusion, we demonstrated that SQV used in combination with AZT and ddC did not add any further immunotoxicity.

HIV-1 gp120 accelerates Fas-mediated activation-induced human lamina propria T cell apoptosis.

Intestinal mucosa represents an important portal of entry of HIV and a site of virus reservoir and active replication. Recently, in HIV patients, an early depletion of intestinal lamina propria T lymphocytes (LPT) has been described. HIV-1 gp120 has been demonstrated to promote apoptosis in noninfected isolated peripheral blood T cells, therefore we investigated whether gpl20 modulates apoptosis of normal human intestinal lamina propria T cells. Purified T cells were obtained by immunomagnetic negative selection from human lamina propria mononuclear cells isolated from surgical specimens by enzymatic procedure. Cells were incubated with or without recombinant gpl20 (10 microg/ml) and cultured either in the absence of any stimulus or in the presence of plate-bound anti-CD3 Ab (OKT3) or soluble anti-CD2 Ab (T11(2) + T11[3]). Apoptosis was assessed by flow cytometric analysis after propidium iodide staining. We demonstrated that preincubation of normal LPT cells with HIV-1 gpl20 accelerates the apoptosis observed during CD2-pathway stimulation of LPT cells. This process is mediated by Fas/Fas ligand interaction and related to an increased induction of Fas ligand mRNA by gpl20. Therefore HIV-1 gp120 could contribute to the depletion of noninfected LPT cells inducing a premature cell death.

Oxidized low-density lipoproteins affect natural killer cell activity by impairing cytoskeleton function and altering the cytokine network.

Several lines of evidence indicate that oxidative imbalance can play an important role in determining an impairment of natural killer (NK) cell activity in a variety of human diseases. Because a specific role for oxidized low-density lipoproteins (LDL) as pro-oxidizing agents has been envisaged, we tested the activity of oxidized LDL (ox-LDL) on NK cell-mediated cytotoxicity, cytokine release, and membrane molecule modulation. Native LDL served as control. Treatment with ox-LDL at noncytotoxic concentrations (0.2 mg/ml) during the NK/target cell (TC) interaction markedly reduced NK cytotoxic activity against U937 tumor cells. This inhibitory activity was also noticed when NK cells were pretreated with ox-LDL. Scanning electron microscopy examination of NK-target cell conjugates failed to reveal any morphological cell damage. In addition, the number of conjugates and the expression of some adhesion molecules (CD11a, CD11b, CD18, CD2, and CD62L) were not modified by ox-LDL. These observations argued against a possible interference of ox-LDL with the binding process leading to the formation of NK/TC conjugates. By contrast, immunocytochemical analyses of cytoskeleton components of NK cells exposed to ox-LDL showed a partial depolymerization and a derangement of the microtubular apparatus. These alterations were accompanied by an evident decrease in their intracellular reduced glutathione content. Owing to the important role played by the microtubular network during the killing process, it is possible to infer that a cytoskeleton alteration underlies the inhibitory activity of ox-LDL on NK cell function. In addition, exposure of mitogen-stimulated peripheral blood mononuclear cells to ox-LDL markedly reduced specific mRNA transcription and release of cytokines relevant for NK cell activity (such as tumor necrosis factor-alpha, interferon gamma, and interleukin 12). These data suggest that the impairment of NK cell activity by ox-LDL likely reflects the concomitant dysregulation of some essential mechanisms of NK cell function.

Interference with cell cycle progression and induction of apoptosis by dideoxynucleoside analogs.

The in vitro effect of single or combined doses of zidovudine (AZT) and dideoxycytidine (ddC) on PHA-activated human peripheral blood mononuclear cells (PBMC) proliferative response and lymphoblastoid T cell line CEM cell growth was evaluated. Clinically relevant amounts (0.1, 1 and 10 microM) of AZT, ddC and AZT/ddC combination (10 + 10 microM) inhibited 3H TdR uptake in both cell models in a dose-dependent manner. The inhibitory effect on cell growth was confirmed by counting the amount of viable CEM cells recovered after 24, 48 and 72 h exposure to the drugs. On equimolar basis, ddC was considerably more efficient than AZT although the latter potentiates the activity of the former Flow cytometric analysis of PBMC and CEM cells exposed to the dideoxynucleosides revealed a decrease in the rate of DNA synthesis (rate of passage through the S phase of the cell cycle) and a reduced number of cell generations, the latter assessed by measuring the halving of the fluorescent probe 5-6-carboxyfluorescein diacetate succinimidyl ester by flow cytometry. The analysis of CEM cells recovered after exposure to ddC or AZT/ddC combination (10 + 10 microM), showed that in addition to perturbing cell cycle progression, ddC, and most efficiently the AZT/ddC combination, induced cell death by apoptosis. The latter was manifested as enhanced side scatter and decreased, sub-G1, DNA content by flow cytometry, and as DNA breakdown in nucleosomal fragments by gel electrophoresis. Present findings indicate that clinically relevant concentrations of dideoxynucleosides reduce cell growth by hampering DNA replication and inducing apoptosis.

Oxidized low density lipoproteins impair peripheral blood mononuclear cell proliferation and cytokine production.

Oxidized low density lipoproteins (ox-LDL) are known to behave as physiological pro-oxidants leading to the formation of intracellular reactive oxygen species. The presence of these altered lipoproteins in the human plasma has been associated with a number of morbid states, including atherosclerosis and immuno-deficiency. Common features of such pathological conditions seem to be represented by several alterations occurring in the immune system. In this work we analyze the in vitro effects of ox-LDL on both proliferative response and cytokine production of normal human peripheral blood mononuclear cells (PBMC). Our results indicate that ox-LDL significantly inhibit proliferative response and modulate cytokine network interfering both at protein secretion and mRNA synthesis level.

Apoptosis induction by human immunodeficiency virus type 1 (HIV-1) gp120 peptides.

We investigated the role of some gp120 peptides on the apoptosis induction in malignant T cell lines. We took advantage of recent findings reporting that three major regions of gp120 are important for CD4 binding. They consist of residues 256-262 in the C2 domain, residues 368-389 in the C3 domain, and residues 421-457 in C4 domain. We used a peptide from C2 domain (aa 250-263) the homologous major histocompatibility complex (MHC) class II peptide (aa 135-155) and three peptides from domain C4 (aa 414-434; 419-430; 428-445). We selected for this study the following human cell lines: CEM and Jurkat, two lymphoblastoid CD4-positive T cell line and U937, a myelomonocytic CD4 positive cell line. We demonstrated that the CD4-positive T cell lines, in the presence of gp120 250-263 peptide and DR 135-155 peptide, can be induced to accelerate apoptosis, while no effect in apoptosis induction was observed in the presence of 414-424 gp120 peptide. Interestingly, we have shown by fluorescence study, that the small sequence 414-419 must be responsible for the inhibition of binding of gp120 to the CD4 molecule. Indeed while 414-424 gp120 peptide is very efficient in CD4-gp120 binding inhibition, no effect is observed in the presence of either 419-430 or 428-445 peptide.

In vitro and in vivo immunomodulatory effects of anti-Pneumocystis carinii drugs.

The anti-Pneumocystis carinii drug effects on mitogen-, antigen-, and interleukin-2-induced proliferative responses and on natural killer (NK) cell-mediated activity were analyzed in vivo (rats) and in vitro (normal human peripheral blood mononuclear cells). Splenocytes derived from in vivo piritrexim- and clindamycin-treated rats showed a significant inhibition of mitogen-induced proliferative responses. In vitro exposure to clindamycin, piritrexim, and pyrimethamine caused an inhibition of human T lymphocyte proliferation in response to mitogen, antigen, and interleukin-2 stimulation. Rat NK cell-mediated cytotoxic activity was not affected by the drugs, and human NK cell activity was reduced only at the highest concentration (10 micrograms/ml) of the drugs. The potential immunotoxicity of the long-term administration of these agents in humans needs further investigation.

Cyclopiazonic acid and aflatoxins production byAspergillus flavus isolated from Argentinian corn.

Thirty-four isolates ofAspergillus flavus obtained from the main Argentinian corn production area were tested for their ability to produce both cyclopiazonic acid (CPA) on corn and on liquid media and aflatoxins on corn.Aflatoxins and CPA were quantified by comparison with standards. The last one was confirmed by mass spectrometry.All but one of the isolates produced CPA on liquid medium in a range between 3120 to 62500 µg/kg, 27/34 isolates produced CPA on corn at levels ranging from 833 to 10000 µg/kg and 5/34 isolates produced aflatoxin B1 in a range between 29 to 115 µg/kg. According to these findings, the percentage ofAspergillus flavus isolates with CPA production ability and their levels of CPA production were higher than the observed elsewhere.It was observed significant differences (p<0,01) between CPA production on corn (median: 1761 µg/Kg) and in liquid medium (median: 27950 µg/Kg). These data represent the first report of the co-production of CPA and aflatoxin B1 by isolates ofAspergillus flavus obtained from corn in Argentina.

N-acetyl-cysteine enhances cell adhesion properties of epithelial and lymphoid cells.

In this paper, we show that the antioxidant N-acetyl-cysteine (NAC) is capable of enhancing the adhesion properties of the epithelial cell line A431 and of the lymphocytic cells with cytotoxic activity from human peripheral blood: the natural killer (NK) cells. This effect leads to an increased efficiency of A431 cells to form a monolayer and of NK cells to kill their targets. In both cases a specific effect of NAC was found in the distribution of those molecules of the cytoskeleton which are generally involved in cell substrate and cell-to-cell contact region formation, e.g., the actin microfilaments. NAC could thus behave as a drug influencing certain cytoskeleton-dependent cell processes in a non-histotype dependent manner.

Down-regulation of interleukin-2 receptor gene activation and protein expression by dideoxynucleoside analogs.

The in vitro effect of single or combined doses of zidovudine (AZT) and dideoxycytidine (ddC) on the production and utilization of interleukin-2 (IL-2) by normal human peripheral blood mononuclear cells (PBMC) was evaluated by measuring IL-2 concentrations in supernatants from PHA-stimulated PBMC cultures. Drugs were added at the beginning of the culture period and left throughout. Whereas AZT alone (1 and 10 microM) caused only a slight increase, ddC alone (1 and 10 microM) and combined AZT/ddC (1 + 1 and 10 + 10 microM) caused a considerable increase. IL-2 gene expression in the drug-treated PBMC did not increase. This finding suggested that the increased supernatant IL-2 accumulations might be caused by a drug-induced down-regulation of the IL-2 receptor alpha (IL-2R alpha, CD25). AZT decreased IL-2R alpha expression, but only slightly. In contrast, ddC alone and combined AZT/ddC decreased the CD25 molecules in a marked and dose-dependent manner. They also markedly reduced IL-2R alpha gene expression. These findings show that the dideoxynucleoside drugs tested left PHA-induced IL-2 gene activation unchanged but decreased IL-2R alpha gene activation, thus down-regulating IL-2R alpha cell-surface protein expression.